3.3.4Organic Chemistry
Electrophilic Addition Mechanism — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 27 of 42
Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Word Equation & Conditions: حمض كربوكسيلي + كحول (في وجود حمض الكبريتيك المركز كعامل حفاز ونازع للماء) ⇌ إستر + ماء
Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
Part 1 of 4Bromine and Ethene
A curly arrow shows the movement of an electron pair, from the source to where the pair ends up.
1
As a bromine molecule approaches the double bond, the π electrons repel the electrons in the Br–Br bond.
2
This induces a dipole in the bromine molecule, so the nearer bromine becomes δ+ and the further one δ−.
3
A pair of electrons from the C=C bond forms a new bond to the δ+ bromine, while the Br–Br bond breaks by heterolytic fission.
4
The bromine leaves as a bromide ion, Br⁻, and the other carbon is left with a positive charge as a carbocation.
5
The bromide ion uses a lone pair to bond to the positive carbon, forming 1,2-dibromoethane.
Part 2 of 4Hydrogen Bromide and Propene
Polar molecule
- Polar molecule HBr is polar, with a δ+ hydrogen that acts as the electrophile.
Step 1
- Step 1 A π electron pair attacks the δ+ hydrogen, the H–Br bond breaks, and a carbocation plus Br⁻ forms.
Two paths
- Two paths Hydrogen can attach to either carbon, giving a primary or secondary carbocation.
Step 2
- Step 2 The Br⁻ ion donates a lone pair to the carbocation to form the bromoalkane.
Major product
- Major product The secondary carbocation is more stable, so the major product is 2-bromopropane.
Part 3 of 4Carbocation Stability
Carbocation Stability
Primary
- Primary A primary carbocation has one alkyl group on the positive carbon and is the least stable.
Secondary
- Secondary A secondary carbocation has two alkyl groups on the positive carbon and is more stable.
Tertiary
- Tertiary A tertiary carbocation has three alkyl groups and is the most stable of the three.
Reason
- Reason Alkyl groups push electron density towards the positive carbon, spreading the charge and stabilising the ion.
Prediction
- Prediction The more stable carbocation forms faster, so it leads to the major product.
| Carbocation | Alkyl groups on C⁺ | Relative stability |
|---|---|---|
| Primary | 1 | Least stable |
| Secondary | 2 | Intermediate |
| Tertiary | 3 | Most stable |
Part 4 of 4Exam Checklist & Tips
3.3.4 – Key Points for the Exam
- Heterolytic fission Breaking a covalent bond so both electrons go to one of the atoms.
- Curly arrow An arrow showing the movement of an electron pair in a mechanism.
- Carbocation An ion with a positive charge on a carbon atom, formed as an intermediate.
- Induced dipole A temporary dipole created in a molecule by the approach of a charged region.
- Intermediate A species formed in one step of a mechanism and used up in the next.
Common Mistakes
- Starting the curly arrow from the C=C carbon atom instead of from the middle of the double bond.
- Drawing arrows to show the movement of atoms rather than electron pairs.
- Omitting the dipole induced in the Br–Br molecule, or drawing it as a permanent dipole.
- Stating that the tertiary carbocation is less stable because it has more carbon atoms.
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