James G . Traynham Louisiana State University Baton Rouge
The Bromonium lon
A lthough the concept of bridged ionic intermediates is eminently convenient and has become widespread in the interpretations of data relati ng to organic reaction mechanisms, it does not yet enjoy challe nge-free status. Two lengthy discussions at recent national meetings 1 illustrate the active concern among organic chemists about the validity of the descriptions of some ionic intermediates at bridged or "nonclassical" ions. Largely through choice by the principal participants in t hese public discu sions, detailed considerations were limited to some proposed ions wit h carbon as the bridging atom: for example, p henonium ions and ions arising from norbornane skeletons. The first bridged ion intermediate to be propo ed was the bromonium ion (1), however, and it seems that it should receive critical evaluation at this time. Some indication of t he need for such an evaluat ion is found in a footnote in a current text on organic mechanisms (2): "Although in the present chapter a number of add itions a re discussed in terms of the 'halogeonium-ion' mechanism, the reader should bear in mi nd t hat few organic reaction mechanisms have been accepted so widely while supported with such limited data." Several criteria have been used in support of bridged ion concepts: rearranged products, stereochemistry, and kinetic acceleration. No one of these alone is likely to be sufficient, ho\\·ever. For example, while an intramolecular rearrangement (such as migration of phenyl) may require a bridged species in the cour e of t he reaction, t hat species may be a transition state with (or without) classical carbonium ion intermediates on either side a long t he reaction coordinate. Sometimes, as in the case of cyclodecyl tosylate solvolysis, cred it for a n unexpectedly rapid reaction has been attributed to anchimeric assistance, without any supporting data other than t he fast rate (Sa), and subsequent study has revealed that such assistance pl~ys little or no role in the reaction (4). An entertaining and sometimes helpful analogy has been drawn between a reaction mechanism study and a murder mystery. 2 The energetics provide the motive; Th is review was supported in part by National ciencc Foundation through Grant No. G 14461. 1 ymposium on Carbonium Ions, Division of Organ ic Chemistry, t. Louis Meeting of the American Chemical ociety, March, 1961; and Conference on R eaction Mechanisms, Brookhaven National Laboratory, September, 1962. Two protagonists in each of these discussions were Professors S. Winstein and H. C. Brown, who do not a lways agree on the interpretation of data relating to the question of " noncalssical" carbonium ions. 2 As a result of informal inquiry among a limited samp le, our chemistry librarian (Miss May Olson ) has concluded that chemists genera lly enjoy murder mysteries; so the ana logy will be used hero in discussing the bromonium ion.
392 / Journal o f Chemical Education
stereochemistry, the alibis; kinetics, the eye-witness accounts; and the prodttcts, the corpse. As in any murder mystery, the ideal mechanism investigation has available information about all of these points. But as in any murder mystery, the investigator may be forced to proceed without information on one or more of them. Motive, for example, or the energetics, may be missing. Certainly all information on this score is helpful, but seldom do we know the important quantities, such as free energy or entropy changes, in a way that will permit a decision between two reasonable formulations of an intermediate. Lack of knowledge about motive, hO\\:ever, does not block inquiry along other lines, and the murder mystery may actually be solved "·ithout such information ever playi ng an important role. The bromonium ion was first suggested by Roberts and Kimball to account for t he well-established stereochemistry of bromine addition to olefins (1) . Even this original paper indicated that the "actual structure of the ion is undoubtedly intermediate between (1) and (2)." Structure (1) was not intended to represent a conventional free carbonium ion, however: "Since R,
"'
R 2/
R3 C- +C/
"' " X
R,
1
R,
R3
"'
C/
C-
R2/
"'-/ X+ 2
"'
R,
the two carbons in either structure a re joined by a single bond and by a halogen bridge, f ree rotation is not to be expected." A clear description of the difference in binding between carbon a nd bromine in (1) and (2) is not given, hut one infers that an electrostatic bond is implied in (1). The binding energy in the intermediate was regarded as relatively weak, weaker than the repulsive forces between the two carboxylate groups in maleate ion, which adds bromine to give the same dibromide as is obtained from fumarate ion (1). Stereochemical Evidence
The bromonium ion was quickly used by other authors to account for new observations on stereoM eso-diacetate of 2,3specific transfotmations. butanediol reacts with hydrobromic acid solution to give nL-2,3-dibromobutane, and DL-diacetate reacts to give meso-dibromide (5) . The corresponding bromohydrins react in equivalent stereospecific manner with hydrobromic acid to form dibromides (5). These results illuminate t he utility of the bromonium ion concept. But stereochemistry, like a libis, may be misleading. D ata from studies on deamination of amino alcohols show in a most persuasive manner that open carbonium ions can react stereospecifically (6) .
11
11
II Br
Br+
CII - 6- t - Clls _o_. Cl!,-6- 6-cH, --'L. CH,- c;:_'c-cH, '
I
I
I
AcO OAr. 1 111 e.•o-rli:ll' to lysis of the two bis-p-bromobenzencsulfonat
