Quiz & Worksheet - Structural Isomers | Study.com - Free Printable

Let’s solve each question step by step, with clear explanations.

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Question 1: What is the correct relationship between the two molecules shown below?

Molecule 1:
Cl attached to the first carbon (leftmost) of a 3-carbon chain:
`Cl-CH₂-CH₂-CH₃` → 1-chloropropane

Molecule 2:
Cl attached to the middle carbon:
`CH₃-CHCl-CH₃` → 2-chloropropane

✔ Both have the same molecular formula: C₃H₇Cl, but the chlorine atom is attached to different carbon atoms.

This is a classic example of positional isomerism — where the functional group (here, Cl) is in a different position on the same carbon skeleton.

> ✘ Not chain isomers — because the carbon chain is the same length and unbranched.
> ✘ Not functional group isomers — because both have the same functional group (chloro).
> ✘ Not “no relationship” — they are definitely isomers.

✔ Correct Answer: They are positional isomers.

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Question 2: Which of the following is a structural isomer of the molecule shown below?

The given molecule is:

```
H H O
| | ||
H - C - C - C - O - H
| | |
H H H
```

This is propanoic acid (CH₃CH₂COOH), molecular formula: C₃H₆O₂

We need to find a structural isomer — same molecular formula (C₃H₆O₂), but different connectivity.

Let’s evaluate each option:

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Option A:
```
O
||
CH₃-C-O-CH₃
```
→ This is methyl ethanoate (ester), formula: C₃H₆O₂ ✔
Same molecular formula, different functional group (ester vs. carboxylic acid) → Structural isomer

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Option B:
```
H H O
| | ||
H - C - C - C - H
| | |
H H H
```
→ This is propanal (aldehyde), formula: C₃H₆O ✘
Only 1 oxygen → not same formula → Not an isomer

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Option C:
```
H O H
| || |
H - C - C - C - H
| |
H H
```
→ This is acetone (propanone), formula: C₃H₆O ✘
Again, only 1 oxygen → Not an isomer

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Option D:
```
H H H
| | |
H - C - C - C - O - H
| | |
H H H
```
→ This is propanol (alcohol), formula: C₃H₈O ✘
Too many hydrogens → Not an isomer

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✔ Only Option A has the same molecular formula (C₃H₆O₂) and different structure → Correct Answer: Option A

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Question 3: All of the following are chain isomers of each other, except one. Which is the exception?

Chain isomers = same molecular formula, but different carbon chain arrangement (branched vs. straight).

Let’s find the molecular formula of each.

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Option 1:
```
CH₃CH₂-CH-CH₂CH₃
|
CH₃
```
→ This is 3-methylpentane
Carbon count: 6 carbons → C₆H₁₄

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Option 2:
```
CH₃
|
CH₃-C-CH₂CH₃
|
CH₃
```
→ This is 2,2-dimethylbutane
Carbon count: 6 carbons → C₆H₁₄

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Option 3:
```
H-C-C-C-C-C-C-H (all single bonds, 6 carbons)
```
→ Straight-chain hexane → C₆H₁₄

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Option 4:
```
CH₃-CH-CH-CH₃
| |
CH₃ CH₃
```
→ This is 2,3-dimethylbutane
Carbon count: 6 carbons → C₆H₁₄

Wait — all seem to be C₆H₁₄?

But look again at Option 3 — it’s drawn as:

```
H-C-C-C-C-C-C-H
```

That’s 7 carbons! Let’s count:

Each “C” in the chain is a carbon. The structure shows 6 bonds between 7 carbons.

So:
C₁ — C₂ — C₃ — C₄ — C₅ — C₆ — C₇

→ Molecular formula: C₇H₁₆

All others are C₆H₁₄ → So this one is not a chain isomer of the others.

✔ Exception: Option 3 (the straight-chain 7-carbon alkane)

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## ✔ Final Answers:

1. They are positional isomers.
2. Option A (methyl ethanoate)
3. Option 3 (the 7-carbon straight chain)

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