Ronald J. Baumgarlen .
University of Illinois at Chicago Circle Chicago, Illinois 60680
I
Textbook Errors, 82
Substitution Products in the Hofmann Elimination
Recently, Freedman discussed the misconceptions which have arisen in connection with the "Hofmann Rule".' Yet another aspect of the Hofmann Elimination reaction needs clarification. Namely, virtually every textbookZstates or implies that alkenes are the only products formed when tetraalkylammonium hydroxides are heated. For example, one classic textbook states that alcohols are formed when no @-hydrogensare available in the quaternary ammonium hydroxide (eqn. (I)), while only alkenes are
formed when @-hydrogensare available. Usually the textbooks are not so explicit on the latter point. Thus, yet another popular text has a list of reactions which give rise to alkenes. For the elimination reaction involving the treatment of alkyl halides with base, the text clearly points out that SN2 substitution products are in competition with EZeliminationproducts; but for the Hofmann elimination listed directly below, there is no statement pertaining to SN2 competition. The same implication by omission occurs in eight (out of eight) other textbooks inve~tigated.~ In actuality only a relatively small proportion of Hofmann eliminations give exclusively alkene products. Therefore, while such tetraalkylammonium hydroxides as tetraethylammonium hydroxide pyrolyze to give ethylene (eqn. (2)), the majority of the tetraalkylammonium hydroxides give varying amounts of the alcoholic substitution products along with the alkene products (eqn. (3))&
In fact, in a few cases, such as with the derivatives of tetrahydroquinoline and pavinemethine, the alcoholic and tertiary amine products are the only ones reported. The substitution competition pathway is discussed in Cope and Trumbull's excellent review of the Hofmann elimination and related reactions.
122 / Journol of Chemical Education
From a theoretical standpoint substitution products are to be expected in the Hofmann pyrolysis, since the reaction of quaternary amine salts with bases is roughly analogous to the nucleophilic displacement of water from protonated primary alcohols (eqn. (4a,b)). XIRCH20H2+] RCH,X H20 (44
+
OH- RCH?Nt(CH,),
heat
+
RCHIOH
+ (CHdrN
(4b)
In future textbook treatments of the "Hofmann elimination" reaction, it is suggested that a statement be made to the effect that alcoholic substitution products may also be formed. Moreover, eqn. (3) rather than eqn. (2) should be the one chosen to exemplify the Hofmann elimination. A further suggestion is to include the trialkylamine group among the common leaving groups discussed under nucleophilic substitution reactions. In relation to this, it is often pointed out that the hydroxide anion is a poor leaving group in SN2 reactions. To remedy this situation, the alcohol may be protonated with acid, wherein the much better leaving group, water, is readily displaced by SN1, SN2, EL,or E2 mechanisms. Similarly the NH2- anion is a very poor leaving group. By analogy the amino function may be exhaustively methylated, wherein the leaving group becomes a trialkylamine, and trialkylamines are much better leaving groups than the NHz- anion. As with the reactions involving water as a leaving group, alkenes and snbstitution products may be observed. Suggestiom of materid suitable for this column and guest columns suitable for publication directly should be sent with as many details as possible, and particu1s;rly with references to modern textbooks, to W. H. Eberbmdt, School of Chemistry, Georgia. Institute of Technology, Atlanta, Ga., 30332. ' FREEDMAN, L. D., J. CHEM.EDUC.,43,662 (1966). Since the purpose of this column is to prevent the spread and the continuation of errors and not the evaluation of individual texts, the sources of errors discussed will not be cited. In order to be presented an error must occur in at least two independent recent standard hooks. The present author was also guilty of this implication by omission in the recent review an deaminations: BAUMGARTEN. R. J., J. CHEM.EDUC.,43,402 (1966). ' COPE,A. C., A N D TRUMUULL, E. R., in "Orgmio Reactions," John Wiley and Sons, Inc., New York, 1960, Vol. 11, pp 350-352; 317493.
