Extending Carbon Chain Length
- Synthetic chemists design new molecules by changing functional groups and changing the length of a carbon chain
- Nucleophilic substitution with cyanide ions can achieve chain extension:
- The solvent used is ethanol, because if water was present OH- would act as the nucleophile, preventing chain extension
- Cyanide ions can also react with carbonyls in nucleophilic addition
- Both reactions form nitriles which can undergo further reactions e.g. reduction and hydrolysis
- Reduction: LiAlH4 is added to the nitrile and heated with a nickel catalyst at high pressure
- RCN + 2H2 → RCH2NH2
- Hydrolysis: Addition of water and a strong acid catalyst to the nitrile under reflux
- RCN + 2H2O + HCl → RCOOH + NH4Cl
- Reduction: LiAlH4 is added to the nitrile and heated with a nickel catalyst at high pressure
Alkylation and Acylation of Aromatic Compounds
- Friedel-Crafts alkylation is used to add a hydrocarbon chain to an aromatic compound
- An alkyl nucleophile is generated by reacting a haloalkane with FeCl3 in situ
- R-Cl + FeCl3 → R+ + FeCl4-
- The R+ reacts with the aromatic species via electrophilic substitution
- Acylation:
- CH3COCl + AlCl3 → CH3CO+ + AlCl4-
- CH3CO+ + C6H6 + AlCl4- → C6H5COCH3 + HCl + AlCl3
Halogenoalkanes
- Haloalkanes are saturated organic compounds that contain at least one halogen atom, e.g. F, Cl, Br, or I.
- The C-X bond has a permanent dipole due to the large difference in electronegativity between the carbon and halogen atoms. With the electrons closer to the halogen atom.
- The δ- on the carbon atom makes it easily attacked by electron-rich nucleophiles (an electron pair donor that is attracted to electron deficient regions).
- Nucleophilic substitution occurs with hydroxide ions, resulting in hydrolysis of the haloalkane:
- The OH- ions can be generated by using an aqueous metal hydroxide solution (e.g. KOH) or by using water in the presence of AgNO3 and ethanol
- Going down Group 7, the electronegativity of the halogen atoms decreases, so the polarity of the C-X bond also decreases. We would expect fluoroalkanes to be the most reactive, however iodoalkanes are the most reactive because bond enthalpy decreases down the group. C-I bond is the longest and weakest
Aldehyde & Ketones
- Aldehydes and ketones are carbonyl compounds containing the C=O functional group.
- Aldehydes can be oxidised to form carboxylic acids by heating under reflux with potassium dichromate (oxidising agent) and concentrated sulphuric acid
- Carbonyls have a permanent dipole making them susceptible to nucleophilic addition reactions.
- Aldehydes can be reduced to primary alcohols and ketones to secondary alcohols using NaBH4 as the reducing agent e.g. reduction of propanal CH3CH2CHO + 2[H] → CH3CH2CH2OH
- Nucleophilic addition of cyanide ions can be used to extend the length of a carbon chain
- Reactions of carbonyls with KCN and dilute acid, produces a hydroxynitrile. Aldehydes and unsymmetrical ketones form two optical isomers, as there is an equal chance of either isomer being formed. This is a racemic mixture.
- Cyanide-containing compounds can be toxic or irritants.