Nov. 20, 1932
ALKALICLEAVAGE O F BENZOINAND
THE
5643
le1 experiment without silica gel, the color of the benzene solution was a very pale yellow and yellow in the case of (IIa) and ( I I I a ) ? respectively. GIZA,CAIRO,EGYPT
above experiments were repeated with dried silica gel Grade “A” in place of activated alumina, the color of the adsorbates on the silica gel was deep red (after 24 hours) and the benzene solution acquired a deep, orange color. In a paral-
[CONTRIBUTION FROM
a-SURSTITUTED BENzoINs
DEPARTMENT OF CHEMISTRY, KANSAS STATECOLLEGE ]
The Rearrangement and Cleavage of Benzoin and a-Substituted Benzoins by Alkali’ BY DEXTERB. SHARP^
AND
EDSELL. MILLER
RECEIVED JULY 23, 1952 The cleavage by alkali of benzoin and six a-substituted benzoins has been examined in detail. a-Phenylbenzoin gave quantitative yields of benzhydrol and benzoic acid. a-( o-Tolyl)-benzoin gave quantitative yields of benzhydrol and o-toluic acid. Benzoin, a-methyl-, a-benzyl-. a-(m-tolyl)- and a-(p-tolyl)-benzoin each gave varying quantities of four products, which indicated partial rearrangement prior to cleavage. Mechanisms are suggested for these reactions.
Acree3 observed that a-phenylbenzoin (I, R = CsH5)was cleaved by hot alcoholic alkali and gave
during the Grignard reaction] and benzoin itself is known to be one molecular species only. Reduction products of a-(o-tolyl)- and a-(m-tolyl)-benzOH 0 oin corresponded with compounds previously reI II C6H:,-C-C-C6Hj ported, and Zerewitinoff determinations of active I hydrogen and carbonyl group content checked R reasonably well for a-keto1 structures. The homoI geneity of the liquid a-substituted benzoins is benzhydrol (11)and benzoic acid (111)in unspecified probably subject to question. amounts. He reported also3”that a-( 1-naphthyl)Cleavage Reaction.-The a-arylbenzoins were benzoin gave benzoic acid and a-naphthylphenyl- cleaved easily in refluxing methanolic alkali and carbinol upon alkaline cleavage. Roger and Mc- gave total‘ product quantities accounting for 70Gregor4 reported similar scissions in which R was 100% of the initial amount of a-ketol. Benzoin, o- and m-tolyl and methyl. Pascual and Cerezo5 a-methyl- and a-benzylbenzoin were unaffected reported the alkaline cleavage of a-benzylbenzoin t o under these conditions, but were cleaved a t 160” benzoic acid and phenylbenzylcarbinol. The results in alkali-containing diethylene glycol with total of Roger and McGregor4 suggested that a rearrange- product quantities accounting for 80-100% of the ment may have occurred, but more complete initial amount of a-ketol. The following formulastudies of this reaction were lacking in the litera- tion represents the general cleavage reaction, and ture. I n contrast, Temnikova6 reported that tri- all corresponding products were obtained from five substituted a-ketols (RzCOHCOR’, R and R’ of the seven compounds examined iR = H, CH?, alternatively methyl and phenyl) did not isomerize C O H ~ C H m~ , and P-CHsCsH4). The products in alkali. Our interest in this reaction developed O H 0 when Acree’ss cleavage of a-phenylbenzoin was I -+(CaH6)tCHOH CtjH,CO.H C,H.,CHOHR repeated and found to give quantitative scission I1 I11 I\‘ into benzhydrol and benzoic acid. Since detection RCOzH of rearrangement is impossible with a-phenylv benzoin, attention was directed toward an examination of the cleavage of representative examples of I. from a-phenylbenzoin were mentioned above. Preparation of a-Substituted Benzoins.-ExCleavage of a-(o-tolyl)-benzoin gave only benzcepting benzoin itself, the starting compounds (I) hydro1 and o-toluic acid, confirming the report were prepared by addition of the appropriate Grig- by Roger and M ~ G r e g o r . ~This suggests that nard reagent to benzil. Benzoin, a-methyl-, complete rearrangement occurred, or that the a-benzyl-, a-phenyl- and a-(o-tolyl)-benzoin were starting compound was diphenyl-o-toluylcarbinol crystalline solids and appeared to be single molec- rather than the expected a-(o-tolyl)-benzoin. The ular species. The samples of a-(m-tolyl)- and a- identities and relative quantities of the cleavage (p-tolyl)-benzoin were viscous liquids obtained by products from each reaction are summarized in fractional distillation a t reduced pressure. It Table I. seemed unlikely that the crystalline a-substituted The cleavage of benzoin itself to give benzhydrol, benzoins were mixtures of isomeric a-ketols formed benzoic acid, benzyl alcohol and formic acid is evidence indicating partial rearrangement prior to (1) Abstracted in part from the Ph.D. thesis submitted by Edsel I.. Miller to the Graduate Faculty of Kansas State College, January, cleavage. Knoevenagel and Arndts’ reported the 1952. Contribution no. 281 from the Department of Chemistry. cleavage of benzoin to benzoic acid and benzyl (2) Monsanto Chemical Co., Dayton, Ohio. alcohol in concentrated alkali, but apparently did (3) (a) S. F. Acree, Am. Chem. J . . 29, 588 (1903); (b) Bcr., 37, not obtain benzhydrol or formic acid. 2753 (190.1). Roger and McGregor4 reported that a-methyl(4) K. Roger a n d A. McGregor, J. Chem. S O C . , 442 (1934). (.5) J. Pascual Vila and J. Cerezo, A n d e s . soc. e s p a n . fix. quim.. 24, benzoin gave only trace amounts of benzhydrol, 395 (1926); c. A , , ai, 3798 (1927). whereas our results indicate that approximately (6) 1’. I. Temnikova, Veslrzik h n i n g r n d Uwiu., 138 (1947); C. A , . I
+
42, 41551 (1948).
+
(7) E. Knoevenagel and J. Amdts, Ber., S6, 1982 (1902).
+
TABLE I P R O D U C T S FROM T H E CLEAV-4GE
OF
0-KETOLSI N ALKALI
or-Keto! hfole PhCRIOH). Products, moles X 108 balCOPh Moles PhzPhPhCHance,n K x 103 CHOH CO?H OHR R C O ~ H m,
H 28 3 5 9 CHy CcH,CH? 9 8 ~-CII~CBH 10~ 1 ~ I I - C H I C ~ 16 H ~t i (J-cFI CbITi
7 -5 24 0
4 3 1 12 99 7 4 2 (i G 0 174 1
1 8 3 2 2
4 10 3 5 4
7 lh 8
7 6
S
4
