Organic Chemistry
Edexcel AS Chemistry — 3.4 Halogenoalkanes — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 3 parts Lesson 41 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.
Lesson summary
Halogenoalkane reactivity: explain how carbon–halogen bond strength affects nucleophilic substitution and the relative reactivity of fluoro-, chloro-, bromo- and iodoalkanes. In nucleophilic substitution, a nucleophile attacks the electron-deficient carbon attached to a halogen.
- C–X bond breaks heterolytically
- CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻
- C–I bond is the weakest
- C–F bond is the strongest
- C–F > C–Cl > C–Br > C–I
- R–I > R–Br > R–Cl > R–F
Section 1 of 3What breaks in substitution
1Nucleophilic substitution
In nucleophilic substitution, a nucleophile attacks the electron-deficient carbon attached to a halogen. The C–X bond breaks heterolytically, and the halogen leaves with both bonding electrons as X⁻.
2CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻
For example, hydroxide can replace bromine in bromoethane: CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻. The organic product is an alcohol.
Section 2 of 3Compare bond strengths
1C–I bond is the weakest
The C–I bond is the weakest carbon–halogen bond in this group, so it needs the least energy to break. Iodoalkanes tend to undergo substitution more readily than corresponding fluoroalkanes under comparable conditions.
2C–F bond is the strongest
The C–F bond is the strongest and requires the most energy to break. Fluoroalkanes are consequently least likely to undergo these substitution reactions.
Section 3 of 3Use the trend correctly
1C–F > C–Cl > C–Br > C–I
For otherwise similar halogenoalkanes, the C–X bond-strength trend is C–F > C–Cl > C–Br > C–I. A typical substitution reactivity trend is the reverse: R–I > R–Br > R–Cl > R–F.
2
Structure also affects mechanism and rate. Primary halogenoalkanes commonly react by a one-step SN2 route, in which the nucleophile attacks as the leaving group departs.
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