Isomers - Worksheet #6 | PDF | Chemistry | Physical Sciences - Free Printable

Let's solve the problems step by step.

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Problem 1: Which of these pairs are structural isomers?

Structural isomers (also called constitutional isomers) are compounds that have the same molecular formula but different connectivity of atoms. That means they differ in how the atoms are bonded together.

We'll examine each pair:

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#### a.
```
H H H H
| | | |
Cl–C–C–Cl vs H–C–C–H
| | | |
H H Cl Cl
```

- Left: 1,2-dichloroethane (ClCH₂–CH₂Cl)
- Right: Also ClCH₂–CH₂Cl — same connectivity.
- Both structures represent the same molecule — just rotated or drawn differently.

✔ Same compound, not isomers.

---

#### b.
```
Br Br Br H
| | | |
H–C=C–H vs H–C=C–Br
| | | |
H H H Br
```

- Left: 1,2-dibromoethene (trans or cis?)
- Double bond between C1 and C2; Br on both carbons.
- This is (E)-1,2-dibromoethene if trans, or (Z) if cis.
- Right: One Br on C1, one Br on C2, but H and Br on same side → this is (Z)-1,2-dibromoethene.

Wait — actually, let’s look closely:
- Left: Br on top of each carbon → could be *trans* (if opposite), but in this drawing, both Br are up → cis.
- Right: Br on left carbon (top), Br on right carbon (bottom) → trans.

But both have the same molecular formula: C₂H₂Br₂

However, they differ in spatial arrangement around the double bond → geometric isomers (a type of stereoisomer), not structural isomers.

✘ Not structural isomers.

---

#### c.
```
H Cl Cl Cl
| | | |
H–C–C–Cl vs H–C–C–H
| | | |
H H H H
```

- Left: CH₃–CHCl₂ → 1,1-dichloroethane
- Right: CH₂Cl–CH₂Cl → 1,2-dichloroethane

Molecular formulas:
- Left: C₂H₄Cl₂
- Right: C₂H₄Cl₂ → same formula

But different bonding:
- Left: Two Cl atoms on same carbon
- Right: Cl atoms on adjacent carbons

✔ Different connectivity → Structural isomers

✔️ Yes, structural isomers

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#### d.
```
H
|
H–C–H
|
H–C–C–H
| |
H H
|
H
```
vs
```
H H
| |
H–C–C–H
| |
H H
|
H
```

Let’s rewrite them clearly.

Left:
```
H
|
H–C–H
|
H–C–C–H
| |
H H
|
H
```

This is a 4-carbon chain with a methyl group attached to the second carbon? Wait — count:

Actually, this seems like butane (C₄H₁₀), but let’s draw it properly.

Better to interpret:

First structure:
- Central carbon bonded to: H, C (with H, H), C (with H, H, H), and another H?
Wait — no.

Let’s count atoms.

Actually, both structures appear to be n-butane (CH₃–CH₂–CH₂–CH₃)

But let’s check:

Left:
- Carbon 1: bonded to 3 H and 1 C → CH₃–
- Carbon 2: bonded to 2 H and 2 C → CH₂–
- Carbon 3: bonded to 2 H and 2 C → CH₂–
- Carbon 4: bonded to 3 H and 1 C → CH₃

So: CH₃–CH₂–CH₂–CH₃ → n-butane

Right:
- Looks like same: CH₃–CH₂–CH₂–CH₃

They are just drawn differently — one is rotated.

✔ Identical molecules.

✘ Not isomers.

---

#### e.
```
H O–H H H
| | | |
H–C–C–C–H vs H–C–C–C–OH
| | | |
H H H H
```

Left: CH₃–CH(OH)–CH₃ → 2-propanol
Right: CH₃–CH₂–CH₂–OH → 1-propanol

Same molecular formula: C₃H₈O

But different functional group positions:
- Left: OH on middle carbon (carbon 2)
- Right: OH on end carbon (carbon 1)

✔ Different connectivity → structural isomers

✔️ Yes, structural isomers

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✔ Answers for Problem 1:

- a. No
- b. No
- c. Yes
- d. No
- e. Yes

So c and e are structural isomers.

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Problem 2: Examine each pair — identical, isomers, or different compounds?

We need to compare the two structures in each row.

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#### a.
```
CH₃
|
CH₃–C–CH₃ vs H₃C–CH₃
|
CH₃
```

Left: A central carbon bonded to three methyl groups → 2-methylpropane? Wait:

Wait — that’s neopentane? No.

Wait: The structure is:

- Central carbon bonded to three CH₃ groups → that’s 2,2-dimethylpropane?

No — wait: If it's CH₃–C(CH₃)₂–CH₃, but here it's written as:

```
CH₃
|
CH₃–C–CH₃
|
CH₃
```

That’s four methyl groups on one carbon → C(CH₃)₄ → 2,2-dimethylpropane? No.

Wait: Actually, the carbon has four bonds:
- Bonded to: CH₃ (left), CH₃ (top), CH₃ (right), CH₃ (bottom) → tetramethylmethane → C₅H₁₂

Wait — that’s 2,2-dimethylpropane, also known as neopentane.

But the right structure is H₃C–CH₃ → ethane, C₂H₆

So:
- Left: C₅H₁₂
- Right: C₂H₆

Different molecular formulas → different compounds

✘ Different compounds

---

#### b.
```
H₃C–CH₂–CH₃ vs H₃C–CH₂–CH₃
```

Both are propane → same structure, same name.

✔ Identical

---

#### c.
```
H₃C–CH₂–OH vs H₃C–CH₂–CH₂–OH
```

Left: ethanol → C₂H₅OH
Right: propanol → C₃H₇OH

Different molecular formulas → different compounds

✘ Different compounds

---

#### d.
```
H₃C–CH=CH₂ vs cyclopropane ring
```

Left: Propene → C₃H₆
Right: Cyclopropane → also C₃H₆

Same molecular formula, but:
- Left: open chain with double bond
- Right: cyclic, no double bond

Different connectivity → structural isomers

✔ Isomers

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✔ Final Answers for Problem 2:

| | Chemical Structure | Chemical Structure | Identical / Isomers / Different compounds |
|---|---------------------|---------------------|-------------------------------------------|
| a. | C(CH₃)₄ (neopentane) | CH₃–CH₃ (ethane) | Different compounds |
| b. | CH₃–CH₂–CH₃ (propane) | CH₃–CH₂–CH₃ (propane) | Identical |
| c. | CH₃–CH₂–OH (ethanol) | CH₃–CH₂–CH₂–OH (1-propanol) | Different compounds |
| d. | CH₃–CH=CH₂ (propene) | cyclopropane | Isomers |

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✔ Final Summary:

#### Problem 1: Structural isomers?
- a. No
- b. No
- c. Yes
- d. No
- e. Yes

#### Problem 2:
- a. Different compounds
- b. Identical
- c. Different compounds
- d. Isomers

Let me know if you'd like drawings or names clarified!