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Reactions of alkenes

Electrophilic addition reactions

Unsaturated molecules such as alkenes readily undergo addition reactions. You may recall from your gcse science lessons that small molecules can "add" across the carbon carbon double bond (C=C) in alkenes to form new saturated molecules. The addition of halogens such as bromine and an explanation of the mechanism for these electrophilic addition reactions was discussed on the page covering simple addition reactions. This page covers the addition of hydrogen halides and sulfuric acid to alkenes as well as the oxidation of alkenes using the oxidising agent potassium permanganate.

1. Addition of hydrogen halides to alkenes

The hydrogen halides are all gases at room temperature; they include hydrogen chloride (HCl), hydrogen bromide (HBr) and hydrogen iodide (HI). Hydrogen chloride and hydrogen bromide both have a permanent dipole due to large differences in the electronegativity values between hydrogen and these halogens. This aids in an easy addition reaction in the gas phase with the carbon carbon double bond in an alkene. This reaction produces halogenalkanes; a mechanism for the gas phase reaction of hydrogen bromide gas with the alkene ethene is shown below, though a similar reaction will occur with hydrogen chloride and hydrogen iodide. This is a typical electrophilic addition reaction.

The mechanism for the addition of hydrogen bromide gas to ethene- a typical electrophilic addition reaction.

Detailed explanation of the mechanism of the electrophilic addition of hydrogen bromide to an alkene.

In the reaction mechanism shown above the partially positively charged hydrogen atom in the hydrogen bromide molecule acts as the electrophile, while the negatively charged bromide ion acts as a nucleophile when it attacks the intermediate carbocation.

Hydrogen chloride, bromide and iodide are all acidic gases and will readily dissolve in water to form acidic solutions So if the reaction with alkenes is carried out in aqueous conditions rather than in the gas phase then the hydrogen ion (H+) from the acidic solution will act as the electrophile and the halide ion will act as the nucleophile and attack the intermediate carbocation to produce the halogenalkane. The relative reactivity in these electrophilic additions reaction is HI > HBr > HCl. This mirrors the strengths of the acids where hydroiodic acid is a stronger acid than hydrobromic acid which in turn is a stronger acid than hydrochloric acid.

2. Preparation of alcohols using alkenes

There are numerous methods of preparing alcohols from alkenes. One method which is often used in industry is called direct hydration. This involves adding a molecule of water across the carbon carbon double bond (C=C) in an alkene to produce an alcohol. This method requires a phosphoric acid (H3PO4) catalyst, a high temperature (570K) and a high pressure (65 atmospheres). The mechanism for this reaction is shown below. The mechanism is similar to the other addition reactions to alkenes and proceeds via three steps:

Mechanism for the hydration of ethene to ethanol using a phosphoric acid catalyst.

We can show this direct hydration using the following equations: Model, word and symbolic equations to show the direct hydration of ethene to produce ethanol.

3. Alkenes and sulfuric acid

Alcohols can also be produced from the reaction of concentrated sulfuric acid with an alkene. The mechanism for this reaction is shown below, hopefully you will notice it is very similar to previous reactions of alkenes that we have seen above. The reaction proceeds as shown below: The mechanism for the reaction of an alkene with concentrated sulfuric acid to form an alcohol.

3. Oxidation of alkenes using potassium permanganate

The structure of the  permanganate ion.

The diols are a family of alcohols which contain two hydroxyl groups (R-OH) per molecule. Perhaps the most widely used diol is ethane-1,2-diol (CH2OHCH2OH) which is used as antifreeze and as a coolant in car engines.

Alkenes can be oxidised to form 1,2-diols using cold dilute solutions of potassium permanganate as an oxidising agent. The permanganate ion (MnO4- ) is a powerful oxidising agent due to the fact that the central manganese atom has an oxidation state of +7. The potassium permanganate can be used in neutral, acidic or alkaline conditions to oxidise the alkene to form the diol. The permanganate ion forms a purple solution when dissolved in water. If it is used to oxidise an alkene the colour changes observed during the reaction depend on whether the reaction is carried out in acidic, alkaline or neutral solutions.

If an alkene such as ethene (CH2=CH2) is bubbled through an acidified solution of the permanganate ion (MnO4-) then the purple permanagnate ion is reduced to form the pale pink Mn2+ ion, which is so pale as to be almost colourless. While the alkene is oxidised to form the diol.

We can show this redox reaction as:
reduction half-equation: : MnO4-(aq) + 8H+(aq) + 5e → Mn2+(aq) + 4H2O(l)

oxidation half-equation: CH2=CH2 (g) + 2H2O(l)HOCH2CH2OH(aq) + 2H+(aq) + 2e

The above two half-equations are clearly not balanced since there are 5 electrons needed to reduce the permanagante ion and only 2 electrons are released by the oxidation of ethene. So to balance it simply multiply the top equation by x2 and the bottom equation by x5 to give:

2MnO4-(aq) + 5CH2=CH2 (g) + 6H+(aq) + 2H2O(l)Mn2+(aq) + 5HOCH2CH2OH(aq)

Whereas if ethene gas is bubbled through an alkaline solution of potassium permanganate we have as a final equation for this redox reaction:

2MnO4-(aq) + 3CH2=CH2 (g) + 4H2O(l)3HOCH2CH2OH(aq) + Mn02(s) + 2OH-(aq)
This time:
MnO4-(aq) + e → MnO42-(aq)
which is then further reduced to form the brown manganese dioxide (MnO2). This appears as a brown precipitate. The ethene is still oxidised to form the diol ethane-1,2-diol.

The permanganate ion adds to one side of the carbon carbon double bond (C=C) in an alkene, this is called syn addition, we can show this as:

Equations to show the oxidation of an alkene by the permanganate ion to form diols.

However the complete equations for the oxidation of alkenes by potassium permanganate are NOT required at A-level and we can simply show the equation for the oxidation reaction as:

Summary equation to show the oxidation of alkenes to diols using an oxidising agent.

Key Points

Practice questions

Check your understanding - Questions on addition reactions