Sn1 (two-step Idea)

Sn1 (two-step Idea)

What is it?

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.

In simple words

• 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

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.

What you should learn

• 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.

Examples

• tert-butyl bromide
• Polar solvent helps
• Ag+ can help X leave named

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.

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.

Full school-level explanation

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.

Who likes SN1, then the rate:

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

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.

Remember

• stable cation

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.

Remember

• k[RX]

Examples

• Nu later
• Two steps
• Racemise named

Remember

• slow ionise, then fast Nu
• tertiary likes SN1
• only RX in the rate
• idea only
• stable cation
• k[RX]
• slow ionise, then fast Nu
• tertiary likes SN1

How the ideas connect

Exam SN1 tell: tertiary haloalkane rate = k [RX] (Nu not in slow step) carbocation intermediate

Deeper understanding

Go beyond memorising Sn1 (two-step Idea). Connect the definition to reasons, relationships, exceptions, and exam wording.

How the ideas connect

Exam SN1 tell: tertiary haloalkane rate = k [RX] (Nu not in slow step) carbocation intermediate

Common mistakes

Exam focus

Core Idea

1. TERTIARY

2. RATE LAW

Exam Example

Exam focus

Quick revision

• slow ionise, then fast Nu
• tertiary likes SN1
• only RX in the rate
• idea only
• SN1 (TWO-STEP IDEA): SN1: one species in the slow step (the haloalkane ionises to a carbocation + X-).
• 1. TERTIARY: 3 haloalkanes.

Mechanism idea. No carbocation cookbooks for harmful products.

For long answers on Sn1 (two-step Idea): define clearly → explain the main parts → give one example → state one exam caution.