Question

Bring out the following conversions:
Acetaldehyde to $But-2-enal$.

Answer 1

Aldol condensation is a condensation reaction of organic chemistry and in which enol or an enolate ion reacts with a carbonyl compound to form a $\beta -hydroxyaldehyde$. If the condensation reaction is present between two different carbonyl compounds it is called crossed Aldol condensation.

Complete step by step answer:
First we will discuss about the mechanism of Aldol condensation,
Mechanism: - In which hydroxide acts as a base and $\alpha -hydrogen$ act as an acid or produced enolate ion. This reaction can be seen as an acid base reaction.
The aldehydes attack at the electrophilic carbonyl carbon and as a result give alkoxide intermediate. The alkaloid deprotonates water molecules that produce hydroxide and $\beta -hydroxyaldehyde$.
Now we will discuss about the chemical reaction which is given in question,
$Ethanal\left( acetaldehyde \right)\to But-2-enal$
$2R-CHO+NaOH\to R\left( OH \right)RCHO\to RCHO+H{}_{2}O$
During the Aldol condensation, when acetaldehyde treated with the dilute alkali$(NaOH)$, ethanal produced $3-hydroxybut-anal$ , then $3-hydroxybut-anal$is heated and the water molecules is liberated, as a result $But-2-enal$ is formed.
$2CH{}_{3}CHO+NaOH\to CH{}_{3}-CH\left( OH \right)-CH{}_{2}-CHO\to H{}_{3}C-CH=CH-CHO+\left( -H{}_{2}O \right)$
Ethanal+ dilute alkali→ $3-hydroxybut-anal$→ $But-2-enal$+ water (liberated)
So according to this theoretical concept, the acetaldehyde is converted into $But-2-enal$ in the presence of dilute alkali and by the liberation of water molecules due to the heating process.

Note: Aldol condensation is important in the organic chemistry for the synthesis of aldehydes and ketones. They provide a good way to form carbon-carbon bonds. The Robinson annulations reaction is a good example of an Aldol condensation.