Alcohols
- Created by: Emily Cartwright
- Created on: 25-05-14 15:04
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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 chain part is non-polar and cannot form hydrogen bonds
- Relatively low volatility. Alcohols are harder to vaporise than hydrocarbons with molecules of a similar size
- To boil these compounds, intermolecular forces must be broken. Alcohols with shorter carbon chains can also form hydrogen bonds, which require more energy to break
- 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
- Preparation of Alcohols
- Hydration of Alkenes
- Alkenes will react with steam in the presence of a 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
- Alkenes will react with steam in the presence of a phosphoric acid catalyst to form alcohols
- Hydration of Alkenes
- Uses and production of alcohol
- Ethanol is an important chemical. Uses are;
- In the form of alcoholic drinks
- As a solvent, most commonly in an impure form called methylated spirit
- As a biofuel
- 1. Hydration of Ethene
- This is the most important method of making industrial ethanol in the U.K as ethene can be obtained easily by cracking of petroleum
- The reaction is: H2C=CH2 + H2O -> C2H5OH
- Conditions: Ethene and steam are passed over a catalyst of phosphoric acid
- Advantages
- Ethene can be obtained easily by cracking of petroleum - so it 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
- Advantages
- Conditions: Ethene and steam are passed over a catalyst of phosphoric acid
- The reaction is: H2C=CH2 + H2O -> C2H5OH
- This is the most important method of making industrial ethanol in the U.K as ethene can be obtained easily by cracking of petroleum
- 2. Fermentation from sugars e.g. Glucose
- Plant materials containing starch or sugars can be fermented into ethanol. This is how drinks are made, but it is also increasingly being used to produce industrial 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 centigrade (warm enough for yeast to grow)
- Water (solvent, needed for yeast growth)
- The necessary conditions are
- The fermentation process can continue until the solution contains about 15% of ethanol, but no more, because ethanol of this concentration kills the yeast
- More concentrated ethanol can be obtained by distillation
- Advantages
- The sugars needed as a feedstock are a renewable resource as we can grow new plants
- Ethanol produced this way is potentially a carbon-neutral fuel as the CO2 released when it burned was taken in from the atmosphere when the plant grew
- Disadvantages
- It produces a dilute aqueous solution, from which ethanol has to be distilled
- It is a batch process: it has to be stopped when the ethanol concentration becomes too high for yeast, This is less efficielt
- The reaction is: C6H12O6 -> 2C2H5OH + 2CO2
- Plant materials containing starch or sugars can be fermented into ethanol. This is how drinks are made, but it is also increasingly being used to produce industrial ethanol
- Ethanol is an important chemical. Uses are;
- Uses of methanol
- Used as an additive to petrol. It improved combustion, allowing the hydrocarbons to burn more smoothly
- Used as a feedstock for the production of other industrially important chemicals e.g. Methanal, which is used in the production of plastics
- 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
- In these cases, since the atoms eliminated form H2O, the reaction is also sometimes called a dehydration
- Elimination of water from an alcohol can be achieved by being heated with a concentrated acid, usually sulphuric acid or phosphoric acid
- In these cases, since the atoms eliminated form H2O, the reaction is also sometimes called a dehydration
- An elimination is a reaction in which atoms are lost from the starting molecule without being replaced by new ones
- Combustion
- Esterification
- An alcohol reacts with a carboxylic acid in the presence of concentrated sulphuric acid catalyst to form an ester
- Carboxylic acids are compounds which contain the group -COOH
- Esters: When carboxylic acids react with alcohols, the hydrogen of the -OH group of the carboxylic acid is replaced by the alkyl group from the alcohol
- 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 esters
- Low molecular weight esters are volatile, with a fruity smell.
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