A Guidebook to Mechanism in Organic Chemistry

Addition to Carbon-Oxygen Double Bonds

This is, indeed, observed in practice; the ketone is in part reduced to

the secondary alcohol in the process, via a cyclic transition state

closely resembling that already encountered in the Meerwein-

Ponndorf reaction (p. 166).

If Grignard additions do proceed via initial attack of one molecule

of the reagent on the carbonyl oxygen, it might be expected that the

reaction would be promoted if a more effective Lewis acid were intro­

duced into the solution, for this would co-ordinate preferentially to

yield an even more positive carbonyl carbon atom. Thus introduction

of MgBr 4 has been observed to double the yield of tg£iary alcohol

in the reactions of some ketones with Grignard reagents.

Grignard reagents are however being increasingly superseded by

other organo-metallic compounds for preparative addition of this

kind, particularly by the more reactive lithium derivatives.

(b) Acetylide ion: A very useful reaction is the addition of acetylide

ion to carbonyl compounds, the reaction often being carried out in

liquid ammonia in the presence of sodamide to convert acetylene

into its carbanion (p. 211). Hydrogenation of the resultant acetylenic

carbinol (XXVII) in the presence of Lindlar catalyst (partially

poisoned palladium) yields the olefine (XXVIII); the latter undergoes

an acid-catalysed^Jlylic rearrangement (p. 29) to the primary alcohol

(XXIX), which, as^he corresponding halide, can be made to undergo

further synthetic reactions. The series constitutes a useful preparative

sequence:

OH OH

f\ NH, / H, /

R,C=M5 —> RSC > R,C

tefeCH C==CH ca,alyst CH=CH,

(XXVII) (XXVIII)

(0+H©"

(ii) -H.O

(i) +H.O

R,C=CH—CH,OH <

(ii)-H®

(XXIX)

e

R 2 C R 2 C

CH—CH 2 CH±CHruli 8

(c) Aldol condensations: The action of bases on an aldehyde having
a-hydrogen atoms results in the formation of a stabilised carbanion