Addition Reactions of Alkenes (Oxford AQA International A Level Chemistry)

Electrophilic Addition Mechanism

• The presence of a carbon-carbon double bond makes alkenes very reactive compared to alkanes

• The C-C double bond is an area of high electron density making it susceptible to attack by electrophiles

• Alkenes therefore typically undergo electrophilic addition reactions

• Electrophilic addition is the addition of an electrophile to a double bond

• The C-C double bond is broken, and a new single bond is formed from each of the two carbon atoms

Reaction with HBr

• A molecule of hydrogen bromide (HBr) is polar as the hydrogen and bromine atoms have different electronegativities

• The bromine atom has a stronger pull on the electrons in the H-Br bond

• As a result of this, the Br atom has a partial negative and the H atom has a partial positive charge

 The polarity of a HBr molecule

• In electrophilic addition:

  • The partially positive (δ+) hydrogen atom acts as an electrophile

  • It is attracted to the high electron density of the C=C double bond in the alkene and accepts a pair of electrons

  • The H-Br bond breaks heterolytically, forming a Br^- ion

  • A highly reactive carbocation intermediate is formed which reacts with the bromide ion, Br^-

• The reaction of ethene with HBr forms bromoethane

Electrophilic addition of HBr mechanism

Reaction with H₂SO₄

• Concentrated sulfuric acid adds across the double bond

• The hydrogen atom in sulfuric acid has a partial positive charge so a sulfuric acid molecule acts as electrophile

The polarity of a H₂SO₄ molecule

• In electrophilic addition:

  • The partially positive (δ+) hydrogen atom acts as an electrophile

  • It is attracted to the high electron density of the C=C double bond in the alkene and accepts a pair of electrons

  • The H-O bond breaks heterolytically, forming a hydrogensulfate ion, HSO₄^-

  • A highly reactive carbocation intermediate is formed which reacts with the HSO₄^-

• The product formed reacts with water to form an alcohol and sulfuric acid

• Essentially, water adds across the double bond with sulfuric acid acting as a catalyst

Reaction with Br₂

• Bromine (Br₂) is a non-polar molecule as both atoms have similar electronegativities and equally share the electrons in the covalent bond

• However, when a bromine molecule gets close to the double bond of an alkene, the high electron density in the double bond repels the electron pair in Br-Br away from the closest Br atom

• As a result of this, the Br atom closest to the double bond has a partial positive charge (δ+) and the further Br atom has a partial negative charge (δ-)

The polarity of a Br₂ molecule

• In an addition reaction:

  • The closest Br atom acts as an electrophile and accepts a pair of electrons from the C=C bond in the alkene

  • The Br-Br bond breaks heterolytically, forming a Br^- ion

  • This results in the formation of a highly reactive carbocation intermediate which reacts with the Br^- (nucleophile)

• The reaction of ethene with Br₂ forms 1,2-dibromoethane

Electrophilic addition of Br₂ mechanism

Testing for unsaturation

• The reaction of an alkene with bromine is used to test for the presence of a carbon-carbon double bond

• Bromine water is an orange / brown solution

• The unknown compound is shaken with the bromine water

• If the compound is unsaturated, an addition reaction will take place and the coloured solution will decolourise

The unsaturation test

Addition Reactions of Unsymmetrical Alkenes

• Carbocations are reaction intermediates that contain positively charged carbon atoms with only three covalent bonds instead of four

• There are three types of carbocations:

  • Primary

  • Secondary

  • Tertiary

The inductive effect

• The alkyl groups attached to the positively charged carbon atoms are ‘electron donating groups’

  • This is also known as the positive inductive effect of alkyl groups

• The inductive effect is illustrated by the use of arrowheads on the bonds to show the alkyl groups pushing electrons towards the positively charged carbon

  • This causes the carbocation to become less positively charged

• As a result of this, the charge is spread around the carbocation which makes it energetically more stable

• Primary carbocations are the least stable as they only have one electron-donating alkyl group to stabilise the carbocation

• Secondary carbocations are more stable as they have two electron-donating alkyl group to stabilise the carbocation

• Tertiary carbocations are the most stable as they have three electron-donating alkyl groups to stabilise the carbocation

• Due to the positive charge on the carbon atom, carbocations are electrophiles

Primary, secondary and tertiary carbocations

Markovnikov’s rule

• Markovnikov’s rule predicts the outcome of electrophilic addition reactions and states that:

  • In the electrophilic addition reaction of a hydrogen halide (HX) to an alkene, the product has the halogen bonded to the most substituted carbon atom

  • In the electrophilic addition reaction of a halogen to an alkene, each halogen atom bonds to one of the C=C carbons

  • In the electrophilic addition reaction of an interhalogen (e.g. Br-Cl) to an alkene, the most electronegative halogen ends up bonded to the most substituted carbon atom

• Markovnikov addition applies to electrophilic addition reactions with unsymmetrical alkenes such as propene and but-1-ene

  • Markovnikov addition favours the formation of the major product

  • Anti-Markovnikov addition favours the formation of the minor product

• In electrophilic addition reactions, an electrophile reacts with the double bond of alkenes (as previously discussed)

• The mechanism for electrophilic addition reactions with unsymmetrical alkenes is slightly different

• The example of propene with HBr is shown below

Step 1 in the electrophilic addition mechanism

• The electrophile can attach in two possible ways:

  1. Breaking the C=C bond and attaching to the least substituted carbon

     • This will give the most stable carbocation as an intermediate that will form the major product

  2. Breaking the C=C bond and attaching to the most substituted carbon

     • This will give the least stable carbocation as an intermediate that will form the minor product

Relative stabilities of primary and secondary carbocations

• The nucleophile will bond to the positive carbon atom of the carbocation

• The more stable carbocation produces the major product

• The less stable carbocation produces the minor product

Formation of major and minor products

Propene + HBr mechanism

• The mechanism for the electrophilic addition of hydrogen bromide to propene, showing the formation of the major and minor products can be shown as: