The Alkenes


The Alkenes

They occur naturally in very small quantities but are obtained from crude oil by the process of cracking. 

Ethene is the starting point for the production of polymers, detergents, solvents and many other chemicals.

Alkenes are hydrocarbons, but are unsaturated. Contain at least one C=C bond.


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C=C, double bond

The C-C single bond (σ bond or sigma bond) is symmetrical along the central axis of the molecule. It can rotate along its axis as it is only a single bond. 

The C=C double bond is flat.

It involves the σ bond plus a second bond which has a density situated above and below the axis. The second bond is the π bond (pi bond). This does not allow rotation around the axis. 

The electron desity in the π is further from the nuclei so the double bond is less than twice as strong.

They are reactive, due to the attraction of the positively polarised groups within the pi bond. - Electrophiles

Many unsaturated compounds show geometric isomerism 

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Geometric Isomerism

Occurs when components of the molecule are arranged on different sides of the molecule. 

Traditional name: cis-trans isomerism

Oftern show different physical properties:

Melting and boiling temperatures

Chemical reactions

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E-Z Isomerism

Groups attached around the C-C bond are ranked by their atomic number. 

The atom with the highest number has the highest priority. 

If the two high prority groups are on the same side it is a Z-isomer.

If the two high prority groups are ondifferent sides it is a E-isomer.

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Reactions of Alkenes

The double bond has two main effects: 

Alkenes exhibit E-Z isomerism.

Alkenes undergo addition reaction. 

Alkenes are attacked by an electrophile, something with high areas of electron density. It form a new covalent bond. 

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Addition Reactions


In the presence of a nickel catalyst, and around 200°alkenes undergo addition reaction. 


Ethene bubbled through bromine water, the bromine is decolourised. - This is the test for alkenes. Addition occurs across the double bond.

The vigour of addition reactions decreases down the halogen groups. 

All occur at room temperature, suggesting there are no free radicals. 


E.g. CH4 + HBr = CH3CH2Br. An alkane is formed. Markovnikov's Rule = The HX adds to the carbon atom in the double bond with the greatest number of H attached. 


Addition across the double bond and oxidation. Purple to colourless. Alkanes do not react, so can be used to distinguish between alkenes and alkanes. 

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Reaction Mechanisms

Curly arrows show the movement of electrons. 

A half-curly arrow is the movement of one electron whereas a full curly arrow shows the movement of a pair of electrons. 

1. An electrophilic attack occurs on the electron-rich Pi bond. The C gains a positive charge and becomes a carbocation

2. The carbocation is then open to attack by nucleophilies. 

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A polymer is a very large molecule made up of long chains of smaller units joined together. - Monomers

Polymers are formed by addition reactions. Two monomers react together and addition occurs across the double bond. 

Low-density polythene - highly branched means it is relatively soft and malleable. 

High-density polythese - few branched chains which makes it more rigid and denser. 

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Properties of Polymers

Most affecting properties are:

The average length of the polymer chain - strength and melting temperatures increase with length. 

Branching of chains - Highly branched chains have low tensile strength

The presence of intermolecular forces between chains - Strong intermolecular forces result in strong chains with high melting points. 

Cross-links between chains - Makes polymers very rigid, hard and brittle, usually with a high melting temperature.

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Recycling Plastics

Mechanical recycling 

Physical changes to the plastics (melted, shredded or turned into granules)

This is very expensive

Chemical recycling

Breakdown of plastic waste into monomers. Which can then be reused again in the formation of other polymers. 

Thermal cracking, burning, and gasification. 

Expensive and can require alot of energy.

Biogradable polymers - Made by modifying natural polymers such as starch or cellulose. 

They are sustainable, renewable and reduce the carbon footprint. 

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Energy Recovery

Recovering some of the energy that is put into producing the polymers. 

Incinerators are used at very high temperatures, polymers contain a great deal of stored energy so this energy can be used to drive incinerators. 

  • Pyrolysis - polymers are broken down by heat in the absence of oxygen.
  • Gasification - the breakdown of solid hydrocarbons in a limited oxgyen atmosphere to make syngas
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