541.643.4

THE STRUCTURE OF ALIPHATIC COMPOUNDS:THE WALDEN INVERSION

BY

WILLIAM TAYLOR.

5.912.H0.94

18-21° 122,40080.3° t 250 (approx)25° 12,200

2.3.4.

From kinetic measurements of the reactions of Me, Et, Pr Pand BuY,phenyl ethers, acetates, alcohols (with hydrogen bromide) and bromides

(with aqueous alcohol) BuY compounds are found to react very muchmore rapidly than the others. The suggestion is made that in primaryand secondary aliphatic compounds. AlkX, X is bound partially by (1­

hydrogen. This would support theories of Pol any i and others toexplain the Walden Inversion as shown by the relative reactivities ofthe corresponding alkyl halides with anions.

During recent years kinetic investigation have revealed similarrelative reactivities of the following alkyl compounds: phenyl ethers 1),acetates 2), alcohols 3) and bromides 4), as shown in the table wherethe figures indicate relative velocities based on PrY = 1.

Reaction Reagents Solvent Temp. BuY prP Et Me

1. Alk OPh + HBr acetic acid 18-21° (BuYOEt 2,090)* I (EtOPhO.l2) 0.29(EtOEt 0.20)

1.02O.H0.38

Alk OAc + HBr acetic acidAlk OH + HBr phenolAlk Br + EtOH aqueous

(60 %) ethyl alcohol(60%)

*) The penyl ether was not studied so ~his figure is included for purposes ofcomparison.

t) Exact measurements were not undertaken for BuYOH but it was found thatthis alcohol reacted with HBr at 40° at least 100 times as fast as MeOH.

The very high reactivity of all BuY" as compared with Me, Et, andprP, compounds towards these reagents must have some relationshipto structure. In the author's opinion it is obvious that the disappearanceof the last u-H atom is responsible for this phenomenon, and thehypothesis is put forward that a-hydrogen binds the group X (inAlk X), which is to suffer substitution. It is unlikely that a truehydrogen bond or bridge connects the C-atom with X (see e.g.,

1) Tronow and Ladigina, Ber. 62, 2844 (1929).2) Tronow and Ssibgatu1lin, Ber. 62, 2850 (1929).3) Bennett and Reynolds,]. Chem. Soc. 131 (1935).4) T a y lor, ]. Chern. Soc. 1937, 992..

William Taylor, The Walden inversion. 899

Me",Me-".C-XMe/

Hug gin s (;) in these compounds and it is postulated that, althoughthe main linkage is between the C~atom and X, in primary andsecondary aliphatic compounds there is also supplementary bindingbetween any «-hydrogen atom and X. so that BuYX and MeX wouldhave formulae:

~'"",-\:.-,-,'/~""7X

H-'

This hypothesis would lend support to the various theories 6) whichhave been brought forward to account for the phenomena of theWalden Inversion. Reagents used in producing such phenomena mayexist as ions or as covalent molecules. According to Pol any i 7)negative particles (anions), attacking the C~atom of the C-X link,cause inversion whilst positive particles (cations), attacking X, causedirect substitution. Presumably covalent molecules may do either- depending on whether C or X is the centre of attack - or both.The effect of anyone reagent on a compound Alk X in a certainsolvent may then be ascribed to the tendency of the reagent todissociate or not in the solution, which tendency will be governed bythe nature of the mixed solvent (Alk X and solvent) and by conditionsof temperature and dilution. Such an interpretation may explain thenumerous anomalous results obtained in this field of study H). Moreespecially. from the point of view of this communication, the supple­mentary uH-X bonds would tend to produce an enhanced positive("unsaturated") condition of the central carbon atom and so renderit more prone to attack by any negative particle (anion). Thus,substitution by an anion should be facilitated on proceeding down theseries BuY, Prfl, Et, Me. This is actually found to occur, as kineticinvestigations !J) have shown for an anion interacting with an alkylhalide.

Lon don. The Polytechnic, Regent St .. June 1937.

(Received June 11th 1937).

5) Huggins. J. Org. Chern. 1,407 (1936).G) Lew i s, Valence and the Structure of Atoms and Molecules. p. 113 (1923);

Kenyon. Phillips and Turley. J. Chern. Soc. 127,406 (1925); Meisen­heimer. Ann. 456,126 (1927).

7) Polanyi. Atomic Reactions. p. 61 (1932).8) Wa Ide n, Salts. Acids and Bases: Electrolytes: Stereochemistry. p. 371.

et seq.9) He c h t, Con r a d and B r u c k n e r. Z. physik. Chern. 4, 288 (1889);

Wislicenus. Ann. 212, 239 (1882).