Aldehydes and Ketones 

    Aldehydes and Ketones are organic compounds. They consists of Carbon (C), Hydrogen (H) and Oxygen (O) atoms. Both aldehydes and ketones has a carbonyl group, and it is located in the end of the carbon chain. Aldehydes are the compounds that has a hydrogen (H) that is directly bonded to the carbonyl carbon. And ketones does not have hydrogen (H) atoms which are directly bonded to the carbonyl carbon. When aldehydes or ketones have a benzene ring bonded to the carbonyl carbon, they are called aromatic aldehydes or ketones. Both aldehydes and ketones have numerous applications in different industries. The main difference between aldehydes and ketones are the location of their carbonyl group.

Properties of aldehydes and ketones

• Both aldehydes and ketones are polar molecules because of their presence of the carbonyl group.
• In their carbonyl group, the electronegativity difference of the carbon atom and the oxygen atom results to this polarity.
• Due to this polarity these molecules usually has dipole-dipole interactions.
• Aldehydes generally have lower boiling points and melting points than ketones.
• Aldehydes tend to be more reactive than ketones due to the presence of the acidic hydrogen (H) atom.
• Aldehydes and ketones has a higher flashing point and a melting point than alkanes and a lower flashing point and a melting point than alcohols.

• Both aldehydes and ketones are soluble in water.
• But when their number carbon (C) atoms in their carbon chain increases, their water solubility will decrease.
  

Preparation of aldehydes and ketones

• Both aldehydes and ketones can be made by the oxidation of alcohols.

• Since aldehydes have one or two hydrogen (H) atoms bonded directly to the carbonyl carbon, when they oxidised using powerful oxidisers such as KMnO₄/H⁺, they tend to oxidise until carboxylic acids.
• So, to get aldehydes, it can be performed by the oxidation of primary alcohols using PCC (Pyridinium chlorochromate)

• Ketones can be prepared by oxidation of secondary alcohols with powerful oxidisers, such as KMnO₄/H⁺.

• Both aldehydes and ketones can also be prepared by hydration of alkynes using [Hg²⁺/H₂SO₄/H₂O]

Reactions of aldehydes and ketones

• Aldehydes and ketones are considered to be electrophilic compounds.
• So both aldehydes and ketones undergo nucleophilic reactions.
• Because aldehydes have hydrogen (H) atoms which are directly bonded to the carbonyl carbon, they can be oxidised to carboxylic acids by using powerful oxidisers.
• Ketones are harder to oxidise because the don’t have hydrogen (H) atoms that are directly bonded to the carbonyl carbon.

• Both aldehydes and ketones reacts with HCN, Grignard, primary amines, NaOH/H₂O, etc.

• Both aldehydes and ketones can be reduced to alkanes using [Zn/Hg/HCl].
• Both aldehydes and ketones can be reduced to alcohols using [① LiAlH₄, ② H₂O]

• Only aldehydes can be oxidised to carboxylic acids using [KMnO₄/H⁺].

• Both aldehydes and ketones undergo aldol condensation reaction while the presence of dil NaOH, OH^- or H⁺.

Additional points

• Aldehydes and ketones shows positional isomers.