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Carbon-carbon bond formation (6.2.4)
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One of the alterations chemists can make to the structure of organic molecules is to increase the size of the carbon skeleton by making additional bonds. This can affect physical properties, such as solubility and melting point. It can also link to a change in functional group and associated reactivities.
In aliphatic compounds, bonds can be formed during:
- substitution of a halogen on a haloalkane by a group
- addition of a group onto a carbonyl.
In aromatic compounds, bonds can be formed during:
- substitution of an alkyl group onto an arene ring
- substitution of an acyl group onto an arene ring.
Reacting a haloalkane with the nucleophile forms a new bond between the alkyl group carbon and the nitrile carbon.
This is a nucleophilic substitution reaction as the attacking reagent acts as a nucleophile using the lone pair to form a new bond to the carbon atom of the polarised bond.
An ethanolic solution of is used as from water acts as a competing nucleophile.
The reaction is completed under reflux.
The nucleophilic addition of the ion to the carbon of a carbonyl forms a new bond.
The reagent is and the reaction is completed under reflux.
Here ethanal reacts with to form a hydroxynitrile.
is a highly toxic gas and cannot be used directly; it is formed in situ from sodium or potassium cyanide and a little sulfuric acid.
Nitrile groups are useful intermediates in chemical synthesis. They are easily formed from haloalkanes and can be converted into amines and carboxylic acids.
Nitriles can be converted to amines by reduction.
The hydrogenation of a nitrile requires hydrogen gas with a nickel catalyst.
Each group requires to saturate the bond. Two hydrogens bond to the carbon and two to the nitrogen.
The product is a primary amine.
Nitriles can be converted to carboxylic acids by hydrolysis.
The hydrolysis of a nitrile requires warming with a dilute strong acid catalyst, such as .
Benzene’s substituents can be substituted via electrophilic substitution reactions.
The formation of a bond to a benzene ring has a high energy barrier, since the aromatic ring is very stable and the aromatic system must be disrupted during the reaction.
Friedel–Crafts reactions, named after chemists Friedel and Crafts, allow for the formation of bonds between a benzene ring and an alkyl or an acyl group.
In Friedel-Crafts alkylation, any alkyl group can be substituted onto an aromatic ring using the appropriate haloalkane, usually a chloroalkane, and an catalyst in anhydrous conditions, under reflux.
The role of the catalyst is to remove the halogen from the haloalkane and create the aliphatic electrophile, .
Formation of the electrophile
The reacts with the chloroalkane to form and the electrophile.
Reaction mechanism: Aromatic electrophilic substitution
Note, the new bond is formed between the benzene ring and the carbon atom of the alkyl group previously bound to the chlorine.
Regeneration of the catalyst
The ion substituted off the benzene ring interacts with from the electrophile formation reaction. This reforms the catalyst and produces .
Friedel–Crafts acylation creates a new bond between a benzene ring and an acyl group, forming an acyl benzene product.
The most reactive source of an acyl group is an acyl chloride. This is used, alongside an catalyst, for acylation of aromatic compounds. The reaction mixture is heated under reflux in anhydrous conditions.
Formation of the electrophile
Reaction between ethanoyl chloride and the catalyst forms the electrophilic acylium ion, with the positive charge on the carbonyl carbon, and the complex ion, .
Reaction mechanism: Aromatic electrophilic substitution
The new bond forms between the benzene ring and the carbon from the carbonyl group, forming an aromatic ketone product.
Regeneration of the catalyst
The ion substituted from the benzene ring reacts with to reform the catalyst and the substitution product .