Question

Which of the following compounds would undergo aldol condensation, which the Cannizzaro reaction and which neither? Write the structures of the expected products of aldol condensation and Cannizzaro reaction.

1. Methanal
2. 2-Methylpentanal
3. Benzaldehyde
4. Benzophenone
5. Cyclohexanone
6. 1-Phenylpropanone
7. Phenylacetaldehyde
8. Butan-1-ol
9. 2, 2-Dimethylbutanal

Answer 1

Aldehydes and ketones having at least one α-hydrogen undergo aldol condensation.

The compounds (ii) 2-Methylpentanal, (v) Cyclohexanone, (vi) 1-phenylpropanone, and (vii) Phenylacetaldehyde contain one or more α-hydrogen atoms. Therefore, these undergo aldol condensation.

Aldehydes having no α-hydrogen atoms undergo Cannizzaro reactions. The compounds (i) Methanal, (iii) Benzaldehyde, and (ix) 2, 2-dimethylbutanal do not have any α-hydrogen. Therefore, these undergo cannizzaro reactions.

Compound (iv) Benzophenone is a ketone having no α-hydrogen atom and compound (viii) Butan-1-ol is an alcohol. Hence, these compounds do not undergo either aldol condensation or cannizzaro reactions.

Aldol condensation:

(ii)

\[\begin{array}{cc}
\phantom{...................................................}\ce{CH3}\\
\phantom{................................................}|\\
\ce{2CH3CH2CH2 - CH - CHO ->[dil NaOH] CH3CH2CH2 - CH - CH - CH - CH2CH2CH3}\\
|\phantom{...................................}|\phantom{......}|\phantom{......}|\phantom{}\\
\phantom{......}\ce{\underset{2-Methylpentanal}{CH3}}\phantom{....................}\ce{\underset{3-Hydroxy-2,4-dimethyl-2-propylheptanal}{\phantom{..}CH3\phantom{...}OH\phantom{...}CHO}}
\end{array}\]

(v)

(vi)

(vii)

Cannizzaro reaction:

(i)
\[\begin{array}{cc}
\ce{H}\phantom{..............................}\ce{H}\phantom{.......}\ce{H}\phantom{......}\\
\backslash\phantom{..............................}|\phantom{.........}\backslash\phantom{....}\\
\ce{2\phantom{...}C = O + conc.KOH -> H - C - OH + C - OK}\\
/\phantom{..............................}|\phantom{.........}/\phantom{....}\\
\ce{\underset{Methanal}{H}}\phantom{..........................}\ce{\underset{Methanol}{H}}\phantom{.}\ce{\underset{methanoate}{\underset{Potassim}{H}}}\phantom{......}
\end{array}\]
(iii)

(ix)

\[\begin{array}{cc}
\phantom{.....}\ce{CH3}\phantom{............................}\ce{CH3}\phantom{........................}\ce{CH3}\phantom{...}\\
\phantom{}|\phantom{...............................}|\phantom{............................}|\phantom{.}\\
\ce{CH3CH2 - C - CHO ->[conc. NaOH] CH3CH2 - C - CH2 - OH + CH3CH2 - C - C - ONa}\\
\phantom{.....}|\phantom{...............................}|\phantom{............................}|\phantom{....}||\phantom{}\\
\phantom{.........}\ce{\underset{2, 2-Dimethylbutanal}{CH3}}\phantom{...............}\ce{\underset{2, 2-Dimethylbutan-1-ol}{CH3}}\phantom{..........}\ce{\underset{Sodium 2, 2-dimethylbutanoate}{CH3\phantom{.}O}}\phantom{.}
\end{array}\]