Q.Write the structures of the following compounds.

Concept: Oxidation Reactions -- but here the focus is on nomenclature to structure, not on any reaction. The key is to interpret the IUPAC name and draw the correct skeletal formula.

1. alpha-Methoxypropionaldehyde Propionaldehyde is propanal (CH3CH2CHO). alpha-position is the carbon next to the aldehyde group. Replace one H on that carbon with a methoxy (-OCH3). Structure: CH3-CH(OCH3)-CHO
2. 3-Hydroxybutanal Butanal with an -OH group on carbon 3. Structure: CH3-CH(OH)-CH2-CHO
3. 2-Hydroxycyclopentane carbaldehyde Cyclopentane ring with an aldehyde group (-CHO) attached to carbon 1, and an -OH on carbon 2. Structure: A pentagon with OH on C2 and CHO on C1.
4. 4-Oxopentanal Pentanal with a keto group (=O) on carbon 4. Structure: CH3-CO-CH2-CH2-CHO
5. Di-sec. butyl ketone …

Let’s break down each name step by step. The logic is always: identify the parent chain (the longest carbon chain containing the principal functional group), number it to give the functional group the lowest locant, then place substituents accordingly.

(i) α-Methoxypropionaldehyde

Parent: Propionaldehyde = propanal (3-carbon chain with –CHO at C1).

Substituent: α-Methoxy means a methoxy group (–OCH₃) attached to the α-carbon (the carbon adjacent to the aldehyde group, i.e., C2).

Structure:

CH₃–CH(OCH₃)–CHO

(ii) 3-Hydroxybutanal

Parent: Butanal = 4-carbon chain with –CHO at C1.

Substituent: –OH (hydroxy) at C3.

Numbering: The aldehyde carbon is always C1, so the chain is numbered from the –CHO end.

Structure:

CH₃–CH(OH)–CH₂–CHO

(iii) 2-Hydroxycyclopentane carbaldehyde

Parent: Cyclopentane carbaldehyde = a cyclopentane ring with a –CHO group attached (the ring is the parent, and the aldehyde is a substituent named as “carbaldehyde”).

Substituent: –OH at C2 of the ring.

Numbering: The carbon bearing the –CHO is C1. Then number the ring to give the –OH the lowest locant (C2).

Structure:

A five-membered ring with –CHO on C1 and –OH on C2 (cis or trans not specified; any stereoisomer is acceptable unless asked).

(iv) 4-Oxopentanal

Parent: Pentanal = 5-carbon chain with –CHO at C1.

Substituent: “Oxo” means a ketone group (=O) at C4.

Numbering: The aldehyde carbon is C1, so the ketone is at C4.

Structure:

CH₃–CO–CH₂–CH₂–CHO

(v) Di-sec. butyl ketone

Parent: Ketone — the functional group is C=O.

Substituents: Two sec-butyl groups attached to the carbonyl carbon. …

Method: Systematic IUPAC Name-to-Structure Conversion

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

Steps

1. Identify the parent chain (alkane, alkene, cycloalkane, or aromatic ring) and the principal functional group (highest priority for naming).
2. Number the parent chain to give the principal functional group the lowest possible locant.
3. Add substituents (alkyl, halo, hydroxy, oxo, alkoxy, etc.) at the correct positions.
4. Draw the complete structure with all bonds and atoms.

(i) α-Methoxypropionaldehyde

• Parent: Propanal (3-carbon aldehyde chain)
• Substituent: Methoxy ($-\text{OCH}_3$) at the α-carbon (carbon adjacent to the aldehyde group)
• Structure:

(ii) 3-Hydroxybutanal

• Parent: Butanal (4-carbon aldehyde chain)
• Substituent: Hydroxy ($-\text{OH}$) at carbon 3
• Structure:

(iii) 2-Hydroxycyclopentane carbaldehyde

• Parent: Cyclopentane ring with a carbaldehyde ($-\text{CHO}$) group attached
• Substituent: Hydroxy ($-\text{OH}$) at carbon 2 of the ring (carbon 1 is the carbaldehyde carbon)
• Structure:

(iv) 4-Oxopentanal

• Parent: Pentanal (5-carbon aldehyde chain)
• Substituent: Oxo ($=\text{O}$) at carbon 4
• Structure:

(v) Di-sec. butyl ketone …

(i) α-Methoxypropionaldehyde

✗ Common Mistakes

• Confusing “α” with “1”: Students often place the methoxy group on carbon-1 (the aldehyde carbon). But α refers to the carbon adjacent to the functional group.
• Forgetting the aldehyde: Some draw a ketone or alcohol instead of –CHO.

✓ How to Avoid

• Step 1: Identify the parent chain: “propionaldehyde” = 3-carbon aldehyde (propanal).
• Step 2: α-carbon = carbon-2 (next to –CHO). Attach –OCH₃ there.
• Structure: $\mathrm{CH_3\text{-}CH(OCH_3)\text{-}CHO}$

(ii) 3-Hydroxybutanal

✗ Common Mistakes

• Wrong carbon numbering: Students number from the wrong end. Aldehyde carbon must be carbon-1.
• Drawing a ketone: “butanal” means aldehyde, not butanone.

✓ How to Avoid

• Rule: For aldehydes, the –CHO carbon is always C-1.
• Parent: Butanal = 4-carbon chain with –CHO at end.
• Position: –OH on C-3.
• Structure: $\mathrm{CH_3\text{-}CH(OH)\text{-}CH_2\text{-}CHO}$

(iii) 2-Hydroxycyclopentane carbaldehyde

✗ Common Mistakes

• Incorrect ring numbering: The aldehyde carbon is not part of the ring — it’s a substituent. Students often number the ring carbon bearing the –CHO as C-1, but the correct IUPAC name uses “carbaldehyde” to indicate –CHO as a suffix on the ring.
• Missing stereochemistry: Not required here, but students sometimes add wedges unnecessarily.

✓ How to Avoid

• Parent: Cyclopentane ring.
• Substituent: –CHO group attached to the ring (named as carbaldehyde).
• Position: –OH on carbon-2 of the ring.
• Structure: A cyclopentane ring with –OH on C-2 and –CHO on C-1.

(iv) 4-Oxopentanal

✗ Common Mistakes

• Misplacing the oxo group: “Oxo” means =O (ketone). Students sometimes put it on C-1 (which is already aldehyde) or on C-5.
• Drawing two aldehydes: “pentanal” means one aldehyde at C-1; the oxo is a separate ketone.

✓ How to Avoid

• Parent: Pentanal (5-carbon chain, aldehyde at C-1).
• Oxo position: C-4 has a =O.
• Structure: $\mathrm{CH_3\text{-}CO\text{-}CH_2\text{-}CH_2\text{-}CHO}$

(v) Di-sec. butyl ketone

✗ Common Mistakes

• Misinterpreting “sec. butyl”: Students draw a straight butyl chain instead of the branched secondary butyl group.
• Forgetting the ketone: “Ketone” means –CO– in the middle. Some draw an aldehyde or alcohol.

✓ How to Avoid

• “Di-sec. butyl” = two identical secondary butyl groups.
• Secondary butyl = –CH(CH₃)CH₂CH₃ (the carbon attached to the ketone is secondary).
• Ketone: The carbonyl carbon is between the two groups.
• Structure: $\mathrm{CH_3CH_2CH(CH_3)\text{-}CO\text{-}CH(CH_3)CH_2CH_3}$

(vi) 4-Fluoroacetophenone

✗ Common Mistakes …