MECHANISMS FOR THE REARRANGEMENTS OF ETHERS

[CONTRIBUTION FROM

THE

CHEMICAL LABORATORY OF NORTHWESTERN UNIVERSITY, EVANSTON, ILLINOIS]

MECHANISMS FOR THE REARRANGEMENTS OF ETHERS : 7-ETHYLALLYL PHENYL ETHER AND 7-ETHYLALLYL VINYL ETHER CHARLES D. HURD

AND

MAXWELL A. POLLACK

Received October 19, 1938

The type of thermal rearrangement represented by the change at about 200’ of phenyl allyl ether into o-allylphenol has been studied considerably’ in the past decade. Phenyl allyl ether contains the skeleton C=C-O-C-C=C, but the simplest compound to contain this skeleton, namely, vinyl allyl ether, was not investigated till very recently. Its rearrangement2 at 250’ into allylacetaldehyde establishes the fact that this is the essential skeleton for the rearrangement. CHz=CHOCHzCH=CH2

+

CHz=CHCH&HzCHO

Vinyl allyl ether, besides being the simplest ether’of its type, also is simpler than phenyl allyl ether in its mode of pyrolysis. Rearrangement of the latter is followed by migration of hydrogen, but no such enolization occurs with the former. It is characteristic of the rearrangement of allyl aryl ethers to involve the ortho position if available, otherwise the para position, but never the meta. When the ortho position is involved, it has been demonstrated conclusively3that the process is intramolecular and that there is inversion of the “wandering” allyl group. It seems, however, that no such inversion occurs4 when rearrangement is forced to the para position. Rearrangement of a-substituted or y-substituted allyl ethers is similar to that of the allyl ethers, but the a,a-disubstituted allyl ethers behave differently. Instead of rearranging, the last-named types split into phenols and dienes,6but a certain amount of splitting into phenol6 has been shown 1 For a summary of the early work in this field see HURD, “The Pyrolysis of Carbon Compounds,” The Chemical Catalog Company, New York, 1929, pp. 214-228. HURDAND POLLACK, J . A m . Chem. SOC.,60, 1905 (1938). a ( a ) HURDAND SCHMERLING, J . Am. Chem. SOC.,69, 107 (1937); ( b ) CLAISENAND TIETZE,Ber., 68, 275 (1925). MUMMAND MBLLER,Ber., 70, 2214 (1937). 6 CLAISENAND COWORKERS, J . prakt. Chem., 106, 67 (1922); HURDA N D COHEN, J . Am. Chem. Soc., 65, 1919 (1931). HURDAND PUTERBAUGH, J. ORG. CHEY., 2, 381 (1937).

660

551

MECHANISMS FOR REARRANGEMENTS OF ETHERS

to take place also with the allyl ether types that do undergo rearrangement. Other pertinent facts are the non-rearrangement of the corresponding propargyl ethers,' and the evolution of propylene from allyl 2,4,6-trialkylphenyl ether.* A related fact is that the ammono analog of phenyl allyl ether, namely, diallylaniline, pyrolyzesQinto propylene and aniline instead of rearranging. Unusual interest attaches itself to the behavior of y-ethylallyl phenyl ether because of its anomalous rearrangement into o- (a-methylcrotyl)phenol : --O-CH&H=CHCH&H3 -CH(CHa)CH=CHCHa The same substance was produced by the normal rearrangement of amethylcrotyl phenyl ether. These two rearrangements were effected by heating in diethylaniline solution. Since this reaction was not covered by any of the existing mechanisms, it seemed important to study it further. To start with, it was necessary to demonstrate the purity of the ether in question. If a-methylcrotyl phenyl ether were present in quantity in Lauer and Filbert's material it would remove the anomaly. These workers did prove that their material contained y-ethylallyl phenyl ether because on oxidation it gave rise to a 28 per cent yield of phenoxyacetic acid (0.32 g. from 1.2 g.), but they did not demonstrate the absence of the isomeric a-methylcrotyl phenyl ether. Their ether was prepared from phenol by reaction with 1-chloro-2-pentene which in turn was synthesized from ethylvinylcarbinol and dry hydrogen chloride. 1-Chloro-2-pentene and 3-chloro-1-pentene were formed and were separated by distillation. A third isomer, 2-chloro-3-pentenej which would have produced a-methylcrotyl phenyl ether, was not mentioned. If the hydrogen chloride functioned as a dehydrating agent, its formation could be anticipated by these steps: CH2=CH-CHOH-CH&Ha 3 ' CHFCH-CH=CHC& CH&H=CH-CHCl--CHa

y'

Some of this chloride was found in the present work, but in quantity insufficient to account for the bulk of the abnormality. The pentenyl chlorides were synthesized according to Lauer and FilHURDAND COHEN,J . A m . Chem. SOC.,63, 1068 (1931). 59, 1686 (1937). 9 CARNAHAN AND HURD,ibid., 62, 4586 (1930). 1 0 LAUER AND FILBERT, ibid., 68, 1388 (1936). 7

* HURDA N D YARNALL, ibid.,

552

CHARLES D. HURD AND MAXWELL A. POLLACK

bert's directions. The following procedure waa developed to analyze each fraction for the three possible chlorides: CHsCH&H=CHCHZCl (A), CH&H&HClCH=CHZ C&CHClCH=CHCHS (C)

(B),

and

Ozonization followed by hydrolytic oxidation with water and silver oxide would yield as volatile acids propionic, formic, and acetic respectively, from A, B, and C, which could be separated by steam distillation from the concurrently formed, non-volatile hydroxy acids (glycolic, a-hydroxybutyric, and lactic acids). The steps with A are illustrative:

A

+

0 3

-+

CHaCHaCH-CHCH2Cl

\ /

fs

>

0 3

CHaCHzCOOH

+ HOCHzCOOH

ANALYSIS OF MIXTURE OF FORMIC, ACETIC, AND PROPIONIC ACIDS

The problem of analyzing the distillate, which contained a mixture of formic, acetic and propionic acids, was solved as follows. Formic acid was determined by oxidation with chromic acid." Propionic and acetic acids, not touched by this reagent, were distilled off, and the aqueous distillate was analyzed by the Duclaux method.12 These operations were practically quantitative. Since the acetic acid represented C it was important to demonstrate its presence or absence even more conclusively. The method selected was baaed on the fact that propionic acid may be oxidized quantitatively to oxalate by hot alkaline permanganate,la whereas acetic acid is relatively unaffected. The latter was distilled off and identified by Duclaux values and by conversion to p-bromophenacyl acetate. ANALYSIS OF THE PENTENYL CHLORIDES

Once the analytical method was established, the pentenyl halides (A and B) were synthesized. Lauer and Filbert's directions were followed closely. Analysis of the higher-boiling fraction, which had been assumed to be pure A, showed it to be 89 per cent A, 11 per cent B, with only a trace of C. Obviously, the anomalous rearrangement could not be traceable to C in the starting material. The lower-boiling fraction (assumed to be pure B) when analyzed similarly was found to contain 62 per cent B, 36 per cent A and 2 per cent C. MACNAIR, Chem. News, 66, 229 (1887). VIRTANEN AND PULKKI, J . Am. Chem. SOC.,60, 3138 (1928). 18 MCNAIR,J . A m . Chem. Soc., 64, 3249 (1932). 11

12


553

The presence of 2 per cent of C indicates that a slight dehydration occurred as pictured. The considerable quantity of A explains the isolation by Lauer and Filbert of a sizeable yield of y-ethylallyl phenyl ether in the reaction of this chloride with phenol. The anomalous rearrangement.-The higher-boiling chloropentene fraction (89 per cent A, 11 per cent B),on condensation with phenol, gave rise to a mixture of pentenyl phenyl ethers which was shown by ozonolysis to consist of 90 per cent of y-ethylallyl phenyl ether (D), and 10 per cent of or-ethylallyl phenyl ether (E). The rearrangement product obtained by heating this mixture of ethers was found to contain 56 per cent of o-(a-ethylal1yl)phenol (F) from the normal “gamma” rearrangement, 42 per cent of the isomer, o-(a,y-dimethylally1)phenol (G) from the abnormal rearrangement, and a small amount of o-(y-ethylally1)phenol (H) from E.

-0-CH&H=CH-CH&H3 CH-CH-CHa

D

+

CH2CHa F

(J-~-CH=CH-CH3 CHa

(

) - O O ~ G T = C H-4 ~

E

G

p

CH2-CH=CH-CH2CH3 H

In an independent check run, a pentenyl phenyl ether mixture (87 per cent D, 13 per cent E), gave rise to phenols in approximately the same proportions (59 per cent of F, 35 per cent of G, and 6 per cent of H). It should be noted that Lauer and Filbert also obtained evidence for the normal rearrangement of D, but failed to recognize it as such. The formaldehyde observed by them among the ozonolysis products of their methylated rearrangement material undoubtedly arose from some o-(a-ethylally1)anisole, but was attributed instead to “deep-seated changes.”

554

CHARLES D. HURD AN'D MAXWELL A. POLLACK 7-ETHYLALLYL VINYL ETHER

Since the rearrangements of vinyl allyl ethers were found to proceed smoothly on heating,2 it seemed desirable to investigate the behavior of vinyl y-ethylallyl ether in order to determine whether the normal or abnormal type of rearrangement would occur. The pentenyl bromide used in the synthesis of vinyl y-ethylallyl ether was a mixture of l-bromo-2-pentene (81-5per cent) and 3-bromo-l-pentene (18.5 per cent) as determined by the ozone method of analysis. It was condensed with sodium glycolate to form the 8-hydroxyethyl pentenyl ether. The latter was converted into the 8-bromo ether with phosphorus tribromide, after which the elements of hydrogen bromide were detached with powdered alkali: HOCHzCHzONa x B A HOCHZCH~OCJIS B ~ C H - Z C H ~ O C ~ - I ~ CHz=CH-O-CJIp

It was assumed that the ratio of isomeric products remained unaltered during these steps and that the ether obtained was a mixture of the two isomers: CHz=CH-O-CH2CH=CHC2Hs (J) and CH?CH-O-CH-CH=CH2 (K), in the ratio of 81.5:18.5. This

I

GHs ether was found to hydrolyze readily into acetaldehyde and pentenyl alcohol : CHFCH-O-C~HS

+ H20 2% CHaCHO + CJI8OH

The thermal stability of this ether was about the same as that of vinyl allyl ether. Short heating of the vapors at 255' produced a 35 per cent conversion into heptenaldehyde. Practically complete conversion was effected by heating in a sealed tube a t 220'. Ozonolysis of the unsaturated aldehyde yielded a mixture of formic, propionic and acetic acids in molar ratios, respectively, of 76.5: 18.9:4.6. This indicated that the aldehyde was a mixture of 76.5 parts of 3-ethyl-4-pentenal (L), 18.9 parts of 4heptenal (M), and 4.6 parts of 3-methyl-bhexenal (N). The normal rearrangement of K is to M: CHFCH-O-CH-CH=CHZ

-+ CzH&H=CHCH2CHzCHO

C2HS The fact that an 18.5 per cent mixture of K gave rise to a mixture containing 18.9 per cent of M is evidence that this reaction occurred.


555

The normal rearrangement of J is to L: CHAH-O-CHaCH4HC&

-+ CHpCH-CH-CHaCHO b2HS

but J is the ether from which abnormal rearrangement to N might occur: CHFCH-O-CH~CH=CHCH,CH~

--+

CH,CH=CH-CH-CH&HO

I

CHs Both of these effects occurred but the chief effect was the former (76.5 per cent of L and 4.6 per cent of N from 81.5 per cent of J). Thus, the abnormal effect which was so prominent in the case of phenyl y-ethylallyl ether resulted also with vinyl y-ethylallyl ether but to a much lesser extent. PREVIOUS MECHANIBMS

From a consideration of the facts at his disposal, chief of which were the inversion of the allyl group and the impossibility of considering chromanes as intermediates, Claisenabsuggested that increase of temperature loosened and finally broke the bond between the allyl group and the oxygen atom. The phenoxy radical was then assumed to resonate to a keto modification: 0

The new bond on the ortho carbon then seized the 7-carbon atom of the allyl group, since that atom was considered to be nearest it in space, after which tautomeriaation to the phenol occurred:

0

OH

This mechanism carries the unjustifiable assumption that radicals retain their positions after formation. Since the y-C of the radical need not necessarily be the “nearest atom,” this becomes an inadequate explanation of the inversion of the allyl group. Hurd and CohenI4 pointed out that if scission into radicals were the first step, inversion might be ex14

HURD AND COHEN,J . Am. Chem. Soc., 63, 1917 (1931).

556


plained by assuming resonance of both the phenoxy and the allyl radicals, followed by recombination of the two new radicals. One objection to radicals as intermediates is the relatively low reaction temperature. Another objection, made by Niederl,I6is the absence of byproducts, reference being made to the absence of such a compound as phenyl peroxide, PhOOPh, in the reaction products (from combination of two PhO radicals). Niederl proposed a mechanism, based on the oxonium mechanism proposed by van Alphenle to explain the catalytic effect of hydrogen chloride in bringing about the rearrangement of benzyl phenyl ether into p-benzylphenol. C1-0-H H PhOCHzPh

+ HC1 -+

‘e*..

(A)

Ph-0-CHzPh

-+

% *..

‘c1

)( H

-+ CHzPh

H I

I

CHSh

I

CHzPh

H

I

Structure A must be polar, namely, (Ph-0-CH2Ph)+

..

..

..

:C1- and the other

oxonium formulas must be regarded similarly. I n extending this mechanism to allyl phenyl ether where heat alone effects the rearrangement, Niederl assumed “the formation of oxonium compounds between identical molecules, then the succeeding transitory bimolecular addition compounds of the quinhydrone type may be formed ~ ~ N I E D EAND R LNATELSON, ibid., 64, 1067 (1932); NIEDERL AND STORCH, ibid., 66, 288 (1933). 16 VAN ALPBEN,Rec. trav. chim., 46, 799 (1927).


557

and finally the substituted phenol is obtained." This would be summarized in the equation:

2cd-Isocas

-

O-ca,

/OH

\

+ CEHKOCBK

CaHs

with I, I1 and I11 as intermediate steps.

I1

I

I11

In terms of electron theory, the structures for I and I1 must be ionic, Le., IV and V respectively.

IV

v

There are several serious objections to this hypothesis. (1) It ignores the actual process of rearrangement, the mechanism of the change of I into 11. (2) It does not explain the inversion of the wandering radical. (3) If phenyl allyl ethers depend on oxonium structures to bring about rearrangement, then any phenyl alkyl ether should do the same because oxonium formation should be general for ethers rather than specific for the allyl ethers. (4) If V is an intermediate, part of it at least should decompose to yield allyl o-allylphenyl ether, o-C~H~C&-O-C~H~, which has not been observed. (5) A mixture of aryl allyl ether and aryl cinnamyl ether should give rise to a mixed addition product

but if so the allyl and the cinnamyl groups should compete for the privilege of rearrangement. The evidenceaa reveals no such competition. (6)

558


Allyl 2,4,6-trialkylphenyl ether should be able to rearrange, because the step from I to II-(or IV to V) is not inhibited, i.e.,

This is not what occurs.8 PROPOBED MECHANISM

It may be assumed that the initial effect of heat on thesystem C=C-0-C-C=C is to alter the position of the pair of electrons which bind the allyl group to the oxygen so that a semi-ionization occurs, such as C=C-0: .. C - G C . Actual separation into ions does not occur, but the semi-ionization promotes other ionic disturbances at the double bonds. This effect, combined with the spatial proximity of the atoms a t the end of the systems, brings about temporary ring closure and readjustment of electrons as shown in the following sequence of steps.

/-*

c

\

//

C

C

T=1

-I :c

IC:

+/

C

..

0:

OF\

*

+e I

\

c+/- +c

0-

II

C

0:

//

/*

- + I

c+ +c -I :C C \ / C

+

I

C

C

C

C

II

\ / C

(or its enol)

It should be emphasized that the unstable, cyclic intermediate is not a chromane. This mechanism provides an explanation for the intramolecular nature of the reaction and for the inversion of the “wandering” radical. The semi-ionic, positive carbon seeks to satisfy its electron deficiency by appropriating electrons from the neighboring double bond. This process is reversible but the next step which involves cyclization is irreversible. To obtain cyclization calls for free rotation of the bonds. This condition is met in the simple cases. If, however, it be assumed that there

559


is restricted rotation with a,a-dialkylallyl aryl ether then a satisfactory picture for its scission into a diene and phenol may be constructed. In the following formulation it shows that the positive a-C of the allyl group may satisfy- its electron deficiency not only by appropriating electrons

f \K

C

II

C

H:C C==C

from the neighboring double bond (reversible) but also from a neighboring bond (irreversible), the released proton being attracted to the oxygen. This gives rise to phenol and a diene. ?-Ethylally1 phenyl ether is capable of existing in cis and trans modifications. The trans form, and to a certain extent the cis form as well, should rearrange normally to o-(a-ethylally1)phenol by the mechanism developed for simple allyl ethers. With the cis form, however, a consideration of

C-H

a

0

B

r

b

t

space models shows that the 6-C of C=G-O--G-C=C-C-C may be in the vicinity of the 0-C. Under these circumstances, the combined attractions of the positive a-C for the 6-Hwith its electrons, and the negative 0-C for the positive 6-C may be sufficient to bring about the abnormal rearrangement which has been observed. It is important to note that the 6-C is ‘Lallylic”and, therefore, inherits a tendency to lose an electron pair.

.. \

/“*

C+

I-.

C:

+CHz

T\

/

H

\

“\“I /H

/O:

C

C

II

I

/A\\ C

C H:C

//cH C

H H

/ -\

CHs

\ /

/

OH CH3

I

C

CH

I/

ll

C

/ \ /CH \ / CH

I

CHI

Para rearrangements.-Two mechanisms suggest themselves for rearrangements which are forced to the para position. Present evidence is not sufficient to decide between them, but both lend themselves to experi-

560


ment. One possibility is that of complete ionization. The other is a modification of the above-described, semi-ionic cyclization. In the former, namely,

0-CHRCH=CHR’

+CHR-CH=CHR’

+

or

+

CHR=CHdCHR’

it is apparent that inversion of the wandering radical may or may not occur. Also, it is evident that by heating a mixture of VI and VI1 there should be formed both VI11 and IX from VI, and the equivalent pair from VII.

OCHRCH=CHR

OCHtCH=CH2 I

VI

Rf@

VI1

OH

OH

I CH&H=CHa VI11

CHRCH=CHR

I

IX

561

IECHANIBMS FOR REARRANGEMENTS OF ETHERS

The following steps are visualized in the semi-ionic mechanism.

__ ..

:0:

\

+CHR

..

..

:O

:O

R-

Lc9

H

H

/ \

CHR

//

CHI-CH

OH

I

I

CH&H=CHR The first step is rearrangement with inversion to the ortho position by way of semi-ionic linkages, to be followed by another semi-ionic cyclization and final enolization. The double inversion demanded by this mechanism gives the final effect of no inversion. In the ethers of this type which have been tested, it will be recalled that inversion did not O C C U ~ . ~Also, the mechanism would not permit of intermolecular interchange by heating such a mixture as VI and VII. It would seem reasonable to believe that the mechanism involving complete ionization is involved in the thermal rearrangement of tert.butyl phenyl ether" into p-ted-butylphenol, or of phenyl a,a,y,y-tetramethylbutyl ether18 into o-(a,a,y,y-tetramethylbutyl)phenol, or of benzyl phenyl etherle at 250" into a mixture of o- and p-benzylphenol. It is characteristic that one of the groups of these thermolabile ethers is related to allyl, benzyl, or tert.-alkyl which are known to be of a low order of electron attraction. a-Methylvinyl phenyl ether is typical of the ethers for which Niederl's oxonium mechanism16 was devised. Essentially, the medium is acidic SMITH,J . Am. Chem. SOC.,66, 3718 (1933). NIEDERL,NATELSON, AND BEECKMAN, ibid., 66, 2571 (1933); NATELSON, ibid., MI, 1583 (1934). 1 9 BEHAGHEL AND FREIENSEHNER, Ber., 67, 1368 (1934). 17

18

562


(sulfuric acid, or boron trifluoride, or similar reagents), and addition yielding an oxonium salt is the first step:

It seems plausible that the next step is resonance at the double bond to -k ..- ..+ H-C-C-0-R following which the C+ (sextet) may attract either the

I l l

H R or the H from the oxygen. The latter would be reversible and not noticed, but the former would be an irreversible rearrangement.

+ ..- ..+ H-C-C-0-R

I

l

l

H

R

1

..- ..++

I

l

-+ H-C-C-0

-4

l

H R

I

I

This modification of Niederl’s mechanism exp1a;ins not only the rearrangements of aryl alkyl ethers to alkylphenols by BF3 or AlC13 or CH3COOH-H2S04 but also, by extension to the “ammonia system,” explains the Hofmann rearrangement of substituted aniline salts :

R H-

&-C-N-R ..- ++ I

l

l

R

R

I

--+

H

I+ l l

C=C-N-R,

I

eto.

R

The acidic nature of these reactions, with ions involved necessarily, is another supporting factor in the belief that ions are involved in this type of rearrangement.

563

MECHANISMS FOR REARRANGEMENTS OF E T H E R S EXPERIMENTAL

Estimation of Mixtures of Volatile Acids Quantitatioe determination.-Chromic acid was used t o oxidize formic acid from the mixture of formic, acetic, and propionic acids. Macnair’s20 chromic acid solution (distilled water, 100 cc.; potassium dichromate, 12 g. ; concentrated sulfuric acid, 30 cc.) was used. I t was found that volatile acids could be distilled quantitatively from such a solution, or from one containing considerable sodium sulfate. In such cases, i t was necessary to distil slowly until crystals began to separate. Definite volumes of 0.1N solutions of formic, acetic, and propionic acids were mixed and refluxed for twenty minutes with an equal volume of the chromic acid solution. Then the solution was distilled, and the distillate was adjusted to a total volume of 110 cc. for Duclaux analysis:*’ Run 1: Ten cc. required 5.45 cc. of 0.0994N sodium hydroxide for neutralization, and the first 30 cc. of distillate required 14.95 cc. of the alkali. Run 2: Ten cc. required 4.22 cc. of 0.0994N alkali, and the first 30 cc. TABLE I ANALYSISOF MIXTUREOF ACIDS RUN

1

-2

ACID

NORMALITY

VOL. TAKEN,

VOL. FOUND,

CC.

CC.

ERROR

Formic Acetic Propionic

0.118 0.117 0.114

13.0 30.5 22.5

14.5 30.0 21.5

1.5 -0.5 -1.0

Formic Acetic Propionic

0.118 0.117 0.114

19.7 22.1 18.2

20.1 22 .o 17.9

0.4 -0.1 -0.3

of distillate required 11.76 cc. of this alkali. In run 1,since the acidity of the original mixture was 0.00767 equiv., and that of the distillate from the oxidation was 0.00596 equiv., the formic acid content was found by difference. The data and findings are assembled in Table I. Isolation of acetic acid.-In the following oxidation by permanganate both formic and propionic acids were affected. The propionic acid52 was oxidized quantitatively t o an oxalate. To a mixture of 9.2 cc. of 0.118N formic acid 9.6 cc. of 0.117N acetic acid and 8.9 cc. of 0.114N propionic acid was added 16 cc. of a 25% sodium hydroxide solution and 60 cc. of a 3% potassium permanganate solution. The 250-cc. Erlenmeyer container was capped with a small glass beaker and the whole was heated on the steam bath for six hours. After cooling, 32 cc. of a 1:2 sulfuric acid solution was added slowly and the resulting solution was distilled. The distillate was made up to 110 cc. The Duclaux values found on this material were 7.0, 7.0, 7.4, which indicated acetic acid only. The 110 cc. of distillate contained 80% of the original acetic acid. In another run, lactic acid (1.2 9 . ) and anhydrous sodium sulfate (10 g.) were added to a mixture MACNAIR, Chem. News, 66, 229 (1887). AND PULKKI, J . Am. Chem. 22 MCNAIR,ibid., 64, 3249 (1932). 20

I’VIHTANEN

S O C . , M),

3138 (1928).

564


of 12.0 cc. of 0.118N formic acid, 24.6 cc. of 0.117N acetic acid, 18.9 cc. of 0.114N propionic acid and the resulting solution was distilled. To the distillate, now containing only formic, acetic and propionic acids, was added 12 g. of anhydrous potassium carbonate and 45 cc. of a 6.7% potassium permanganate solution (prepared and added while hot). The whole was heated as above for six hours on the steam bath. Thirty-two cc. of a 1:2 sulfuric acid solution was used to acidify and the acid solution was distilled. This distillate was refluxed for a short time t o eliminate any dissolved carbon dioxide, cooled and made u p to 110 cc. Duclaux values, 6.9, 7.2, 7.6, again indicated only acetic acid. After neutralization of the distillate in the Duclaux determination, the solutions were evaporated t o dryness. Treatment of the resulting sodium salt with p-bromophenacyl bromide yielded p-bromophenacyl acetate, m.p. 85".

The Pentenyl Halides Ethylvinylcarbinol and hydrogen chloride.-Ethylvinylcarbinol2a was prepared from acrolein and ethylmagnesium bromide, was freshly distilled (b.p., 111-113.5") and treated with dry hydrogen chloride according to Lauer and Filbert's directions.10 The weight of carbinol taken was 33.7 g. and the total gain in weight was 20.7 g. It was twice fractionally distilled through a 33-cm. Widmer column, constant pressure being maintained by use of the Munch pressure regulator.*' These fractions were collected, all a t 149 mm.: (1) b.p. 42-45.5", 1.7 g.; (2) 45.5-51°, 9.1 g.; (3) 5159.5", 3.0 g.; (4) 59.5-62", 6.2 g.; (5) 62-62.5", 5.7 g.; residue, 1.7 g. Fractions 4 and 5 should be pure l-chloro-2-pentene, and fraction 2 has been regarded as pure 3-chloro-1-pentene. It was not found possible to limit the boiling range of fraction 2 t o less than five degrees although Lauer and Filbert reported a 0.2" range. Refluxing under atmospheric pressure for one hour produced no observable change in composition. All the material distilled a t 43-48" (147 mm.). From the work of Winstein and Young*' i t would be expected that either of the allylic isomers would rearrange into an equilibrium mixture if heated briefly a t atmospheric pressure. Since the boiling range of fraction 2 did not change appreciably on refluxing, its composition probably approximated that of the equilibrium mixture. Ethylvinylcarbinol and hydrobromic acid.-To a mixture of 272 g. of 48% hydrobromic acid and 80 g. of concentrated sulfuric acid was added slowly with stirring, 94 g. of ethylvinylcarbinol. The mixture was stirred a t 20-25" for twenty-four hours; the upper layer waa separated, washed thrice with water, and dried over sodium sulfate. I t was twice vacuum-distilled through an efficient column. FRACTION

1 2

B.P., "c.

PREBSURE, MM.

WT.,Q.

23-30 30-34

20 20-19

9.5 118.2

20

nD

1.4654 1.4769

Structure of the Pentenyl Halides Pentenyl chloride, fraction b.-An excess of a 5% ozone-in-oxygen stream was passed through an ice-cold solution of 100 cc. of dry carbon tetrachloride and 1.4 g.

HURDAND MCNAMEE, ibid., 69, 104 (1937). ~'MUNCEI, J . Chem. Educ., B, 1275 (1932). *6 WINSTEINAND YOUNQ, J . Am. Chem. Soc., 68, 104, (1936). 25


565

of pentenyl chloride, fraction 5 (b.p. 62-62.6" at 149 mm.). Then the ozonide was hydrolyzed by refluxing one hour with 100 cc. of pure water, distilling off the carbon tetrachloride, refluxing another thirty minutes, and then refluxing for f e u hours in the presence of freshly-prepared silver oxide. I t was cooled, filtered, the alkaline filtrate was evaporated t o half-volume, and acidified with phosphoric acid. Then 2 g. of anhydrous sodium sulfate was added, the volatile acids were distilled off, and the distillate was divided into two equal parts of 30.5 cc. each. Titration of one part showed it to contain 0.00405 equivalent of acid. After refluxing with chromic acid mixture and distillation, 0.00362 equivalent of volatile acid remained. From this, 0.00043 equivalent of formic acid was oxidized away. Duclaux values (11.9, 1 1 . 6 , l l . l ) on the distillate indicated practically pure propionic acid. The other part was refluxed for six hours with the alkaline permanganate solution, acidified, and distilled. The Duclaux values (8.2, 8.4, 7.9) indicated slightly impure acetic acid. This acid was neutralized; the sodium salt solution was evaporated t o dryness and treated with p-bromophenacyl bromide. The ester thus produced, after three recrystallizations from alcohol, melted a t 77-79'. When mixed with authentic p-bromophenyl acetate, m.p. 85", the map. was 82-84". The three acids were present in these amounts: propionic 89%, formic 11%, and a trace of acetic acid. Pentenyl chloride, fraction 8.-A part of this fraction (1.85 g., b.p. 4348" a t 147 mm.) was subjected to ozonolysis as described above. There was 0.00630 equivalent of total volatile acids. The formic acid content, determined by the chromic acid method, was 0.00393 equivalent. The unoxidized volatile acids, made up to 110 cc. for Duclaux titrations with 0.0314N acid, gave these data: 10 cc. of the 110 cc. required 6.86 cc. of alkali; the first three 10-cc. portions of distillate required 7.96, 7.92, 7.53 cc. of the alkali. Duclaux values: 11.6, 11.5, 11.0. The 30% distillation values indicated the presence of 0.00224 equivalent of propionic acid and 0.00013 equivalent of acetic acid. The percentage of acids present in the original distillate, therefore, was formic 62.3, acetic 2.1, and propionic 35.6. Pentenyl bromide, fraction $.--One gram of the higher-boiling fraction (b.p. 30-34' a t 20-1!3 mm.) was subjected to the same technique of ozonolysis. The volatile acids produced were diluted to 110 cc., of which 10 cc. required 14.43 cc. of 0.0314N sodium hydroxide. The first three 10-cc. portions of distillate from the remaining 100 cc. required successively 15.12, 14.94, 14.44 cc. of the alkali. Duclaux values: 10.5, 10.3, 10.0. By calculation, these data show 18.5% formic acid (0.00092 equiv.) and 81.5% propionic acid (0.00406 equiv.). The portions of volatile acids were combined, neutralized, evaporated t o 30 cc., and oxidized by the chromic acid procedure. The volatile acid which remained after this treatment was practically pure propionic acid, judged by the Duclaux values of 12.0, 11.7, 11.3 which were obtained. Pure propionic acid gives 11.9, 11.7, 11.3. y-Ethylallyl Phenyl Ether Preparation from pentenyl chloride (89% I-chloro-2-pentene) .-Lauer and Filbert's directions10 were followed exactly with one-third quantities. A 61% yield was collected a t 123-125" (25 mm.). This ether will be referred t o as X. Preparation from pentenyl bromide (88% I-bromo-9-pentene).-The same general directions of synthesis were followed. The sample investigated had these properties: b.p. 109-110' (17 mm.), n: 1.5176, d: 0.975, M.R. (found) 50.39, M.R. (calc'd) 59.57. This ether will be referred to as XI.

566


0zonoEysis.-Essentially the same technic of ozonization, hydrolysis and oxidation with silver oxide as that described earlier was applied to 0.7 g. of X and to 1.0 g. of XI. From the former, phenoxyacetic acid, m.p. and mixture m.p. 98-99', was isolated from the residue after distilling off the aliphatic acids. The distillate was redistilled from 5 g. of anhydrous sodium sulfate. Titration revealed 0.00256 equivalent (or 60% yield) of volatile acids. The total quantity was refluxed for half an hour with Macnair's chromic acid mixture; the remaining volatile acids were distilled and redistilled from sodium sulfate. There was 0.00230 equivalent of acid: Duclaux values 11.9, 11.9, 11.2. I t was, therefore, practically pure propionic acid. The formic acid lost by oxidation was 0.00026 equivalent. Formic and propionic acids were present in the ratio 10:90. From X I there was obtained 0.00498 equivalent (81% yield) of total volatile acid, shown by Duclaux analysis to be 86.9% propionic and 13.1% formic from the following data. When the acid was made up to 110 cc., 10 cc. required 14.71 cc. of 0.0308N sodium hydroxide and the first three 10-cc. portions of distillate (of the remaining 100 cc.) required 16.07, 15.91, 15.09 cc., from which 10.9, 10.7, 10.3 are the Duclaux values. After the chromic acid treatment the Duclaux values were 11.9, 11.9, 11.6 indicating practically pure propionic acid. Pyrolysis.-The rearrangement of 4.1 g. of X (in nitrogen atmosphere) or 4.0 g. of X I (in carbon dioxide atmosphere) was carried out after the directions of Lauer and Filbert by heating with 2.1 g. of diethylaniline. These investigators extracted the pentenylphenol from ether solution with 20% sodium hydroxide solution but we obtained much more effective extraction with 4% solution. The pentenylphenol prepared from X was collected a t 72-74' (1 mm.); n: 1.5337; dy 0.990; M.R. 50.86 (calc'd 50.46). That from X I was collected in 55% yield a t 122' (20 mm.); n! 1.5320. Ozonolysis of the pentenylphenoL-The pentenylphenols from both X and XI were ozonized, 0.8 g. in 80 cc. of dry carbon tetrachloride a t 0' being used with an excess of ozone in each case. After hydrolysis of the ozonide and oxidation by silver oxide, the volatile acids were prepared for Duclaux analysis by distillation and redistillation from sodium sulfate. Total acid in distillate: 0.00395 equivalent, or 80% yield, from X, and 0.00338 equivalent, or 69% yield, from XI. Duclaux analysis of acid from X: 5.3, 5.5, 5.8, 6.0,6.3; that from X I was 5.2, 5.8,6.2. These results indicated formic acid, mixed with others. The total acid of each was neutralized, concentrated to 60 cc., the formic acid was removed by refluxing with an equal volume of chromic acid mixture. The distillate of the remaining volatile acids was made up to 110 cc. and analyzed. Ten cubic centimeters of the solution from X required 5.06 cc. of 0.0314N alkali. The first three 10-cc. distillates required 3.58,3.69,3.91cc. from which Duclaux values of 7.1, 7.3, 7.7 are obtained. Ten cubic centimeters of the solution from X I required 4.03 cc. of 0.0308N alkali. The first three 10-cc. distillates required 3.01, 3.13, 3.18 cc. from which follow the Duclaux values of 7.5, 7.8, 7.9. The data from X point to the presence of 0.00165 equivalent of acetic acid and 0.00010 equivalent of propionic acid. The formic acid oxidized was 0.00220 equivalent. Therefore, formic, acetic and propionic acids existed in the relative amounts of 55.7, 41.8, 2.5. The data from X I show 0.00118 equivalent of acetic acid, O.OOO19 equivalent of propionic acid. The formic acid lost by oxidation was 0.00201 equivalent. Hence, formic, acetic, and propionic acids were in the proportions: 59.4, 35.0, 5.6. The sodium salt from the Duclaux determinations was treated with p-bromophenacyl bromide. The ester obtained melted a t 78-80". The melting point of a mixture with authentic p-bromophenacyl acetate (m.p. 85') was 82-84'.


567

r-Ethylallyl Vinyl Ether 8-Hydroxyethyl -pethylallyl ether.-Twenty grams of sodium in small pieces was added slowly t o 145 g. of redistilled ethylene glycol, with gentle heating on the steam bath. When the reaction was over, 117 g. of "1-bromo-2-pentene" (18.5% of 3-bromo-1-pentene) was added slowly with constant stirring and gentle heating on the steam bath. After the addition, the heating and stirring were continued for two hours, and the reaction mixture then allowed t o stand for four hours. After removal of the sodium bromide by filtration, the filtrate was distilled under reduced pressure. Since refractionation did not suffice for the complete separation of the hydroxy ether and ethylene glycol, advantage was taken of the relative solubilities of the two in ether and water. All of the material was shaken with 400 cc. of water and 100 cc. of ether. The ether layer was extracted with 100 cc. of water. The combined aqueous solutions were then extracted three times with ether; the ether portions were combined and dried over anhydrous sodium sulfate. After filtration, and distillation of the ether on the steam bath, the residue was distilled and refractionated through a good column. The yield of this ether (crude) was 72.5 g. (71%). The pure @-hydroxyethyl y-ethylallyl ether (45.3 g. or 44.3% yield) was collected at 85-87" (13 mm.): n:, 1.4452; d:', 0.925; M.R. (found), 37.48; M.R. (calc'd), 37.23. Anal. Calc'd for C,H11O2: C, 64.57; H, 10.84. Found: C, 64.67; H, 10.85. @-Bromoethyl y-ethylallyl ether.-A mixture of 43.5 g. of pure 8-hydroxyethyl 7-ethylallyl ether and 5.9 g. of dry pyridine was added slowly to 36.2 g. of phosphorus ' 0 After thirty minutes the crude bromotribroniide in a fractionating flask a t . ether was distilled under reduced pressure. The distillate was washed twice with dilute sodium hydroxide, twice with dilute sulfuric acid, then once with water, and was dried over anhydrous sodium sulfate. After filtration, the pure bromide was obtained by distillation through a 55-cm. electrically-heated Vigreux column with partial condensation head. The yield was 18.4 g. or 28.5%. In other runs, the yield was comparable. Physical constants of 8-bromoethyl 7-ethylallyl ether: b.p., 79' a t 11 mm.; n: = 1.4705; dy = 1.234; M.R. (found) = 43.69; M.R. (calc'd) = 43.47. Anal. Calc'd for CvHlaBrO: Br, 41.39. Found: Br, 41.51. Since the original pentenyl bromide was a mixture, and all of the intermediate product was used in these runs, the substance here obtained is undoubtedly a mixture of the y- and the a-ethylallyl @-bromoethers (approximately in the ratio of 80:20). y-Ethylallyl vinyl ether.-Into a 50-cc. distilling flask was placed 23.2 g. of the bromoethyl pentenyl ether. Nineteen grams of finely-powdered, technical potassium hydroxide was then added, whereupon a n exothermic reaction ensued. The mixture was heated slowly by an oil bath kept a t 160-170". A small amount of material distilled over during the first hour, after which the bath temperature was lowered, and 5 g. more of potassium hydroxide was added. Further heating to 170174' for one and one-half hours caused only a few more drops of material t o come over. Very slight suction was used t o assist in the distillation for a small part of the time. The distillate consisted of two layers; an upper one, weighing 4 g., and a small amount of water. Upon taking up the residue in the flask with water, 6 g. of a darkbrown oil separated, which was mainly unchanged ether.

568

CHARLES D. HURD AND -WELL

A. POLLACK

The water-insoluble distillate was distilled through the 55 cm. Vigreux column t o obtain fractions 1and 2, but fraction 3 was distilled through a short Vigreux column: FRACTION

____

B.P.,

~ 1

2 3 Residue

"c. _

94.5-97 97-101 39-40

PRESSURE

WT., 0.

_

Atmos. Atmos. 15 mm.

0.3 1.2 0.6 0.7

1

72;

1.4317 1.4307

'I

d:

0.813 0.856

The highest temperature reached by the bath during the distillation was 190". Fraction 2 was the desired y-ethylallyl vinyl ether: M.R. (found), 35.77; M.R. (calc'd), 35.24. Anal. Calc'd for ClHlnO: C, 74.93; H, 10.78. Found: C, 74.24; H, 10.77. Fraction 3 appears to be the heptenaldehyde rearrangement product of the vinyl ether: M.R. (found), 33.92; M.R. (calc'd), 34.07. That i t was somewhat impure was indicated by carbon and hydrogen analyses, wherein the carbon values found were 1.7-2.1% low. The 2,4-dinitrophenylhydrazone of this material melted, after recrystallization, a t 94". Acid hydrolysis of y-ethylallyl vinyl ether.-One drop of this ether was dissolved in 5 cc. of alcohol, to which was added 5 cc. of a saturated solution of 2,4-dinitrophenylhydrazine in alcohol and a few drops of hydrochloric acid. The mixture was heated to boiling, and water was added to incipient cloudiness. After two crystallizations from dilute alcohol, the yellow powder melted at 140-145". Further recrystallization lowered the melting point to 129-131", where i t remained fairly constant. T h a t this was impure acetaldehyde 2,4-dinitrophenylhydrazone,contaminated with some more insoluble impurity, was indicated by the fact that a mixture of this material with authentic acetaldehyde derivative (m.p. 165-166") melted at 146-149 '. in Pyrolysis of y-ethylallyl vinyl ether.-The apparatus used in previous which the vapors of the ether were passed through a tube heated to 255", was employed. One and three-tenths grams of 7-ethylallyl vinyl ether was distilled through the tube during a period of twenty minutes. One gram of material with the constants n: = 1.4307 and d:' = 0.826 was recovered. This material was then heated slowly to 220" in a sealed tube and kept at that temperature for eight minutes. Upon cooling, i t was observed that only an extremely faint yellow tint had been developed. The constants of this resulting material, which had an odor strongly resembling that of allylacetaldehyde were n: = 1.4314; diO= 0.850; M.R. (found), 34.20; M.R. (calc'd for heptenaldehyde), 34.07. Ozonolysis of the heptena1dehyde.-The reaction of 0.6 g. of the aldehyde with ozone (5y0) was carried out with the technic described earlier. The volatile acids produced were made up t o 110 cc. and analyzed. The volumes of 0.0314N alkali required for 10 cc. of the original 110 cc. and the first three 10-cc. portions of distillate were respectively: 8.12, 4.88, 4.87, 4.97. Duclaux values: 6.0, 6.0, 6.1-indicating a mixture of formic with one or more volatile acids. Total acid in distillate, 0.00281 equivalent (62% yield). All portions used in the determination were combined, neutralized, and evap*e

HURDAND POLLACK, J . Am. Chew,. SOC.,80, 1905 (1938).


569

orated down to a volume of 35 cc. An equal volume of Macnair’s chromic acid mixture was added and the whole was refluxed for one-half hour. The volatile acids were distilled and redistilled from sodium sulfate. This distillate was made up t o 110 cc. and a Duclaux determination made as before. Cubic centimeters of alkali required (in the same order as before): 1.90, 2.17, 2.05, 1.92. The corresponding Duclaux values (11.4,10.8,10.1)indicate propionic acid with a small amount of acetic acid. Using the 30% distillation values and calculating in the usual way, the relative amounts of propionic and acetic acids were found to be 0.00053 and 0.00013 equivalent. Since the total amount of acid here present is 0.00066 equivalent, the amount of formic acid which has been oxidized is 0.00281 - 0.00066 = 0.00215equivalent. T o confirm the acetic acid, all portions of the Duclaux analysis were combined, neutralized, and evaporated to 30 cc. Two grams of anhydrous potassium carbonate and 30 cc. of a 3% potassium permanganate solution were added, and the mixture was heated on the steam bath in a capped Erlenmeyer flask for five and one-half hours. After cooling, dilute sulfuric acid was used to acidify, and the mixture was distilled. The distillate was refluxed for ten minutes and distilled from sodium sulfate. The final distillate was made up to 110 cc., and a determination of the Duclaux values was made. In these analyses, the indicator used for each titration was one drop of a 1% solution of phenolphthalein in a 1:l dioxane-water mixture. Cubic centimeters of 0.0314N sodium hydroxide required (in same order as above): 0.57, 0.40, 0.41, 0.44. The corresponding Duclaux values are: 7.0, 7.2, 7.7-confirming the presence of acetic acid. Therefore, the proportions of formic, acetic and propionic acids were 76.5, 4.6, 18.9. SUMMARY

The rearrangement of yethylallyl phenyl ether into o-pentenylphenol was subjected to critical study. It was established that the pentenylphenol was a mixture of o-(ar-ethylally1)phenol and o-(a,-y-dimethylallyl)phenol in about the ratio of 3:2. The second of these products comes by an “abnormal” type of rearrangement. The same type of abnormality, but to a far lesser degree, was found in the pyrolytic rearrangement of 7-ethylallyl vinyl ether. The normal and abnormal products, namely, 3-ethyl-4-pentenal and 3-methyl-4-hexenal, were found in a 1 7 : l ratio. The analytical method was based on ozonolysis and subsequent determination of the volatile acids which were formed. A reliable analytical method for the determination of a mixture of formic, acetic, and propionic acids was developed. A survey of the various mechanisms to account for rearrangements of ethers has been presented and the limitations of each have been noted. A mechanism is proposed for these rearrangements which is based on semi-ionic (and sometimes, ionic) intermediates rather than on radicals. This mechanism accounts for both the normal and abnormal effects.

MECHANISMS FOR THE REARRANGEMENTS OF ETHERS

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