[FROMT H E KEDZIECHEMICAL LABORATORY, MICHIGANSTATE
COLLEGE]
FRAIGMENTATION OF ALCOHOLS I N THE PRESENCE OF ALUMINUM CHLORIDE. I. 2,3,4-TRIh!IETHYLPENTANOL-3 AND 2,4-DIMETHYL-3-ETHYLPENTANOL-3 RALPH C. I-IUSTOS
AND
JORGE AWUAPARA
Received April 22, 1944
Earlier work in this laboratory (1, 2) has shown that the accumulation of methyl groups on the carbon atom adjacent t o the carbinol carbon of tertiary alcohols causes a marked reduction in the yield of the expected tertiary alkylbenzene when these alcohols are condensed with benzene in the presence of aluminum chloride. Isolation of tertiary butylbenzene as a reaction product in a number of cases lcd us to suspect that the low yields resulted from fragmentation of the carbon chain rather than from a depressive influence on the condensing capability of the alcohol. This suspicion was strengthened by the observation that 2,4,4trimethylpentanol-2, in which there is no branching on the carbon adjacent to the carbinol carbon, gave large yields of tertiary butylbenzene and very low yields of 2,4 ,4-trimethyl-2-pheny1pentanea This is the first of a series of reports on the fragmentation of tertiary alcohols, in which we hope to establish relationships between types of chain branching and tendency to rearrange and split. The addition of 2,3,4-trimethylpentanol-3to a suspension of aluminum chloride in benzene gave the following products: ..An oetylbenzene, 2,3,4-trimethyl-3-phenylpentane(9%) ; a butylbenzene, 3-methyl-3-phenylpropane (48%) ; alkyl halides, CsHI,C1, probably 2,3,4trimethyl-3-chloropentane, with some 2,3,4-trimethyl-2-chloropentane;unsaturated hydrocarbons, CsHla, which were formed by elimination of hydrochloric acid from the alkyl halides, mostly 2,3,4-trimethylpentene-2. It is proposed that the initial step in the reaction is the combination of alcohol and aluminum chloride by a dative bond and that this is followed by the formation of a n alkyl cation.
H Ha C CC H3
I I
HJCCOH HI C C C HJ H
H
H Ha C C CH,
+ Alcl3 -+
H3CCCHj
I+ -
I I
HsCCO*AlCls -+ H3CC+
IH
+
Ha C C CH3 H
Hs C C CHs H
‘The complex anion may lose either a chlorine ion or a hydroxyl ion. Al(0H)Clz AlCls 401
+ C1-
+ (OH-)
402
RALPH C. HUSTON AND JORGE AWUAPARA
The presence of protons, formed either by the condensation of the cation with benzene or by the formation of an alkene, will form as end products aluminum chloride and water. The formation of
[izA1 ::]-and [izA1 :;]-as
inter-
mediates is also possible. In any case, the reaction will terminate when the aluminum chloride is hydrated to such an extent that it no longer forms a dative bond with the alcoholic oxygen. When one mole of alcohol is added to one-third of a mole of aluminum chloride in benzene, the addition of the first one-third mole must be drop-wise. The second third may be added more rapidly while the last third may be added at one time, without causing the temperature to rise. During the addition of this last third there is definite evidence of the developnient of back pressure in the system. The further reactions of the cation will depend upon its stability and its environment. In normal alkylation, it will unite with the benzene which has been activated by aluminum chloride and a proton mill be eliminated (3).
,
?J /,j + \\
I1
€33
+CCITa I
H Ha C C CH3
il
c c CIIB
H 3 ------f
HP C C CH3
f]:----((c& I \
JI
CCCII3
()---7CH3
---i
I HBCCCH3 I1
\/
I
+ H+
cc
€13 c 143 11
The significance of the relative ability of the benzene ring to react nith the cation is shown by comparing the ease of alkylation of benzene and phenol. The highly reactive phenolic ring condenses with 2, 3,4-triniethylpentano1-3to give a sixty per cent yield of the alkylphenol (4)with no eyidence of rearrangement or fragmentation. Under similar conditions, benzene yields about ten per cent of t-octylbenzene, while the yield of products resulting from rearrangement and fragmentation is high. The fraction of the cation which fails to condense with benzene may unite with ionic chlorine from the complex aluminum anion to form the alkyl chloride, or it may lose a proton to form either or both of the alkenes:
c' €I3
i'Ii;
I
ITaCC---C=CI12 14 I Ha C C CH3 TT
or
I HC--C=C I l
c €13 I l
CII3 CITa CH3
The first of these, 3-methyl-2-isopropylbutene-1 could add a proton, or hydrocloric acid, and the resulting cation or alkyl chloride could condense with benzene, or the unsaturated hydrocarbon could condense directly (5). In any case the product would be 2 , 3 ,4-trimethyl-3-phenylpentane. On the other hand, the addition of proton to 2 ,3,4-trimethylpentene-2 mould give a mixture of two cations which may be considered in equilibrium.
403
FRAGMENTATION OF ALCOHOLS
€I HsCCCHs
I
HaCC+ I
F’,
+
HsCCCHj
HsCCCI-Ts
HsCC I
H3CCII I
I
II
Of these, the first would condense to give 2,3,4-trimethyl-3-phenylpentane, while the second would give 2 , 3 ,4-trimethyl-2-pheny1pentaneaIt has been found that 2,3,4-trimethylpentanol-2rapidly undergoes fragmentation (2). The experiments now under consideration gave no evidence of the formation of 2,3,4-trimethyl-2-phenyIpentane. ‘The transient existence of the second cation (above) may be attributed to its tendency to rearrange ( 6 ) to a cation which by loss of a proton would yield di-isobut#ylene. cI-Ts C H3 H3C6CHS
I H3CCH I
-
H3CCCH3 H
I I +CH I
I
H3CCCH3
HaCCCltI3
C H3
I
CH
--+
---+ 2 CH,C=CH*
1I
Ha c C CII3
Ha C C CHs H
‘The formation of a tertiary alkyl (butyl) group, with its low energy level, adjacent to a positive (unsaturated) carbon, sets up a condition highly favorable t o chain rupture. One of the fragments could well be in the form of a cation, while the other could form a cation by proton addition. It may be significant, that the yield of tertiary butyl benzene from 2,3,4-trimethylpentanol-3 is about the same as from 2,4,4-trimethylpentanol-2.In neither case was there evidence of an appreciable amount of tertiary butyl chloride in the reaction mixture, but in both cases there was evidence of the formation of gaseous hydrocarbon. The Condensation of 2,4-dimethyl-3-ethylpentanol-3with benzene was undertaken for the purpose of observing the effect of the size of the migrating group. Nonylbenzene was isolated in a yield of twenty-six per cent. By analogy, this was tentatively assigned the structure of 2,4-dimethyI-3-phenylpentane. Both 2-methyl-2-phenylpropane and 2-methyl-2-phenylbutane were isolated w split products in practically equal amounts, 3%. CHs
I
CH2
H HaCCCII3
I I
C&HSC+ HsCCCHs H
Ha CdCHa
I I
Tr! CzHsCH
H3CCCHj I€
CHS
I
I H3 CC CII3 I_.I. I............... -P
+CH
I
CH3 CH2 C=CH2 --+
€€3C(’C€€3 I1
+
CH3
I
CHsC=CHZ
404
RALPH C. RUSTON AND JORGE AWUAPARA
The nonyl chloride fraction (12%) which by analogy contained mostly 3,4dimethyl-3-ethyl-3-chloropentanewas decomposed, partly by boiling and completely by dilute sodium hydroxide, to a constant-boiling alkene fraction, probably 2,3-dimethyl-3-ethylpentene-2. The formation of tertiary butylbenzene and tertiary amylbenzene givm definite information as to the mechanism of fragmentation. The small yields of split products result because the ethyl group has less tendency to migrate than does the methyl group. The formation of two alkyl benzenes proves that both fragments are capable of uniting with the benzene ring. If the theory is correct, 2,3,4-trimethylpentanol-3condenses with benzene to form 2,3,4-trimethyl-3-phenylpentane,because a cation is formed which is sufzciently stable to allow condensation to occur before unsaturation. It was, therefore, of interest to determine the behavior of 2 ,4-dimethylpentanol-3 in which the hydroxyl is part of a secondary alcoholic group, adjacent to a carbon atom on which there is branching. Distillation of the reaction products gave two fractions and a high-boiling residue. The first fraction consisted of an unstable alkyl chloride (or chlorides), 15%, which waa decomposed by boiling with dilute sodium hydroxide to 2 4-dimethylpentene-2. The second fraction proved to be 2,4-dimethyl-2-phenylpentaneas shown by the melting points of the a-naphthylurethan and the benzoyl ester of its p-hydroxyl derivative (7). There was no evidence of fragmentation and it is apparent that the critical intermediate in this reaction is the unsaturated compound. The hydroxyl of the secondary alcohol is less readily released as a part of the complex anion than that of a tertiary alcohol. An unsaturated compound may be formed either by direct decomposition of the intermediate addition complex or through the aluminate. )
CHI
C Ha
CHI
I H3CCH
I
HaCCH
HaCCH
I
I+ -
I
HCO.AlC13 or HCOAlClz -+ HC+ I IH I H3 C-CH HSCCH HaCC:
I
I
A
CHa
I
CH3
CH3
H --C---C+ €I
I
CH3
I
HICCH
I
AlCl
HC:
I
H3CCCHa
HgCCCHs
HOAlClz HC1 H+ CHI
+
+
+ /-\ \ d /\>F-l +
(!lI4
-P
I HaCCH I HC II
CHa
+
CHs **
CH*
I
(activated)
I
CH3 e.
C--C--CH H CH a
I I
CHa
-+
405
FRAGMENTATION OF ALCOHOLS
As in the formation of cations from tertiary alcohols, it is essential t o the theory that the A1 (OH) Clz react with hydrochloric acid to formA1Cl3and water, or that Al(0H)Clz and Al(OH)%Clform complexes by means of the dative bond with a second and third molecule of alcohol. It has been shown that the condensation of secondary alcohols with benzene must take place through the unsaturated hydrocarbon as an intermediate (S), also that aluminates may be formed (9) when aluminum chloride reacts with primary or secondary alcohols. Tertiary alcohols react with aluminum chloride a t room temperature, or below, to give excellent yields of alkyl chloride. EXPERIMENTAL
Condensation. The same technique was used in all condensations. High temperatures were avoided because the advantages of increased fragmentation by heating were more than offset by irregularities i n the course of the reaction and yields of products. On the other hand, low temperatures are not desirable because they decrease the amount of fragmentation and increase the yield of alkyl chlorides. One-half mole of the alcohol, dissolved in 50 ml. of benzene, was added drop by drop t o a vigorously stirred suspension of 50 g. of anhydrous aluminum chloride in 400 ml. of anhydrous thiophene-free benzene. During this addition, the temperature was maintained at 20-30". The reaction mixture was stirred for one hour, allowed t o stand overnight and then poured on ice. The benzene layer was separated, washed with water, then with a dilute solution of sodium carbonate and again with water. After drying over anhydrous sodium sulfate, the benzene was removed and the residual liquid was fractionated. b,9,~-Trimethylpentunol-S.This was prepared from di-isopropyl ketone and methyl magnesium bromide (lo), b.p. 156-157"/750. Fractionation of the condensation product gave : Fraction
B.P (10mm.)
Yield (grams)
1
46-50' 52-54' 102-105° residue
5 30 9 18
2 3
4
The first fraction proved t o be a mixture. When nitrated i t gave a small amount of p-nitro-t-hutylbenzene. Determination of chlorine (Carius) indicated about 80% of CSIII?CI. Treatment of 2,3,4-trimethylpentanol-3 in ether solution with dry hydrochloric acid gave an alkyl chloride which boiled a t 47-49" (10 mm.). Fractions of this range from several condensations were combined and refluxed with dilute sodium hydroxide. Extraction with ether and repeated distillation of the extract gave an unsaturated hydrocarbon boiling at 112-116'; d:" 0.733; n: 1.4204 (11). The fraction boiling a t 52-54" (10nim.) consisted of tert.-butylbenzene. It was identified by the melting point (168-170") and mixed melting point of its acetamino derivative which was prepared by a modification of the method of Ipatieff and Schmerling (12). After several fractionations the third fraction came over at 104-105" (11 mm.). It was nitrated, reduced, and diazotized t o 2,3,4-trimethyl-3-p-hydroxyphenylpentaneand this was identified by the melting points of the 3,5-dinitrobenzoyl derivative and the a-naphthylurethan (4). @.~-l)imethyl-d-ethylpentanoZ-S.This was prepared from di-isopropyl ketone and ethylmagnesium bromide (lo), b.p., 176-177"/750, d r 0.8485; n: 1.4388. Four fractions were obtained from the condensation: Fraction
1
2 3 4
6
B.P. (750 mm.)
146-160' 165-172" 188-193" 106-114' (10 mm.) Residue
Yield (grams)
10 2.5 3 25 10
406
RALPH C. HUSTON AND JORGE AWUAPARA
The first fraction consisted of an alkyl chloride (or alkyl chlorides) which liberated prepared from hydrochloric acid at the boiling point (2,4-dimethy1-3-ethyl-3-chloropentane the alcohol and hydrochloric acid boiled with decomposition at 156-160" under atmospheric pressure). The fractions from these condensations were combined and decomposed by refluxing with dilute sodium hydroxide. The unsaturated compound came over after three distillations a t 126-128". The density ( d r 0.7332) and refractive index (n: 1.4148) gave the molecular refraction 43.02 as compared with 43.14 calculated for 2,4-dimethyl-3-ethylpentene-2. There is no record in the literature of the constants of this hydrocarbon. The second fraction was identified as tertiary butylbeneene by means of its acetamino derivative (12). The third fractions from three condensations were combined and refractionated several times. Most of the product came over a t 188-190'. It was identified by the melting point of its acetaniino derivative (12) as tertiary amylbeneene. The fourth fraction had, after several refractionations, a constant boiling point, 251-253" (750 mm.) and 112-114° (10 mm.). Its analysis and physical constants proved i t t o be R nonylbenzene; df 0.8706, n: 1.4920. MR, 68.01 (Calc'd 67.88). Anal. Calc'd for CllH24: C, 88.16; H, 11.84. Found C, 87.83; H, 12.05. .t?,4-DimethyZpentanoZ-S.This was prepared by reducing di-isopropyl ketone by means of di-isopropylmagnesium bromide (13) ; b.p. 136-137"/745; d: 0.8157, n: 1.4159. Fractionation of the condensation products gave: Fraction
B.P.
Yield (grams)
1
112-118" (745 mm.) 101-104" (20 mm.) Residue
10 30
2 3
10
The alkyl halide (or halides) of the first fraction was decomposed by boiling with dilute sodium hydroxide. The unsaturated compound came over at 83-85'. Oxidation with acid potassium dichromate gave a small yield of isobutyric acid. Physical constants checked closely with those given (11) for 2,4-dimethylpentene-Z; d: 0.6961, T Z 1.4016. ~ The second fraction proved t o be 2,4-dimethyl-2-phenylpentane (I), b.p. 216-217' (715 mm.), d: 0.8724, n: 1.4928. When nitrated, seduced, and diazotized i t gave 2,4-dimethyl-2-p-hydroxylphenylpentane as proved by the melting points of the benzoyl ester (70-71") and the a-naphthylurethan (7) (114-115'). SUMMARY
1. As a part of an investigation of the fragmentation of tertiary alcohols, 2,3,4-trimethylpentanol-3was added to a suspension of aluminum chloride in benzene. The yield of 2,3,4-trimethyl-3-phenylpentane was about ten per cent. 2. The principal product of fragmentation, tert-butylbenzene, was obtained in a yield of approximately fifty per cent (calculated on the 1:l basis). A mechanism is suggested. Alkyl halides, CsHI7C1,and alkenes, C8Hl8,were byproducts. 3. Similar treatment of 2,4-dimethyl-3-ethylpentanol-3gave a fair yield of nonylbenzene (30%) and, as fragmentation products, small yields of tert-butylbenzene and tert-amylbenzene. Nonyl chloride and nonene fractions were isolated. EASTLANSING, MICH
FRAGMENTATION OF ALCOHOLS
407
REFERENCES HUSTON, Fox, AND BINDER,J. Org. Chem., 3,251 (1938). HUSTON, GUILE,SCULATI, AND WASSON, J. O r g . Chem., 6, 252 (1941). PRICE,Chem. Revs., 29, 44 (1941). HUSTON AND MELOY, J. Am. Chem. Soc., 64,2656 (1942). H U N T E ARN D YOHE,J . Am. Chem. SOC.,66, 1248 (1933). PRICEAND CISROWSKI, J . Am. Chem. SOC., 60,2499 (1938). (6) WHITMORE,J . Am. Chem. Soc., 64, 3274 (1932). J. Am. Chem. SOC.,69,2002 (1937). (7) HUSTONA N D HEDRICK, ( 8 ) HUSTON AND KAYE,J . Am. Chem. Soe., 64, 1576 (1942). (9) NORRXS AND INGRAHAM, J. A m , Chem. Soc., 60, 1421 (1938). EVART, ? S. * Thesis, I. Michigan State College (1938). (10) STAS,Bull. S O C . chim. Belg., 36, 379 (1926). AND LAUGHLIN, J . Am. Chem. SOC.,64, 2392 (1932). (11) WHITNORE E G L O I ~“Physical F, Constants of Hydrocarbons,” Vol. I, pg. 233. A. C. S. Monograph, No. 78. (12) IPATIEFF A N D SCHMERLING, J . Am. Chem. Soc., 69, 1056 (1937). AND BLATT,J. Am. Chem. SOC., 61, 1233 (1929). (13) CONAXT (14) EGLOFF, Eoc. cit. page 210. (15) HUSTON AND GUILE,J . Am. Chem. Soc., 61, 69 (1939). (1) (2) (3) (4) (5)
