Naming Alkynes by IUPAC Nomenclature Rules with Practice Problems - Free Printable

Let’s go step by step to understand how the names 6-ethyl-2-methylnonane and 7-methyl-3-propyloct-1-yne (or 7-methyl-3-propyl-1-octyne) are derived from their structures.

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For the Alkane: 6-ethyl-2-methylnonane

#### Step 1: Identify the parent chain
We look for the longest continuous carbon chain. In this case, it’s 9 carbons long → so the base name is nonane.

#### Step 2: Identify substituents
There are two branches:
- A methyl group (–CH₃) attached at one point.
- An ethyl group (–CH₂CH₃) attached at another.

#### Step 3: Number the parent chain
We number the chain from left to right or right to left — whichever gives the smallest numbers to the substituents.

If we number left to right:
- Methyl is on carbon #2
- Ethyl is on carbon #6
→ Locants: 2 and 6

If we number right to left:
- Methyl would be on carbon #8
- Ethyl would be on carbon #4
→ Locants: 4 and 8 → worse!

So we choose left-to-right numbering → locants 2 and 6.

#### Step 4: Alphabetize substituents
“Ethyl” comes before “methyl” alphabetically? Wait — no! Actually, “e” comes before “m”, so ethyl should come first in the name.

But wait — let’s check the final name given: 6-ethyl-2-methylnonane

That means they wrote ethyl first (correctly alphabetical), even though its locant (6) is higher than methyl’s (2). That’s okay — alphabetical order ignores the numbers.

✔ So:
Substituents: ethyl at 6, methyl at 2 → alphabetized as “ethyl” then “methyl” → 6-ethyl-2-methylnonane

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For the Alkyne: 7-methyl-3-propyloct-1-yne (or 7-methyl-3-propyl-1-octyne)

#### Step 1: Identify the parent chain
Important rule: The triple bond must be included in the parent chain, even if it’s not the absolute longest chain.

Here, the longest chain that includes the C≡C bond has 8 carbons → so base name is octyne.

The triple bond is between carbons 1 and 2 → so it’s an oct-1-yne.

#### Step 2: Identify substituents
Branches:
- A methyl group (–CH₃)
- A propyl group (–CH₂CH₂CH₃)

#### Step 3: Number the parent chain
Rule: Give the triple bond the smallest possible number.

So we must start numbering from the end closest to the triple bond.

In the diagram, the triple bond is at the top-right of the main chain. If we number from that end:

- Triple bond starts at carbon #1 → good!
- Then moving down the chain:
- Propyl group is attached at carbon #3
- Methyl group is attached at carbon #7

Locants: triple bond at 1, propyl at 3, methyl at 7.

If we numbered from the other end, the triple bond would be at position 7 or 8 — which violates the rule.

So correct numbering: triple bond gets lowest number → 1.

#### Step 4: Alphabetize substituents
Substituents: methyl and propyl.

Alphabetical order: m comes before p → so “methyl” before “propyl”.

Even though methyl is at position 7 and propyl at 3, we still write them in alphabetical order.

So:
→ 7-methyl-3-propyloct-1-yne

Alternative naming: sometimes people write the number before the suffix like “1-octyne” instead of “oct-1-yne”. Both are accepted.

So also valid: 7-methyl-3-propyl-1-octyne

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Final Check:

Alkane:
Parent: nonane (9C)
Substituents: ethyl at 6, methyl at 2 → alphabetized → 6-ethyl-2-methylnonane ✔

Alkyne:
Parent: octyne with triple bond at 1 → oct-1-yne
Substituents: methyl at 7, propyl at 3 → alphabetized → 7-methyl-3-propyloct-1-yne ✔

All steps follow IUPAC rules correctly.

Final Answer:
6-ethyl-2-methylnonane and 7-methyl-3-propyloct-1-yne (or 7-methyl-3-propyl-1-octyne)