Alkenes

A covalent bond is a shared pair of electrons. More specifically, when two atoms form a covalent bond: 'a singly occupied orbital on one atom overlaps with a singly occupied orbital on another atom' 

When the atomic orbitals overlap, the resulting molecular orbital concentrates the electrons mainly in the region of space inbetween the nuclei, which is energetically favourable

There are different types of bond, depending on the type of atomic orbitals involved and the way in which they overlap;

s-orbitals

• 2 s-orbitals overlap, each with one electron to form sigma bond

p-orbitals

• 2 p-orbitals, each with one electron overlap end to end to form a sigma bond
• 2 p-orbitals,each with one electron overlap sideways to form a pi bond

• When carbon forms a single bond, this will be a sigma bond since this type of overlap leads to a lower energy orbital, and hence a stronger bond than a pi-bond
• In a double bond, one electron pair forms a sigma-bond and the other forms a pi-bond

Key features of a double bond

Shape;

• Each C has one double bond and two other bonded pairs repelling each other in it's outer shell, so the geometry around each carbon is trigonal planar (bond angle 120 degrees)

Rigidity

• Unlike a single bond, rotation is not possible around a double bond. This is because rotation would destroy the sideways overlap of the p-orbitals and hence break the pi-bond

Bond lengths and energies

• A double bond is shorter and stronger than a single bond
• Within the double bond, the pi-bond is weaker than the sigma bond, so in reactions it is the pi-bond than tends to break

• Alkenes are unsaturated hydrocarbons
• They form a homologous series, with general formula CnH2n
• Every alkene contains a C=C double bond. Compounds containing more than one C=C double bond are called diene's

Isomerism in Alkenes

• From C4H8 onwards, the alkenes exist as a number of different isomers. There are two sorts of isomerism in alkenes;

Structural Isomerism

• Alkenes can be straight or branched and the position of the double bond may also vary

Stereoisomerism

• Stereoisomers are compounds with the same structural formula, but with a different arrangement in space

E/Z Isomerism

• These isomers have the same structural formula but differ in their 3-dimensional arrangement - they are stereoisomers
• To distinguish between the isomers, we use the prefixes E-(the two hydrogens are on opposite sides of the double bond) and Z-(the two hydrogens are on the same side of the double bond)
• For E/Z isomerism to occur, each of the doubly bonded C atoms must also be bonded to two different groups

Preparation of alkenes

Cracking of hydrocarbons

• This is the main industrial source of alkenes. Long chain alkanes from petroleum are cracked to provide a variety of alkenes. These are major industrial chemicals
• Industrial uses of alkenes: Polymerisation to form plastics (e.g. poly(ethene)) and Conversion to other organic chemicals e.g. alcohols

Dehydration of alcohols

• In the laboratory, and in some industrial processes, alkenes can be made starting from alcohols, which are compounds that contain an -OH group
• If a water molecule is lost from this molecule by eliminating the -OH from one carbon and a hydrogen from the adjacent carbon, it leaves a double bond between the two carbons
• This type of reaction is called elimination or dehydration
• Conditions: Dehydration requires heating in the presence of an acid catalyst, usually concentrates sulphuric acid or concentrated phosphoric acid

• In alkenes, the functional group is the C=C double bond and we call this the alkene group
• Since they all contain the same functional group, all alkenes have similar chemical properties
• The characteristic reaction of alkenes is addition. An addition reaction is one in which two reactant molecules combine together to form one product molecule. In this case, the pi-bond of the double bond breaks and each carbon atom forms a new sigma bond to another atom

Addition of hydrogen

• Alkenes react with hydrogen gas in the presence of a nickel catalyst
• This reaction is the basis of the manufacture of margarine from liquid vegetable oils. These oils are unsaturates. On hydrogenation, some of these double bonds react with hydrogen and are converted into single bonds
• The more saturated hydrocarbons obtained have higher melting points and are solid at room temperature, so can be used as margarine

Addition of hydrogen

• Alkenes react readily with chlorine or bromine at room temperature and in the dark
• The reaction with bromine is used as a test for unsaturation, because the orange colour of bromine is immediately decolourised in the dark and at room temperature

Addition of Steam

• This is the reverse of the dehydration reaction
• The reaction requires H2O in the form of steam and a phosphoric catalyst
• The product of addition is an alcohol
• Hydration is ethene is as important industrial process used to manufacture ethanol. Ethene and steam are passed at a high temperature and pressure (about 300 degrees and 60 atmospheres) over a phosphoric acid catalyst

Addition of hydrogen halides

• Hydrogen chloride, iodide and bromide undergo addition reactions with alkenes, forming the corresponding halogenoalkane

Polymerisation of alkenes

• A polymer is a long molecule formed by chemically linking together many smaller molecules called monomers. The polymer is named simply by writing poly(ethene). So if ethene is polymerised, the product is poly(ethene)

• Alkene polymers are not biodegradable, so if we throw them away after use, waste plastics accumulate in the environment either on land or at sea. They will remain in landfill for hundreds of years

1. Combusition

• Advantage = They are good fuels and burning them releases energy and reduces the demand for landfill sites
• Disadvantage = Toxic combustion products may be formed. This is particularly an issue for chlorinated plastics such as PVC

2. Recycling

• The different types of polymers have to be seperated if recycling is to produce a useful product and separation is a costly, labour-intensive process

3. Feedstock for cracking

• Polymers can be cracked to give a mixture of alkenes and alkanes. Alkanes can be used for petrol and alkenes can be polymerised to give more polymers
• At present, this is still an expensive route

Biodegradable polymers

• Polymers are now being developed which are biodegradable 
• One example is where granules of starch are built into the plastic. Microbes in soil feed on the starch, breaking the plastic into small bits that will rot away more quickly

Arenes

• The simplest arene is benzene. It has the molecular formula C6H6 
• Benzene does not react as if it had three C=C double bonds; the pi bonds interfere with each other and as a result have very different properties