Alkyl group

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Different forms of presentation of the structural formula of an alkyl group using the example of the isobutyl group (= isobutyl group) marked in blue .

An alkyl group is part of a molecule made up of linked carbon and hydrogen atoms . The simplest alkyl group is the methyl group -CH 3 ; it occurs relatively frequently. Further examples are the ethyl group -CH 2 -CH 3 and the n -propyl group -CH 2 -CH 2 -CH 3 . The general formula for alkyl groups in the form of a chain is C n H 2n + 1 , that for the cyclic cycloalkyl groups is C n H 2n − 1 .

nomenclature

Alkyl groups occur in many molecules, especially in the saturated hydrocarbons , the alkanes . The name alkyl group is derived from this, and the naming of the various alkyl groups also depends on the underlying alkane (or alkene or alkyne ): The stem of the name is extended with the suffix "-yl", that is, the ending -an is replaced by -yl replaced (or a -yl is appended). The simplest alkyl groups are alkanes that lack a hydrogen atom: e.g. B. the methyl, ethyl and propyl groups are derived from methane , ethane and propane . Alkyl groups are often abbreviated when they appear in structural or empirical formulas. For example, MeOH stands for methanol , Et 2 O for diethyl ether . When referring to any alkyl group, -R is often used. Since the abbreviation -R can also stand for -H or even for residues with functional groups, their meaning is usually also indicated.

Depending on the length of the alkyl chain, the following systematic names and common names are used:

Systematic and common names for the alkyl groups marked in blue .
X is a single bond, e.g. B. a halogen, a hydroxy group or an aryl radical
Number of
carbon atoms 		Systematic
name 		Common
name 		Abbre-
Zung 		Semi-structural formula Isomers
01 Methyl- Me Alkylgroup1 Structural Formulas V.1.png no
02 Ethyl Et Alkylgroup2 Structural Formulas V.1.png no
03 Propyl Alkylgroup3 Structural Formulas V.1.png Alkylgroup6 Structural Formulas V.1.png
04th Butyl Alkylgroup4 Structural Formulas V.1.png 3 isomers
05 Pentyl Valeryl Alkylgroup5 Structural Formulas V.1.png 8 isomers
06th Hexyl Capron multiple isomers
07th Heptyl Enanthyl multiple isomers
08th Octyl Caprylic multiple isomers
09 Nonyl Pelargonyl numerous isomers
10 Decyl Caprine numerous isomers
12 Dodecyl Lauryl numerous isomers
14th Tetradecyl Myristyle numerous isomers
16 Hexadecyl Cetyl numerous isomers
18th Octadecyl Stearyl numerous isomers

The aryl groups are to be distinguished from the mostly aliphatic alkyl groups, in which case an aryl group is expediently understood to mean a group which is linked directly to the aromatic ring to the remainder of the molecule, depending on the influence on the rest of the molecule. There are also combinations of alkyl radicals and aryl radicals, an example is the benzyl radical [-CH 2 -C 6 H 5 ].

Influence of an alkyl group on the rest of the molecule

  • Alkyl groups have a weak electron-donating effect, called the + I effect . So they act as weak electron donors .
  • In particular, branched alkyl groups influence adjacent functional groups due to their space requirements and reduce their reactivity due to this steric hindrance .
  • The longer and more numerous the alkyl groups in a compound, the poorer its water solubility and the better its solubility in non-polar solvents .

Web links

Commons : alkyl group  - collection of pictures, videos and audio files

Individual evidence

  1. ^ Brockhaus ABC chemistry. VEB FA Brockhaus Verlag, Leipzig 1965, p. 46.