Alkenes: Structure, Isomerism & Reactions (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching Chemistry, and this video is about the structure and bonding of alkenes, isomerism in alkenes, their reactions, and the test for unsaturation.

All of it comes back to the pi bond in the carbon-carbon double bond.

A carbon-carbon double bond is one sigma bond and one pi bond. The pi bond does two things. It stops the groups on the double bond rotating, so they stay fixed in position and causes the existence of E/Z stereoisomerism. And it is a centre of high electron density, which is what makes the double bond susceptible to electrophilic attack, and what makes alkenes decolourise bromine water.

We'll take the bonding first, then isomerism, then the electrophilic addition reactions and what decides the major product, and finally the bromine water test.

Alkenes are unsaturated hydrocarbons, and bonding in alkenes involves a double covalent bond. In a carbon-carbon double bond, each carbon forms three sigma bonds using three of its outer electrons. A sigma bond is formed by the direct overlap of orbitals along the line between two nuclei. The remaining electron on each carbon occupies an unhybridised p orbital, and when those p orbitals overlap sideways, a pi bond is formed. The pi bond contains two electrons and lies above and below the plane of the atoms.

The three regions of electron density around each carbon repel equally, giving a trigonal planar shape with bond angles of approximately 120 degrees. That pi bond is the feature the rest of this topic depends on.

In unsaturated compounds, the groups attached to the carbons of the double bond remain fixed in their position, because free rotation about the carbon-carbon double bond is not possible due to the presence of the pi bond. That restriction is what gives rise to E/Z stereoisomers. Stereoisomerism itself is covered in full in the structural isomerism and stereoisomerism video; here it is the pi bond doing the work.

Alkenes are more reactive than alkanes because of their carbon-carbon double bonds. The double bond has high electron density, which makes it susceptible to electrophilic attack, and electrophilic addition is the addition of an electrophile to that double bond: the carbon-carbon double bond is broken, and a new single bond is formed from each of the two carbon atoms.

Hydrogen bromide is polar, because bromine pulls electrons more strongly, so the partially positive hydrogen acts as the electrophile. Bromine is non-polar, but the high electron density of the double bond repels the electron pair in the bromine-bromine bond and polarises it as it approaches. Concentrated sulfuric acid adds across the double bond in the cold, with its partially positive hydrogen acting as the electrophile. In each case the bond breaks heterolytically and a highly reactive carbocation intermediate is formed, which then reacts with the negative ion.

Carbocations are positively charged carbon atoms with only three covalent bonds instead of four, and there are three types: primary, secondary and tertiary. The alkyl groups attached to the positively charged carbon are electron-donating groups: they push electrons away from themselves towards the positively charged carbon, which spreads the charge around the carbocation and makes it energetically more stable. Tertiary carbocations are therefore the most stable, because they have three electron-donating alkyl groups.

That stability decides the product. The electrophile adds to the carbon that gives the most stable carbocation, so the nucleophile ends up bonded to the carbon atom with the highest number of alkyl groups. This is Markovnikov's rule: in an addition reaction of a hydrogen halide to an alkene, the halogen ends up bonded to the most substituted carbon atom. It is still the same addition across the same pi bond; the alkyl groups only decide which way round it happens.

Halogens can be used to test whether a molecule is unsaturated, that is, whether it contains a double bond. Bromine water is an orange or yellow solution of Br2, and it is the halogen most commonly used. The unknown compound is shaken with the bromine water, and if the compound is unsaturated an addition reaction takes place and the coloured solution decolourises. The test works because the pi bond is there to be added across.

A double bond consists of one sigma bond and one pi bond, and a triple bond consists of one sigma bond and two pi bonds. Bond strength increases in the order single, double, triple.

The stability of the carbocation intermediate runs tertiary, then secondary, then primary. When more than one carbocation can be formed, the major product of the reaction is the one that results from the nucleophilic attack on the most stable carbocation.

A carbon-carbon double bond is one sigma bond and one pi bond, formed by the sideways overlap of p orbitals, and each carbon is trigonal planar at 120 degrees. The pi bond prevents rotation about the carbon-carbon double bond, so the attached groups stay fixed and E/Z stereoisomers exist. Its high electron density draws in electrophiles, so hydrogen bromide, bromine and sulfuric acid all add across the double bond through a carbocation intermediate, with the most stable carbocation giving the major product. And it is that same addition across the double bond which decolourises bromine water.

Everything alkenes do comes back to the pi bond.