Chapter 14: Alcohols Flashcards
What is the functional group of an alcohol?
- The hydroxyl group
- OH
What is the simplest alcohol?
What is it used for?
Methanol
- It is used as a high performance fuel because of its efficient combustion
What are other important uses of methanol?
- It is an important chemical feedstock
- The starting material in many industrial synthesis
- It can be converted into polymers, paints solvents, insulation, adhesives and many other useful products
State some properties of alcohols (compared to alkanes)
- Less volatile
- Have higher boiling points
- Greater solubility
Explain the properties of alcohols (and alkanes)
- The alkanes have non-polar bonds because the electro-negativity of hydrogen and carbon are very similar
- The alkane molecules are therefore non-polar
- The intermolecular forces between non-polar molecules are very weak non-polar London forces
- Alcohols have a polar O-H bond because of the difference in electro-negativity of the oxygen between the oxygen and hydrogen atoms
- Alcohol molecules are therefore polar
- The intermolecular forces will be very weak London forces but there will also be much stronger hydrogen bond between the polar O-H
Volatility and boiling points (alkanes vs alcohols)
- Intermolecular hydrogen bonds hold the alcohol molecules together
- These bonds must be broken in order to change the liquid alcohol into a gas
- This requires more energy than overcoming the weaker London forces in alkanes so alcohols have a lower volatility than alkanes of the same number of carbon atoms
Solubility in water (Alkanes vs alcohols)
- Alkanes are non-polar molecules and cannot form hydrogen bonds with water
- Alcohols are completely soluble in water, as hydrogen bonds form between the polar -OH group of the alcohol and the water molecules
What happens as the hydrocarbon chain increases in size
-The influence of the -OH group becomes relatively smaller, and the solubility of the longer-chain alcohols becomes more like that of hydrocarbons-solubility decreases
How are alcohols classified?
- Depends on the number of hydrogen atoms and alkyl groups attached to the carbon atom that contains the alcohol functions group
What are primary alcohols?
- The OH group is attached to a carbon atom that is attached to one alkyl group
What are secondary alcohols?
- The OH group is attached to a carbon atom that is attached to two alkyl groups
What are tertiary alcohols?
- The OH group is attached to a carbon atom that is attached to three alkyl groups
What do alcohols burn in a plentiful supply of oxygen to produce?
- carbon dioxide and water
What kind of reaction is the combustion of alcohols?
- Exothermic
- It releases a large quantity of energy in the form of heat
Name an oxidising agent
- Potassium dichromate
What will be observed if the alcohol is oxidised using potassium dichromate?
A colour change from orange to green
What can primary alcohols be oxidised to?
Alcohols or carboxylic acids
Preparation of aldehydes:
On gentle heating of primary alcohols with acidified potassium dichromate, an aldehyde is formed
How do you ensure that a aldehyde is produced instead of a carboxylic acid?
- The aldehyde is distilled out of the reaction mixture as it forms
- This prevents any further reaction with the oxidising agent
Preparation of carboxylic acids?
- If a primary alcohol is heated strongly under reflux, with an excess of acidified potassium dichromate, a carboxylic acid is formed
How can you ensure that all of the alcohol is oxidised?
- Use of an excess of acidified potassium dichromate
What does heating under reflux ensure?
- It ensures that any aldehyde formed initially in the reaction also undergoes oxidation to carboxylic acid
What does oxidation of a secondary alcohol produce?
A Keytone
What can be done to ensure that oxidation of secondary alcohol goes to completion?
It is heated under reflux with the oxidising agent
