Question

What type of reaction is the synthesis of crotonaldehyde from acetaldehyde
(i) Nucleophilic Addition
(ii) Elimination
(iii) Electrophilic Addition
(iv) Nucleophilic Addition-Elimination

Answer 1

Nucleophiles are electron rich molecules, they donate their extra pair of electrons to an electron deficient species to undergo a chemical reaction. Acetaldehyde is an aldehyde and it undergoes an aldol condensation reaction to form crotonaldehyde. Consider this fact and try to interpret what type of reaction it is.

Complete step-by-step answer: The structure of acetaldehyde is given as ${H_3}C - CHO$.
It is an aldehyde, where the $C$ atom is electron deficient and electrophilic in nature.
For the formation of crotonaldehyde from acetaldehyde, the latter undergoes aldol condensation reaction. In case of aldol condensation the reagent used is $dil\,NaOH$. The $O{H^ - }$ion which is nucleophilic in nature gets attached to the electron deficient carbon center via nucleophilic addition reaction.
$2\,{H_3}C - CHO\,\,\xrightarrow{{dil\,NaOH}}\,\,{H_3}C - CHOH - C{H_2} - COC{H_3}$
When heated the compound loses a molecule of water to form crotonaldehyde via elimination.
${H_3}C - CHOH - C{H_2} - COC{H_3}\,\,\xrightarrow{\Delta }\,\,{H_3}C - CH = CH - COC{H_3}$
So acetaldehyde undergoes nucleophilic addition elimination reaction to form crotonaldehyde.

Additional Information: Aldol condensation reaction is defined as an organic reaction in which enolate ion reacts with a carbonyl compound to form $\beta$-hydroxy ketone or $\beta$-hydroxy aldehyde, followed by dehydration to give a conjugated enone. This reaction plays a vital role in organic synthesis, creating a path to form carbon-carbon bonds. The first part of this reaction is an aldol reaction, the second part is dehydration—an elimination reaction which Involves removal of a water molecule or an alcohol molecule. Dehydration may be accompanied by decarboxylation when an activated carboxyl group is present.

Hence the correct answer is (iv) Nucleophilic Addition-Elimination.

Note: Name reactions can be tricky so it would be better if you know the starting compound and the reagents used for name reactions. You should also be careful with the terms which one is electrophile and which one is nucleophile. Also properly check if a compound is undergoing addition or elimination reaction.