601. Which statement best explains why formaldehyde does not undergo ordinary aldol reaction?
ⓐ. It has no carbonyl group.
ⓑ. It lacks \(\alpha\)-hydrogen.
ⓒ. It is a ketone.
ⓓ. It contains a \(COOH\) group.
Correct Answer: It lacks \(\alpha\)-hydrogen.
Explanation: Aldol reaction requires an \(\alpha\)-hydrogen so that an enolate ion can form. Formaldehyde, \(HCHO\), has no carbon atom adjacent to the carbonyl carbon. Therefore it has no \(\alpha\)-hydrogen. Because of this, formaldehyde does not undergo ordinary self-aldol reaction and can undergo Cannizzaro reaction instead.
602. Which assertion-reason pair is correctly evaluated?
Assertion: Benzaldehyde undergoes Cannizzaro reaction in concentrated alkali.
Reason: Benzaldehyde lacks \(\alpha\)-hydrogen and cannot form an enolate for ordinary aldol reaction.
ⓐ. Assertion is true, but Reason is false.
ⓑ. Both are true, but Reason does not explain Assertion.
ⓒ. Both are true, and Reason explains Assertion.
ⓓ. Assertion is false, but Reason is true.
Correct Answer: Both are true, and Reason explains Assertion.
Explanation: Benzaldehyde has no \(\alpha\)-hydrogen because the \(CHO\) group is directly attached to the benzene ring. Without \(\alpha\)-hydrogen, benzaldehyde cannot form an enolate ion for ordinary self-aldol reaction. In concentrated alkali, such aldehydes undergo Cannizzaro reaction. Thus benzaldehyde gives benzyl alcohol and benzoate salt.
603. Which product is obtained after acidification of sodium benzoate formed in Cannizzaro reaction of benzaldehyde?
ⓐ. Benzoic acid
ⓑ. Benzyl chloride
ⓒ. Benzyl alcohol
ⓓ. Benzene
Correct Answer: Benzoic acid
Explanation: Cannizzaro reaction of benzaldehyde in \(NaOH\) gives sodium benzoate as the oxidised product. Sodium benzoate is a carboxylate salt. On acidification, the benzoate ion accepts a proton and forms benzoic acid. Thus \(C_6H_5COONa\) can be converted into \(C_6H_5COOH\) by acid treatment.
604. Which set contains only aldehydes capable of Cannizzaro reaction?
ⓐ. \(CH_3CHO\), \(CH_3CH_2CHO\)
ⓑ. \(CH_3CHO\), \(C_6H_5CHO\)
ⓒ. \(HCHO\), \(CH_3CHO\)
ⓓ. \(HCHO\), \(C_6H_5CHO\)
Correct Answer: \(HCHO\), \(C_6H_5CHO\)
Explanation: Formaldehyde and benzaldehyde both lack \(\alpha\)-hydrogen. Therefore both can undergo Cannizzaro reaction in concentrated alkali. Ethanal and propanal contain \(\alpha\)-hydrogens and generally undergo aldol reaction under alkaline conditions. The key selection rule is absence of \(\alpha\)-hydrogen in an aldehyde.
605. What is the directing nature of the \( -CHO \) group in electrophilic aromatic substitution?
ⓐ. Deactivating and meta-directing
ⓑ. Activating and ortho-directing
ⓒ. Activating and para-directing
ⓓ. Deactivating and ortho-directing
Correct Answer: Deactivating and meta-directing
Explanation: The \( -CHO \) group withdraws electron density from the benzene ring. It does this through both \(-I\) and \(-M\) effects. The ring becomes less reactive toward electrophilic substitution, so the group is deactivating. The electron withdrawal also makes the meta position relatively favoured compared with ortho and para positions.
606. Which product is mainly formed when benzaldehyde undergoes nitration?
ⓐ. \(o\)-nitrobenzaldehyde
ⓑ. \(p\)-nitrobenzaldehyde
ⓒ. \(m\)-nitrobenzaldehyde
ⓓ. \(2,4\)-dinitrobenzaldehyde
Correct Answer: \(m\)-nitrobenzaldehyde
Explanation: Benzaldehyde contains the \( -CHO \) group attached to the benzene ring. The \( -CHO \) group is deactivating and meta-directing in electrophilic aromatic substitution. During nitration, the incoming \(NO_2\) group is therefore directed mainly to the meta position. Ortho and para products are less favoured because the corresponding intermediates are less stable.
607. Which electronic effect mainly explains the meta-directing nature of \( -CHO \) in benzaldehyde?
ⓐ. Strong \(+I\) effect only
ⓑ. Strong \(+M\) effect only
ⓒ. Hyperconjugation from \( -CHO \)
ⓓ. \(-I\) and \(-M\) withdrawal
Correct Answer: \(-I\) and \(-M\) withdrawal
Explanation: The formyl group, \( -CHO \), withdraws electron density from the aromatic ring. It withdraws inductively because the carbonyl group is polar. It also withdraws by resonance because the carbonyl group can pull \(\pi\)-electron density from the ring. These withdrawing effects reduce electron density especially in ways that disfavor ortho and para attack, so meta substitution is favoured.
608. What is the major product when benzaldehyde reacts with bromine in the presence of a Lewis acid?
ⓐ. \(o\)-bromobenzaldehyde
ⓑ. \(m\)-bromobenzaldehyde
ⓒ. \(p\)-bromobenzaldehyde
ⓓ. benzyl bromide
Correct Answer: \(m\)-bromobenzaldehyde
Explanation: The \( -CHO \) group in benzaldehyde is a meta-directing group. In electrophilic bromination of the aromatic ring, the incoming bromine is directed mainly to the meta position. The reaction is ring substitution, not side-chain bromination under these conditions. Therefore the major aromatic substitution product is \(m\)-bromobenzaldehyde.
609. Why is the benzene ring of benzaldehyde less reactive than benzene toward electrophilic substitution?
ⓐ. The \( -CHO \) group withdraws ring electron density.
ⓑ. The \( -CHO \) group donates electron density to the ring.
ⓒ. The benzene ring loses aromaticity permanently.
ⓓ. The aldehyde group changes into \(COOH\) first.
Correct Answer: The \( -CHO \) group withdraws ring electron density.
Explanation: Electrophilic aromatic substitution is favoured by electron-rich aromatic rings. The \( -CHO \) group withdraws electron density through the carbonyl system. This makes the ring less electron-rich and less reactive toward electrophiles. The group therefore deactivates the ring compared with unsubstituted benzene.
610. Which group attached to benzene is deactivating and meta-directing like \( -CHO \)?
ⓐ. \( -OH \)
ⓑ. \( -NHCOCH_3 \)
ⓒ. \( -CH_3 \)
ⓓ. \( -COCH_3 \)
Correct Answer: \( -COCH_3 \)
Explanation: The group \( -COCH_3 \) is an acyl group attached through the carbonyl carbon. Like \( -CHO \), it withdraws electron density from the ring through \(-I\) and \(-M\) effects. Such withdrawal deactivates the ring and directs incoming electrophiles mainly to the meta position. Groups such as \( -OH \), \( -NH_2 \), and \( -CH_3 \) generally direct to ortho and para positions.
611. Which statement correctly compares toluene and benzaldehyde in electrophilic aromatic substitution?
ⓐ. Both are strongly meta-directing.
ⓑ. Toluene is deactivating, while benzaldehyde is activating.
ⓒ. Toluene directs ortho-para; benzaldehyde directs meta.
ⓓ. Both direct incoming groups only to the para position.
Correct Answer: Toluene directs ortho-para; benzaldehyde directs meta.
Explanation: The methyl group in toluene donates electron density to the ring through hyperconjugation and \(+I\) effect. It activates the ring and directs incoming electrophiles to ortho and para positions. The \( -CHO \) group in benzaldehyde withdraws electron density and deactivates the ring. It directs incoming electrophiles mainly to the meta position.
612. Which equation best represents nitration of benzaldehyde at the major position?
ⓐ. \[C_6H_5CHO \xrightarrow{HNO_3/H_2SO_4} o\text{-}NO_2C_6H_4CHO\]
ⓑ. \[C_6H_5CHO \xrightarrow{HNO_3/H_2SO_4} m\text{-}NO_2C_6H_4CHO\]
ⓒ. \[C_6H_5CHO \xrightarrow{HNO_3/H_2SO_4} p\text{-}NO_2C_6H_4CHO\]
ⓓ. \[C_6H_5CHO \xrightarrow{HNO_3/H_2SO_4} C_6H_5CH_2OH\]
Correct Answer: \[C_6H_5CHO \xrightarrow{HNO_3/H_2SO_4} m\text{-}NO_2C_6H_4CHO\]
Explanation: Benzaldehyde contains the \( -CHO \) group.
The \( -CHO \) group is deactivating and meta-directing.
Nitration introduces the \(NO_2\) group into the aromatic ring.
The major product is therefore the meta-substituted derivative.
The correct reaction pattern is:
\[C_6H_5CHO \xrightarrow{HNO_3/H_2SO_4} m\text{-}NO_2C_6H_4CHO\]
613. Which compound is mainly formed by nitration of acetophenone, \(C_6H_5COCH_3\)?
ⓐ. \(m\)-nitroacetophenone
ⓑ. \(o\)-nitroacetophenone
ⓒ. \(p\)-nitroacetophenone
ⓓ. \(2,4\)-dinitroacetophenone
Correct Answer: \(m\)-nitroacetophenone
Explanation: Acetophenone contains the group \( -COCH_3\) attached to the benzene ring. This carbonyl-containing group withdraws electron density from the ring. It is deactivating and meta-directing in electrophilic aromatic substitution. Hence nitration gives \(m\)-nitroacetophenone as the major product.
614. Which assertion-reason pair is correctly evaluated?
Assertion: The \( -COCH_3\) group in acetophenone directs electrophiles mainly to the meta position.
Reason: The carbonyl group withdraws electron density from the aromatic ring.
ⓐ. Assertion is true, but Reason is false.
ⓑ. Assertion is false, but Reason is true.
ⓒ. Both are true, but Reason does not explain Assertion.
ⓓ. Both are true, and Reason explains Assertion.
Correct Answer: Both are true, and Reason explains Assertion.
Explanation: The \( -COCH_3\) group contains a carbonyl group directly attached to the benzene ring. This group withdraws electron density by inductive and resonance effects. Electron withdrawal deactivates the ring and makes ortho and para attack less favoured. Therefore electrophilic substitution occurs mainly at the meta position.
615. Which statement is incorrect about aromatic aldehydes in electrophilic substitution?
ⓐ. The \( -CHO \) group withdraws electron density.
ⓑ. The \( -CHO \) group strongly activates the ring.
ⓒ. The \( -CHO \) group directs mainly to meta position.
ⓓ. The ring is less reactive than benzene.
Correct Answer: The \( -CHO \) group strongly activates the ring.
Explanation: The \( -CHO \) group is not an activating group. It withdraws electron density from the aromatic ring through the carbonyl system. This makes the ring less reactive toward electrophiles than benzene. Its directing effect is mainly meta, not ortho-para.
616. Which starting aromatic carbonyl compound would give \(m\)-bromoacetophenone on bromination?
ⓐ. Benzaldehyde
ⓑ. Benzoic acid
ⓒ. Acetophenone
ⓓ. Toluene
Correct Answer: Acetophenone
Explanation: \(m\)-bromoacetophenone contains a bromine atom meta to the \( -COCH_3\) group. Acetophenone has the structure \(C_6H_5COCH_3\). Since \( -COCH_3\) is meta-directing, bromination of acetophenone gives the meta-bromo product as the major ring-substitution product. Toluene would direct mainly to ortho and para positions.
617. Which group makes an aromatic ring less reactive and directs incoming electrophiles to the meta position?
ⓐ. \( -CHO \)
ⓑ. \( -CH_3 \)
ⓒ. \( -OH \)
ⓓ. \( -NH_2 \)
Correct Answer: \( -CHO \)
Explanation: The \( -CHO \) group is a carbonyl-containing substituent. It withdraws electron density from the ring and decreases the ring’s reactivity toward electrophiles. It also directs electrophilic substitution mainly to the meta position. Electron-donating groups such as \( -OH \), \( -NH_2 \), and \( -CH_3 \) are generally ortho-para directors.
618. Which comparison is correct for benzene and acetophenone during nitration?
ⓐ. Acetophenone reacts faster and gives mainly ortho product.
ⓑ. Acetophenone reacts slower and gives mainly meta product.
ⓒ. Benzene gives only meta product due to \( -COCH_3\).
ⓓ. Both react at identical rates and give the same product.
Correct Answer: Acetophenone reacts slower and gives mainly meta product.
Explanation: Acetophenone contains the electron-withdrawing \( -COCH_3\) group. This group deactivates the aromatic ring, so acetophenone reacts more slowly than benzene toward electrophilic substitution. It also directs the incoming \(NO_2\) group mainly to the meta position. Benzene has no directing substituent, so it gives nitrobenzene as the substitution product.
619. Which explanation best accounts for the lower stability of ortho and para attack intermediates in benzaldehyde?
ⓐ. The benzene ring becomes non-aromatic before attack.
ⓑ. The \( -CHO \) group donates electrons strongly at ortho and para positions.
ⓒ. Positive charge is unfavourable near the withdrawing carbonyl.
ⓓ. The aldehydic hydrogen leaves as \(H^-\) before substitution.
Correct Answer: Positive charge is unfavourable near the withdrawing carbonyl.
Explanation: During electrophilic aromatic substitution, the ring forms a positively charged sigma complex. For ortho and para attack in benzaldehyde, resonance forms place positive charge at positions strongly affected by the electron-withdrawing \( -CHO \) group. This is energetically unfavourable. Meta attack avoids the most destabilising resonance arrangement, so the meta product is favoured.
620. Which product is mainly formed when \(C_6H_5COCH_3\) undergoes bromination on the ring?
ⓐ. \(o\)-bromoacetophenone
ⓑ. \(p\)-bromoacetophenone
ⓒ. \(2,4\)-dibromoacetophenone
ⓓ. \(m\)-bromoacetophenone
Correct Answer: \(m\)-bromoacetophenone
Explanation: \(C_6H_5COCH_3\) is acetophenone. The group \( -COCH_3\) is deactivating and meta-directing. Therefore, in electrophilic bromination of the aromatic ring, bromine enters mainly at the meta position relative to \( -COCH_3\). The ortho and para products are less favoured.
