Question

When benzaldehyde is oxidised to give benzoic acid then the oxidation state of carbon of aldehydic group is changed from:

• A. $+2$ to $+3$
• B. $+1$ to $+3$
• C. Zero to $+2$
• D. No change

Answer 1

The correct option is B $+1$ to $+3$

Bonds between same atoms donot affect the oxidation state. Therefore only functional group affects the oxidation state of $C$

In benzaldehyde:

$=O$ has$-2$ and $H$ has $+1$, therefore in $CHO$ O.S. of $C$ is: $x-2+1=0$

$C$ is $+1$

In Benzoic acid $=O$ has$-2$ and $OH$ has $-1$, therefore in $COOH$ O.S. of $C$ in $CO-OH$ is: $x-2-1=0$

$C$ is $+3$

Thus O.S. of $C$ changes from $+1$ to $+3$. Option B is correct