Aldehydes and Ketones

Structure of Aldehydes and Ketones

Structure of Aldehydes and Ketones

Aldehydes and ketones incorporate a carbonyl functional group, C=O. These are organic compounds with structures -CHO and RC(=O)R’ where R and R’ represent carbon-containing substituents respectively. Aldehydes and Ketones are often called as methanoyl or formyl group. The carbon atom of this group has 2 remaining bonds that might be occupied by aryl or alkyl or substituents. If neither of these substituents is hydrogen, the compound is a Ketone. If at least one is hydrogen, the compound is an Aldehyde.

Examples of Aldehydes and Ketones

Examples of Aldehydes and Ketones

Aldehydes

In aldehydes, the carbonyl group has one hydrogen atom attached to it together with either a 2nd hydrogen atom or a hydrogen group which may be an alkyl group or one containing a benzene ring.

Example:

Examples of Aldehydes

Examples of Aldehydes

One can notice that all these have the exact same end to the molecule. The only difference is the complexity of the other attached group.

Ketones

What are Ketones?

In ketones, the carbonyl group has 2 hydrocarbon groups attached to it. These can be either the ones containing benzene rings or alkyl groups. Ketone does not have a hydrogen atom attached to the carbonyl group.

Example:

Examples of Ketones

Examples of Ketones

Propane is generally written as CH3COCH3.  In pentanone, the carbonyl group could be in the middle of the chain or next to the end – giving either pentan-3-one or pentan-2-one.

Occurrence of Ketones and Aldehydes

Combined with other functional group aldehydes and ketone are widespread in nature. Compounds such as cinnamaldehyde (cinnamon bark), vanillin (vanilla bean), Citra (lemongrass), helminthosporal (a fungal toxin), carvone (spearmint and caraway), camphor (camphor trees) are found chiefly in microorganisms or plants. Whereas, compounds such as muscone (musk deer), testosterone (male sex hormone), progesterone (female sex hormone), cortisone (adrenal hormone) have animal and human origin.

Uses of Aldehydes and Ketones

Formaldehyde is the simplest aldehyde whereas acetone is the smallest ketone. There are a number of aldehydes and ketones which find application due to their chemical properties. A few uses of Aldehydes and Ketones are listed below.

Uses of Aldehydes

  • Formaldehyde is a gas. With 40% solution in water, it forms Formalin which is used in preserving biological specimens.
  • Formaldehyde is used in embalming, tanning, preparing glues and polymeric products, as germicides, insecticides, and fungicides for plants. It is also used in drug testing and photography.
  • When reacted with phenol, formaldehyde forms Bakelite, which is used in plastics, coatings, and adhesives.
  • Acetaldehyde is largely used for the production of acetic acid and pyridine derivatives.
  • Benzaldehyde is used in perfumes, cosmetic products, and dyes. It is added to provide almond flavour to food products and also used as a bee repellent.

Uses of Ketones

  • The most common ketone is acetone which is an excellent solvent for a number of plastics and synthetic fibres.
  • In the household, acetone is used as a nail paint remover and paint thinner.
  • In medicine, it is used in chemical peeling and for acne treatments.
  • Methyl ethyl ketone (MEK), chemically butanone, is a common solvent. It is used in the production of textiles, varnishes, plastics, paint remover, paraffin wax, etc.
  • MEK is also used as a welding agent for plastics due to its dissolving properties.
  • Cyclohexanone is another important ketone which is primarily used in the production of nylon.

Preparation of Ketones and Aldehydes

Aldehydes and Ketones are obtained from products of many reactions. Some reactions for the synthetic preparation of Aldehyde and Ketone is mention below.

Preparation of Aldehydes and Ketones

Preparation of Aldehydes and Ketones

Stay tuned with BYJU’S to learn more about different types of aldehydes and ketones, their physical and chemical properties.

Frequently Asked Questions

How are Aldehydes and Ketones Different?

The carbonyl carbon of an aldehyde has a hydrogen atom attached to it whereas that of a ketone is attached to two alkyl or aryl groups. The C-H bond in aldehydes makes them easily oxidizable (they are strong reducing agents).

Why are Aldehydes more Reactive towards Nucleophilic Substitutions than Ketones?

The two alkyl/aryl groups in ketones offer steric hindrance during substitution reactions. Since the hydrogen atom is relatively small, it barely offers any steric hindrance. This is the primary reason why aldehydes are more susceptible to nucleophilic substitutions. Also, the partially positive charge on the carbonyl carbon is stabilized by the two R groups in ketones.

Why are the Boiling Points of Ketones Higher than those of Aldehydes?

The presence of two electron-donating R-groups in ketones makes them more polar than aldehydes. The dipole moments that arise from this polarity account for the higher boiling points of ketones.

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