- Created by: charley
- Created on: 11-09-18 11:37
Properties or Alkanes
General formula: CnH2n+2
Alkanes are saturated hydrocarbons joined together by sigma bonds.
Alkanes have a tetrahedral shape with bond angles of 109.5 degrees.
Boiling point increases as chain length increases beacause theres a larger surface area for london forces to act on.
Branched isomers have a lower boiling point than their straight chained counterparts because theres fewer surface points of contact so fewer london forces.
Alkanes are unreactive beacuse its bonds are non-polar and sigma bonds are strong.
Reactions of Alkanes
Alkanes combust completely or incompletely.
Alkanes react with halogens in radical substitution. There are three stages; initiation, propagation and termination.
- The covalent bond is broken by homolytic fission with energy provided by UV radiation.
- The reaction propagates through propagation steps forming a chain reaction where the radical reacts with a molecule forming a new radical and a new molecule.
- Where two radicals collide forming a molecule with all electrons paired. This stops the reaction.
There are limitations to radical subsitiution as further substitutions can occur until all hydrogens have been substituted and isomers can form when there is more than one carbon in the chain.
Properties of Alkenes
Alkenes are unsaturated hydrocarbons.
Alkenes have a trigonal planar shape with bond angles of 120 degrees.
Alkenes show E/Z isomerism and cis-trans isomerism (when each carbon in the double bond is attached to a hydrogen).
Atoms attached to the carbons in the double bond are assigned priorities depending on their atomic number. The higher the atomic number, the higher the priority.
Alkenes are more reactive than alkanes due to the pi bond. The pi electrons are more exposed so a pi bond readily breaks and undergos addition reactions.
Alkenes undergo addition polymerisation.
Reaction of Alkenes
- Hydrogenation of alkenes:
- Hydrogen is added across the double bond in the presence of a nickel catalyst
- Halogenation of alkenes:
- Either bromine or chlorine is added across the double bond at room temperature
- Bromine water is used to test for unsaturation. Compounds containing C=C bonds decolourise bromine water.
- Alkenes and hydrogen halides:
- Alkenes react with hydrogen halides to form haloalkanes
- There are two possible products
- Hydration of alkenes:
- Water, as steam, is added across the double bond in the presence of an acid catalyst e.g. phosphoric acid, forming an alcohol
- There are two possible products
Alkenes undergo electrophilic addition. Stability increases as the number of alkyl groups attached to a carbon increases, explaining why two products are formed from unsymetrical reactants as the major product is formed via the most stable route.
Properties of Alcohols
Alcohols are polar and can form hydorgen bonds.
As the chain length increases, the boiling point increases. The higher boiling point, the lower the volatility.
Alcohols are soluble in water. Solubility decreases as the chain length increases.
Primary alcohols have the -OH group attached to a carbon that is attached to two or three hydrogens.
Secondary alcohols have the -OH group attached to a carbon that is attached to one hydrogen.
Tertiary alcohols have the -OH group attached to a carbon that is attached to no hydrogens.
Reactions of Alcohols
Alcohols undergo combustion reactions, oxidisation, dehydration and substitution reactions.
- Oxidation of primary alcohols:
- Primary alcohols can be oxidised to aldehydes with the addition of acidified potassium dichromate under distillation
- Primary alcohols can be oxidised to carboxylic acids with the addition of an excess of potassium dichromate under reflux
- Oxidation of secondary alcohols:
- Secondary alcohols are oxidised to ketones with the addition of acidified potassium dichromate under reflux
- Oxidation of tertiary alcohols:
- Tertiary alcohols cannot be oxidised
- Dehydration of alcohols:
- An alcohol is heated under reflux in the presence of an acid catalyst forming an alkene
- Substitution of alcohols:
- Alcohols react with hydrogen halides when heated under reflux to form haloalkanes. The hydrogen halide is formed in situ from sulfuric acid and a sodium halide.
Haloalkanes can be classified into primary, secondary and tertiary.
The carbon-halogen bond is polar and attracts nucleophiles.
Haloalkanes undergo nucleophilic substitution.
Haloalkanes can be hydrolysed to form alcohols using aqueous sodium hydroxide which is heated under reflux. The rate of hydrolysis increases as reactivity decreases. Rate of reaction also increases with an increase in stability of the molecule.