Alkenes and Electrophilic Addition

Detailed Explanations

A. Why alkenes react (the key “because…therefore…” chain)

Because the π bond is a region of high electron density, therefore it attracts electrophiles and can donate an electron pair to form a new bond.

B. Workflow: predicting products of addition reactions

1. Identify the alkene (locate the C=C).
2. Identify the reagent and what adds across C=C (e.g. H and Br from HBr).
3. Add across the double bond to form two new single bonds (saturated product).
4. If the alkene is unsymmetrical and the reagent is HX, consider the more stable carbocation (major product).

Mini example: CH₃CH = CH₂ + HBr → CH₃CHBrCH₃ Major product is 2-bromopropane via the more stable secondary carbocation.

C. Addition of bromine (test for unsaturation)

Overall: CH₂ = CH₂ + Br₂ → CH₂BrCH₂Br

Explanation (what to write):

• The π bond polarises Br₂, inducing δ + on one Br.
• The alkene attacks the δ + Br (electrophile) and the Br-Br bond breaks.
• A second bromide ion adds, forming the 1,2-dibromoalkane.

D. Addition of hydrogen halides (e.g. HBr)

Example: CH₃CH = CH₂ + HBr → CH₃CHBrCH₃

Key idea to state:

• The π bond attacks H⁺ first (electrophilic attack), forming a carbocation intermediate.
• Br⁻ then attacks the carbocation.

If the question expects a “major product”:

• Addition tends to form the more stable carbocation (often giving the Markovnikov product).

E. Hydration (steam) to form alcohols

Example: CH₂ = CH₂ + H₂O → CH₃CH₂OH

Conditions to quote:

• steam, acid catalyst (e.g. H₃PO₄), high temperature and pressure

F. Hydrogenation to form alkanes

Example: CH₂ = CH₂ + H₂ → CH₃CH₃

Conditions to quote:

• H₂, Ni catalyst, heat

G. Oxidation with manganate(VII)

Cold, dilute alkaline MnO₄- oxidises an alkene to a vicinal diol: an -OH group is added to each carbon of the former C=C. The purple solution is decolourised and brown MnO₂ may form under alkaline conditions.

Hot, concentrated acidified MnO₄- causes oxidative cleavage of the C=C. Treat each alkene carbon separately:

• a carbon with no attached H gives a ketone;
• a carbon with one attached H gives a carboxylic acid;
• a terminal = CH₂ carbon is oxidised ultimately to CO₂.

For example, hot acidified manganate(VII) cleaves but-2-ene to two molecules of ethanoic acid. Draw the alkene, cut the double bond, then decide the oxidation product from the number of hydrogens originally on each alkene carbon.