ORGANIC 2. ALIPHATIC COMPOUNDS ALKENES

Physical properties

1. The simplest alkenes are gases at room temperature, then liquids, finally solids, due to increased molecular mass. This decrease in volatility is due to increasing Van der Waal's forces.
2. They have typical covalent, physical properties (i.e. almost insoluble in water, soluble in organic solvents)
3. They have the General Formula: C_nH_2n with a C=C bond.

For example: C₂H₄ - Ethene

Structure of C=C bond

In the formation of the C=C bond each carbon atom is sp² hybridised. One of the two s² electrons is promoted to the vacant 2p orbital. Then the 2s orbitals combine together to give three new orbitals that have 120^o bond angles and are planar.

Two of these orbitals then overlap with the 1s¹ orbital of a hydrogen atom to form two C-H bonds. The other overlaps with one of the sp² orbitals of the other carbon atom in the double bond.

The C-C single bond is called a sigma (σ) bond. These are formed by end-on overlap of orbitals. This still leaves a singularly occupied p orbital on each carbon atom.

These then overlap side on to produce a pi (π) bond which occupies a position above and below the plane of the molecule.

It is this π bond that is responsible for the majority of reactions of alkenes and also geometric isomerism that occurs in alkenes.

Geometric isomerism occurs due to there being no rotation around the C=C bond. Groups bonded to the C=C bond are held in fixed positions in the same plane at an angle of 120^o.

This means that if two different groups/atoms are bonded onto the same sides of the double bond (cis), then two isomers exist. One in which the same group is on the same side of the double bond, and one where they are on opposite sides (trans).

Chemical properties

The electron charge cloud of the C=C is the centre of attraction for electrophiles. In most reactions involving alkenes, electrophiles are added to the C=C to give saturated compounds. This means that the characteristic reactions of alkenes are electrophilic addition reactions.

The general mechanism is:

Addition of Bromine

This is a test for C=C. The orange bromine decolourises if C=C is present.

Mechanism:

Reaction with Hydrogen Bromide

Reaction with Hydrogen

Reaction with Steam

Oxidation by Manganate (VII) ions

Polymerisation