Naming branched alkanes worksheet with six chemical structures.

Here is the solution to naming the branched alkanes in your worksheet, following IUPAC nomenclature rules:

---

🔍 General Rules Recap:

1. Find the longest continuous carbon chain → this gives the parent name (e.g., methane, ethane, propane, butane, pentane, hexane, heptane, octane, etc.).
2. Number the chain so that substituents get the lowest possible numbers.
3. Name and number substituents (methyl, ethyl, propyl, etc.).
4. List substituents in alphabetical order (ignoring prefixes like di-, tri-).
5. Use commas between numbers and hyphens between numbers and words.

---

## ✔ Solutions:

---

1.

```
H₃C—CH—CH₃
|
CH₃
```

- Longest chain: 3 carbons → propane
- One methyl group on carbon #2
- Name: 2-methylpropane

> 📝 Note: This is also called *isobutane*, but IUPAC name is 2-methylpropane

---

2.

```
H₃C—CH—CH₃
|
CH₂—CH₃
```

- Longest chain: 4 carbons → butane
- Ethyl group? No — actually, if you count the branch as part of the main chain, you can get a longer chain!
- Let’s reorient: The branch is ethyl (–CH₂CH₃), but the longest chain is actually 4 carbons if you go through the branch.
- Actually, let’s number it properly:

If we number left to right: C1–C2–C3, with ethyl on C2 → total chain length = 4 (C1–C2–C3 + ethyl’s C1–C2)

So longest chain = 4 carbons → butane
- Substituent: methyl group on C2? Wait — no.

Wait — let’s redraw mentally:

The central carbon is bonded to:
- CH₃ (left)
- CH₃ (right)
- CH₂CH₃ (down)

That’s a carbon with 3 alkyl groups: two methyls and one ethyl → that’s a 4-carbon chain if you take the ethyl as part of the main chain.

✔ Correct approach:

Start from the ethyl group: CH₃–CH₂–CH–CH₃
|
CH₃

So longest chain is 4 carbons: CH₃–CH₂–CH–CH₃ → numbered as:
C1 (CH₃) – C2 (CH₂) – C3 (CH) – C4 (CH₃), with a methyl on C3.

Wait — that’s 2-methylbutane?

Actually, let’s number to give lowest numbers:

If we number the chain as:

CH₃–CH(CH₃)–CH₂–CH₃ → that’s 4 carbons, methyl on C2 → 2-methylbutane

But in the original drawing, it's written as:

H₃C–CH–CH₃
|
CH₂–CH₃

That’s the same as: CH₃–CH(CH₂CH₃)–CH₃ → which is 2-ethylpropane? But that’s wrong because ethyl on a 3-carbon chain makes the longest chain 4 carbons.

✔ So correct IUPAC: 2-methylbutane

> Why? Because the longest chain is 4 carbons: CH₃–CH₂–CH–CH₃ with methyl on C2? Wait — let’s draw it clearly.

Actually, the structure is:

Carbon 1: CH₃–
Carbon 2: CH– (with CH₃ on one side, CH₂CH₃ on the other)
Carbon 3: CH₃

So the longest chain is: CH₃–CH–CH₂–CH₃ → 4 carbons, with a methyl on C2? No — wait, the central carbon is connected to:

- CH₃ (group A)
- CH₃ (group B)
- CH₂CH₃ (group C)

So the longest chain is CH₃–CH₂–CH–CH₃ → where the middle CH has a methyl group → that’s 2-methylbutane

✔ Final Answer: 2-methylbutane

---

3.

```
H₃C—CH₂—CH₂—CH—CH₂—CH₂—CH₃
|
CH₂—CH₃
```

- Longest chain: 7 carbons → heptane
- Ethyl group on carbon #4
- Numbering: from left to right → ethyl on C4
- From right to left: ethyl on C4 → same
- Name: 4-ethylheptane

> ✔ Correct: 4-ethylheptane

---

4.

```
H₃C—CH₂—CH₂—CH—CH—CH₂—CH₃
| |
CH₃ CH₂—CH₃
```

- Longest chain: 7 carbons → heptane
- Substituents:
- Methyl on C4
- Ethyl on C5
- Numbering: try both directions.

Left to right:
- C1–C2–C3–C4–C5–C6–C7
- Methyl on C4, ethyl on C5 → positions 4 and 5

Right to left:
- C1’–C2’–C3’–C4’–C5’–C6’–C7’
- Ethyl on C3’, methyl on C4’ → positions 3 and 4 → better!

So we number from the right:

CH₃–CH₂–CH–CH–CH₂–CH₂–CH₃
| |
CH₃ CH₂CH₃

Numbered as:
C1: CH₃ (rightmost)
C2: CH₂
C3: CH (with ethyl)
C4: CH (with methyl)
C5: CH₂
C6: CH₂
C7: CH₃

So:
- Ethyl on C3
- Methyl on C4

Alphabetical order: ethyl before methyl → 3-ethyl-4-methylheptane

✔ Final Answer: 3-ethyl-4-methylheptane

---

5.

```
H₃C—CH₂—CH—CH₂—CH—CH₂—CH₃
| |
CH₃ CH₂—CH₂—CH₃
```

- Longest chain: 7 carbons → heptane
- Substituents:
- Methyl on C3
- Propyl on C5
- Numbering: left to right → methyl on C3, propyl on C5 → positions 3,5
- Right to left: propyl on C3, methyl on C5 → same set of numbers → choose alphabetical order

Substituents: methyl and propyl → “m” before “p” → so list methyl first

Name: 3-methyl-5-propylheptane

> ❗ BUT — is there a longer chain? The propyl group is –CH₂CH₂CH₃ → if we include it, could we get an 8-carbon chain?

Let’s check:

If we go: CH₃–CH₂–CH–CH₂–CH–CH₂–CH₃
| |
CH₃ CH₂–CH₂–CH₃

From the propyl end: start at the end of propyl: CH₃–CH₂–CH₂–CH–CH₂–CH–CH₂–CH₃
| |
CH₃ CH₃? Wait — no, the propyl is attached to C5.

Actually, the longest chain is still 7 carbons — because the propyl is a branch.

✔ So correct: 3-methyl-5-propylheptane

---

6.

```
H₃C—CH₂—CH₂—CH₂—CH₂
|
H₃C—CH₂—CH₂—CH₂—C—CH₂—CH₃
|
CH₃
```

This is a bit complex. Let’s parse:

The central carbon (let’s call it C*) is bonded to:
- CH₂–CH₂–CH₂–CH₃ (top)
- CH₂–CH₂–CH₂–CH₃ (left)
- CH₂–CH₃ (right)
- CH₃ (bottom)

So C* is a carbon with 4 alkyl groups → it’s a quaternary carbon

Now, find the longest chain:

Option 1: Go from top: CH₃–CH₂–CH₂–CH₂–CH₂–C*–CH₂–CH₃ → that’s 8 carbons (C1 to C8)

C1: CH₃ (top)
C2: CH₂
C3: CH₂
C4: CH₂
C5: CH₂
C6: C*
C7: CH₂
C8: CH₃

And C* also has:
- CH₂–CH₂–CH₂–CH₃ (left) → that’s 4 more carbons → but not in chain
- CH₃ (bottom) → methyl

So longest chain is 8 carbons → octane

Now, substituents on C6 (the central carbon):

- A methyl group (bottom)
- A butyl group (left: CH₂–CH₂–CH₂–CH₃)

But wait — the left group is CH₂–CH₂–CH₂–CH₃ → that’s a butyl group

So on C6, we have:
- Methyl
- Butyl

But numbering: we need to assign numbers so substituents get lowest numbers.

If we number the chain as:

C1: CH₃ (top)
C2: CH₂
C3: CH₂
C4: CH₂
C5: CH₂
C6: C*
C7: CH₂
C8: CH₃

Then substituents are on C6: methyl and butyl

But we can number from the other end? Same thing — C6 is still the center.

Alternatively, if we number from the right:

C1: CH₃ (right end)
C2: CH₂
C3: C* ← now C3
C4: CH₂
C5: CH₂
C6: CH₂
C7: CH₂
C8: CH₃ (top)

Then substituents on C3: methyl and butyl → position 3 is lower than 6 → better!

So number from the right:

Chain: CH₃–CH₂–C*–CH₂–CH₂–CH₂–CH₂–CH₃

C1: CH₃ (right)
C2: CH₂
C3: C* (with methyl and butyl)
C4: CH₂
C5: CH₂
C6: CH₂
C7: CH₂
C8: CH₃ (top)

Substituents on C3:
- Methyl
- Butyl

Alphabetical: butyl before methyl → 3-butyl-3-methyloctane

> ❗ BUT — according to IUPAC, we list substituents alphabetically, ignoring prefixes. “b” for butyl comes before “m” for methyl → so 3-butyl-3-methyloctane

However — there’s a problem: the butyl group is actually a propyl? Wait, let’s count:

The left group is: H₃C–CH₂–CH₂–CH₂– → that’s 4 carbons → butyl

Yes.

But wait — is there a longer chain? What if we go through the butyl group?

Try: Start from the left butyl: CH₃–CH₂–CH₂–CH₂–C*–CH₂–CH₃ → that’s 7 carbons (C1 to C7), but C* also has another butyl? No — the top is CH₂–CH₂–CH₂–CH₂–CH₃ → that’s 5 carbons.

Actually, the longest chain is 8 carbons as above.

✔ Final Answer: 3-butyl-3-methyloctane

> ⚠️ However, note that “butyl” and “methyl” are both on the same carbon, and we write them in alphabetical order: butyl before methyl → so 3-butyl-3-methyloctane

---

## 🧾 Final Answers Summary:

| # | IUPAC Name |
|---|--------------------------|
| 1 | 2-methylpropane |
| 2 | 2-methylbutane |
| 3 | 4-ethylheptane |
| 4 | 3-ethyl-4-methylheptane |
| 5 | 3-methyl-5-propylheptane |
| 6 | 3-butyl-3-methyloctane |

---

✔ All names follow IUPAC rules: longest chain, lowest numbers, alphabetical order of substituents.

Let me know if you want structural drawings or explanations for any specific one!