Alkenes

DEEPSHIKHA

DEPARTMENT OF CHEMISTRY

• Alkenes contain carbon-carbon double bonds. –

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General formula: CnH2n (for one double bond) –

Suffix = -ene • In the carbon-carbon double bond, two pairs of electrons are being shared, leaving the carbon free to bond to two other things.

C₂H₄

ethene

C₃H₆

propene

Isomerism in the alkenes

Structural isomerism

All the alkenes with 4 or more carbon atoms in them show structural isomerism. This means that there are two or more different structural formulae that you can draw for each molecular formula.

For example, with C₄H₈, it isn't too difficult to come up with these three structural isomers:

Geometric (cis-trans) isomerism

The carbon-carbon double bond doesn't allow any rotation about it. That means that it is possible to have the CH₃ groups on either end of the molecule locked either on one side of the molecule or opposite each other.

These are called cis-but-2-ene (where the groups are on the same side) or trans-but-2-ene (where they are on opposite sides).

Preparation of alkenes ALKENES IN THE LAB

Dehydration of alcohols using aluminium oxide as catalyst

The dehydration of ethanol to give ethene

This is a simple way of making gaseous alkenes like ethene. If ethanol vapour is passed over heated aluminium oxide powder, the ethanol is essentially cracked to give ethene and water vapour.

Dehydration of alcohols using an acid catalyst

The acid catalysts normally used are either concentrated sulphuric acid or concentrated phosphoric(V) acid, H₃PO₄.

The concentrated sulphuric acid is a catalyst. Write it over the arrow rather than in the equation.

Cyclohexanol is heated with concentrated phosphoric(V) acid and the liquid cyclohexene distils off and can be collected and purified.

Phosphoric(V) acid tends to be used in place of sulphuric acid because it is safer and produces a less messy reaction.

The dehydration of cyclohexanol to give cyclohexene

The dehydration of more complicated alcohols

When you dehydrate an alcohol, you remove the -OH group, and a hydrogen atom from the next carbon atom in the chain. With molecules like butan-2-ol, there are two possibilities when that happens.

With molecules like butan-2-ol, there are two possibilities when that happens.

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Physical Properties of Alkenes

• Many of the physical properties of alkenes are similar to those of alkanes. –

Alkenes are nonpolar compounds. • insoluble in water. • soluble in nonpolar solvents. –

They are less dense than water.

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Range of physical states: –  4 C's — gases

– 5 - 17 C's — liquids

–  18 C's — solids

• The chemical properties of alkenes, however, are completely different from those of alkanes.

Reactions of Alkenes —

Addition Reactions •

Most reactions of alkenes can be classified as addition reactions, in which both parts of a reactant are added to the carbon-carbon double bond:

Halogenation of Alkenes

• In a halogenation reaction, an alkene reacts with molecular bromine (Br2) or chlorine (Cl2) to form an alkyl halide (or haloalkane).

Addition of Acids

• Acids (HF, HCl, HBr, and HI) can add to a double bond to produce an alkyl halide

ALKENES and SULPHURIC ACID

The addition of sulphuric acid to alkenes

The reaction with ethene

Alkenes react with concentrated sulphuric acid in the cold to produce alkyl hydrogensulphates.

Ethene reacts to give ethylhydrogensulphate..

The reaction with propene

This is typical of the reaction with unsymmetrical alkenes. An unsymmetrical alkene has different groups at either end of the carbon-carbon double bond.

If sulphuric acid adds to an unsymmetrical alkene like propene, there are two possible ways it could add. You could end up with one of two products depending on which carbon atom the hydrogen attaches itself to.

However, in practice, there is only one major product.

This is in line with Markovnikov's Rule which says:

When a compound HX is added to an unsymmetrical alkene, the hydrogen becomes attached to the carbon with the most hydrogens attached to it already.

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Using these reactions to make alcohols

Making ethanol

Ethene is passed into concentrated sulphuric acid to make ethyl hydrogensulphate (as above). The product is diluted with water and then distilled.

The water reacts with the ethyl hydrogensulphate to produce ethanol which distils off.

Making propan-2-ol

More complicated alkyl hydrogensulphates react with water in exactly the same way. For example:

THE DIRECT HYDRATION OF ALKENES

This page looks at the production of alcohols by the direct hydration of alkenes - adding water directly to the carbon-carbon double bond.

Manufacturing ethanol

Ethanol is manufactured by reacting ethene with steam. The reaction is reversible.

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Only 5% of the ethene is converted into ethanol at each pass through the reactor. By removing the ethanol from the equilibrium mixture and recycling the ethene, it is possible to achieve an overall 95% conversion.

Manufacturing other alcohols

If you start from an unsymmetrical alkene like propene, you have to be careful to think about which way around the water adds across the carbon-carbon double bond.

Markovnikov's Rule says that when you add a molecule HX across a carbon-carbon double bond, the hydrogen joins to the carbon atom which already has the more hydrogen atoms attached to it.

That means that in the propene case, you will get propan-2-ol rather than propan-1-ol.

ALKENES and POTASSIUM MANGANATE(VII)

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Oxidation of alkenes with cold dilute potassium manganate(VII) solution

 The overall equation for the formation of this from the manganate(VII) ions is:

Oxidation of alkenes with hot concentrated acidified potassium manganate(VII) solution

If you are using hot concentrated acidified potassium manganate(VII) solution, what you finally end up with depends on the arrangement of groups around the carbon-carbon double bond.

The acidified potassium manganate(VII) solution oxidises the alkene by breaking the carbon-carbon double bond and replacing it with two carbon-oxygen double bonds.

The products are known as carbonyl compounds because they contain the carbonyl group, C=O. Carbonyl compounds can also react with potassium manganate(VII), but how they react depends on what is attached to the carbon-oxygen double bond. So we need to work through all the possible combinations.

If both attached R groups in the products are alkyl groups

If the groups attached either side of the original carbon-carbon double bond were the same, then you would end up with a single ketone. If they were different, then you would end up with a mixture of two.

For example:

If a product has one hydrocarbon group and one hydrogen

In this case, the first product molecule has a methyl group and a hydrogen attached to the carbonyl group. This is a different sort of compound known as an aldehyde.

Aldehydes are readily oxidised to give carboxylic acids, containing the -COOH group. So this time, the reaction will go on a further step to give ethanoic acid, CH₃COOH.

The overall effect of the potassium manganate(VII) on this kind of alkene is therefore:

If a product has two hydrogens but no hydrocarbon group

You might have expected that this would produce methanoic acid, as in the equation:

But it doesn't! That's because methanoic acid is also easily oxidised by potassium manganate(VII) solution. In fact, it oxidises it all the way to carbon dioxide and water.

So the equation in a case like this might be, for example:

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