OCR Chemistry - Alcohols

Revision notes for OCR Chemistry A for spreads 2.2.1 - 2.2.4

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Ethanol

Ethanol can be made using two different processes:

  • The hydration of ethene, C2H4
  • The fermentation of sugars.

Hydration

Ethanol is industrially manufactured by the catalytic hydration of ethene. Ethene and stram are passed over a phosphoric acid catalyst at a high temperature and a moderate pressure. The process is operated continously.

The reaction however is reversible so the conversion rate is only about 5%. Any unreacted products are recycled and passed back into the reactor to form ethanol.

Fermention

Fermentation is the process of converting glucose into ethanol and carbon dioxide. Yeast and a temperature of 37 degrees is used. The yeast ceases to function when the concentration of ethanol is 14% so and higher concentration requires distillation of the mixture. The process must be also in the absense of air as it an anaerobic reaction.

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Uses Of Ethanol

The main uses of ethanol is as a fuel and in alcoholic drinks.

Ethanol can also be mixed together with methanol and coloured dye to make methylated spirits. These are much cheaper than alcoholic drinks as they are undrinkable because they are toxic, meaning they are exempt from being taxed.

Ethanol can also be mixed together with octane to increase the octane rating of the fuel as it make it burn much cleaner. This is the key use for ethanol in America.

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Proprerties Of Alcohols

Because of hydrogen bonding between O-H groups of alcohol molecules, alcohols have a lower volatility and boiling point than other molecules with similar molecular mass. This is because hydrogen bonds are the strongest type of molecular bond.

Alcohols dissolve in water  because of the hydrogen bonds being the polar groups of the water and alcohol molecules. The first three alcohols are soluable in water however solubility decreases as chain length increases as a larger part of the molecule is made up on a non-polar chain and this does not form hydrogen bonds with water molecules.

Alcohols can be classes as primary, secondary or tertiary.

In a primary alcohol, the -OH group is attached to a carbon atom with no alkyl groups or bonded to one alkyl group.

In a secondary alcohol, the -OH group is attached to a carbon atom bonded to two alkyl groups.

In a tertiary alcohol, the -OH group is attached to a carbon atom bonded to three alkyl groups.

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Combustion and Oxidation Of Alcohols (1)

In a plentiful supply of oxygen, alcohols burn to form carbon dioxide and water.

Primary and secondary alcohols can be oxidised using an oxidising agent. A suitable oxidising agent is a solution containing acidified dichromate ions (H+, Cr2O72-). This oxidising mixture can be made from potassium dichromate (K2Cr2O7) and sulfuric acid (H2SO4). During the reaction, the acified potassium dichromate changes colour from orange to green.

Primary Alcohols

On gentle heating with acidified potassium dichromate, a primary alcohol can be oxidised to produce an aldehyde.

On stronger heating with excess acidified dichromate the alcohol will be completely oxidised, passing through the aldehyde stage to form a carboxylic acid.

When preparing aldehydes, you need to distill the aldehydes as it is formed to avoid them being oxidised thurther to form a carboxylic acid. When making a carboxylic acid, the reaction mixture is usually heated under reflux before distilling off the product. 

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Combustion and Oxidation Of Alcohols (2)

Secondary Alcohols

Secondary alcohols are oxidised by acidified dichromate ions to form ketones. Ketones cannot be oxidised thurther.

Tertiary Alcohols

Tertiary alcohols are resistant to oxidisation. The oxidising agent remains the colour orange.

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