Q.Give the structure of the following compounds:

Concept: IUPAC Nomenclature — identifying the parent chain, principal functional group, and substituents from the given name.

(a) 4-Nitropropiophenone

• Parent: propiophenone (a 3-carbon chain attached to a phenyl ring and a ketone).
• Substituent: nitro group at position 4 of the phenyl ring.
• Structure: $\text{O}_2\text{N} \!-\! \text{C}_6\text{H}_4 \!-\! \text{CO} \!-\! \text{CH}_2\text{CH}_3$ (para-nitro on benzene, ethyl ketone).

(b) 2-Hydroxycyclopentanecarbaldehyde

• Parent: cyclopentanecarbaldehyde (a cyclopentane ring with an aldehyde group at position 1).
• Substituent: hydroxy group at position 2.
• Structure: A 5-membered ring with $\text{CHO}$ at C1 and $\text{OH}$ at C2.

(c) Phenyl acetaldehyde …

Let’s build each structure step by step, focusing on the logic behind the name.

The Core Idea: Decoding IUPAC Names

IUPAC nomenclature is a systematic way to translate a compound’s structure into a name, and vice versa. The name tells you three things in order:

1. The parent chain or ring (the longest carbon chain or the principal ring system).
2. The principal functional group (the one with highest priority, which gives the suffix like -one, -al, -oic acid).
3. Substituents (other groups attached, with their positions indicated by locant numbers).

The trick is to read the name from the end backwards: the suffix tells you the functional group, the root tells you the carbon count, and the prefix tells you what’s attached where.

(a) 4-Nitropropiophenone

1. Identify the parent compound. The name ends in “-phenone”. This suffix indicates a ketone where one of the alkyl groups attached to the carbonyl ($\ce{C=O}$) is a phenyl ring ($\ce{C6H5-}$). The root “propi-” tells us the other alkyl group is a propyl group ($\ce{-CH2CH2CH3}$). So, propiophenone is simply 1-phenylpropan-1-one. The carbonyl carbon is always carbon 1 in such ketones, and the phenyl ring is attached to it.

2. Locate the substituent. The propyl chain in propiophenone has only 3 carbons total (the carbonyl carbon plus one $-\text{CH}_2-$ and one $-\text{CH}_3$), so it has no “carbon 4” at all. The locant “4” in “4-nitro” must therefore name a position on the phenyl ring instead: counting the ring carbon attached to the carbonyl as ring-C1, “4-nitro” places the $-\ce{NO2}$ group at the ring’s para position.

3. Assemble the structure. Draw a benzene ring with a $-\text{CO-CH}_2\text{-CH}_3$ (propanoyl) group at ring-C1 and a $-\ce{NO2}$ group at the para (ring-C4) position.

The structure is:

4. Final structure in condensed form: $\ce{4-O2N-C6H4-CO-CH2CH3}$ (para-nitrophenyl ethyl ketone)

(b) 2-Hydroxycyclopentanecarbaldehyde

1. Identify the parent compound. The name ends in “-carbaldehyde”. This suffix means the principal functional group is an aldehyde ($\ce{-CHO}$) attached directly to a ring. The root “cyclopentane” tells us the ring is a five-membered carbon ring. So, cyclopentanecarbaldehyde is a cyclopentane ring with a formyl group ($\ce{-CHO}$) attached to one of its carbons. The carbon of the formyl group is not part of the ring; it’s a substituent. The ring carbon to which it is attached is considered carbon 1 for numbering the ring.

2. Locate the substituent. The prefix “2-hydroxy” tells us a hydroxyl group ($\ce{-OH}$) is attached to carbon number 2 of the cyclopentane ring. Since the aldehyde carbon is not part of the ring, the ring carbons are numbered starting from the carbon bearing the aldehyde group as carbon 1.

3. Assemble the structure. Draw a cyclopentane ring. Attach a $\ce{-CHO}$ group to one carbon (call it C1). Then attach an $\ce{-OH}$ group to the adjacent carbon (C2). The stereochemistry (cis or trans) is not specified, so any relative orientation is acceptable.

The structure is a cyclopentane ring with $\ce{-CHO}$ at position 1 and $\ce{-OH}$ at position 2 (adjacent ring carbons).

4. Final structure in condensed form: A cyclopentane ring with $\ce{-CHO}$ and $\ce{-OH}$ on adjacent carbons. …

IUPAC Nomenclature — Structure Drawing Method

Method: Functional Group Priority + Parent Chain Expansion

This method works by identifying the principal functional group (highest priority), then building the parent chain with correct numbering, and finally attaching all substituents.

Steps (General)

1. Identify the principal functional group (the one with highest priority — e.g., aldehyde > ketone > alcohol > nitro).
2. Find the parent chain that includes this functional group.
3. Number the chain to give the principal group the lowest locant.
4. Draw the carbon skeleton with the correct number of carbons.
5. Place the principal functional group at the correct position.
6. Attach all substituents (alkyl, nitro, hydroxy, etc.) at the numbered positions.
7. Check valency — each carbon must have 4 bonds.

(a) 4-Nitropropiophenone

Step-by-step

• Principal group: Ketone (‑one suffix) — propiophenone means a 3‑carbon chain (prop‑) attached to a phenyl ring (phen‑), with the ketone on the carbon next to the ring.
• Parent structure: Propiophenone = $\ce{Ph-CO-CH2-CH3}$ (phenyl ring + carbonyl + ethyl group).
• Substituent: 4‑Nitro means a nitro group ($\ce{-NO2}$) at the 4‑position on the phenyl ring (numbering starts from the carbon attached to the carbonyl as position 1).

Structure

In condensed form:

$\ce{O2N-C6H4-CO-CH2-CH3}$

(with nitro at the para position relative to the carbonyl).

(b) 2-Hydroxycyclopentanecarbaldehyde

Step-by-step

• Principal group: Aldehyde (‑carbaldehyde suffix) — this means the aldehyde is attached to a ring (cyclopentane).
• Parent chain: Cyclopentane ring (5 carbons).
• Numbering: The carbon bearing the aldehyde is position 1 (by IUPAC rules for carbaldehyde).
• Substituent: 2‑Hydroxy means an $\ce{-OH}$ group at position 2 on the ring.

Structure

In condensed form:

$\ce{OH-C5H8-CHO}$

(with OH on the carbon adjacent to the aldehyde carbon).

(c) Phenyl acetaldehyde

Step-by-step …

(a) 4-Nitropropiophenone

✗ Common Mistake

Students often draw a propiophenone (ethyl phenyl ketone) and attach the nitro group at the 4-position of the ethyl chain — or worse, they misplace the nitro on the phenyl ring without checking numbering.

✓ Correct Structure

• Propiophenone = ethyl phenyl ketone: $\text{C}_6\text{H}_5\text{—CO—CH}_2\text{—CH}_3$
• The 4-nitro refers to the 4-position on the phenyl ring (since the carbonyl carbon is considered position 1 on the ring).
• So the structure is:

In line notation:

4-NO₂–C₆H₄–CO–CH₂–CH₃

🛡️ How to Avoid

• Remember: In aryl ketones, the ring is numbered starting from the carbon attached to the carbonyl (position 1). The substituent number (here 4) refers to the ring, not the side chain.
• Always identify the parent functional group first (here: propiophenone = ketone with ethyl + phenyl).

(b) 2-Hydroxycyclopentanecarbaldehyde

✗ Common Mistake

Students often draw a cyclopentane ring with an aldehyde group and a hydroxyl group — but they place the aldehyde as a substituent (e.g., –CHO attached to C1) and the –OH at C2. That’s actually correct in structure, but the naming confusion arises because they forget that “carbaldehyde” means the –CHO is not the parent — it’s a suffix on the ring name.

✓ Correct Structure

• Parent = cyclopentane
• Suffix = carbaldehyde (means –CHO is attached to the ring, and the carbon of –CHO is not part of the ring numbering)
• Numbering: start at the carbon bearing the –CHO (that’s C1), then –OH at C2.

In line notation:

2-HO–C₅H₈–CHO (with –CHO at C1, –OH at C2)

🛡️ How to Avoid

• “Carbaldehyde” is a special suffix: the aldehyde carbon is not part of the ring count. So the ring is numbered 1, 2, 3... starting from the ring carbon attached to –CHO.
• Never number the –CHO carbon as C1 — that carbon is outside the ring.
• Practice: “cyclohexanecarbaldehyde” = C₆H₁₁–CHO, not CHO directly on C1 of ring as a substituent.

(c) Phenyl acetaldehyde

✗ Common Mistake

Students often draw acetophenone (C₆H₅–CO–CH₃) instead of the correct aldehyde. They confuse “acetaldehyde” (CH₃–CHO) with “acetophenone” (methyl phenyl ketone).

✓ Correct Structure

• Acetaldehyde = CH₃–CHO
• Phenyl acetaldehyde = a phenyl group attached to the methyl carbon of acetaldehyde. …