Carbon—Carbon Bond Formation
Nucleophilic substitution with cyanide ions
If we want to extend the carbon chain of a molecule, we can react a molecule with a nucleophilic carbon (like cyanide ions) with a molecule containing an electrophilic carbon (like haloalkanes). A carbon—carbon bond is synthesised, extending the length of the carbon chain.
The cyanide ion consists of carbon triple bonded to nitrogen. The carbon has a negative charge, so it will react with anything that is electron deficient and donate its electrons. Anything that releases cyanide ions can be used as the nucleophile, such as hydrogen cyanide (HCN), potassium cyanide (KCN) and sodium cyanide (NaCN).
Haloalkanes contain a carbon with a slightly positive carbon atom, since halogens are electronegative and pull the bonding electrons towards themselves and establishing a polar C-X bond.
Here’s the mechanism for the reaction:
- The CN- nucleophile attacks the slightly positive carbon atom of the C-X bond. A carbon-carbon bond is formed.
- The bonding electrons in the C-X bond move onto the halogen atom, breaking the C-X bond and releasing a halogen ion. A nitrile is formed.
Notice that it’s the same as the nucleophilic substitution reactions between hydroxide ions and haloalkanes to form alcohols. Instead of OH- acting as the nucleophile, it’s CN-, but it’s happening in the same way.
Nucleophilic addition
Cyanide ions can also react with aldehydes and ketones, since the carbonyl group (C=O) contains an electronegative oxygen, creating a polar bond with a slightly positive carbon. Hydrogen cyanide is used as the reagent, which dissociates to form the negative cyanide ion and a positive hydrogen ion.
The cyanide ion attacks the slightly positive carbon of the C=O bond. A carbon-carbon bond is formed.
The pi bond of the carbonyl (C=O) group breaks, with the pi electrons jumping onto oxygen. Oxygen is now singly bound to carbon and is negatively charged.
The oxygen donates its extra electrons to the hydrogen ion, to form an OH group. A hydroxynitrile is formed.
Nucleophilic substitution or addition reactions with cyanide ions are useful in chemistry because nitriles and hydroxynitriles are really reactive. This means that they can be easily converted into other compounds.
Reduction of nitriles to amines
Nitriles and hydroxynitriles can be reduced by reacting them with a reducing agent (such as lithium aluminium hydride) in the presence of dilute acid. A primary amine is formed.
Instead of lithium aluminium hydride (LiAlH4), a mixture of sodium and ethanol can be used. On a large scale though, both reducing agents are too expensive to make industrial reactions economically viable. In practice, hydrogen gas is used as a reducing agent, in the presence of a nickel catalyst at high temperatures and pressures.
Hydrolysis of nitriles to carboxylic acids
Nitriles and hydroxynitriles can be hydrolysed by refluxing with dilute hydrochloric acid. A water molecule is used to break the carbon-nitrogen triple bond. The carbon then reacts with water to form –COOH. An ammonium salt is also formed.
Friedel-Crafts reactions
Friedel-Crafts reactions are used to form carbon-carbon bonds with benzene rings (aromatic compounds). It’s done by refluxing benzene in the presence of a halogen carrier e.g. AlCl3 or FeBr3. You’ll also need to add either a haloalkane (for alkylation) or an acyl chloride (for acylation).
This is an electrophilic substitution reaction. A carbon is formed between a carbon in the benzene ring and the carbon that is attached to the halogen in the haloalkane / acyl chloride.
