Side chain

In organic chemistry, a side chain is a substituent (radical, abbreviated R ) of a main chain or cyclic group , e.g. B. a short carbon chain ( alkyl group ) branching off from a longer carbon chain or ring.

Side chains in an alkane: 3-ethyl-2-methyloctane with two side chains marked in blue ( methyl group and ethyl group ).

Side chains in alkanes

Even with simple hydrocarbons, side chains (or the total degree of branching) can be decisive for the molecule. Isooctane in gasoline engines especially anti-knock , cetane in diesel engines particularly ignitable.

Side chains in aromatics: ethylbenzene (left) with one side chain marked in blue ( ethyl group ) and p- xylene (right) with two side chains marked blue ( methyl groups ).

Side chains in aromatics

Side chains have a special property in aromatic chemistry. The hydrogen atoms on the carbon atom adjacent to the aromatic are characterized by a particular reactivity towards radicals (easy formation of the mesomeric-stabilized benzyl radical ). They can be substituted, for example by chlorine (see benzyl chloride ) or by oxygen ( phenol synthesis according to Hock , cumene hydroperoxide process ; phenol as the starting material for the plastic polyamide 6, ε-caprolactam ).

The substitution in side chains of aromatics takes place according to the SSS rule , e.g. B. by the side chain halogenation, in which a hydrogen atom of the side chain is replaced by a halogen atom. If, as an alternative, a substitution reaction should take place on the aromatic nucleus and at the same time should be avoided in the side chain, the reaction must be carried out according to the KKK rule .

Side chains in amino acids

Basic structure of proteinogenic amino acids with the side chain R marked in blue (above) and as a specific example the natural amino acid ( S ) -alanine (below) with the blue methyl group as side chain.

Side chains are also of great importance in amino acids . All proteinogenic amino acids are derived from glycine, in which a hydrogen atom (more precisely the pro-SH) has been replaced by a characteristic residue. The remainder can be a simple alkyl substituent (with alanine , possibly also branched as with leucine ), but also other functional groups (e.g. hydroxy (for serine ), amino (for proline ), thio (for cysteine ) and / or carboxy groups (for aspartic acid )). Also, aromatics and heteroaromatics may be incorporated into the side chain ( tryptophan ).

In the case of simple main chains, the side chain is dominant for the structural properties (basic amino (for proline ), acidic amino acids (for aspartic acid )).

Side chains in organic polymers

Polyethylene (above) a polymer without a side chain compared to a polymer with the blue side chain R (middle) and, as a specific example, polypropylene (below) with the blue methyl group as side chain.

Tacticity of polymers: (a) atactic, the blue marked side chains R (top) are spatially arranged irregularly relative to one another, (b) isotactic, the blue marked side chains R (top) are spatially arranged regularly relative to one another towards the same side and (c ) syndiotactic, the blue- marked methyl groups (side chain CH ₃ ) (bottom) are spatially arranged relative to each other regularly on alternating sides.

By modifying the side chains on polymers , their properties can be manipulated. For X = H, the non-polar polyethylene is obtained ; for X = -COOH one arrives at superabsorbents , a class of compounds with extremely hydrophilic properties. The relative arrangement of the side chains plays an important role in the tacticity of a polymer and influences its properties.

Side chains in inorganic polymers

Side chains can also be built into inorganic polymers. Silica gels are esterified with organic residues . The polarity of the silica gels can be varied from strongly polar to very non-polar. These products are widely used in high performance liquid chromatography (HPLC), especially in reverse phase chromatography .

Individual evidence

^ Siegfried Hauptmann : Organic Chemistry , 2nd edition, VEB Deutscher Verlag für Grundstoffindustrie, Leipzig, 1985, p. 297, ISBN 3-342-00280-8 .

[Hauptmann-1] Siegfried Hauptmann : Organic Chemistry , 2nd edition, VEB Deutscher Verlag für Grundstoffindustrie, Leipzig, 1985, p. 297, ISBN 3-342-00280-8 .