Question

Answer the following questions:
(i) Draw structure of the methyl hemiactetal of methanal.
(ii) There are two -NH$_2$ groups in semicarbazide. However, only one is involved in the formation of semicarbazones. Give reason.
(iii) How will you convert ethanol to 3-hydroxybutanal?

Hint:

For reactions involving aldehydes and alcohols, keep in mind their reactivity with nucleophiles and the common reactions they undergo.

Answer 1

(i) The structure of the methyl hemiactetal of methanal can be written as follows:
\[ \text{CH}_3\text{OH}_2\text{C=O} \longrightarrow \text{CH}_3\text{O}\text{CH}_2\text{OH} \]
(ii) Only one -NH$_2$ group of semicarbazide participates in the formation of semicarbazones, as the other -NH$_2$ group is in a sterically hindered position and does not react under normal conditions.
(iii) Ethanol can be converted to 3-hydroxybutanal by an aldol condensation. The steps involve:
- First, oxidize ethanol to acetaldehyde.
- Then, carry out a condensation reaction with acetaldehyde and another molecule of acetaldehyde under basic conditions to form 3-hydroxybutanal.