Question

In Nef-Carbonyl synthesis, primary nitroalkanes on treatment with $ NaOH $ followed by acidification with $ 50%~,{{H}_{2}}S{{O}_{4}} $ ​ gives:
(A) Aldehydes
(B) Ketones
(C) Ketoacids
(D) Esters

Answer 1

Hint: The carbon atom next to the carbonyl group in an aldehyde or a ketone is called carbon and the hydrogen’s attached to it are called $ \alpha $ hydrogen’s. These $ \alpha $ hydrogens are acidic due to the electron withdrawing inductive effect of the carbonyl group and so they can be easily abstracted by strong bases to emulate ions.

Complete step by step solution:
Based on this acidity of $ \alpha $ hydrogens, the aldol condensation reaction involves the reaction of aldehyde or ketone with dilute alkali to form a $ \beta $ hydroxy aldehyde or a $ \beta $ hydroxy ketone. These $ \beta $ hydroxy aldehyde or $ \beta $ hydroxy ketone are called aldols.
Acetaldehyde is an aldehyde containing - hydrogen atoms and so they can be easily abstracted by a base. So acetaldehyde will undergo aldol condensation reaction with sodium hydroxide to give $ 3- $ hydroxybutanal which is an aldol.
Nef reaction or synthesis involves acid hydrolysis of a salt of primary or secondary nitroalkane to yield carbonyl compounds. Or, we can say that , sodium or potassium salt of primary $ \left( 1{}^\circ \right) $ and secondary $ \left( 2{}^\circ \right) $ nitroalkanes gives carbonyl compound on acidification with 50% concentrated solution of sulphuric acid $ \left( {{H}_{2}}S{{O}_{4}} \right).~ $
Therefore, correct answer is option A i.e. Aldehydes.

Note:
Aldehydes which do not have $ \alpha $ hydrogen atoms will not undergo aldol condensation reaction. For example, formaldehyde does not contain any $ \alpha $ hydrogen atom. So there are no acidic hydrogens available for abstraction by bases and hence it cannot undergo an aldol reaction to give beta hydroxy aldehyde or aldol. Pivaldehyde also does not contain any - hydrogen atom. So there are no acidic hydrogens available for abstraction by bases and hence it cannot undergo an aldol reaction to give aldol.