Electrophilic addition reactions

Alkenes undergo a type of reaction called electrophilic addition across their carbon-carbon double bond. In this process:

• The π bond of the C=C double bond breaks.
• Atoms or groups add across the carbon atoms.

This happens because the C=C double bond is rich in electrons, making it a target for electrophiles - electron pair acceptors that are attracted to areas with a high electron density.

Common electrophiles that add to alkenes include:

• Polar molecules, where the positive δ⁺ atom is attracted to the C=C electrons, such as the H^δ+ of H-Br.
• Halogens such as chlorine and bromine.

Addition of hydrogen

Alkenes react with hydrogen gas (H₂) in the presence of a nickel catalyst at 150°C, forming alkanes.

For example, ethene reacts with hydrogen to form ethane:

H₂C=CH_2(g) + H_2(g) ➔ CH₃CH_3(g)

In this reaction, the double bond breaks, allowing hydrogen atoms to add across the carbon atoms, resulting in a single C-C bond.

Addition of halogens

Halogens react with alkenes in an electrophilic addition reaction to form dihalogenoalkanes.

The mechanism for addition of halogens to alkenes, using bromine and ethene as an example, is:

Step 1 - The high electron density of the C=C double bond repels electrons in the bromine molecule, polarising the Br-Br bond.

Step 2 - This leads to heterolytic fission of the Br-Br bond, with one bromine atom taking the bonding pair of electrons, creating a positively charged bromine which bonds to one of the carbon atoms.

Step 3 - A positively charged carbocation intermediate forms. The negatively charged bromide ion then bonds to the other carbon atom.

Step 4 - The final product, 1,2-dibromoethane, has a bromine atom bonded to each of the original alkene carbon atoms.

The overall equation for this reaction is:

H₂C=CH₂ + Br₂ ➔ BrCH₂CH₂Br

Bromine water tests for carbon-carbon double bonds

The reaction of bromine with alkenes is a useful test for C=C bonds.

Bromine water, an orange solution of bromine, reacts with an alkene:

• The alkene reacts with bromine via electrophilic addition.
• This reaction removes the orange colour as the bromine is consumed.
• The product is a colourless dibromoalkane solution.

The rapid decolourisation of orange bromine water indicates the presence of carbon-carbon double bonds, offering a simple test for alkenes.

Hydration to form alcohols

Under the influence of a phosphoric(V) acid catalyst, alkenes react with steam at 300°C and 60-70 atm pressure, undergoing hydration to form alcohols.

For example, ethene is hydrated to produce ethanol:

H₂C=CH_2(g) + H₂O_(g) ⇌ CH₃CH₂OH_(g)

This reaction is significant in the industrial production of ethanol from ethene.

A limitation is the reaction's reversible nature, with only about 5% yield. However, recycling unreacted ethene can increase overall yields to over 95%.

Addition of hydrogen halides

Hydrogen halides like HCl and HBr add across the C=C double bond in an electrophilic addition, forming halogenoalkanes.

In symmetrical alkenes, the double bonded carbons are identical, leading to a single monosubstituted product.

For example, ethene reacts with HBr to form bromoethane:

H₂C=CH₂ + HBr ➔ CH₃CH₂Br

The reaction mechanism is:

In unsymmetrical alkenes with two different carbon groups, hydrogen halides can form two positional isomers.

For example, propene reacts with HBr to form a mixture of 1-bromopropane and 2-bromopropane:

CH₃CH=CH₂ + HBr ➔ CH₃CH₂CH₂Br (1-bromopropane)

CH₃CH=CH₂ + HBr ➔ CH₃CHBrCH₃ (2-bromopropane)

The ratio of these products depends on the stability of the intermediate carbocation.

Predicting major addition products

The major product in hydrogen halide addition to unsymmetrical alkenes can be predicted by carbocation stability.

Carbocation stability increases with more alkyl groups

The reaction generates a positively charged carbocation intermediate. Its stability varies based on its structure:

• Primary carbocations - These have one alkyl group attached to the positively charged carbon amd are the least stable.
• Secondary carbocations - These have two alkyl groups at the carbocation centre and offer moderate stability.
• Tertiary carbocations - These have three alkyl groups and are the most stable.

Alkyl groups stabilise the positive charge through inductive effects, with more groups providing greater stability.

Most stable carbocation forms major product

The major halogenoalkane product forms via the most stable carbocation intermediate.

For example, in the reaction of propene with HBr, the secondary carbocation forms preferentially, making 2-bromopropane the major product.

Worked example 1 - Determining the major addition product

Predict the major product when 2-methylbut-2-ene reacts with HCl.

Step 1: Determine structure of 2-methylbut-2-ene

2-methylbut-2-ene: (CH₃)₂C=CHCH₃

Step 2: Determine potential carbocation intermediates

When HCl adds to 2-methylbut-2-ene, two types of carbocations can form:

1. Carbocation at C²: (CH₃)₂C⁺CH₂CH₃ (tertiary carbocation).
2. Carbocation at C³: (CH₃)₂CHC⁺H(CH₃) (secondary carbocation).

Step 3: Determine the most stable carbocation

Tertiary carbocations (like the one at C²) are more stable than secondary carbocations (like the one at C³) due to greater alkyl group substitution.

Therefore, the major product will be formed via the more stable carbocation. In this case, it's the tertiary carbocation at C².

Step 4: Predict major product

The major product of the addition of HCl to 2-methylbut-2-ene is 2-chloro-2-methylbutane:

(CH₃)₂C=CHCH₃ + HCl ➔ (CH₃)₂C(Cl)CH₂CH₃

Oxidation to form diols

When alkenes react with cold, dilute acidified potassium manganate(VII), KMnO₄, they undergo a chemical change known as oxidation.

• The pale purple KMnO₄ solution changes to colourless as the permanganate ions (MnO₄^-) are reduced to manganese(II) ions (Mn²⁺).
• In this reaction, each carbon atom in the C=C double bond gains an -OH group. This forms a compound called a diol, which has two alcohol groups.

For instance, ethene reacts with KMnO₄ to produce ethane-1,2-diol:

H₂C=CH₂ + [O] + H₂O ➔ HO-CH₂CH₂-OH

This reaction is significant because it can be used to test for the presence of a C=C double bond in a compound, which is a characteristic feature of alkenes.