A2-Level Advanced Organic Chemistry
A carbonyl group is a carbon double bonded to an oxygen atom (C=O).
Aldehydes have a carbonyl group at the end of a carbon chain; ketones have a carbonyl group in the middle of a carbon chain.
Aldehydes can be formed from the oxidation of primary alcohols; ketones can be formed from the oxidation of secondary alcohols.
The slightly positive carbon in an aldehyde and a ketone reacts with nucleophiles in addition reactions.
Aldehydes can be reduced with hydride ions (H⁻) to primary alcohols; ketones can be reduced with hydride ions to secondary alcohols.
A common source of hydride ions for reduction is the compound lithium tetrahydridoaluminate (LiAlH )
Carbonyls (Formation and Reduction)
A carbonyl group is a carbon atom double bonded to an oxygen atom.
There are two possible ways of having a C=O bond in a hydrocarbon chain – either at the end of the chain (this is an aldehyde) or in the middle of the chain (this is a ketone).
A C=O bond is polar as the oxygen atom is highly electronegative and pulls electron density in the covalent bond towards itself.
As a result, the carbon atom gains a slight positive charge and the oxygen atom a slight negative charge.
A carbon atom with a slight positive charge is vulnerable to nucleophilic attack, which makes carbonyl compounds likely to undergo nucleophilic addition (and addition-elimination) reactions.
Forming aldehydes and ketones
The easiest way to form aldehydes and ketones is through the oxidation of alcohols. A primary alcohol can be oxidised to an aldehyde; a secondary alcohol can be oxidised to a ketone. For more information on this, see alcohols section.
Reducing aldehydes and ketones
The opposite of oxidation is reduction. Remember in organic chemistry – oxidation means increasing the number of oxygen bonds to a carbon atom (and therefore decreasing the number of hydrogen bonds to the carbon atom). Reduction means the opposite, decreasing the number of oxygen bonds to a carbon atom, or increasing the number of hydrogen bonds to the carbon atom.
The carbon atom in a carbonyl has a slight positive charge, this means anything wanting to form a bond with it will need a negative charge or lone pair of electrons (nucleophile). To add hydrogen atoms to this carbon, they will have to be negatively charged. Negatively charged hydrogen ions are called hydride ions and are a little special in chemistry.
The hydride ions are attracted to the electron deficient carbon atom in the carbonyl, causing the C=O bond to break (reduction).
The negatively charged oxygen can now pick up a proton form a water molecule.
There are different sources of hydride ions, but the most common source at A-level chemistry is from lithium tetrahydridoaluminate (LiAlH ).
The mechanism for this process and the steps involved are beyond the level required for A-level knowledge. What you need to understand is that the LiAlH provides a H⁻ ion that acts as a nucleophile and binds to the carbon in the carbonyl group.
When writing out the reaction, [H] represents a source of hydrogen from a reducing agent (in this case, LiAlH ).
If an aldehyde is reduced it forms a primary alcohol.
If a ketone is reduced it forms a secondary alcohol.