A Guidebook to Mechanism in Organic Chemistry

Addition of Thiols

An ^ with the hemi-acetal is often set up on dissolving the aldehyde
in the alcohol but conversion to the acetal proper does not take place
in the absence of added catalysts.
With ketones the carbonyl carbon atom is not sufficiently positive
to undergo initial attack by R- OH and ketals cannot readily be made
in this way. Both acetals and ketals may, however, be made by reaction
with the appropriate alkyl orthoformate, HC(OR) 3 , in the presence of
NHjCl as catalyst. These derivatives may be used for protecting
carbonyl grojips for they are extremely resistant to alkali, but the
carbonyl compound may be recovered readily on treatment with
dilute acid.

(iii) RSH

Mercaptans will react with aldehydes and ketones to yield thioacetals,

R'CH(SR) 2 , and thioketals R' 2 C(SR) 2 , respectively. The successful

attack on the carbonyl carbon atom of ketones indicates the greater

tendency of R-SH than R-OH to form an effective nucleophile,

RSe, i.e. the greater acidity of thiols than the corresponding alcohols.

These derivatives offer, with the acetals, differential protection of the

carbonyl group for they are stable to acid but readily decomposed

by HgCl 2 /CdCOs. They may also be decomposeaoy Raney nickel

NI/H,

R',C=0 > R',C(SR), > R',CH,

the overall reaction offering a preparative method of value for the

reduction of-CHO-* -CH 3 and)>CO->-)>CH 2.

(iv) ®CN, HS0 3 e, etc.

These are both normal addition of anions:

OH

o

II

ecN/*

/

1

R—C—H

HCN

> R-

or H.O

1

-C—H

1

CN

II

R—C-

/

-H

V

• 
  


• \
  HSO,©\

o

e

1

R—C—H > R-

OH

1

-C—H

SO, OH SO,Oe