Q.Draw the structures of the following compounds.

Concept: IUPAC Nomenclature of carbonyl compounds, acids, and substituted aromatic systems.

Reasoning:

1. 3-Methylbutanal: Aldehyde group at C1, methyl substituent at C3 of a 4-carbon chain → $\text{CH}_3\text{CH}(\text{CH}_3)\text{CH}_2\text{CHO}$.
2. p-Nitropropiophenone: Propiophenone (ethyl phenyl ketone) with a nitro group at the para position → $\text{O}_2\text{N}\text{-}\text{C}_6\text{H}_4\text{-}\text{COCH}_2\text{CH}_3$.
3. p-Methylbenzaldehyde: Benzaldehyde with a methyl group at the para position → $\text{CH}_3\text{-}\text{C}_6\text{H}_4\text{-}\text{CHO}$.
4. 4-Methylpent-3-en-2-one: Pent-3-en-2-one (methyl ketone with a C3–C4 double bond) with a methyl at C4 → $\text{CH}_3\text{COCH}=\text{C}(\text{CH}_3)_2$.
5. 4-Chloropentan-2-one: Pentan-2-one with a chloro substituent at C4 → $\text{CH}_3\text{COCH}_2\text{CHClCH}_3$.
6. 3-Bromo-4-phenylpentanoic acid: Pentanoic acid with bromo at C3 and phenyl at C4 → $\text{C}_6\text{H}_5\text{CH}(\text{CH}_3)\text{CHBrCH}_2\text{COOH}$.
7. p,p'-Dihydroxybenzophenone: Benzophenone (diphenyl ketone) with hydroxy groups at both para positions → $\text{HO}\text{-}\text{C}_6\text{H}_4\text{-}\text{CO}\text{-}\text{C}_6\text{H}_4\text{-}\text{OH}$.
8. Hex-2-en-4-ynoic acid: Hexanoic acid with a double bond at C2–C3 and a triple bond at C4–C5 → $\text{CH}_3\text{C}\equiv\text{CCH}=\text{CHCOOH}$.

(i) 3-Methylbutanal

Butanal (4-C aldehyde, $-CHO$ at C1) with a methyl at C3:

(ii) p-Nitropropiophenone

Propiophenone is $C_6H_5\text{-}CO\text{-}CH_2CH_3$; a nitro group sits at the para position of the ring:

(iii) p-Methylbenzaldehyde

Benzaldehyde ($C_6H_5CHO$) with a methyl at the para position:

(iv) 4-Methylpent-3-en-2-one

Pent-3-en-2-one (ketone at C2, double bond C3=C4) with a methyl at C4:

(v) 4-Chloropentan-2-one

Pentan-2-one (ketone at C2) with $Cl$ at C4:

(vi) 3-Bromo-4-phenylpentanoic acid

Pentanoic acid ($-COOH$ at C1) with $Br$ at C3 and phenyl at C4:

(vii) p,p'-Dihydroxybenzophenone

Benzophenone is $C_6H_5\text{-}CO\text{-}C_6H_5$; one $-OH$ at the para position of each ring:

(viii) Hex-2-en-4-ynoic acid

6-C acid ($-COOH$ at C1), a double bond C2=C3 and a triple bond C4-C5:

IUPAC Nomenclature — Structure Drawing Method

Method: Retrosynthetic Name-to-Structure Decoding

This method works by breaking the IUPAC name into its functional group, parent chain, substituents, and locants — then assembling the structure step by step.

General Steps

1. Identify the parent chain (longest carbon chain containing the principal functional group).
2. Identify the principal functional group (highest priority group — determines suffix).
3. Number the parent chain to give the functional group the lowest possible locant.
4. Identify and place substituents (alkyl, halo, nitro, etc.) at the correct carbon numbers.
5. Draw the carbon skeleton with correct bond order (single, double, triple).
6. Add the functional group (aldehyde, ketone, acid, etc.) at the correct position.
7. Check stereochemistry if indicated (E/Z, R/S) — not needed here.

Solutions

(i) 3-Methylbutanal

• Parent: butanal (4-carbon aldehyde chain)
• Functional group: aldehyde (–CHO) at C1
• Substituent: methyl (–CH₃) at C3
• Structure:

Key: Aldehyde carbon is always C1.

(ii) p-Nitropropiophenone

• Parent: propiophenone (ethyl phenyl ketone)
• Functional group: ketone (C=O) attached to benzene and ethyl group
• Substituent: nitro (–NO₂) at para position on benzene
• Structure:

Key: “p-” means substituent is opposite the carbonyl on the benzene ring.

(iii) p-Methylbenzaldehyde

• Parent: benzaldehyde (benzene with –CHO)
• Substituent: methyl (–CH₃) at para position
• Structure:

Key: Aldehyde carbon is directly attached to benzene ring.

(iv) 4-Methylpent-3-en-2-one

• Parent: pent-3-en-2-one (5-carbon chain with C=C at C3, C=O at C2)
• Substituent: methyl at C4
• Structure:

Key: Numbering gives ketone (C2) the lowest number; double bond at C3.

(v) 4-Chloropentan-2-one

• Parent: pentan-2-one (5-carbon chain, ketone at C2)
• Substituent: chloro (–Cl) at C4
• Structure:

Key: Ketone at C2, chloro at C4.

(vi) 3-Bromo-4-phenylpentanoic acid

• Parent: pentanoic acid (5-carbon chain, –COOH at C1)
• Substituents: bromo at C3, phenyl at C4
• Structure:

Key: Carboxylic acid carbon is C1; phenyl is a benzene ring.

(vii) p,p'-Dihydroxybenzophenone

• Parent: benzophenone (two benzene rings connected by C=O)
• Substituents: hydroxy (–OH) at para position on both rings
• Structure:

Key: “p,p'” means para on both rings.

(viii) Hex-2-en-4-ynoic acid

• Parent: hex-2-en-4-ynoic acid (6-carbon chain)
• Functional group: carboxylic acid (–COOH) at C1
• Double bond: C2=C3
• Triple bond: C4≡C5
• Structure:

H—C≡C—CH=CH—COOH
  |     |
  CH₃   H

Correct structure:

Key: Numbering from COOH; double bond at C2, triple bond at C4.

Quick Exam Tips

Always number the parent chain to give the principal functional group the lowest possible number.

Here are the common mistakes students make with these specific IUPAC nomenclature problems, along with the conceptual fixes to avoid them.

General Mistake: Ignoring the Priority of Functional Groups

The Mistake: Forgetting that the principal functional group (e.g., aldehyde, ketone, acid) determines the suffix and the numbering priority, not the double bond or substituent.

How to Avoid: Always identify the Principal Functional Group first using the priority order: Carboxylic Acid > Aldehyde > Ketone > Alcohol > Alkene > Alkyne > Alkane.

(i) 3-Methylbutanal

Common Mistake: Drawing a 5-carbon chain with the aldehyde in the middle (e.g., CH3-CH2-CH(CHO)-CH3).

• Why it’s wrong: The suffix "-al" means the aldehyde ($-CHO$) must be at the end of the chain (carbon 1). The name implies a butane chain (4 carbons) with a methyl on carbon 3.
• Correct Structure: (CH3)2CH-CH2-CHO
• How to Avoid: Remember that for aldehydes, carbon 1 is always the carbonyl carbon. Number the parent chain from that end.

(ii) p-Nitropropiophenone

Common Mistake: Drawing the nitro group on the wrong carbon or misidentifying the parent.

• Why it’s wrong: "Propiophenone" means a phenyl ring attached to a propanone (3-carbon ketone). The "p-" (para) means the nitro ($-NO_2$) is opposite the ketone group on the benzene ring.
• Correct Structure: O2N-C6H4-CO-CH2-CH3 (Nitro at para position relative to the carbonyl).
• How to Avoid: Recognize "phenone" as a ketone attached to a benzene ring. "Propiophenone" = ethyl phenyl ketone. The "p-" tells you the substituent's position on the ring.

(iii) p-Methylbenzaldehyde

Common Mistake: Drawing the aldehyde group as a substituent ($-CHO$) instead of the parent suffix.

• Why it’s wrong: The suffix "-aldehyde" indicates the $-CHO$ group is the principal functional group. The parent is benzaldehyde (benzene ring + aldehyde).
• Correct Structure: CH3-C6H4-CHO (Methyl at para position relative to the aldehyde).
• How to Avoid: The name ends in "-aldehyde," so the $-CHO$ group defines the parent chain (benzaldehyde). The "p-methyl" is just a substituent on the ring.

(iv) 4-Methylpent-3-en-2-one

Common Mistake: Numbering the chain from the wrong end (e.g., starting from the methyl group).

• Why it’s wrong: The ketone ($-one$) has higher priority than the alkene ($-en$). The carbonyl carbon must get the lowest possible number.
• Correct Structure: CH3-CO-CH=C(CH3)-CH3
  • Numbering: C1 (carbonyl), C2, C3 (double bond), C4 (methyl), C5.
• How to Avoid: Always number the parent chain to give the principal functional group (here, the ketone) the lowest number, even if it means the double bond gets a higher number.

(v) 4-Chloropentan-2-one

Common Mistake: Drawing the chlorine on carbon 4 but forgetting the ketone position.

• Why it’s wrong: Students often draw a straight chain but misplace the carbonyl group.
• Correct Structure: CH3-CO-CH2-CHCl-CH3
  • Numbering: C1 (methyl), C2 (carbonyl), C3, C4 (chlorine), C5.
• How to Avoid: The suffix "-2-one" tells you the carbonyl is on carbon 2. Draw the 5-carbon chain first, place the $C=O$ on carbon 2, then add the chlorine on carbon 4.

(vi) 3-Bromo-4-phenylpentanoic acid

Common Mistake: Misidentifying the parent chain or numbering incorrectly.

• Why it’s wrong: The suffix "-oic acid" means the parent is pentanoic acid (5-carbon chain with $-COOH$ at C1). The phenyl group ($-C_6H_5$) is a substituent, not the parent.
• Correct Structure: CH3-CH(C6H5)-CHBr-CH2-COOH
  • Numbering: C1 (COOH), C2, C3 (Br), C4 (Phenyl), C5 (Methyl).
• How to Avoid: The carboxylic acid carbon is always C1. Number the chain from that end. The phenyl group is just a substituent (like bromo or methyl).

(vii) p,p'-Dihydroxybenzophenone

Common Mistake: Drawing only one benzene ring or misplacing the hydroxyl groups.

• Why it’s wrong: "Benzophenone" is a specific molecule: diphenyl ketone ($(C_6H_5)_2C=O$). The "p,p'-" means there is a hydroxyl ($-OH$) at the para position on both benzene rings.
• Correct Structure: (HO-C6H4)-CO-(C6H4-OH) (Two benzene rings connected by a carbonyl, each with an OH at the para position).
• How to Avoid: Memorize that "benzophenone" = two phenyl rings attached to a single carbonyl. The "p,p'-" refers to the 1,4 positions on both rings.

(viii) Hex-2-en-4-ynoic acid

Common Mistake: Confusing the numbering priority between the alkene and alkyne.

• Why it’s wrong: The suffix "-oic acid" means the carboxylic acid is the principal group (C1). The chain must be numbered to give the lowest locants to the multiple bonds as a set.
• Correct Structure: CH3-C≡C-CH=CH-COOH
  • Numbering: C1 (COOH), C2 (double bond), C3, C4 (triple bond), C5, C6.
• How to Avoid: When both an alkene and alkyne are present, number the chain from the end nearest the first multiple bond (alkene or alkyne). Here, the double bond gets the lower number (2 vs 4).