TEXTBOOK ERRORS:'
GUEST COLUMN
XIX: The Relative Reactivity of Acetylenes and Ole$ns Toward Bromine RALPH DANIELS and LUDWIG BAUER College of Pharmacy, University of Illinois, Chicago
MANY contemporary textbooks2and laboratory manuals in organic chemistry either state explicitly or imply that bromine adds much faster to acetylenes than to olefins. Some authors consider the triple bond of acetylenes more unsaturated3 than the double bond of olefins and hence attribute greater reactivity to acetylene^.^ Often the reaction with bromine is recommended as a qualitative test for both ethylenic and acetylenic unsaturation. Contrary to this popular belief, alkynes in fact react very slowly with bromine, especially when compared to the readiness with which alkenes undergo a similar reaction. We have found that acetylene (generated from calcium carbide) did not discharge the brown color of bromine, either from hromine water or from a 5% solution of bromine in carbon tetrachloride, whereas ethylene decolorized the test solutions instantaneously. Other acetylenic compounds, such as phenylacetylene, propargyl alcohol, acetylenedicarhoxylic acid, 2-hutyne1.4diol. and 3-methvl-3-hvdroxv-l-butvne reacted Suggestions of msterirtl suitable for thk column and guest columns suitable for publication directly are eagerly solicited. They should be sent with as many details as possible, and particulerly with references to modern textbooks, ta K m l J. Mysels, Department of Chemistry, University of Southern California, Los Angeles 7, California. Since the purpose of this column is to prevent the spread and continuation of errors and not the evaluation of individual texts, the source of errors discussed will not be cited. The error must occur in at least two independent standard books to be presented. 'Throughout this discussion, the term unsaturation refers strictly to the reactivity of a multiple band, and not to its quantitative behavior. 4 A similar situation once existed in the benzene problem. Benzene, as written by conventional Xekul6 structures, with three conjugated double bonds, represents a molecule which should add bromine avidly-ertainly as easily as olefins. [See KIPPING, F. S., AND F. B. KIPPING, "Organic Chemistry," Revised edition, W. R. Chambers, Ltd., London, 1943, Part I, p. 355, et seq., for a historical review of this subject.]
sluggishly with bromine compared to a similar reaction of olefins. That bromine reacts preferentially with a double bond rather than with the triple bond-if a choice e x i s t s m a y be illustrated by the following reported reactions (1, 8):
CH&-CHBlcCH,Ca2H (90% yield)
Rr.lCUC11.
Robertson and his co-workers' (5) have compared the rates of addition of bromine to alkenes as well as their corresponding alkynes and their results are summarized in the table. Their data clearly show that bromine in acetic acid a t 25" adds much faster to alkenes than to alkynes. MECHANISM O F BROMINE ADDITION TO UNSATURATED COMPOUNDS
Alkenes. There is a considerable body of evidence which indicates that the polar mechanism for the addition of bromine to olefins proceeds by a two stage process (4-7) : (1) The reaction is initiated by an electrophilic attack on the olefin (I) by the halogen to forms. resonance stabilized bromonium ion intermediate (11) :
JOURNAL OF CHEMICAL EDUCATION
Alkme
Alkyne
CH8(CHa)7-CH=CH-(CH~)1C01H
CHJ(CH2)~-CeC-(CH,),COtH
Rale-ratio' 50,000
C28
Ratio of the seoand-order rate constant for the addition of bromine to the alkene to that far the alkyne. (2) The rertetion is completed by combination of (11) with a bromide ion to give (111).
R Bre R
\I \/ G C
R/
R Br
+ Bra
R
L
\R (11)
/
R
I\
Br R
catalyzed by light (9) and inhibited by nitric oxide (Job). This is consistent with the general characteristics of a free radical mechanism (22). Sinn studied the rate of addition of bromine to stilbene (IV). tolane (V), p,pl-dinitrostilbene (VI), and p,pl-dinitrotolane (VII). NO*
(111)
Alkynes. It is misleadimg to associate greater unsaturation (ergo, greater reactivity) with the two ?rbonds in a triple bond compared to the me =-bond in a double bond. This assumption ignores the conclusions drawn by Walsh (8) from the far ultraviolet spectlvm of acetylene. This datum indicates that both ?relectron pairs in acetylene are more tightly bound than the single ?r-electron pair in ethylenethus inferring that the r-electrons in acetylenic molecules are less vulnerable to an attack by an electrophiiic reagent than those in olefins. The extension of this concept to a comparison of other reactions of alkynes and alkenes with other electrophilic reagents is discussed by Bohlmann (9). The intrinsic mechanism of the addition of bromine to alkynes has not been established and much speculation exists on this subject. Robertson et al. (3) believed that the addition proceeds similarly to the addition to alkenes, viz., initiation by electrophilic attack; but he stated that some acetylenic compounds may add bromine by a nucleophilic mechanism. In neither case was a stepwise mechanism presented. Bohlmann (9) also preferred an ionic mechanism, initiated by a nucleophilic attack by the bromine molecule. for the reaction carried out in the absence of light. Sinn (IOU)has advanced argunents for a free radical mechanism and reported some preliminary work to support such a theory. It was found that alkynes react exceedingly slowly with bromiue in the dark in marked contrast with alkenes which react very rapidly in the dark. The reaction of alkynes is strongly
LITERATURE CITED
NOn
The reactions were carried out in glacial acetic acid a t 68.5", and three important conclusions may be drawn from Sinn's work: (1) The data show that the alkyne, tolane, reacts much slower than the corresponding alkene, stilbene. (2) Furthermore, it was found that p,pldinitrotolane reacts faster than tolane. Sinn pointed out this is best interpreted on the basis of a free radical reaction, for it is known that a para nitro group enhances such reactions (if?). (3) Sinn also demonstrated that p,pl-diuitrostilbene reacted much slower than stilbene. If the addition proceeds by an electrophilic mechanism, this diminution in rate is expected because of the strong electron-withdrawing nature of the para nitro group (13). Therefore, it is apparent that a tacit extrapolation of the polar mechanism of alkenes to that of alkynes is untenable. Thus, it is clear that alkenes and alkynes of similar molecular environment exhibit great differences in their reactivity toward bromine.
ed., John Wiley & Sons, Inc., New York, 1957, pp. 2 9 s
""". W?
(1) PREVOST,C., P. SOUCHAY, AND J. CHAUVELIER, Bull. soe. Aim., [5], 18, 715 (1951). (2) PETROT,A. A,, AND Yu. I. PORFIR'EVA,Zhw. Obshehel Khim., 23, 1867 (1953) C. A,, 49, 147 (1955). (3) ROBERTSON, P.,W., W. E. DASENT,R. M. MILBWN, AND W. H. OLIVER,J . Chem. Soc., 1950, 1628. (4) DE LA MARE. P. B. D., Q w r l . Revs. (London), 3, 1 2 M 5 (1949). (5) HINE, J., "Physical Organic Chemistry," McGraw-Hill Book Co., Ino., New York, 1956, pp: 202-13. (6) WHELAND, G. W., "Advanced Organlo Chemistry," 2nd
VOLUME 35, NO.9, SEPPEMBER, 1958
(7) INQO~D, C. K., "Structure and Mechanism in Organic Chemistry," Cornell University Press, Itbaca, 1953, pp. 65%70. (8) WAL~E, A. D., Quart. Revs. ( L n d n ) , 2 , 78-9 (1948). (9) BOHLMANN, F., Angew. Chem., 69, 82-6 (1957). (10) (a) SINN,H., Angew. Chem., 69, 754 (1957). (b) SCHILOW, E., quoted by Sinn. (11) WALLING, C., "Free Radicals in Solution," John Wiley & Sons, Inc., New York, 1957, pp. 35-6. (12) Ref. (5), pp. 439-42. (13) Ref. (61,p. 352. 445
