Chapter 14 (4.2.1) Flashcards
Polarity of alcohols
Have a polar O-H bond thus are polar molecules. Weak London Forces but also hydrogen bonds so have stronger intermolecular forces than alkanes.
Properties of alcohols compared with alkanes
- less volatile
- higher melting points
- greater water solubility
Differences between alcohols and alkanes become smaller as the length of the carbon chain increases
What is volatility?
Volatile = low boiling point - easily converted from a liquid to a gas. The higher the boiling point, the lower the volatility.
Volatility of alcohols
- In the liquid state, hydrogen bonds as well as London Forces must be broken in order to change to the gas state
- Requires more energy than overcoming London Forces in alkanes, so alcohols are less volatile than alkanes with the same number of carbon atoms
Solubility in water of alcohols
- Some alcohols are completely soluble in water, as hydrogen bonds form between the polar -OH group of the alcohol + water molecules (know how to draw)
- As the hydrocarbon chain increases in size, the influence of the -OH group becomes relatively smaller, so solubility decreases.
Classifying alcohols
- Primary: -OH group is attached to a carbon atom that is attached to 2 hydrogen atoms and 1 alkyl group (except methanol)
- Secondary: -OH group is attached to a carbon atom that is attached to 1 H and 2 alkyl
- Tertiary: -OH is attached to a carbon atom that is attached to 0 H and 3 alkyl.
Combustion of alcohols
- burn completely in a plentiful supply of oxygen to produce carbon dioxide and water
- exothermic - releases lots of energy
- as number of carbon atoms in alcohol chain is increases, quantity of heat released per mole also increases
Oxidising agent of alcohols
- oxidising mixture: acidified potassium dichromate (VI) - K₂Cr₂O₇/H₂SO₄
- oxidising agent: Cr₂O₇²⁻/H⁺
- use [O] to represent the oxidising agent in equations
Observation for oxidation
Orange solution containing dichromate (VI) ions is reduced to a green solution containing chromium (III) ions:
Cr₂O₇²⁻ —-> Cr³⁺
Oxidation of primary alcohols (distil)
- forms aldehydes: gentle heating of alcohols with acidified potassium dichromate (VI)
- to ensure preparation of an aldehyde, it is distilled out of the reaction mixture as it forms - preventing any further reaction with the oxidising agent.
(know equations)
Oxidation primary alcohols (reflux)
- forms carboxylic acids: strong heating under a reflux with an excess of acidified potassium dichromate (VI)
- excess K₂Cr₂O₇/H₂SO₄ ensures all of the alcohol is oxidised
- reflux ensures any aldehyde formed initially also oxidises
- equation has 2[O]
Oxidation of secondary alcohols
- forms ketones: heated under reflux with the oxidising mixture
- dichromate (VI) ions change from orange to green
Oxidation of tertiary alcohols
- do not undergo oxidation reactions
- acidified dichromate (VI) ions remain orange
Elimination of alcohols
- an alcohol is heated under a reflux/heat in the presence of an acid catalyst such as concentrated sulfuric acid, H₂SO₄, or concentrated phosphoric acids, H₃PO₄.
- product is an alkene (double bond forms where -OH group is removed)
Substitution of alcohols
- alcohols react with hydrogen halides to form haloalkanes (substitution with halide ions)
- alcohol heated under a reflux with sulfuric acid and a sodium halide. Hydrogen halide is formed in situ.
e. g. NaBr + H₂SO₄ —-> HBr + NaHSO₄ - the HBr formed reacts with the alcohol to produce the haloalkane
- overall: CH₃CHOHCH₃ + NaBr + H₂SO₄ —-> CH₃CHBrCH₃ + NaHSO₄ + H₂O
