alcohols- chapter 14 Flashcards
explain why the polarity affects the bonds in both alkanes and alcohols
- the alkanes have non-polar bonds because the electronegativity of hydrogen and carbon are very similar, therefore the alkanes molecules are non-polar
- the intermolecular forces between non-polar molecules are very weak in London forces
- alcohols have a polar O-H bond because of the difference in electronegativity of the oxygen and hydrogen atoms
- alcohols molecules are therefore polar
- the intermolecular forces will be very weak London forces but there will also be much stronger hydrogen bonds between the polar O-H groups
explain the difference in boiling points of alkanes and alcohols
- in a liquid state, the 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 weak London forces in alkanes, so alcohols have a lower volatility than the alkanes with the same number of carbon atoms
explain the difference of solubility in water between alcohols and alkanes
- a compound that can form hydrogen bonds with water is more water-soluble than a compound that cannot
- alkanes are non-polar molecules and cannot form hydrogen bonds with water
- alcohols such as methanol and ethanol are completely soluble in water, as hydrogen bonds form between the polar -OH group of the alcohol and the water molecules
- as the hydrocarbon chain increases in size, the influence of the -OH group becomes relatively smaller and the solubility of longer-chain alcohols becomes more like that of the hydrocarbons= solubility decreases
what are primary alcohols
the -OH group is attached to a carbon atom that is attached to two hydrogen atoms and one alkyl group
what is a secondary alcohol
the -OH group is attached to a carbon atom that’s attached to one hydrogen atom and two alkyl groups
what is a tertiary alcohol
the -OH group is attached to a carbon atom that’s attached to no hydrogen atoms and three alkyl groups
how are primary and secondary alcohols oxidised
by potassium dichromate (VI) acidified with dilute sulfuric acid, H2SO4
if its oxidised, the orange solution is reduced to a green solution
what are primary and secondary alcohols oxidised to
either aldehydes or carboxylic acids
the product depends on the reaction conditions because aldehydes themselves also oxidised to carboxylic acids
what is the preparation of aldehydes
- on gentle heating of primary alcohols with acidified potassium dichromate, an aldehyde is formed
- to ensure an aldehyde is formed, it is distilled out of the reaction mixture as it forms, this prevents any further reaction
- orange to green
how is a carboxylic acid prepared
- primary alcohol is heated strongly under reflux, with an excess of acidified potassium dichromate
- excess ensures alcohol is oxidised
- heating ensure that any aldehyde initially formed in the reaction undergoes oxidation to the carboxylic acids
oxidation of secondary alcohols
- oxidised to ketones
- secondary alcohol is heated under reflux with dichromate
- orange to green
what is the oxidation of tertiary alcohols
don’t undergo oxidation reaction, remains orange
what is dehydration (and of an alcohol)
- any reaction in which a water molecule is removed from the starting material
- an alcohol is heated under reflux in the presence of an acid catalyst e.g H2SO4 or H3PO4
- product= alkene
- elimination reaction
what is the substitution of alcohols
- alcohols react with hydrogen halides to form haloalkanes
- alcohol is heated under reflux with sulfuric acid and a sodium halide the hydrogen bromide is formed in situ (in place)
