F to M

Continued from terms starting with D and E

Contents

Fiducial Group; Fischer Projection (or Fischer-Tollens Projection); Fischer-Rosanoff Convention (or Rosanoff Convention); Flagpole; Free Rotation, Hindered Rotation, Restricted Rotation; Gauche; Gauche Effect; Geometric Isomerism ; Half-chair; Handedness; Helicity; Heterotopic; Hindered Rotation; Homochiral; Homomorphic; Homotopic; In-Out Isomerism; Induction, Stereochemical; Internal Compensation; Inversion; Isomer; Isomeric; Isomerism; Isometric, Anisometric; Isotopomers; Kinetic Resolution; L; l; l, u; (Lambda); M; meso ; meso-Compound; Minus; Molecular Entity; Mutarotation

Fischer Projection (or Fischer-Tollens Projection)

A projection formula in which vertically drawn bonds are considered to lie below the projection plane and horizontal bonds to lie above that plane. Thus for the molecule Cabcd:

Fischer-Rosanoff Convention (or Rosanoff Convention)

An arbitrary convention according to which (+)-glyceraldehyde, now known to be (R)-2,3-dihydroxypropanal, was named D-glyceraldehyde (with the enantiomer L-glyceraldehyde and its racemate DL-glyceraldehyde) and taken to have the absolute configuration represented by the Fischer projection formula shown below.

D-glyceraldehyde

L-glyceraldehyde

The atom numbered 1 according to normal nomenclature rules is conventionally placed at the top of the main chain, which is drawn vertically and other groups are drawn on either side of that main chain. The convention is still in use for -amino acids and for sugars; see 'Nomenclature and Symbolism for Amino Acids and Peptides, Recommendations 1983', Pure Appl. Chem. 56, 595-624 (1984); and 'Tentative Rules for Carbohydrate Nomenclature', Eur. J. Biochem. 21, 455-477 (1971).

Flagpole See bowsprit, flagpole.

Free Rotation, Hindered Rotation, Restricted Rotation

In a stereochemical context the rotation about a bond is called "free" when the rotational barrier is so low that different conformations are not perceptible as different chemical species on the time scale of the experiment.

The inhibition of rotation of groups about a bond due to the presence of a sufficiently large rotational barrier to make the phenomenon observable on the time scale of the experiment is termed hindered rotation or restricted rotation.

Gauche

Synonymous with a synclinal alignment of groups attached to adjacent atoms. See also torsion angle.

Gauche Effect

1. The stabilisation of the gauche (synclinal) conformation in a two carbon unit bonded vicinally to electronegative elements e.g. 1,2-difluoroethane.

2. The destabilisation of the gauche (synclinal) conformation in a two carbon unit bonded vicinally to large, soft and polarisable elements such as sulfur and bromine.

Geometric Isomerism (usage strongly discouraged)

Obsolete synonym for cis-trans isomerism.

Half-chair

The conformation of a six-membered ring structure in which four contiguous atoms are in a plane and the other two atoms lie on opposite sides of the plane is called a half-chair. See also chair, boat, twist.

Handedness

This term has been used in two ways, either chirality or chirality sense.

Helicity

The chirality of a helical, propeller or screw-shaped molecular entity. A right-handed helix is described as P (or plus), a left-handed one as M (or minus).

(M)-hexahelicene

The application of this system to the description of conformations considers the torsion angle between two specified (fiducial) groups that are attached to the atoms linked by that bond. The sign of the smaller torsion angle between the fiducial groups defines the chirality sense of the helix. Rules for the selection of fiducial groups according to priority are given by R.S. Cahn, C.K. Ingold and V. Prelog, Angew. Chem. 78, 413-447 (1966), Angew. Chem. Internat. Ed. Eng. 5, 385-415, 511 (1966). See also axial chirality; (delta), (lambda).

Heterotopic See stereoheterotopic.

Hindered Rotation See free rotation, hindered rotation, restricted rotation.

Homochiral See enantiomerically pure/enantiopure.

Homomorphic

Superposable ligands are called homomorphic.

Homotopic

Atoms or groups of a molecule which are related by an n-fold rotation axis (n = 2, 3, etc) are called homotopic. For example, chiral tartaric acid (C₂ axis), chloroform (C₃ axis) and cyclohexaamylose (-cyclodextrin, C₆ axis) have respectively two homotopic carboxyl groups, three homotopic chlorine atoms and six homotopic D-glucose residues. See prochirality.

In-Out Isomerism

Isomerism found in bicyclic systems having long enough bridges to allow the bridgehead exocyclic bond or lone pair of electrons to point either inside the structure or outside.

Induction, Stereochemical See asymmetric induction.

Internal Compensation (usage strongly discouraged) See meso-compound.

Inversion

1. See Walden inversion.

2. See pyramidal inversion.

3. See ring inversion.

4. A symmetry operation involving a centre of inversion (i ).

Isomer

One of several chemical species (or molecular entities) that have the same stoichiometric molecular formula but different constitutional formulae or different stereochemical formulae and hence potentially different physical and/or chemical properties.

Isomeric

Adjective derived from isomer.

Isomerism

The relationship between isomers.

Isometric, Anisometric

Two molecular entities that are superposable or can be made superposable by reflection of one of them in a mirror are isometric; otherwise they are anisometric.

Isotopomers

Isomers having the same number of each isotopic atom but differing in their positions. The term is a contraction of isotopic isomer.

Isotopomers can either be constitutional isomers (e.g. CH₂DCHO and CH₃CDO) or isotopic stereoisomers (e.g. (R)- and (S)-CH₃CHDOH).

Kinetic Resolution

The achievement of partial or complete resolution by virtue of unequal rates of reaction of the enantiomers in a racemate with a chiral agent (reagent, catalyst, solvent, etc.).

L See Fischer-Rosanoff Convention.

l

1. An obsolete prefix for (-)-, see d, l, dl.

2. See l,u for like and unlike diastereoisomers.

l, u

Stereodescriptors of diastereoisomers with two chirality elements. When the two chirality elements are both R or both S the molecular entity is described as having an l relationship (l = like), when one chirality element is R and the other S the molecular entity is described as having a u relationship (u = unlike). The notation can be extended to molecules with more than two chirality elements whereby the l,u relationships of the chiral elements are considered in pairs. See V. Prelog and G. Helmchen, Angew. Chem. 94, 614-631 (1982), Angew. Chem. Internat. Ed. Eng. 21, 567-583 (1982).

(Lambda) See (delta).

M See helicity.

meso

A prefix to indicate the presence of a symmetry element of the second kind (see chirality) in a chemical species which is usually one of a set of diastereoisomers that also includes a chiral member. See meso-compound.

meso-Compound

A term for the achiral member(s) of a set of diastereoisomers which also includes one or more chiral members. For example:

Minus See plus, minus

Molecular Entity

Any chemically or isotopically distinct atom, molecule, ion, ion pair, radical, radical ion, complex, conformer etc., which can be identified as a separately distinguishable entity.

It depends on the context with what degree of precision a molecular entity is to be described. For example 'hydrogen molecule' is an adequate definition of a certain molecular entity for some purposes, whereas for others it is necessary to distinguish the electronic state and/or vibrational state and/or nuclear spin, etc. of the hydrogen molecule.

['Glossary of Terms Used in Physical Organic Chemistry', Pure Appl. Chem. 66, 1077-1184 (1994)].

Mutarotation

The change in optical rotation accompanying epimerisation. In sugar chemistry this term usually refers to epimerisation at the hemiacetal carbon atom.

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