axial (chemistry)

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The term axial is used in chemistry to denote bonds in molecules , the position of substituents on cycles, the direction of attack by an electrophile in a chemical reaction and to describe a certain form of chirality that occurs in allenes and biarenes . It is in contrast to the term equatorial .

Organic chemistry

Axial bonds

The chair form of cyclohexane (C 6 H 12 ) contains six axial C – H bonds (left) and six equatorial C – H bonds (middle), for a total of twelve C – H bonds (right).

In the chair form ( chair conformation ) of cyclohexane there are two types of C – H bonds, the axial ( a ) and the equatorial ( e from English equatorial ). The six axial C – H bonds run parallel to the threefold axis C 3 of the cyclohexane ring. The six equatorial C – H bonds lie in the “equator” of the cyclohexane and run parallel to the next but one C – C bonds. When flipped over, the axial C – H bonds transform into the equatorial C – H bonds and vice versa.

Position of cyclohexane substituents

The two possible chair conformations of methylcyclohexane: The methyl group is preferably equatorial (left). In equilibrium there is only a small concentration of the chair conformer with an axial methyl group (right).

Monosubstituted cyclohexane can exist in two different conformal forms, with a substituent R being able to assume either an equatorial or an axial position. The more conformal with the equatorial position of the radical R is preferred. The height of the inversion barrier (approx. 42 to 46 kJ · mol −1 ) is low, so that the conversion of the conformers takes place at 25 o C at high speed (there are about 100,000 transitions per second, corresponding to a half-life of 10 −6 s) and so the separation of the conformers is not possible. The nature of the residue R affects the equilibrium of conformers. For example, in methylcyclohexane at 25 ° C., about 95% of the molecules have the methyl group in an equatorial and only 5% in an axial conformation .

Axial electrophilic attack on six-membered rings with one or two sp 2 -hybridized carbon atoms

Six-membered rings with an sp 2 -hybridized carbon atom in the ring are attacked by an electrophile either axially or equatorially. If the six-membered ring contains two or more sp 2 -hybridized carbon atoms, the electrophile attacks axially. The product that is finally formed can contain the newly entered remainder in an axial or equatorial position, but this has nothing to do with the dynamics of the addition.

Enantiomeric axially chiral compounds

Allenenes (left) and biarenes (right) are axially chiral compounds.

In allenes and biarenes, axially chiral enantiomers occur.

Inorganic chemistry

In inorganic chemistry, the pair of equatorial and axial terms is used to classify the types of positions of ligands or residues in trigonal bipyramids.

Trigonal bipyramidal structure with equatorial (eq) and axial (ax) positions.

Individual evidence

  1. ↑ Otto-Albrecht Neumüller (Ed.): Römpps Chemie-Lexikon. Volume 1: A-Cl. 8th revised and expanded edition. Franckh'sche Verlagshandlung, Stuttgart 1979, ISBN 3-440-04511-0 , p. 344.
  2. ↑ Joachim Buddrus: Fundamentals of Organic Chemistry , 4th edition, de Gruyter Verlag, Berlin, 2011, p. 109, ISBN 978-3-11-024894-4 .
  3. ^ Siegfried Hauptmann : Organic Chemistry , 2nd revised edition, VEB Deutscher Verlag für Grundstoffindustrie, Leipzig, 1985, pp. 112–115, ISBN 3-342-00280-8 .
  4. ↑ Jonathan Clayden, Nick Greeves, Stuart Warren, Peter Wothers: Organic Chemistry , Oxford University Press, 2001, pp. 858-861, ISBN 978-0-19-850346-0 .
  5. ^ Ulrich Lüning: Organic reactions , 2nd edition, Elsevier GmbH, Munich, 2007, p. 202, ISBN 978-3-8274-1834-0 .