W4L1: Electrophilic addition reactions: the basics

Addition of hydrogen to alkenes makes alkanes

• Alkene is reduced and hydrogen is oxidised - redox reaction

• Overall delta G is negative - favourable, barrier is very high and requires a catalyst

Hydrogenation: chemoselectivity

Alkenes can be hydrogenated in the presence of other functional groups

• This is called chemoselectivity – the selection of one functional group over another

• The molecule containss both an alkene and a ketone, then under mild conditions can hydrogenate the alkene while leaving the ketone untouched. If, however, we run the reaction under more forcing conditions, by increasing the pressure, then we can reduce the ketone also

Hydrogenation: stereospecificity

• The two hydrogen atoms always add syn to one another

• This is called stereospecificity – the reaction mechanism only allows a specific stereochemical outcome

Alkynes can be hydrogenated to alkanes

propyne to propane

Alkynes can be partially hydrogenated to alkenes

Can get cis or trans product with different reactants

Electrophilic addition to alkenes

Alkenes can act as nucleophiles, and suitable electrophiles can add to them

• Can react with HCl or HBr to form halogenoalkanes

• React with water to form alcohols

• React with alcohols to form ethers

• Electrophilic addition because the species that adds to the alkene initially acts as an electrophile

• Alkenes are electron-rich, so they can act as nucleophiles

Electrophilic addition: mechanism

Not stereoselective - forms a racemic mixture

Electrophilic addition: selectivity

• The acid always adds to form the more stable carbocation

• This is called regioselectivity – the selection of one region of the molecule over another

• More stable secondary carbocation is more likely to form - more of the 2-halogenopropane is formed

Electrophilic addition: energies

• Alkene and hydrohalic acid are reactants

• Forms the intermediate carbocation and retains halide anion - slow

• Second barrier to form product

Formation of alcohols and ethers is slightly more complex

Alkene isomerisation

A low concentration of acid will isomerise a cis alkene, rather than adding to it:

Electrophilic addition to alkynes

Alkynes can also act as nucleophiles, and suitable electrophiles can add to them

• Germinal - compound with the functional groups on the same carbon atom