Carbonyl compounds

The carbon atom is δ+ causing the C=O bond to be polar. This allows it to be attacked by nucleophiles (a species that donates a lone pair of electrons).

Nucleophilic addition

An example of this is the addition of hydrogen cyanide, HCN. 

Mechanism for addition of HCN

Another example is the addition of hydrogen. 

Mechanism for addition of hydrogen

This reaction is a reduction reaction as the carbonyl compound is reduced to an alcohol.  

Condensation reaction

This type of reaction is an addition then an elimination. It occurs in substances with a H₂N-NH group. An example of this reaction is using 2,4-DNPH.

Oxidation  

Aldehydes can be oxidised to form carboxylic acids.

RCHO + [O] --> RCOOH

1. When using potassium dichromate, the CrO ions (orange) are reduced to Cr ions (green) when mixed with aldehydes but not ketones.
2. When warming carbonyl compounds with Fehling's/Benedict's solution (copper(II) complex), aldehydes form a red precipitate whereas the ketone solution would stay blue.
3. When warming carbonyl compounds with Tollens' reagent [Ag(NH₃)₂]⁺(aq), a silver mirror is formed with aldehydes as the silver mirror complex is reduced. Ketones cannot reduce this complex.
    

One last reaction ... The iodoform reaction

A solution of iodine in sodium hydroxide is warmed gently with ketones containing the CH₃CO group. A pale yellow precipitate forms (CHI₃).