Alkanols and Alkanoic Acids
Alkanols
Alkanols (alcohols) are organic compounds with the -OH group. General formula CnH2n+1OH.
| Name | Formula |
|---|---|
| Methanol | CH3OH |
| Ethanol | C2H5OH |
| Propanol | C3H7OH |
| Butanol | C4H9OH |
Naming: take the alkane name, drop the final -e and add -ol, giving the position of -OH where needed (propan-2-ol).
Physical properties
- Lower members are colourless liquids, soluble in water due to hydrogen bonding.
- Solubility falls as chain length increases.
- Boiling points rise with molar mass and are higher than alkanes of similar size.
Preparation of ethanol
Fermentation: C6H12O6 -> 2C2H5OH + 2CO2, using yeast zymase at about 25-35 C without air. Kenyan examples include brewing from sugarcane molasses at Kisumu and Nzoia, and traditional brews from millet. Fractional distillation raises the alcohol content.
Hydration of ethene: C2H4 + H2O -> C2H5OH (steam, phosphoric acid, 300 C, high pressure).
Chemical properties of ethanol
- Combustion: C2H5OH + 3O2 -> 2CO2 + 3H2O. Ethanol is a clean fuel; blended biofuel is used in some countries.
- Oxidation with acidified potassium dichromate(VI): orange turns green; ethanol becomes ethanoic acid.
- Reaction with sodium: 2C2H5OH + 2Na -> 2C2H5ONa + H2 (hydrogen gas).
- Dehydration with concentrated sulphuric acid at 170 C gives ethene.
- Esterification with carboxylic acids.
Alkanoic acids
Contain the -COOH group. General formula CnH2n+1COOH.
- Methanoic acid HCOOH (ant stings)
- Ethanoic acid CH3COOH (vinegar)
- Propanoic acid C2H5COOH
Naming: drop -e from alkane and add -oic acid.
Properties of ethanoic acid
Weak acid; partly ionises: CH3COOH <=> CH3COO- + H+.
- Turns blue litmus red.
- With magnesium: 2CH3COOH + Mg -> (CH3COO)2Mg + H2.
- With sodium carbonate: gives salt, water and carbon dioxide (effervescence).
- With bases forms ethanoates.
Esters
Alkanol + alkanoic acid -> ester + water (reversible, concentrated H2SO4 catalyst). Ethanol + ethanoic acid gives ethyl ethanoate, which has a fruity smell. Esters are used in perfumes, food flavours and solvents.
Worked example
Name the ester from methanol and propanoic acid. Alcohol part gives the first name (methyl); acid part gives the second (propanoate): methyl propanoate, CH3CH2COOCH3.
Soap link
Esters of fatty acids with glycerol (fats and oils) are hydrolysed with sodium hydroxide to give soap and glycerol. This is saponification, used in local soap making.
Safety
Methanol is poisonous and illicit brews contaminated with it have caused deaths in Kenya. Ethanol is flammable.
Try it
- Draw structures of butan-1-ol and butan-2-ol.
- Write the equation for sodium reacting with propanol.
- Describe a test to distinguish ethanoic acid from ethanol.