Butyl group

The butyl group is a grouping of atoms in organic chemistry . It belongs to the alkyl groups and is formally derived from the structure of butane by abstraction of a hydrogen . It therefore has the empirical formula C ₄ H ₉ . The butyl group can appear in four isomeric forms: as n -butyl, iso- butyl (also “isobutyl”), sec- butyl and tert- butyl. While in the case of the n -butyl and isobutyl groups the radical is bonded to a primary carbon atom, in the case of the sec -butyl group it is bonded to a secondary one , and in the case of the tert -butyl group to a tertiary one .

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  n -Butanol,
  n -butyl group marked in blue .

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  isobutanol ,
  isobutyl group marked in blue .

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  sec -butanol,
  sec -butyl group marked in blue .

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  tert -Butanol,
  tert -butyl group marked in blue .

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  n -Butyl radical,
  n -butyl group marked in blue .

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  iso- butyl radical,
  iso- butyl group marked in blue .

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  sec -butyl radical,
  sec -butyl group marked in blue .

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  tert -Butyl radical,
  tert -butyl group marked in blue .

properties

Steric properties

With increasing branching, the steric demand of the butyl groups increases. Tert- butyl groups are sterically demanding residues. They can be used to stabilize chemical compounds , since attack can be significantly prevented by filling them up. This hindrance also affects the reaction kinetics , which can be both accelerated and slowed down as a result. If reactions that require the attack of a reactant in the vicinity of the tert-butyl group take place, then these reactions usually take place much more slowly, since the attack is prevented by the space-filling residue. Conversely, the conformation of a molecule can also be frozen by the tert-butyl radical in a position that is beneficial to the reaction kinetics and thus accelerates the reaction. The tert-butyl group acts as an anchor .

Anchor groups

Iso- butyl and tert-butyl groups can be used as anchor groups. This means that they hinder the change in the conformation of the compound through their steric effect. For example, they restrict the free rotation of alkyl radicals or prevent the conversion of the two chair conformations of cyclohexane rings into one another. The bulky tert-butyl radical here preferably remains in the equatorial position, since this results in the lowest interactions with other substituents. In the second possible chair conformation, the remainder is in an axial position, which results in interactions with other axial substituents ( Prelog stress ). Since these are energetically unfavorable, this equilibrium lies strongly on the side on which the tert-butyl group takes an equatorial position.

Electronic properties

The different positions with which the butyl group binds to the molecule result in different electronic properties, which are caused by the inductive effects of the alkyl radicals. Among the isomeric butyl radicals, for example, the electron density is highest on the tertiary carbon of the tert-butyl group and lowest on the n- butyl group. This is also reflected in the increasing basicity of the lithiated species from n -butyllithium via sec -butyllithium to tert -butyllithium . The same effect is also for decreasing acidity of the alcoholic proton of the butyl alcohols, 1-butanol , 2-butanol and tert -butanol responsible.

use

Structure of n -butyllithium with the polarized carbon-lithium bond. The n -butyl group is marked blue , including the polarization.

Lithium alkyls are strong organometallic bases . They can be used for deprotonating weakly acidic protons and for lithiation . Three butyllithium bases are commercially available as standard reagents : n -butyllithium, sec -butyllithium, and tert -butyllithium (increasing reactivity). The tert-butyl group is part of the important BOC protective group .

The hydrogenation agent diisobutyl aluminum hydride (DIBAL) has two isobutyl radicals.

Connection groups

The following groups of substances contain all four isomeric forms of the butyl group as a structural element:

• Butanols
• Butylamines
• Chlorobutane
• Bromobutanes
• Butylbenzenes
• Leucine (butylglycine)
• Pentanals (butyl carbaldehydes)
• Pentanoic acids (butyl carboxylic acids)
• Butyl acetate

Individual evidence

1. ^ Brockhaus ABC chemistry. VEB FA Brockhaus Verlag, Leipzig 1965, p. 46.
2. ^ Hans Beyer and Wolfgang Walter : Organic Chemistry. S. Hirzel Verlag, Stuttgart 1984, ISBN 3-7776-0406-2 , p. 792.