The first four alcohols and their formulae
Definition
Alcohol
A member of the homologous series whose functional group is -OH.
- Methanol is CH3OH\text{CH}_3\text{OH}CH3OH.
- Ethanol is C2H5OH\text{C}_2\text{H}_5\text{OH}C2H5OH.
- Propan-1-ol is C3H7OH\text{C}_3\text{H}_7\text{OH}C3H7OH.
- Butan-1-ol is C4H9OH\text{C}_4\text{H}_9\text{OH}C4H9OH.
- Each has one more CH2\text{CH}_2CH2 than the one before, so they form a homologous series.
Key Idea
Writing the formula as CnH2n+1OH\text{C}_n\text{H}_{2n+1}\text{OH}CnH2n+1OH keeps the −OH-\text{OH}−OH group visible.
The functional group is -OH
Definition
Functional group
The group of atoms in a molecule that gives a homologous series its characteristic reactions.
- Every alcohol contains an oxygen joined to a hydrogen, written −OH-\text{OH}−OH.
- That group is attached to a carbon atom in the chain.
- It is what gives every alcohol its characteristic reactions.
- The 1 in propan-1-ol says the group sits on the first carbon of the chain.
- The rest of the molecule is an ordinary hydrocarbon chain.
Note
The −OH-\text{OH}−OH in an alcohol is covalently bonded, and is not the hydroxide ion of an alkali.
Drawing the structures
- Each carbon still shows four bonds and each hydrogen one.
- The oxygen shows two bonds, one to a carbon and one to a hydrogen.
- Methanol is one carbon carrying three hydrogens and one −OH-\text{OH}−OH.
- Ethanol is two carbons joined by a single bond, with the −OH-\text{OH}−OH on the end carbon.
- Counting the bonds on the oxygen as well as the carbons checks the structure.
Practical
- Method: weigh a capped spirit burner, burn the alcohol under a metal can holding a measured mass of water, then cap it, let it cool and reweigh it.
- The four alcohols used are ethanol, propanol, butanol and pentanol, with the mass of water, the wick length and the distance to the flame kept the same.
- Compare them by the energy transferred per gram of fuel burned, found from the temperature rise and the mass of alcohol used up.
- Heat loss to the surroundings is the largest source of error, so a draught shield and a lid help.
- Soot on the can shows incomplete combustion, which lowers the energy released.
Alcohols can be dehydrated to alkenes
Definition
Alkene
An unsaturated hydrocarbon containing a carbon to carbon double bond, with the general formula CnH2n.
- Dehydration removes a molecule of water from the alcohol.
- The water comes from the −OH-\text{OH}−OH group and a hydrogen from the next carbon.
- The two carbons left behind form a double bond.
- Ethanol is dehydrated to ethene: C2H5OH→C2H4+H2O\text{C}_2\text{H}_5\text{OH} \rightarrow \text{C}_2\text{H}_4 + \text{H}_2\text{O}C2H5OH→C2H4+H2O
- The product is an alkene, so a saturated molecule has become unsaturated.
Self review
- Give the formulae of methanol, ethanol, propan-1-ol and butan-1-ol.
- What is the functional group in an alcohol?
- Describe the structure of ethanol.
- What is formed when ethanol is dehydrated?
- Why are the alcohols a homologous series?
