Alcohols
- Created by: Emily Cartwright
- Created on: 29-05-14 19:29
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
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
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
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
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
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
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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