Edexcel iGCSE Chemistry -4.26 Structures and Naming of Alkenes- Study Notes- New Syllabus
Edexcel iGCSE Chemistry -4.26 Structures and Naming of Alkenes- Study Notes- New syllabus
Edexcel iGCSE Chemistry -4.26 Structures and Naming of Alkenes- Study Notes -Edexcel iGCSE Chemistry – per latest Syllabus.
Key Concepts:
4.26 understand how to draw the structural and displayed formulae for alkenes with up to four carbon atoms in the molecule, and name the unbranched-chain isomers
(knowledge of cis/trans or E/Z notation is not required)
4.26 Drawing Structural and Displayed Formulae of Alkenes (Up to Four Carbon Atoms) and Naming Unbranched-Chain Isomers
Alkenes are unsaturated hydrocarbons containing the functional group:
>C=C<
They have the general formula:
\( \mathrm{C_{n}H_{2n}} \)
You must be able to:
- Draw structural formulae.
- Draw displayed formulae.
- Name unbranched-chain isomers (up to 4 carbons).
- No cis/trans (E/Z) notation is required.
1. Structural Formula
Shows condensed groups but includes the double bond.
Ethene
\( \mathrm{CH_2=CH_2} \)
Propene
\( \mathrm{CH_2=CHCH_3} \)
But-1-ene
\( \mathrm{CH_2=CHCH_2CH_3} \)
2. Displayed Formula
Shows all atoms and all covalent bonds.
Ethene
Unbranched Alkenes (Up to 4 Carbons)
| Number of Carbons | Formula | Name |
|---|---|---|
| 2 | \( \mathrm{C_2H_4} \) | Ethene |
| 3 | \( \mathrm{C_3H_6} \) | Propene |
| 4 | \( \mathrm{C_4H_8} \) | But-1-ene / But-2-ene |
For 4 carbons, two position isomers exist:
- But-1-ene (double bond at carbon 1)
- But-2-ene (double bond at carbon 2)
Naming Rules
- Find the longest carbon chain.
- Number from the end nearest the double bond.
- Indicate the position of the double bond using a number.
- Names end in -ene.
GCSE Points
- Alkenes must contain a C=C double bond.
- Displayed formula shows all bonds clearly.
- Unbranched means no side chains.
- No stereochemistry required.
Example 1 (Conceptual):
Why does butene have two unbranched-chain isomers?
▶️ Answer/Explanation
The double bond can be in two different positions.
It can be at carbon 1 or carbon 2.
This produces but-1-ene and but-2-ene.
Example 2 (Application):
Write the structural formula for but-2-ene.
▶️ Answer/Explanation
\( \mathrm{CH_3CH=CHCH_3} \)
Example 3 (Hard):
A compound has the molecular formula \( \mathrm{C_4H_8} \) and is unbranched. Draw the possible structural formulae and name them.
▶️ Answer/Explanation
Using the alkene formula \( \mathrm{C_{n}H_{2n}} \), \( \mathrm{C_4H_8} \) is an alkene.
The double bond can be in two positions.
But-1-ene:
\( \mathrm{CH_2=CHCH_2CH_3} \)
But-2-ene:
\( \mathrm{CH_3CH=CHCH_3} \)
Therefore there are two unbranched-chain isomers.