L2 - ELECTROPHILIC ADDITION

objectives

Understandings

• An electrophile is an electron-deficient species that can accept electron pairs from a nucleophile. Electrophiles are Lewis acids.
• Markovnikov’s rule can be applied to predict the major product in electrophilic addition reactions of unsymmetrical alkenes with hydrogen halides and interhalogens. The formation of the major product can be explained in terms of the relative stability of possible carbocations in the reaction mechanism.

Applications and Skills

• Deduction of the mechanism of the electrophilic addition reactions of alkenes with halogens/interhalogens and hydrogen halides.

ELECTROPHILIC ADDITION MECHANISM

Electrophillic Addition - Mechanism

Electrophilic Addition of Halogen Halides to Alkenes

Reagent:

• Hydrogen bromide... it is electrophilic as the H is slightly positive

Condition: Room temperature. NO Catalyst

Equation:

C₂H₄(g) + HBr(g) ———> C₂H₅Br(l) bromoethane

Mechanism

Electrophilic Addition of Bromine to an Alkene

Reagent:

• Bromine (Neat liquid or dissolved in tetrachloromethane, CCl₄ )

Condition:

• Room temperature. No catalyst or UV light required!

Equation:

C₂H₄(g) + Br₂(l) ——> CH₂BrCH₂Br(l) 1,2 - dibromoethane

Mechanism

Because the bromine adds to the alkene, it no longer exists as molecular bromine and the typical red-brown colour disappears

Reaction of Halogens with Alkenes

• Alkenes are much more reactive than alkanes because the C=C moiety is a reactive functional group.
• A π bond, being a weaker bond, is disrupted much more easily than a σ bond.
• Thus, alkenes undergo a characteristic reaction in which the π bond is broken and replaced by two σ bonds.
• This reaction is called an addition reaction.
• The hybridization of the carbon atoms in the double bond in an alkene changes from sp2 to sp3 during an addition reaction.
• For example, halogens add to the double bond in an alkene instead of replacing hydrogen, as occurs in an alkane:

Electrophilic Addition of HBr to Propene

• addition of HBr to propene gives two isomeric brominated compounds

• HBr is unsymmetrical and can add in two ways

• products are not formed to the same extent

• the problem doesn't arise in ethene because it is symmetrical.

Possibility 1

Possibility 2

Markownikoff's Rule

• A Russian scientist, Markownikoff, investigated the products of the addition of hydrogen halides to alkenes.
• He found that, when two products were formed, one was formed in a larger quantity.
• His original rule was based only on this reaction.
• The modern version uses carbocation stability as a criterion for predicting the products.

In the electrophilic addition to alkenes the major product is

formed via the more stable carbocation (carbonium ion)

Carbocation Stability

• Build up of charge in one place leads to instability.
• If it can be spread around or neutralised in some way, stability is increased.
• Alkyl groups are electron releasing and can “push” electrons towards the carbocations thus reducing the charge density.

In the addition to propene, path A involves a 2° carbocation, path B a 1° carbocation.

As the 2° ion is more stable, the major product (i.e. 2-bromopropane) is formed this way.