Emam D. HWGW
VOl. 57
[CONThIBVTION FROM UNWBRSITY c h L E G I t , LONDON, ENGLAND]
Hydrolysis of Secondw and Tertiary Alkyl Halides BY EDWARD D. HUGHES Comment has recently been passed112 on the substitution in accordance with mechanism s N 1 . relative proportions in which alcohols and olefins In the simplest examples of hydrolysis this is the are formed by the hydrolysis of sec6ndary and fate of most of the cations, but increasing alkylatertiary alkyl halides. We wish to direct attention tion will favor the rearrangement with proton to some considerations relating to the same topic. ejection. One of the reactions to be considered involves French, McShan and Johler' examined the substitution by a nucleophilic reagent. Hughes, hydrolysis of s-butyl bromide in a variety of Ingold and Pate13 recognized two mechanisms of aqueous alkaline solutions, and found a regular this type of substitution, namely, a bimolecular variation of the percentage of olefin with the mechanism (SN2) involving attack by hydroxide concentration of the Quantitative inions, and a unimolecular mechanism (SN~) kineti- terpretation of the data is rendered impossible by callydependent on the ionizationof the alkyl halide. the presence of two liquid phases, but tbe figures are consistent with the operation of the combinaAlk. Hal OH- +Alk. OH Hal- ( S N ~ ) tion of mechanisms SN1 and E2. Alk. Hal +Alk+ Hal-,followed by Alk+ + OH- ----f Alk. OH (instantaneously) (SN1) The writer has examined the hydrolysis of isoIt was anticipated that there would be a change- propyl and s-butyl bromides in homogeneous over from 'the bimolecular to the unimolecular solution in 60% alcohol. In dilute alkaline solumechanism toward the right-hand end of the tions, when the reaction is almost exclusively series: CHI, CHIMe, CHMe2, CMe3. This was hydrolytic, decomposition follows a unimolecular later r e a l i ~ e d . ~The writer has now located the law, and the velocity is identical with that obtainchaage-over (in dilute aqueous alcoholic solution) ed in acid solution. In more strongly alkaline solutions, when the o l e h formation becomes between the ethyl and isopropyl groups. The other reaction to be discussed involves the impartant, the velocity is partly dependent on the elimiaation of a p-hydrogen atom. Hughes, hydroxide-ion concentration, and a comparison Ingold and Patel showed that this readion would with the data relating to acid media supports the normally possess a bimolecular mechanism which assumption that the elimination process is bifor alkyl halides may be formulated as follows molecular. French, McShan and Johler have also shown : -Hal + OHH -CRL!Rs that the hydrolpis of t-butyl bromide proceeds H20 C R d R t Hal- (E2) mnch more rapidly than that of the secondary The dotted lines indicate the direction of the compound, and that the production of olefin, electron tremfers. If the halogen atom is first under the conditions employed, is very small. ioriized, however, another mechanism intervenes. The writer has conf'nmed these statements, and No detailed study of this elimination mechanism has shown that the hydrolysis of t-butyl chloride has yet been published from these Laboratories, in homogeneous solation is unimolecular.' In tbut it may be stated that it leads k a unimolecular butyl halides ionization has been so far increased that reaction SNl, which involves ionization, ,reaction of the type predominates over all reactions such as H-CR2CR, i -Hal +H--CR&+R* Hal-, followed by E2, which do not. With further alkylation, as in t-amyl H i -CR2CfR2 +H + CR2=CR2 (instantaneously) chloride, reaction E l attains importance. The intermediate cation may, of course, effect a Woodburn and Whitmore2 have shown that the (I) French, McShan and Johler, THISJOURNAL, 66,1346 (1934). hydrolysis of t-amyl chloride proceeds with almost
+
+
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+
+
(2) Woodburn and Whitmore, ibid.. 68, 1394 (1934). (3) Hughes, Ingold and Patel, J . Chem. Soc., 526 (1933). (4) Cf. Hughes and Ingold, Nature, 180,933 (1933). (5) The position of the point of mechanistic change must depend, inkr olio. on the medium and the concentration, for non-ionizing media and high concentrations will favor mechanism SN2.
(6) Silver oxide was exceptional, but this may be omitted from consideration as i t is well known that the reactions between alkyl halides and silver compounds are heterogeneously catalyzed by Dre. cipitating rilver halide (cy. Baker, J . Chcm. SOL.,987 (1934)). (7) Hughes, J . Chem. SOC.,255 (1935).
TEE ALKYLATION oP PHENOLWITH
April, 1935
the same velocity whether tke reagent is dilute sodium carbonate, water or dilute! sulfuric acid, although both alcohol and o l e b are formed in large amount. This surely indimtes that the total decomposition is unimolecular, and that the speed is independent of reagent anions. The mechanisms a t work here are s N 1 and El. The unimolecular character of the reaction in homopeneous solution has been confirmed by the writer.
Experimental The bimolecular character of the alkaline hydrolysis of methyl and ethyl halides has been previously established.' The point of mechanistic change in the alkyl series is illustrated by the figures in the subjoined table, selected from dynamical experiments which will be published in detail later. They refer to experiments in which the concentration of the alkyl halide does not appreciably change during the reaction. The velocity in dilute alkaline solution is then constant and is identical with that in acid solution. The first five lines are self-explanatory. The (8) Bruyn and Steger, Rcc. Irav. chim., 18, 311 (1699); Grant and Hinshelwood, J . Chcm. Soc., 258 (1933).
[CONTRmUTION FROM THE
709
ALCOHOLS
figures in lines 6 and 7 show the average changes in titer per hour; they relate to 10 cc. of the reaction mixtures and are expressed in cc. of 0.005 N acid and. alkali, respectively. i-PrBr s-BuBr 1-AmCl Halide Moles halide per mole 38 'io 63 KOH 90 60 Medium, % EtOH 60 25 45 45 Temperature, "C. 0. 0043Range of readings alka- 0.0030- 0.0029,0030 line N --f acid N ,0040 ,0035 2.52 2.60 3.00 Velocity in alk. s o h . 3.12 2.44 2.58 Velocity in acid s o h .
The author wishes to express his indebtedness for his invaluto Professor C. K. Ingold, F.R.S., able help. summary The dynamics of the hydrolysis of certain secondary and tertiary alkyl halides have been investigated and the mechanism of the reaction is discussed. LONDON,
ENGLAND
DEPARTMENT OF CHEMISTRY,
RECEIVEDNOVEMBER 30, 1934
UNIVERSITY O F NOTRE D A M k
Organic Reactions with Boron Fluoride. M. The Alkylation of Phenol with Alcohols' BY F. J. SOWA, G. F. HENNIONAND J. A. NIEUWLAND In a recent publication2 it was shown by conductivity measurements that the system phenolboron fluoride possessed acid properties considerably stronger than phenol. Evidence was also presented for compound formation from one mole of boron fluoride and two moles of phenol. Boron fluoride has been used as an agent in the formation of alkyl phenyl ethers from olefins and phenols,* from ethers and phenols3 and as an esteifying agent.* These properties should make this agent particularly adaptable in the reaction between alcohols and phenols. Altogether neither methyl nor ethyl alcohol reacted with phenol in the presence of boron fluoride by heating at atmospheric pressure, they did react quite readily when heated in a sealed tube. The products were mainly anisole and phenetole, respectively. Small quantities of substituted (1) Paper V I I I , Jlorri,
bowa and Nieuwland, THISJ O U R \ A I
66, 2880 (1934)
(2) Sowa, Hinton d u d Nieuwland, r b r d , SS,a402 (1933) i 3) Ilennion, Hinton and Nieuwland, h d , 66, 285': (197 1) (4) IIiriton and Nieuwland. r b r d , 84, 2017 (1932)
phenols and alkyl phenyl ethers were also formed. Isolated cases of reactions between alcohols and phenols using various catalysts have bceii rcp o ~ - t e d . ~ ' ~Some - ' ~ ~ of these experhntnters claim to have obtained fi-isobutylphenol from the condensation of isobutyl alcohol with phenol. It has since been demonstrated9 that isobutyl phenyl ether rearranges to tertiary butylphenol. In none of these investigations was there any attempt made to prove the mechanism of condensation between the alcohol and phenol. Attempts are reported in this paper to determine, first, if the reaction between alcohols and phenols involves the removal of the elements of water, as in most esterification reactions, equation I CdijOH
+ CnHJu+~OH
----f
C~Hj0CnH2.+1 f H20 (1)
(5) Liebmanii, Be?., 16, 150 (1882,. ( 8 ) Senhwski, ibid., 44, 2974 (1891). ( 7 ) hlazzara, J. Chenr. Soc., 84, W 8 (1882). ( 8 ) Mazzura? Guz:. chim. i t a [ . , 12, 505 (1882) (9) Smith, Taro J O U R N A L , 86, 3518 (1933).
