ChemistryChapter 8: Organic Chemistry
8.2.2 Naming Alkenes
**Alkenes:** Alkenes contain a carbon-carbon double bond
Explanation level
Introduction
In the previous section, we learned how to name alkanes — hydrocarbons with only single bonds. Now we turn to alkenes, which contain at least one carbon-carbon double bond.
The naming of alkenes follows a similar pattern to alkanes, but with one important difference: the ending is "-ene" instead of "-ane". This tells us that the compound contains a carbon-carbon double bond.
This section explains how to name alkenes using the stem and suffix system, and how to indicate the position of the double bond.
Definition
Alkenes: Alkenes contain a carbon-carbon double bond. This is shown in their name by the ending "-ene".
In simple words: Alkenes are hydrocarbons with a double bond, and their names end in -ene.
Main Idea
- Alkenes contain a carbon-carbon double bond.
- Their names end in "-ene".
- For example:
- A 2-carbon chain containing a carbon-carbon double bond is named ethene, CH2 = CH2.
- A 3-carbon chain containing a carbon-carbon double bond is named propene, CH3CH = CH2.
- With larger chains, the position of the double bond is indicated by numbering the chain.
- Numbering is started from the end nearest to the double bond, noting which carbon the double bond starts from.
Key point: Alkene names end in -ene, and the position of the double bond is indicated by a number.
Core Concept
| Feature | Description |
|---|---|
| Suffix | -ene |
| Functional group | Carbon-carbon double bond (C=C) |
| General formula | CnH2n |
| Position of double bond | Indicated by a number in the name |
| Numbering rule | Start from the end nearest to the double bond |
Key point: The suffix -ene tells you it is an alkene, and the number tells you where the double bond is.
Detailed Explanation
What are Alkenes?
Alkenes contain a carbon-carbon double bond. This is shown in their name by the ending "-ene".
Examples:
- A 2-carbon chain containing a carbon-carbon double bond is named ethene: CH2 = CH2
- A 3-carbon chain containing a carbon-carbon double bond is named propene: CH3CH = CH2
Question from the source: What will be the name of the alkene containing 4-carbon atoms?
Answer: A 4-carbon chain containing a carbon-carbon double bond is named butene.
Key point: The name of an alkene is based on the number of carbon atoms in the chain and the presence of a double bond.
Naming Alkenes with Larger Chains
With larger chains, the position of the double bond is indicated by numbering the chain.
Numbering rule: Numbering is started from the end nearest to the double bond, noting which carbon the double bond starts from.
Example: A 4-carbon chain with a double bond starting on carbon number 1 is named but-1-ene:
1CH2 = 2CH - 3CH2 - 4CH3
Condensed formula: CH2 = CH - CH2 - CH3
Example: If the double bond is starting on the second carbon, then the alkene is named but-2-ene:
1CH3 - 2CH = 3CH - 4CH3
Condensed formula: CH3 - CH = CH - CH3
Key point: The position of the double bond is indicated by a number in the name.
The Numbering Rule
Important rule from the source: 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.
Example:
- In but-1-ene, numbering from the left gives the double bond at position 1.
- In but-2-ene, numbering from the left gives the double bond at position 2.
- If we numbered from the right, we would get but-3-ene (for the same compound as but-1-ene) or but-2-ene (for the same compound as but-2-ene).
Key point: Numbering always starts from the end that gives the smaller number to the double bond.
Examples of Alkenes
| Name | Molecular Formula | Condensed Formula |
|---|---|---|
| Ethene | C2H4 | CH2 = CH2 |
| Propene | C3H6 | CH3CH = CH2 |
| But-1-ene | C4H8 | CH2 = CH - CH2 - CH3 |
| But-2-ene | C4H8 | CH3 - CH = CH - CH3 |
| Pent-1-ene | C5H10 | CH2 = CH - CH2 - CH2 - CH3 |
| Pent-2-ene | C5H10 | CH3 - CH = CH - CH2 - CH3 |
Key point: The name of the alkene is determined by the number of carbon atoms and the position of the double bond.
Summary Table
| Stem | Number of C-atoms | Alkene Name (with double bond at position 1) |
|---|---|---|
| Eth | 2 | Ethene |
| Prop | 3 | Propene |
| But | 4 | But-1-ene |
| Pent | 5 | Pent-1-ene |
| Hex | 6 | Hex-1-ene |
Intermediate Level
Naming alkenes:
- Count the number of carbon atoms in the chain → choose the correct stem.
- Add the suffix -ene.
- Number the chain from the end nearest to the double bond.
- Write the position of the double bond as a number before the suffix.
- Write the full name (e.g., but-1-ene, but-2-ene).
Examples:
| Formula | Name |
|---|---|
| CH2 = CH2 | Ethene |
| CH3CH = CH2 | Propene |
| CH2 = CH - CH2 - CH3 | But-1-ene |
| CH3 - CH = CH - CH3 | But-2-ene |
| CH3 - CH = CH - CH2 - CH3 | Pent-2-ene |
Key point: The position of the double bond is indicated by a number, and numbering starts from the end nearest to the double bond.
Solved Example
Example: Name the Following Alkene
Question: Name the following alkene:
CH3 - CH = CH - 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 double bond → suffix = -ene
Step 3: Determine the position of the double bond.
- Number the chain from the end nearest to the double bond.
- From the left: 1CH3 - 2CH = 3CH - 4CH2 - 5CH3 → double bond starts at carbon 2.
- From the right: 5CH3 - 4CH = 3CH - 2CH2 - 1CH3 → double bond starts at carbon 2.
- Either way, the double bond is at position 2.
Step 4: Write the name.
- Name: Pent-2-ene
Final Answer: The name of the alkene is pent-2-ene.
Concept Assessment Exercise 8.2
1. Write the name and structural formula of an alkane containing 5-carbon atoms.
Answer:
- Name: Pentane
- Structural formula: CH3CH2CH2CH2CH3
2. Write the name and structural formula of an alkene containing 6-carbon atoms with a double bond starting on the third carbon.
Answer:
- Name: Hex-3-ene
- Structural formula: CH3CH2CH = CHCH2CH3
3. Which name is correct for the following compound CH3CH2CH = CHCH3? Pent-2-ene or Pent-3-ene?
Answer:
- Numbering from the left: 1CH3 - 2CH2 - 3CH = 4CH - 5CH3 → double bond starts at carbon 3.
- Numbering from the right: 5CH3 - 4CH2 - 3CH = 2CH - 1CH3 → double bond starts at carbon 2.
- The smaller number is 2, so the correct name is Pent-2-ene.
4. Write the structural formula of an alkene 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
- - Alkenes = hydrocarbons with at least one carbon-carbon double bond.
- - General formula: CnH2n.
- - Names end in "-ene".
- - The name has two parts:
- 1. Stem — number of carbon atoms.
- 2. Suffix — -ene indicates an alkene.
- - For larger chains, the position of the double bond is indicated by a number.
- - Numbering starts from the end nearest to the double bond.
- - Examples:
- - Ethene: CH2 = CH2
- - Propene: CH3CH = CH2
- - But-1-ene: CH2 = CH - CH2 - CH3
- - But-2-ene: CH3 - CH = CH - CH3
- - Pent-2-ene: CH3 - CH = CH - CH2 - CH3
Remember
- - Alkenes = hydrocarbons with a double bond.
- - Names end in -ene.
- - Stem tells the number of carbon atoms.
- - Position of double bond is indicated by a number.
- - Numbering starts from the end nearest to the double bond.
- - Examples:
- - Ethene: CH2 = CH2
- - Propene: CH3CH = CH2
- - But-1-ene: CH2 = CH - CH2 - CH3
- - But-2-ene: CH3 - CH = CH - CH3
- - General formula: CnH2n.
Common Mistakes
| Mistake | Correction |
|---|---|
| Thinking alkene names end in -ane | Alkene names end in -ene |
| Forgetting to indicate the position of the double bond | The position is indicated by a number in the name |
| Numbering from the wrong end | Numbering starts from the end nearest to the double bond |
| Thinking the double bond is between any two atoms | The double bond is between two carbon atoms |
| Confusing alkenes with alkanes | Alkenes: -ene; Alkanes: -ane |
| Forgetting the general formula of alkenes | Alkenes: CnH2n |
| Thinking but-1-ene and but-2-ene are the same | They are isomers with different structures |
Quick Revision
Naming Alkenes:
Alkenes contain carbon-carbon double bond. This is shown in their name by the ending "ene". For example, 2-carbon chain containing a carbon-carbon double bond is named ethene, CH2 = CH2, 3-carbon chaining alkene is named propene, CH3CH2 = CH2. What will be the name of alkene containing 4-carbon atoms?
With larger chains, the position of double bond is indicated by numbering the chain. Numbering is started from the end nearest to the double bond, noting which carbon the double bond starts from. For example a 4-carbon chain with a double bond starting on carbon number 1 is named as;
- 1CH2 = 2CH3CH2CH3
If the double bond is starting on the second carbon, then the alkene is named;
- 1CH3CH = 3CH4CH3
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:
- Ethene: CH2 = CH2
- Propene: CH3CH = CH2
- But-1-ene: CH2 = CH - CH2 - CH3
- But-2-ene: CH3 - CH = CH - CH3
Concept Assessment Exercise 8.2:
- Write the name and structural formula of an alkane containing 5-carbon atoms.
- Write the name and structural formula of an alkene containing 6-carbon atoms with a double bond starting on the third carbon.
- Which name is correct for the following compound CH3CH2CH = CHCH3? Pent-2-ene or Pent-3-ene?
- Write the structural formula of an alkene containing 6-carbon atoms with a triple bond starting on the second carbon.
60-Second Revision
- Alkenes = hydrocarbons with a double bond.
- Names end in -ene.
- Stem tells the number of carbon atoms.
- Position of double bond is indicated by a number.
- Numbering starts from the end nearest to the double bond.
- Examples:
- Ethene: CH2 = CH2
- Propene: CH3CH = CH2
- But-1-ene: CH2 = CH - CH2 - CH3
- But-2-ene: CH3 - CH = CH - CH3
- Pent-2-ene: CH3 - CH = CH - CH2 - CH3
- General formula: CnH2n.
- ⚠️ Warning: numbering always starts from the end that gives the smaller number to the double bond.
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Quick revision
Revision Zone
Review the key facts, formulas and checks before you move on.8.2.2 Naming Alkenes
Sticky Notes · Quick Revision
Key Point 1
- Alkenes = hydrocarbons with a double bond.
Key Point 2
- Names end in -ene.
Key Point 3
- Stem tells the number of carbon atoms.
Key Point 4
- Position of double bond is indicated by a number.
Key Point 5
- Numbering starts from the end nearest to the double bond.
Key Point 6
- Examples:
Key Point 7
- Ethene: CH2 = CH2
Key Point 8
- Propene: CH3CH = CH2
Key Point 9
- But-1-ene: CH2 = CH - CH2 - CH3
Key Point 10
- But-2-ene: CH3 - CH = CH - CH3
Key Point 11
- Pent-2-ene: CH3 - CH = CH - CH2 - CH3
Key Point 12
- General formula: CnH2n.
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