Alkyl Halides Sn1 Reactions

Recap of Sn2 Reactions

  • “backside attack“ pushes electron from left to right.

  • transition state - shows whats happening, starting material to product.

  • one step reaction

The Sn1 reaction with 3deg alkyl halides

  • Generally, tertiary (3°) alkyl halides undergo SN1 reactions with nucleophiles

  • SN1: Substitution, Nucleophilic, Unimolecular

  • The general SN1 reaction has a two-step mechanism

Steps

  1. C-X bond breaks to form a carbocation (R3C+) and a halide (X-)

    • step one is slow, RDS Rate Determining Step (RDS)

    • C-X bond cleaves to give a 3° carbocation intermediate (R3C+) and the leaving group, a halide (X-)

  1. Step 2: Now the nucleophile (Nu) attacks the carbocation (R3C+) intermediate to form a new bond (Nu-C)

    • step 2 is fast

    • Nu attacks the 3° R3C+ intermediate to give SN product

Overview

  • Remember: Mechanism is deduced from kinetic studies

  • In a Rate Equation, the molecularity of a reaction is the

    number of molecules taking part in the RDS of the reaction In this case, the nucleophile is not involved in the RDS

    So we can delete this from our equation


    rate = k [R3C-X][Nu ] rate = k [R3C-X]


    Rate of this Nucleophilic Substitution (SN) depends on the

    concentration of only RX (not the Nu)
    So, the reaction is: first order = Sn1 (unimolecular)

Why do 3° RX undergo SN1 rather than SN2 reactions?

Steric effects

Remember: SN2 requires Nu to approach electrophilic C of R3C-X close enough to form a bond & displace the halogen leaving group, X

• Must approach C from the opposite side to the halide (X) for: • orbital overlap AND to avoid negative charge of halogen

• Recall that alkyl groups are bigger than H’s
• Approach by the nucleophile to the electrophilic C of R3C-X is hindered

by the three bulky R (alkyl) groups
So, with 3° RX, SN2 is difficult to achieve and SN1 is favoured
Also, with the release of steric strain, with loss of LG & the formation of carbocation, drives reaction forward “steric acceleration”

Electronic effects

  • Alkyl groups help stabilise positive charge on carbocation & electrophilic C of R3C-X

  • More alkyl groups = more stabilisation

  • If electrophilic C of R3C-X is stabilised, easier to cleave C-X bond

  • If carbocation intermediate is stabilised, helps reaction move forward

  • More alkyl groups = SN1 is more favoured

  • methyl, primary, secondary

  • tertiary

Example of a tertiary alkyl halide undergoing a SN1 reaction with a nucleophile

  1. C-Br bond breaks to form carbocation (R3C+) and a halide (Br-)

  2. the nucleophile (Nu) attacks the carbocation (R3C+) to form a new bond (Nu-C)

Elimination reaction with a base

  • using a base instead of a nucleophile

  • still losing halide, also loses a hydrogen (H+) and (X-)

  • The base captures H+ and this is generally shown as B-H on the reaction schemes.

  • forming an alkene

  • identify secondary alkyl halide (one carbon away, does it have a hydrogen attached?)

  • base does not attack carbon - attacks hydrogen (proton) and pushes to form alkene

  • different from Sn reactions - forms alkene

  • pink shows where lost hydrogen comes from

Competition may occur between Sn and elimination reactions

  • can act as a base or a nucleophile

Example of E2 reaction

  • The E2 reaction has a one-step (concerted) mechanism

  • E2: Elimination, Bimolecular rate = k [R-X][base]

  • {E2 Goes through a transition state but we will not explore these details}

  • imagine what the product will look like

Alternative elimination products

  • For elimination there MUST be a H on the carbon adjacent to (next to) the C-X

  • Overall: Eliminate H+ and X- from alkyl halide

  • But what happens if there are more than one adjacent carbons, each with a H? You can get a variety of elimination products

  • May get a mixture of both E2 and SN products, depending on reaction conditions & compounds

  • Therefore, purification is important in organic chemistry

Grignard Reagents

  • Alkyl halides are used to create Grignard Reagents

  • Grignard Reagents are organomagnesium compounds with the general formula RMgX Victor Grignard awarded a Nobel prize in 1912

  • How to make a Grignard Reagent?
    Reaction of Mg with alkyl halides (in dry diethyl ether)

  • Mg inserts itself into the bond, flipping the electronegativity.

  • carbon becomes nucleophilic