Alcohols

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Alcohols

An alcohol is a compound which contains the functional group -OH, which is called an alcohol group

Physical properties

Solubility in water

  • Alcohols with short carbon chains are very soluble in water. This is because the -OH group is able to form hydrogen bonds to water
  • However, alcohols with longer carbon chains are less soluble because the carbon part is non-polar and cannot form hydrogen bonds

Relatively low volatility

  • Alcohols are harder to vapourise than hydrocarbons with molecules of a similar size
  • This is because alcohols can also form hydrogen bonds which are stronger than van der Waal forces
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Alcohols

Preparation of alcohols

Hydration of alkenes

  • Alkenes will react with steam in the presence of phosphoric acid catalyst to form alcohols
  • This is an electrophilic addition. Because it is H2O that is added, it is often referred to as hydration

Uses and production of ethanol

Ethanol is used;

  • In the form of alcoholic drinks
  • As a solvent, most commonly in an impure form called methylated spirit 
  • As a biofuel
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Alcohol

1. Hydration of ethene

  • This is the most important method of making industrial ethanol in the UK, as ethene can be easily obtained by cracking of petroleum
  • Conditions = Ethene and steam are passed over a phosphoric acid catalyst

Advantages

  • Ethene can be easily obtained by cracking of petroleum - so is convenient in countries with a petrochemical industry
  • It can be operated as a continuous process, so it can be fuel and time efficient
  • Pure ethanol is easily obtained, rather than a dilute solution

Disadvantages

  • The petroleum needed is non-renewable
  • If the ethanol produced is used as fuel, it will add new CO2 to the atmosphere, contributing to climate change
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Alcohols

2. Fermentation from sugars e.g. Glucose

  • Plant material containing sugars or starch can be fermented into ethanol. This is how drinks are made, but is also increasingly being used to produce inductrial ethanol
  • The reaction is: C6H12O6 -> 2C2H5OH +2CO2

The necessary conditions are;

  • Yeast (which uses the sugars in anaerobic respiration)
  • Absence of air (in the presence of oxygen, the yeast respires aerobically so little ethanol is produced)
  • Temperature of about 30-35 degrees (warm enough for yeast to grow)
  • Water (solvent, needed for yeast growth)

The fermentation process can continue until the solution contains about 15% of ethanol, but no mroe because ethanol of this concentration kills the yeast. More concentrated ethanol can be obtained by distillation

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Alcohols

Advantages

  • The sugars needed as the feedstock are a renewable resource - because we can always grow new plants
  • Ethanol produced this way is (potentially) a carbon neutral fuel. This is because the CO2 released when it burns was taken in from the atmosphere when the plants grew

Disadvantages

  • It produced a dilute aqueous solution, from which the ethanol has to be distilled
  • It is as a batch process - it has to be stopped when the ethanol concentration becomes too high for the yeast. This is less efficient

Uses of methanol

Methanol (CH3OH) is used

  • As an additive to petrol. It improves combustion, allowing the hydrocarbons to burn more smoothly
  • As a feedstock for the production of other industrially important chemicals
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Alcohols

Reactions of alcohols

Combustion

  • Ethanol and other alcohols readily burn in air, forming carbon dioxide and water

Elimination of water to form an alkene

  • An elimination is a reaction in which atoms are lost from the starting molecule without being replaced by new ones. 
  • Elimination of H2O from an alcohol can be achieved by being heated with a concentrated acid or phosphoric acid is used
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Esterification

An alcohol reacts with a carboylic acid in the presence of concentrated sulphuric acid (catalyst) to form an ester

Carboxylic acids are compounds which contain the group -COOH

Esters: When a carboxylic acid reacts with an alcohol, the hydrogen of the -OH group of the carboxylic acid is replaced by the alkyl group from the alcohol

Conditions for esterification: The carboxylic acid and the alcohol are heated together with a concentrated sulphuric acid catalyst

Properties and uses of esters: Low molecular weight esters are volatile, with a fruity smell. They occur in fruits and other natural products and esters are used as artificial fruit flavourings and in perfumes. Some esters are also important as solvents

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