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 were 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.
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 the halogens atom in the molecule. This aids in an easy addition reaction in the gas phase with the carbon carbon double bond (C=C) in an unsaturated alkene molecule. 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.
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 an alkene 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.
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 electrophilic addition reactions to alkenes and proceeds via three steps:
We can show this direct hydration using the following equations:
The diols are a family of alcohols
which contain two hydroxyl functional 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 and when 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 permanganate 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:The above two half-equations are clearly not balanced since there are 5 electrons needed to reduce the permanganate 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:
Whereas if ethene gas is bubbled through an alkaline solution of potassium permanganate we have as a final equation for this redox reaction:
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:
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:
