Atoms and Molecules

Biochemically important groups

Alkanes, e.g. ethane, CH₃-CH₃
The most important reaction is the removal of two hydrogen atoms from neighbouring carbons to form an alkene. This is an oxidation, e.g.

CH₃-CH₃ CH₂=CH₂ + 2[H]

The hydrogen atoms are not free but must be used to reduce another molecule, e.g. convert oxygen to hydrogen peroxide O₂ + 2[H] H₂O₂

Alkenes, e.g. ethene, CH₂=CH₂
There are two important reactions:
(a) formation of alkanes by the addition of two hydrogen atoms to the double bond. This is a reduction, e.g.

CH₂=CH₂ + 2[H] CH₃-CH₃

(b) formation of alcohols by the addition of a water molecule to the double bond. This is an addition reaction, e.g.

CH₂=CH₂ + H₂O CH₃-CH₂-OH

Alcohols, e.g. ethanol, CH₃-CH₂-OH
Alcohols have different properties dependent upon the number of hydrogen atoms attached to the carbon atom holding the alcohol group; the more hydrogen atoms, the more reactive.

                     1-butanol                          2-butanol                     2-methyl-propan-2-ol
               (a primary alcohol)       (a secondary alcohol)         (a tertiary alcohol)

There are three important reactions
(a) the removal of a water molecule to form an alkene. This is an elimination reaction, e.g.

CH₃-CH₂-OH CH₂=CH₂ + H₂O

(b) oxidation to an aldehyde (from primary alcohols) or ketone (from secondary alcohols)

CH₃-CH₂-OH CH₃-CH=O + 2[H]

(c) formation of esters with carboxylic acids. This is a condensation reaction

Carboxylic acids, e.g. ethanoic acid (acetic acid), CH₃-CO₂H

There are five important reactions:
(a) its acidity             CH₃-CO₂H CH₃-CO₂^- + H⁺
(b) formation of esters with alcohols
(c) formation of amides with amines. This is a condensation reaction

(d) formation of anhydrides with other acids. This is a condensation reaction.

(e) it may eliminate carbon dioxide

Anhydrides, e.g. acetic anhydride, CH₃-CO-O-CO-CH₃
There is one important reaction, acylation. This is a transfer reaction involving the transfer of an acyl (R-C=O) group.

Esters, e.g. ethyl ethanoate (ethyl acetate), CH₃-CH₂-O-CO-CH₃
There is one important reaction, hydrolysis forming an alcohol and a carboxylic acid.

Amines, e.g. ethylamine, CH₃-CH₂-NH₂
Amines have different properties dependent upon the number of non-hydrogen atoms attached to the nitrogen atom; the more non-hydrogen atoms, the less reactive.   
methylamine     dimethylamine     trimethylamine     quaternary methyl ammonium ion
primary amine  secondary amine  tertiary amine         quaternary ammonium ion

There are three important reactions:
(a) its basicity         CH₃-CH₂-NH₂ + H⁺ CH₃-CH₂-NH₃⁺
(b) formation of amides with carboxylic acids
(c) Reaction with aldehydes to form secondary imines. This is an addition reaction followed by the elimination of water.

Amides, e.g. N-ethylacetamide, CH₃-CH₂-NH-CO-CH₃
There is one important reaction, hydrolysis forming an amine and a carboxylic acid.

Aldehydes, e.g. ethanal (acetaldehyde), CH₃-CH=O
There are four important reactions:
(a) they may be reduced to alcohols

CH₃-CH=O + 2[H] CH₃-CH₂-OH

(b) they may be oxidised to carboxylic acids

CH₃-CH=O + H₂O CH₃-CO₂H + 2[H]

(c) they may form hemiacetals

(d) they may form acetals

(e) they reaction with primary amines to form secondary imines (see above)

Ketones, e.g. acetone,
Ketones react similarly to aldehydes, but generally less well. They form ketals and hemiketals with alcohols rather than the acetals and hemiacetals formed by aldehydes.

Hemiacetal, e.g. glucose

Acetal e.g. maltose

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This page was last updated by Martin Chaplin
on 10 February, 2005