ChemistryChapter 17: Halogenoalkanes
Telling Sn1 From Sn2 (exam)
Look at the class (1 vs 3).
Learning objectives
- Explain telling sn1 from sn2 (exam) in simple chemical language.
- Connect 1. structure with 2. data using equations, observations, or trends where needed.
- Answer a short Federal Board question with the correct terms and a clear reason.
Definition
Look at the class (1 vs 3). Look at the rate law if data given. Look at solvent (polar protic helps SN1). Look at Nu strength (strong Nu helps SN2). Secondary is the messy middle — say 'both possible' if that is the book line. Meaning: you are matching a story to evidence, not designing a process. Primary + good Nu -> SN2; tertiary + weak Nu -> SN1 — classify substrate first. Classify the halogen-bearing carbon as 1, 2, or 3 before choosing SN1, SN2, or elimination.
Important terms
- TELLING SN1 FROM SN2 (EXAM)
- Look at the class (1 vs 3).
- 1. STRUCTURE
- Primary: SN2.
- 2. DATA
- If rate stays when you double OH-, SN1.
Overview
Structure, then kinetics:
Structure
Primary: SN2. Tertiary: SN1. Iodo faster than chloro (leaving group). Crowding vs cation stability. Rate law: SN2 second order; SN1 first order in halide only.
Examples CH3X SN2 · tert-RX SN1 · Leaving group I > Br > Cl school
Data
If rate stays when you double OH-, SN1. If rate doubles, SN2 (or mixed). Product optical: inversion vs racemic named. Stereochemistry: SN2 inversion; SN1 racemic mix if chiral.
Examples k[RX][Nu] SN2 · k[RX] SN1 · 2 is mixed
WHAT — TELLING SN1 FROM SN2 (EXAM)
Look at the class (1 vs 3). Look at the rate law if data given. Look at solvent (polar protic helps SN1). Look at Nu strength (strong Nu helps SN2). Secondary is the messy middle — say 'both possible' if that is the book line. Meaning: you are matching a story to evidence, not designing a process. Primary + good Nu -> SN2; tertiary + weak Nu -> SN1 — classify substrate first. Classify the halogen-bearing carbon as 1, 2, or 3 before choosing SN1, SN2, or elimination.
WHY — Why this topic matters
TELLING SN1 FROM SN2 (EXAM) 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
STRUCTURE: Primary: SN2. Tertiary: SN1. Iodo faster than chloro (leaving group). Crowding vs cation stability. Rate law: SN2 second order; SN1 first order in halide only. Examples: CH3X SN2; tert-RX SN1; Leaving group I > Br > Cl school. DATA: If rate stays when you double OH-, SN1. If rate doubles, SN2 (or mixed). Product optical: inversion vs racemic named. Stereochemistry: SN2 inversion; SN1 racemic mix if chiral. Examples: k[RX][Nu] SN2; k[RX] SN1; 2 is mixed. Worked: tertiary RX, rate unchanged when [OH-] doubles. OH- is not in the slow step. SN1 carbocation then fast OH- attack
Worked example
- tertiary RX, rate unchanged when [OH-] doubles.
- OH- is not in the slow step.
- SN1
- carbocation then fast OH- attack
In simple words
Exam tell-apart. No restricted-substance procedures.
Key points
- 1 -> SN2, 3 -> SN1
- rate law is evidence
- class first
- change one conc
Exam preparation
Exam Focus
Quick revision
Revision Zone
Review the key facts, formulas and checks before you move on.Telling Sn1 From Sn2 (exam)
Sticky Notes · Quick Revision
Core Idea
- Look at the class (1 vs 3). Look at the rate law if data given. Look at solvent (polar protic helps SN1). Look at Nu strength (strong Nu helps SN2). Secondary is the messy middle — say 'both possible' if that is the book line. Meaning: you are matching a story to evidence, not designing a process. Primary + good Nu -> SN2; tertiary + weak Nu -> SN1 — classify substrate first. Classify the halogen-bearing carbon as 1, 2, or 3 before choosing SN1, SN2, or elimination.
1. STRUCTURE
- Primary: SN2. Tertiary: SN1. Iodo faster than chloro (leaving group). Crowding vs cation stability. Rate law: SN2 second order; SN1 first order in halide only.
- CH3X SN2
- tert-RX SN1
- Leaving group I > Br > Cl school
2. DATA
- If rate stays when you double OH-, SN1. If rate doubles, SN2 (or mixed). Product optical: inversion vs racemic named. Stereochemistry: SN2 inversion; SN1 racemic mix if chiral.
- k[RX][Nu] SN2
- k[RX] SN1
- 2 is mixed
Exam Example
- Worked: tertiary RX, rate unchanged when [OH-] doubles.
- OH- is not in the slow step.
- SN1
- carbocation then fast OH- attack
Common Misconceptions
- Do not confuse 1. STRUCTURE with 2. DATA; state the difference before giving an example.
- Writing only “1 -> SN2, 3 -> SN1” is not enough. Explain what it means in a complete sentence.
FBISE Exam Check
- Explain telling sn1 from sn2 (exam), compare its two main parts, and include the correct chemical evidence.
- Exam tell-apart. No restricted-substance procedures.
60-second revision
- Look at the class (1 vs 3).
- 1. STRUCTURE: Primary: SN2.
- 2. DATA: If rate stays when you double OH-, SN1.
- Exam reminder: Worked: tertiary RX, rate unchanged when [OH-] doubles.
Did you understand this topic?
Your answer is saved with your learning record. It does not mark the topic as mastered.