Alkane Nomenclature Worksheet for 9th - 12th Grade | Lesson Planet - Free Printable

Let’s go through each problem one by one. We’re naming alkanes using IUPAC rules — that means we find the longest carbon chain, number it to give substituents the lowest numbers, and name them in alphabetical order.

---

1.
Structure: A 4-carbon chain with a methyl group on carbon #2.
Longest chain = 4 carbons → “butane”
Methyl on carbon 2 → “2-methyl”
Name: 2-methylbutane

---

2.
Structure: A 5-carbon chain with a methyl group on carbon #3.
Longest chain = 5 carbons → “pentane”
Methyl on carbon 3 → “3-methyl”
Name: 3-methylpentane

---

3.
Structure: A 4-carbon chain with two methyl groups — one on carbon #2, one on carbon #3.
Longest chain = 4 carbons → “butane”
Two methyls → “dimethyl”
Positions: 2 and 3 → “2,3-dimethyl”
Name: 2,3-dimethylbutane

---

4.
Structure: Looks like a 6-carbon chain with an ethyl group on carbon #3 and a methyl on carbon #4? Wait — let’s check again. Actually, if you trace the longest chain, it’s 7 carbons! Let me redraw mentally:

Start from left: CH₃–CH₂–CH(CH₃)–CH(C₂H₅)–CH₂–CH₂–CH₃? No — better way: The main chain is actually 7 carbons long. Substituents: methyl on carbon 3, ethyl on carbon 4? But we want lowest numbers.

Actually, correct longest chain: 7 carbons → heptane
Substituents: methyl on C3, ethyl on C4 → but alphabetically ethyl comes before methyl. So we number to get lowest set of numbers.

If we number from right: ethyl would be on C4, methyl on C5 → worse.

So number from left: methyl on C3, ethyl on C4 → positions 3 and 4.

But wait — maybe there’s a longer chain? No, 7 is max.

Alphabetical: ethyl before methyl → so “4-ethyl-3-methylheptane”

Wait — let’s double-check structure. Original drawing: it’s branched at two points. Yes, 7-carbon chain with ethyl on 4 and methyl on 3.

Name: 4-ethyl-3-methylheptane

---

5.
Given name: 3-ethyl-2,4,5-trimethylheptane
We need to draw the structure.

Heptane = 7-carbon chain.
Numbering: C1–C2–C3–C4–C5–C6–C7
At C2: methyl
At C3: ethyl
At C4: methyl
At C5: methyl

So:
C1 – C2(CH₃) – C3(C₂H₅) – C4(CH₃) – C5(CH₃) – C6 – C7

Draw as:
CH₃–CH(CH₃)–CH(C₂H₅)–CH(CH₃)–CH(CH₃)–CH₂–CH₃

---

6.
Given name: 4-ethyl-2,7-dimethyloctane
Octane = 8-carbon chain.
C1–C2–C3–C4–C5–C6–C7–C8
At C2: methyl
At C4: ethyl
At C7: methyl

Structure:
CH₃–CH(CH₃)–CH₂–CH(C₂H₅)–CH₂–CH₂–CH(CH₃)–CH₃

---

7.
Structure: Looks like a 6-carbon chain with methyl groups on C2, C3, and C4? Let’s count.

Main chain: 6 carbons → hexane
Substituents: methyl on C2, C3, C4 → three methyls → trimethyl
Positions: 2,3,4 → “2,3,4-trimethylhexane”

Check numbering: If we number from other end, it becomes 3,4,5 — which is higher, so 2,3,4 is correct.

Name: 2,3,4-trimethylhexane

---

8.
Structure: Longest chain? Let’s see — it looks like 7 carbons with branches.

Actually, tracing: start from leftmost CH₃–CH₂–CH(CH₃)–CH₂–CH(C₂H₅)–CH₂–CH₃? That’s 7 carbons.

Substituents: methyl on C3, ethyl on C5.

Numbering: if we go left to right: methyl on 3, ethyl on 5 → positions 3,5
If we go right to left: ethyl on 3, methyl on 5 → same numbers, but ethyl comes first alphabetically.

So we should number so that ethyl gets lower number? Wait — both ways give 3 and 5. Alphabetical order decides prefix order.

Ethyl before methyl → so “5-ethyl-3-methylheptane”? No — if we number from right, ethyl is on C3, methyl on C5 → then it’s “3-ethyl-5-methylheptane”

Which is better? Lower number for first-named substituent? Rule: assign lowest numbers to substituents regardless of name, then list alphabetically.

In this case, whether we number left-to-right or right-to-left, the substituent positions are 3 and 5. So we choose the direction that gives the lowest number to the first-named group (alphabetically).

Ethyl comes before methyl → so we want ethyl to have lower number → number from right: ethyl on C3, methyl on C5 → “3-ethyl-5-methylheptane”

Yes.

Name: 3-ethyl-5-methylheptane

---

9.
Structure: Central carbon with three methyls and one propyl? Wait — let’s look.

It’s a central carbon attached to:
- One CH₃
- One CH₃
- One CH₃
- One CH₂–CH₂–CH₃ (propyl)

That’s 4 groups → tetrahedral carbon.

Longest chain: from propyl through central carbon to one methyl? That’s 4 carbons: CH₃–CH₂–CH₂–C(CH₃)₃ → but wait, the central carbon has three methyls and one propyl.

So longest chain is 4 carbons: the propyl plus the central carbon → butane? But the central carbon is also attached to three methyls.

Actually, the longest continuous chain is 4 carbons: e.g., CH₃–CH₂–CH₂–C(CH₃)₃ → that’s 4 carbons in a row, with three methyls on the last carbon.

So base chain: butane
Substituents: three methyls on carbon #2? Wait — if we number the chain as:

C1 – C2 – C3 – C4
Where C4 is the central carbon with three methyls? Then C4 has three methyls → that’s 2,2-dimethyl? No.

Better: make the longest chain include the propyl and one methyl? Still 4.

Standard way: this is 2,2-dimethylbutane? No.

Wait — actual structure: central carbon bonded to three methyl groups and one n-propyl group.

So the longest chain is from the end of propyl through central carbon to one methyl → that’s 4 carbons: CH₃–CH₂–CH₂–C(CH₃)₃ → but the C at the end has three methyls, so it’s C with four bonds: one to CH₂CH₂CH₃, and three to CH₃.

So the longest chain is 4 carbons: let’s call it C1–C2–C3–C4, where C4 is the central carbon, and C4 has two additional methyl groups (since one is part of the chain? No — if C4 is the central carbon, and we take the chain as C1–C2–C3–C4, then C4 is only bonded to C3 and three H’s normally — but here it’s bonded to C3 and three CH₃ groups.

So actually, the chain is C1–C2–C3–C4, and C4 has two extra methyl groups (because one methyl is considered part of the chain? No — confusion.

Correct approach: the molecule is (CH₃)₃C–CH₂–CH₂–CH₃

Longest chain: from one end of propyl to the end of a methyl on the tert-butyl? That would be CH₃–CH₂–CH₂–C(CH₃)₃ → but the C in C(CH₃)₃ is quaternary.

The longest continuous chain is 4 carbons: for example, CH₃–CH₂–CH₂–C(CH₃)₃ — but the last carbon has three methyls, so we consider the chain as butane with two methyl substituents on carbon 2? Let's number properly.

Set the chain as:
C1 – C2 – C3 – C4
Where C1 is CH₃– (from propyl), C2 is CH₂, C3 is CH₂, C4 is C(CH₃)₃

But C4 is carbon with three methyl groups — so it has three identical methyl substituents.

So the base chain is butane (C1 to C4), and on C4, there are two additional methyl groups (since one methyl is already counted in the chain? No — in standard naming, when you have (CH₃)₃C–R, the R is the main chain if longer.

Here, R is propyl (3 carbons), and the tert-butyl has 4 carbons including the central one? Total carbons: central C + 3 CH₃ + CH₂CH₂CH₃ = 1+3+3=7 carbons.

Longest chain: from one methyl of tert-butyl through central C to end of propyl: CH₃–C(CH₃)₂–CH₂–CH₂–CH₃? That’s 5 carbons: C1 (methyl) – C2 (central) – C3 (CH₂) – C4 (CH₂) – C5 (CH₃)

And on C2, there are two methyl groups.

So chain: pentane
Substituents: two methyls on C2 → “2,2-dimethylpentane”

Yes! Because:
CH₃–C(CH₃)₂–CH₂–CH₂–CH₃
Carbon 1: CH₃– (end of what was propyl? Wait no.

Let’s define:
Start from the end of the propyl:
C1: CH₃– (of propyl)
C2: CH₂
C3: CH₂
C4: C (central)
Attached to C4: three CH₃ groups.

But if we take C4 as part of the chain, and go to one of its methyls, say C5: CH₃

Then chain is C1–C2–C3–C4–C5 → pentane
On C4, there are two additional methyl groups (since one is C5).

So substituents: two methyls on C4 → “4,4-dimethylpentane”? But we want lowest numbers.

If we number from the other end:
C1: the methyl that was attached to central C
C2: central C
C3: CH₂
C4: CH₂
C5: CH₃ (end of propyl)

Then on C2, there are two methyl groups → “2,2-dimethylpentane”

Yes, and 2,2 is lower than 4,4, so we use this numbering.

Name: 2,2-dimethylpentane

Wait — but in the structure given, it’s drawn as a central carbon with three methyls and one propyl — yes, that’s exactly (CH₃)C–CH₂CH₂CH₃, which is 2,2-dimethylpentane.

Confirm: pentane chain: C1–C2–C3–C4–C5
C1: CH₃– (one of the methyls from tert-butyl)
C2: C (central) with two additional CH₃ groups
C3: CH₂
C4: CH₂
C5: CH₃

Yes.

Name: 2,2-dimethylpentane

---

10.
Structure: Looks complex. Let’s break it down.

There’s a central chain. From left: CH₃–CH₂–C(CH₃)₂–CH(CH₃)–CH₂–CH(CH₃)–CH₂–CH₃? Let’s count atoms.

Actually, tracing the longest chain: start from far left CH₃–CH₂– then a carbon with two methyls (so tertiary butyl-like), then CH, then CH₂, then CH with methyl, then CH₂, then CH₃.

Better: identify all carbons.

Total carbons: let’s count branches.

From the drawing:
- Left: ethyl group? Or part of chain.

Assume the main chain is horizontal.

Left end: CH₃–CH₂–
Then a carbon that has two methyl groups attached (so it’s C with three alkyl groups: ethyl, and two methyls, and next in chain) — so that carbon is quaternary? No, in alkane, carbon has four bonds.

In the drawing: after CH₃–CH₂–, there’s a carbon that has:
- Bond to previous CH₂
- Bond to next CH
- Bond to CH₃
- Bond to another CH₃
So yes, quaternary carbon.

Then next is CH, which has:
- Bond to previous C
- Bond to next CH₂
- Bond to CH₃ (branch)
Then CH₂, then CH, which has:
- Bond to previous CH₂
- Bond to next CH₂
- Bond to CH₃ (branch)
Then CH₂, then CH₃.

So longest chain: from left CH₃ to right CH₃:
C1: CH₃– (left end)
C2: CH₂
C3: C (quaternary, with two methyl branches)
C4: CH (with one methyl branch)
C5: CH₂
C6: CH (with one methyl branch)
C7: CH₂
C8: CH₃

So 8-carbon chain → octane

Substituents:
- On C3: two methyl groups → "3,3-dimethyl"
- On C4: one methyl → "4-methyl"
- On C6: one methyl → "6-methyl"

Now, list substituents in alphabetical order: dimethyl, methyl, methyl — but we combine same prefixes.

So: 3,3-dimethyl, 4-methyl, 6-methyl → but alphabetically, "d" for dimethyl comes before "m" for methyl? No — we ignore di-, tri- for alphabetizing.

Rule: alphabetize by the name of the substituent, ignoring prefixes like di-, tri-.

So "methyl" and "methyl" — all are methyl groups.

We have four methyl groups total: two on C3, one on C4, one on C6.

So we list them as: 3,3,4,6-tetramethyloctane? But we need to specify positions.

The substituents are all methyl, so we can combine: tetramethyl

Positions: 3,3,4,6

But we write in numerical order: 3,3,4,6-tetramethyloctane

Is that correct? Let’s confirm numbering.

Could we number from the other end to get lower numbers?

Current numbering: substituents at 3,3,4,6

If we number from right:
C1: CH₃ (right end)
C2: CH₂
C3: CH (was C6, with methyl)
C4: CH₂
C5: CH (was C4, with methyl)
C6: C (was C3, with two methyls)
C7: CH₂
C8: CH₃ (left end)

Substituents:
On C3: methyl
On C5: methyl
On C6: two methyls → so positions 3,5,6,6

Compare sets: original 3,3,4,6 vs new 3,5,6,6

To compare, list in order: 3,3,4,6 vs 3,5,6,6

First different number: 3 vs 3 (same), then 3 vs 5 — 3<5, so original is better.

Thus, name: 3,3,4,6-tetramethyloctane

But wait — in the structure, is the chain really 8 carbons? Let me recount.

From left:
- C1: CH₃–
- C2: CH₂–
- C3: C (with two CH₃) — so this carbon is bonded to C2, C4, CH₃, CH₃
- C4: CH– (bonded to C3, C5, CH₃)
- C5: CH₂–
- C6: CH– (bonded to C5, C7, CH₃)
- C7: CH₂–
- C8: CH₃

Yes, 8 carbons in chain.

Substituents: methyl on C3 (two of them), methyl on C4, methyl on C6 → total four methyl groups.

Positions: 3,3,4,6

Alphabetical: all methyl, so tetramethyl.

Numerical order: 3,3,4,6

Name: 3,3,4,6-tetramethyloctane

---

Now, compiling all answers:

1. 2-methylbutane
2. 3-methylpentane
3. 2,3-dimethylbutane
4. 4-ethyl-3-methylheptane
5. Structure for 3-ethyl-2,4,5-trimethylheptane: CH₃–CH(CH₃)–CH(C₂H₅)–CH(CH₃)–CH(CH₃)–CH₂–CH₃
6. Structure for 4-ethyl-2,7-dimethyloctane: CH₃–CH(CH₃)–CH₂–CH(C₂H₅)–CH₂–CH₂–CH(CH₃)–CH₃
7. 2,3,4-trimethylhexane
8. 3-ethyl-5-methylheptane
9. 2,2-dimethylpentane
10. 3,3,4,6-tetramethyloctane

For problems 5 and 6, since they ask to draw the structure, but in text we describe it. In the answer, we can write the condensed formula.

Final Answer:
1. 2-methylbutane
2. 3-methylpentane
3. 2,3-dimethylbutane
4. 4-ethyl-3-methylheptane
5. CH₃CH(CH₃)CH(C₂H₅)CH(CH₃)CH(CH₃)CH₂CH₃
6. CH₃CH(CH₃)CH₂CH(C₂H₅)CH₂CH₂CH(CH₃)CH₃
7. 2,3,4-trimethylhexane
8. 3-ethyl-5-methylheptane
9. 2,2-dimethylpentane
10. 3,3,4,6-tetramethyloctane