A Guidebook to Mechanism in Organic Chemistry

Stereochemical Implications of Mechanism

R'»-C—OH

R

R* (+)

p-Me-C.HcSO.-a

an)

(IV) MeCO,® (VII)

R R

HO—C-R'

\

R*

eOH

MeCOO—C-R'

(VI) (-)• • (V)

formation of an ester with toluene /^-sulphonyl chloride is known not

to break the C—O bond of the alcohol,* hence the tosylate (IV) must

have the same configuration as the original alcohol. Reaction of tl.'s

ester with acetate ion is known to be a replacement in which p-

Me-C 6 H 4 -S<VOe is expelled and MeCOO® introduced,* hence

the C—O bond is broken in this reaction and inversion of configura­

tion can take place in forming the acetate (V). Alkaline hydrolysis of

the acetate (V->VI) can be shown not to involve fission of the alkyl-

oxygen C—O linkage.f so the alcohol (VI) must have the same con­

figuration as the acetate (V). As (VI) is found'to b?the mirror image of

the starting material, an inversion of configuration must have^aken

place during the series of reactions and it can only have taken place

during the reaction of acetate ion with the tosylate (IV). A number of

further reactions with the tosylate show that an inversion of configura­

tion takes place with a variety of anions and hence it may be concluded

with a considerable degree of confidence that it takes place on reaction

with chloride ion, so that the chloride (VII), like the acetate (V), has

the opposite configuration to the original alcohol (HI). Now that it is

thus possible to show that SN 2 reactions are normally attended by

inversion of configuration, independent demonstration that a par­

ticular reaction takes place by an SN 2 mechanism is often used to

relate the configuration of product and starting material in that

reaction.

* That such is the case may be shown by using an alcohol with wO in the -OH

group and showing that this atom is not eliminated on forming the tosylate; it

is, however, eliminated when the tosylate is reacted with Me - CO,®.

lst Hydrolysis of an acetate in which the alcohol-oxygen atom is labelled with

O fails to result in its replacement, showing that the alkyl-oxygen bond is not

broken in the hydrolysis (cf. p. 35).