A Guidebook to Mechanism in Organic Chemistry

Addition to Carbon-Carbon Double Bonds

KTO©

R.CHC1IO >- R..C Clio > RjCCHO

C1I. 0:11—C^N CH 2 —CH^ci-N

T

R„CCHO

CH 2 CH 2 CN

The value of cyanoethylation is that three carbon atoms are added,

of which the terminal one may be further modified by reduction,

hydrolysis, etc., preparatory to further synthetic operations.

ADDITION TO ap-UNSATURATED CARBONYL COMPOUNDS

The most important electron-withdrawing group is probably

)>C=0, found irr*fl)f-unsaturated aldehydes, ketones, esters, etc.

TheSfc*Sy»tems will acWon hydrogen halide, etc., by a 1:4-mechanism

involving initial protonation of oxygen:

>C=C—C=0 F=± >C=tciC—OH ~ ^C—C=C—OH

I I V I I / I I J

(XXXVIII)

Br<=

Br

\.

/

(XL)

nr at

>C—CH—C=0 - Nc—C=C-

II /II

Attack by Br^0 on the ion (XXXVIII) at Cl (1:2-addition) would

lead to formation of a ge/M-bromohydrin which is hjghly unstable,

losing HBr, hence preferential attack at C 3 (1:4-addition) yields

(XXXIX) which is, of course, the enol of the /?-bromdketone (XL).

Addition to ajS-unsaturated acids proceeds somewhat similarly.

The reaction is normally carried out in the presence of base in order
to obtain an anion from the would-be adduct. Carbon-carbon bonds
may also be formed: