ChemistryChapter 8: Organic Chemistry
8.2.3 Naming Alkynes
**Alkynes:** Alkynes contain a carbon-carbon triple bond
Explanation level
Introduction
In the previous sections, we learned how to name alkanes (single bonds only) and alkenes (at least one double bond). Now we turn to alkynes, which contain at least one carbon-carbon triple bond.
The naming of alkynes follows a similar pattern to alkanes and alkenes, but with one important difference: the ending is "-yne" instead of "-ane" or "-ene". This tells us that the compound contains a carbon-carbon triple bond.
This section explains how to name alkynes using the stem and suffix system, and how to indicate the position of the triple bond.
Definition
Alkynes: Alkynes contain a carbon-carbon triple bond. This is shown in their name by the ending "-yne".
In simple words: Alkynes are hydrocarbons with a triple bond, and their names end in "-yne".
Main Idea
- Alkynes contain a carbon-carbon triple bond.
- Their names end in "-yne".
- For example:
- A 2-carbon chain containing a carbon-carbon triple bond is named ethyne, H - C ≡ C - H.
- A 3-carbon chain containing a carbon-carbon triple bond is named propyne, CH3C ≡ CH.
- With larger chains, the position of the triple bond is indicated by numbering the chain.
- Numbering is started from the end nearest to the triple bond, noting which carbon the triple bond starts from.
Key point: Alkyne names end in -yne, and the position of the triple bond is indicated by a number.
Core Concept
| Feature | Description |
|---|---|
| Suffix | -yne |
| Functional group | Carbon-carbon triple bond (C≡C) |
| General formula | CnH2n-2 |
| Position of triple bond | Indicated by a number in the name |
| Numbering rule | Start from the end nearest to the triple bond |
Key point: The suffix -yne tells you it is an alkyne, and the number tells you where the triple bond is.
Detailed Explanation
What are Alkynes?
Alkynes contain a carbon-carbon triple bond. This is shown in their name by the ending "-yne".
Examples:
- A 2-carbon chain containing a carbon-carbon triple bond is named ethyne: H - C ≡ C - H
- A 3-carbon chain containing a carbon-carbon triple bond is named propyne: CH3C ≡ CH
Question from the source: What will be the name of the alkyne containing 4-carbon atoms?
Answer: A 4-carbon chain containing a carbon-carbon triple bond is named butyne.
Key point: The name of an alkyne is based on the number of carbon atoms in the chain and the presence of a triple bond.
Naming Alkynes with Larger Chains
With larger chains, the position of the triple bond is indicated by numbering the chain.
Numbering rule: Numbering is started from the end nearest to the triple bond, noting which carbon the triple bond starts from.
Example: A 4-carbon chain with a triple bond starting on carbon number 1 is named but-1-yne:
1CH ≡ 2CH2 - 3CH2 - 4CH3
Condensed formula: HC ≡ C - CH2 - CH3
Example: If the triple bond is starting on the second carbon, then the alkyne is named but-2-yne:
1CH3 - 2C ≡ 3C - 4CH3
Condensed formula: CH3 - C ≡ C - CH3
Key point: The position of the triple bond is indicated by a number in the name.
The Numbering Rule
Important rule from the source: Numbering is started from the end which gives the smaller number to the name.
Example:
- In but-1-yne, numbering from the left gives the triple bond at position 1.
- In but-2-yne, numbering from the left gives the triple bond at position 2.
- If we numbered from the right, we would get but-3-yne (for the same compound as but-1-yne) or but-2-yne (for the same compound as but-2-yne).
Key point: Numbering always starts from the end that gives the smaller number to the triple bond.
Examples of Alkynes
| Name | Molecular Formula | Condensed Formula |
|---|---|---|
| Ethyne | C2H2 | HC ≡ CH |
| Propyne | C3H4 | CH3C ≡ CH |
| But-1-yne | C4H6 | HC ≡ C - CH2 - CH3 |
| But-2-yne | C4H6 | CH3 - C ≡ C - CH3 |
| Pent-1-yne | C5H8 | HC ≡ C - CH2 - CH2 - CH3 |
| Pent-2-yne | C5H8 | CH3 - C ≡ C - CH2 - CH3 |
Key point: The name of the alkyne is determined by the number of carbon atoms and the position of the triple bond.
Summary Table
| Stem | Number of C-atoms | Alkyne Name (with triple bond at position 1) |
|---|---|---|
| Eth | 2 | Ethyne |
| Prop | 3 | Propyne |
| But | 4 | But-1-yne |
| Pent | 5 | Pent-1-yne |
| Hex | 6 | Hex-1-yne |
Intermediate Level
Naming alkynes:
- Count the number of carbon atoms in the chain → choose the correct stem.
- Add the suffix -yne.
- Number the chain from the end nearest to the triple bond.
- Write the position of the triple bond as a number before the suffix.
- Write the full name (e.g., but-1-yne, but-2-yne).
Examples:
| Formula | Name |
|---|---|
| HC ≡ CH | Ethyne |
| CH3C ≡ CH | Propyne |
| HC ≡ C - CH2 - CH3 | But-1-yne |
| CH3 - C ≡ C - CH3 | But-2-yne |
| CH3 - C ≡ C - CH2 - CH3 | Pent-2-yne |
Key point: The position of the triple bond is indicated by a number, and numbering starts from the end nearest to the triple bond.
Solved Example
Example: Name the Following Alkyne
Question: Name the following alkyne:
CH3 - C ≡ C - CH2 - CH3
Solution:
Step 1: Count the number of carbon atoms in the chain.
- The chain has 5 carbon atoms → stem = pent-
Step 2: Identify the suffix.
- The compound contains a triple bond → suffix = -yne
Step 3: Determine the position of the triple bond.
- Number the chain from the end nearest to the triple bond.
- From the left: 1CH3 - 2C ≡ 3C - 4CH2 - 5CH3 → triple bond starts at carbon 2.
- From the right: 5CH3 - 4C ≡ 3C - 2CH2 - 1CH3 → triple bond starts at carbon 3.
- The smaller number is 2, so the triple bond is at position 2.
Step 4: Write the name.
- Name: Pent-2-yne
Final Answer: The name of the alkyne is pent-2-yne.
Concept Assessment Exercise 8.2 (Part 4)
Write the structural formula of an alkyne containing 6-carbon atoms with a triple bond starting on the second carbon.
Answer:
- Name: Hex-2-yne
- Structural formula: CH3 - C ≡ C - CH2 - CH2 - CH3
Important Points
- - Alkynes = hydrocarbons with at least one carbon-carbon triple bond.
- - General formula: CnH2n-2.
- - Names end in "-yne".
- - The name has two parts:
- 1. Stem — number of carbon atoms.
- 2. Suffix — -yne indicates an alkyne.
- - For larger chains, the position of the triple bond is indicated by a number.
- - Numbering starts from the end nearest to the triple bond.
- - Examples:
- - Ethyne: HC ≡ CH
- - Propyne: CH3C ≡ CH
- - But-1-yne: HC ≡ C - CH2 - CH3
- - But-2-yne: CH3 - C ≡ C - CH3
- - Pent-2-yne: CH3 - C ≡ C - CH2 - CH3
Remember
- - Alkynes = hydrocarbons with a triple bond.
- - Names end in -yne.
- - Stem tells the number of carbon atoms.
- - Position of triple bond is indicated by a number.
- - Numbering starts from the end nearest to the triple bond.
- - Examples:
- - Ethyne: HC ≡ CH
- - Propyne: CH3C ≡ CH
- - But-1-yne: HC ≡ C - CH2 - CH3
- - But-2-yne: CH3 - C ≡ C - CH3
- - General formula: CnH2n-2.
Common Mistakes
| Mistake | Correction |
|---|---|
| Thinking alkyne names end in -ane or -ene | Alkyne names end in -yne |
| Forgetting to indicate the position of the triple bond | The position is indicated by a number in the name |
| Numbering from the wrong end | Numbering starts from the end nearest to the triple bond |
| Thinking the triple bond is between any two atoms | The triple bond is between two carbon atoms |
| Confusing alkynes with alkanes or alkenes | Alkynes: -yne; Alkenes: -ene; Alkanes: -ane |
| Forgetting the general formula of alkynes | Alkynes: CnH2n-2 |
| Thinking but-1-yne and but-2-yne are the same | They are isomers with different structures |
Quick Revision
Naming Alkynes:
Alkynes contain a carbon-carbon triple bond. This is shown in their name by the ending "yne". For example, 2-carbon chain containing a carbon-carbon triple bond is named ethyne,
- H - C ≡ C - H
3-carbon chaining alkyne is named propyne, CH3C ≡ CH. What will be the name of the alkene containing 4-carbon atoms?
With larger chains, the position of the triple bond is indicated by numbering the chain. Numbering is started from the end nearest to the triple bond, noting which carbon the triple bond starts from. For example, a 4-carbon chain with a double bond starting on carbon number 1 is named;
- 1CH ≡ 2CH2CH3
If the triple bond is starting on the second carbon, then the alkene is named;
- 1CH3C ≡ 3C4CH3
NOTE: How do you know from which end of the longest chain numbering is done? For this, you should remember the rule that numbering is started from the end which gives the smaller number to the name.
Examples:
- Ethyne: HC ≡ CH
- Propyne: CH3C ≡ CH
- But-1-yne: HC ≡ C - CH2 - CH3
- But-2-yne: CH3 - C ≡ C - CH3
Concept Assessment Exercise 8.2 (Part 4):
Write the structural formula of an alkyne containing 6-carbon atoms with a triple bond starting on the second carbon.
Answer: Hex-2-yne: CH3 - C ≡ C - CH2 - CH2 - CH3
60-Second Revision
- Alkynes = hydrocarbons with a triple bond.
- Names end in -yne.
- Stem tells the number of carbon atoms.
- Position of triple bond is indicated by a number.
- Numbering starts from the end nearest to the triple bond.
- Examples:
- Ethyne: HC ≡ CH
- Propyne: CH3C ≡ CH
- But-1-yne: HC ≡ C - CH2 - CH3
- But-2-yne: CH3 - C ≡ C - CH3
- Pent-2-yne: CH3 - C ≡ C - CH2 - CH3
- General formula: CnH2n-2.
- ⚠️ Warning: numbering always starts from the end that gives the smaller number to the triple bond.
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Sticky Notes · Quick Revision
Key Point 1
- Alkynes = hydrocarbons with a triple bond.
Key Point 2
- Names end in -yne.
Key Point 3
- Stem tells the number of carbon atoms.
Key Point 4
- Position of triple bond is indicated by a number.
Key Point 5
- Numbering starts from the end nearest to the triple bond.
Key Point 6
- Examples:
Key Point 7
- Ethyne: HC ≡ CH
Key Point 8
- Propyne: CH3C ≡ CH
Key Point 9
- But-1-yne: HC ≡ C - CH2 - CH3
Key Point 10
- But-2-yne: CH3 - C ≡ C - CH3
Key Point 11
- Pent-2-yne: CH3 - C ≡ C - CH2 - CH3
Key Point 12
- General formula: CnH2n-2.
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