Sign in
Please select an account to continue using cracku.in
↓ →
Join Our JEE Preparation Group
Prep with like-minded aspirants; Get access to free daily tests and study material.
In the Hoffmann bromamide degradation reaction, the number of moles of NaOH and $$Br_2$$ used per mole of amine produced are:
In the Hoffmann bromamide degradation, an unsubstituted amide $$RCONH_2$$ is converted into an amine with one carbon atom less. The general reaction in basic medium is
$$RCONH_2 + Br_2 + NaOH \;\rightarrow\; RNH_2 + \dots$$
To find the exact stoichiometry, we write the complete ionic reaction and then balance every atom and charge. Let us begin by recalling that:
1. Each molecule of $$Br_2$$ is reduced to two bromide ions $$2\,Br^-$$.
2. The amide carbonyl carbon finally appears in the carbonate ion $$CO_3^{2-}$$ produced from hydroxide.
First, write the skeletal ionic equation showing the essential species:
$$RCONH_2 + Br_2 + OH^- \;\rightarrow\; RNH_2 + Br^- + CO_3^{2-} + H_2O$$
Now we balance step by step.
Balancing bromine. One mole of $$Br_2$$ gives two bromide ions, so we need the coefficient 2 in front of $$Br^-$$ on the product side:
$$RCONH_2 + Br_2 + OH^- \;\rightarrow\; RNH_2 + 2\,Br^- + CO_3^{2-} + H_2O$$
Balancing nitrogen and the organic part. Nitrogen already balances: one $$NH_2$$ in the amide and one $$NH_2$$ in the amine.
Balancing carbon. The carbonyl carbon of the amide ends up in the carbonate ion. One $$RCONH_2$$ gives one $$CO_3^{2-}$$, so the carbon atoms are balanced.
Balancing oxygen and hydrogen. Count oxygen atoms on each side:
Left side: from one $$OH^-$$ there is 1 oxygen; from $$RCONH_2$$ there is 1 oxygen. Total = 2.
Right side: in $$CO_3^{2-}$$ there are 3 oxygens; in $$H_2O$$ there is 1 oxygen. Total = 4.
We are short by 2 oxygens on the left. Each additional $$OH^-$$ contributes one oxygen. Therefore we add 2 more $$OH^-$$ to the left:
$$RCONH_2 + Br_2 + 3\,OH^- \;\rightarrow\; RNH_2 + 2\,Br^- + CO_3^{2-} + H_2O$$
Now recount oxygens:
Left: 3 oxygens from $$3\,OH^-$$ + 1 oxygen from amide = 4.
Right: 3 in $$CO_3^{2-}$$ + 1 in $$H_2O$$ = 4.
Oxygens balance. Next, balance hydrogen atoms. Hydrogen count:
Left: in $$RCONH_2$$ there are 2 hydrogens; in $$3\,OH^-$$ there are 3 hydrogens. Total = 5.
Right: in $$RNH_2$$ there are 2 hydrogens; in $$H_2O$$ there are 2 hydrogens. Total = 4.
We are short by 1 hydrogen on the right. To supply one more hydrogen (and one more oxygen so that oxygen balance stays intact) we must place another $$H_2O$$ on the right and correspondingly add another $$OH^-$$ on the left. Therefore we write:
$$RCONH_2 + Br_2 + 4\,OH^- \;\rightarrow\; RNH_2 + 2\,Br^- + CO_3^{2-} + 2\,H_2O$$
Now, hydrogen count:
Left: 2 (from amide) + 4 (from $$4\,OH^-$$) = 6.
Right: 2 (in amine) + 4 (in $$2\,H_2O$$) = 6.
Oxygen count:
Left: 4 oxygens in $$4\,OH^-$$ + 1 in amide = 5.
Right: 3 in carbonate + 2 in $$2\,H_2O$$ = 5.
Everything balances, and the charges also balance: left side charge $$= 4(-1) = -4$$; right side charge $$= 2(-1) + (-2) = -4$$.
Thus the balanced ionic equation is confirmed. Converting the ions back to the molecular form using sodium ions as counter-ions, we multiply every ionic species by $$Na^+$$ as required:
$$RCONH_2 + Br_2 + 4\,NaOH \;\rightarrow\; RNH_2 + 2\,NaBr + Na_2CO_3 + 2\,H_2O$$
From this balanced molecular equation we read directly:
Per mole of amine $$RNH_2$$ produced,
moles of $$NaOH$$ required $$= 4$$,
moles of $$Br_2$$ required $$= 1$$.
This matches Option B, which states “Four moles of NaOH and one mole of $$Br_2$$.”
Hence, the correct answer is Option B.
Create a FREE account and get:
Educational materials for JEE preparation