ChemistryChapter 17: Halogenoalkanes
Sn1 (two-Step Idea)
SN1: one species in the slow step (the haloalkane ionises to a carbocation + X-).
Learning objectives
- Explain sn1 (two-step idea) in simple chemical language.
- Connect 1. tertiary 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
SN1: one species in the slow step (the haloalkane ionises to a carbocation + X-). Then Nu grabs the cation fast. Favoured by tertiary (cation stabilised) and by polar solvents that help ions. Rate = k [haloalkane] only. Racemisation named if a chiral 3 centre is taught (Nu either face). Meaning: adding more OH- does not speed the slow step. Still school ideas only. Step 1: slow ionisation to carbocation; step 2: fast Nu attack — rate depends on [RX] only. Classify the halogen-bearing carbon as 1, 2, or 3 before choosing SN1, SN2, or elimination.
Mathematical form
rate = k [RX] (Nu not in slow step)
Rate = k
rate = k
Important terms
- SN1 (TWO-STEP IDEA)
- SN1: one species in the slow step (the haloalkane ionises to a carbocation + X-).
- 1. TERTIARY
- 3 haloalkanes.
- 2. RATE LAW
- First order in RX.
Overview
Who likes SN1, then the rate:
Tertiary
3 haloalkanes. 2 can do both (book). 1 rarely SN1 in school. Carbocation order of stability 3 > 2 > 1 named. Tertiary favours SN1 — stable carbocation; racemisation possible at chiral centre.
Examples tert-butyl bromide · Polar solvent helps · Ag+ can help X leave named
Rate Law
First order in RX. Nu not in the slow step. Two-peak profile: first peak higher (slow). Intermediate: carbocation (not in the overall? it is an intermediate). Weak Nu acceptable because carbocation is electrophile.
Examples Nu later · Two steps · Racemise named
WHAT — SN1 (TWO-STEP IDEA)
SN1: one species in the slow step (the haloalkane ionises to a carbocation + X-). Then Nu grabs the cation fast. Favoured by tertiary (cation stabilised) and by polar solvents that help ions. Rate = k [haloalkane] only. Racemisation named if a chiral 3 centre is taught (Nu either face). Meaning: adding more OH- does not speed the slow step. Still school ideas only. Step 1: slow ionisation to carbocation; step 2: fast Nu attack — rate depends on [RX] only. Classify the halogen-bearing carbon as 1, 2, or 3 before choosing SN1, SN2, or elimination.
WHY — Why this topic matters
SN1 (TWO-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
TERTIARY: 3 haloalkanes. 2 can do both (book). 1 rarely SN1 in school. Carbocation order of stability 3 > 2 > 1 named. Tertiary favours SN1 — stable carbocation; racemisation possible at chiral centre. Examples: tert-butyl bromide; Polar solvent helps; Ag+ can help X leave named. RATE LAW: First order in RX. Nu not in the slow step. Two-peak profile: first peak higher (slow). Intermediate: carbocation (not in the overall? it is an intermediate). Weak Nu acceptable because carbocation is electrophile. Examples: Nu later; Two steps; Racemise named. Exam SN1 tell: tertiary haloalkane rate = k [RX] (Nu not in slow step) carbocation intermediate
In simple words
Mechanism idea. No carbocation cookbooks for harmful products.
Key points
- slow ionise, then fast Nu
- tertiary likes SN1
- stable cation
- k[RX]
Exam preparation
Exam Focus
Quick revision
Revision Zone
Review the key facts, formulas and checks before you move on.Sn1 (two-Step Idea)
Sticky Notes · Quick Revision
Core Idea
- SN1: one species in the slow step (the haloalkane ionises to a carbocation + X-). Then Nu grabs the cation fast. Favoured by tertiary (cation stabilised) and by polar solvents that help ions. Rate = k [haloalkane] only. Racemisation named if a chiral 3 centre is taught (Nu either face). Meaning: adding more OH- does not speed the slow step. Still school ideas only. Step 1: slow ionisation to carbocation; step 2: fast Nu attack — rate depends on [RX] only. Classify the halogen-bearing carbon as 1, 2, or 3 before choosing SN1, SN2, or elimination.
1. TERTIARY
- 3 haloalkanes. 2 can do both (book). 1 rarely SN1 in school. Carbocation order of stability 3 > 2 > 1 named. Tertiary favours SN1 — stable carbocation; racemisation possible at chiral centre.
- tert-butyl bromide
- Polar solvent helps
- Ag+ can help X leave named
2. RATE LAW
- First order in RX. Nu not in the slow step. Two-peak profile: first peak higher (slow). Intermediate: carbocation (not in the overall? it is an intermediate). Weak Nu acceptable because carbocation is electrophile.
- Nu later
- Two steps
- Racemise named
Exam Example
- Exam SN1 tell:
- tertiary haloalkane
- rate = k [RX] (Nu not in slow step)
- carbocation intermediate
Common Misconceptions
- Do not confuse 1. TERTIARY with 2. RATE LAW; state the difference before giving an example.
- Writing only “slow ionise, then fast Nu” is not enough. Explain what it means in a complete sentence.
FBISE Exam Check
- Explain sn1 (two-step idea), compare its two main parts, and include the correct chemical evidence.
- Mechanism idea. No carbocation cookbooks for harmful products.
60-second revision
- SN1: one species in the slow step (the haloalkane ionises to a carbocation + X-).
- 1. TERTIARY: 3 haloalkanes.
- 2. RATE LAW: First order in RX.
- Exam reminder: Exam SN1 tell: tertiary haloalkane rate = k [RX] (Nu not in slow step) carbocation intermediate
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