ChemistryChapter 17: Halogenoalkanes
Sn2 (one-Step Idea)
SN2: substitution, nucleophilic, two species in the slow step.
Learning objectives
- Explain sn2 (one-step idea) in simple chemical language.
- Connect 1. primary with 2. rate law using equations, observations, or trends where needed.
- Answer a short Federal Board question with the correct terms and a clear reason.
Definition
SN2: substitution, nucleophilic, two species in the slow step. Nu attacks from the back as X leaves (concerted). Favoured by primary C-X (less crowding) and by a small strong Nu. Rate = k [haloalkane][Nu]. Inversion of arrangement named if the book shows a tetrahedral flip. Meaning: both concentrations matter. Not a protocol for making restricted substances. One step: Nu attacks backside, X leaves — inversion of configuration at chiral C. Classify the halogen-bearing carbon as 1, 2, or 3 before choosing SN1, SN2, or elimination.
Mathematical form
rate = k [RX][Nu]
Rate = k
rate = k
Important terms
- SN2 (ONE-STEP IDEA)
- SN2: substitution, nucleophilic, two species in the slow step.
- 1. PRIMARY
- Methyl and 1 haloalkanes.
- 2. RATE LAW
- Double Nu, double rate.
Overview
Who likes SN2, then the rate:
Primary
Methyl and 1 haloalkanes. Crowding blocks the back of a 3 centre. Polar aprotic solvents named if the book has them (idea). Favoured by primary halide, good Nu, polar aprotic solvent — school factors list.
Examples CH3Br SN2 · 1-bromobutane · 3 too crowded
Rate Law
Double Nu, double rate. Double RX, double rate. Mechanism cartoon: one transition state. Curly arrows if asked: pair from Nu to C, pair from C-X to X. Rate depends on both [RX] and [Nu-] — second order overall.
Examples k[RX][Nu] · Backside · Inversion named
WHAT — SN2 (ONE-STEP IDEA)
SN2: substitution, nucleophilic, two species in the slow step. Nu attacks from the back as X leaves (concerted). Favoured by primary C-X (less crowding) and by a small strong Nu. Rate = k [haloalkane][Nu]. Inversion of arrangement named if the book shows a tetrahedral flip. Meaning: both concentrations matter. Not a protocol for making restricted substances. One step: Nu attacks backside, X leaves — inversion of configuration at chiral C. Classify the halogen-bearing carbon as 1, 2, or 3 before choosing SN1, SN2, or elimination.
WHY — Why this topic matters
SN2 (ONE-STEP IDEA) is an independent FBISE learning unit because students must use its definitions, conditions, equations, or trends separately in explanations and calculations.
HOW — How it works
PRIMARY: Methyl and 1 haloalkanes. Crowding blocks the back of a 3 centre. Polar aprotic solvents named if the book has them (idea). Favoured by primary halide, good Nu, polar aprotic solvent — school factors list. Examples: CH3Br SN2; 1-bromobutane; 3 too crowded. RATE LAW: Double Nu, double rate. Double RX, double rate. Mechanism cartoon: one transition state. Curly arrows if asked: pair from Nu to C, pair from C-X to X. Rate depends on both [RX] and [Nu-] — second order overall. Examples: k[RX][Nu]; Backside; Inversion named. Exam SN2 tell: primary haloalkane, strong Nu rate = k [RX][Nu] one step, backside attack
In simple words
Mechanism idea. No illicit SN2 cookbooks.
Key points
- one step, back-attack
- rate depends on both
- space at the back
- second order
Exam preparation
Exam Focus
Quick revision
Revision Zone
Review the key facts, formulas and checks before you move on.Sn2 (one-Step Idea)
Sticky Notes · Quick Revision
Core Idea
- SN2: substitution, nucleophilic, two species in the slow step. Nu attacks from the back as X leaves (concerted). Favoured by primary C-X (less crowding) and by a small strong Nu. Rate = k [haloalkane][Nu]. Inversion of arrangement named if the book shows a tetrahedral flip. Meaning: both concentrations matter. Not a protocol for making restricted substances. One step: Nu attacks backside, X leaves — inversion of configuration at chiral C. Classify the halogen-bearing carbon as 1, 2, or 3 before choosing SN1, SN2, or elimination.
1. PRIMARY
- Methyl and 1 haloalkanes. Crowding blocks the back of a 3 centre. Polar aprotic solvents named if the book has them (idea). Favoured by primary halide, good Nu, polar aprotic solvent — school factors list.
- CH3Br SN2
- 1-bromobutane
- 3 too crowded
2. RATE LAW
- Double Nu, double rate. Double RX, double rate. Mechanism cartoon: one transition state. Curly arrows if asked: pair from Nu to C, pair from C-X to X. Rate depends on both [RX] and [Nu-] — second order overall.
- k[RX][Nu]
- Backside
- Inversion named
Exam Example
- Exam SN2 tell:
- primary haloalkane, strong Nu
- rate = k [RX][Nu]
- one step, backside attack
Common Misconceptions
- Do not confuse 1. PRIMARY with 2. RATE LAW; state the difference before giving an example.
- Writing only “one step, back-attack” is not enough. Explain what it means in a complete sentence.
FBISE Exam Check
- Explain sn2 (one-step idea), compare its two main parts, and include the correct chemical evidence.
- Mechanism idea. No illicit SN2 cookbooks.
60-second revision
- SN2: substitution, nucleophilic, two species in the slow step.
- 1. PRIMARY: Methyl and 1 haloalkanes.
- 2. RATE LAW: Double Nu, double rate.
- Exam reminder: Exam SN2 tell: primary haloalkane, strong Nu rate = k [RX][Nu] one step, backside attack
Did you understand this topic?
Your answer is saved with your learning record. It does not mark the topic as mastered.