(À). This D and L system is common in biology/biochemistry, especially
with sugars and amino acids (with amino acids, the –NH 2 configuration is
compared with the –OH of glyceraldehyde). This system is particularly used
to designate various carbohydrate or sugar molecules, e.g. D-glucose, Lrhamnose and L-alanine.
(R) and (S) system (Cahn–Ingold–Prelog system) Three chemists, R. S.
Cahn (UK), C. K. Ingold (UK) and V. Prelog (Switzerland), devised a
system of nomenclature that can describe the configuration of enantiomers
more precisely. This system is called the (R) and (S) system, or the Cahn–
Ingold–Prelog system.
According to this system, one enantiomer of 2-hexanol should be
designated (R)-2-hexanol, and the other (S)-2-hexanol. R and S came from
the Latin words rectus and sinister, meaning right and left, respectively.
CH 3
H
O
H
CH 2 CH 2 CH 3
CH 3
OH
H
CH 2 CH 2 CH 3
*
*
1
2
3
4
5
Enantiomers of 2-hexanol
1
2
3
4
5
The following rules or steps are applied for designating any enantiomer as R
or S.
(a) Each of the four groups attached to the chiral carbon is assigned 1–4
(or a–d) in terms of order of priority or preference, 1 being the highest
and 4 being the lowest priority. Priority is first assigned on the basis of
the atomic number of the atom that is directly attached to the chiral
carbon.
Higher atomic number gets higher priority. This can be shown by the
structure of 2-hexanol.
CH 3
H
O
H
CH 2 CH 2 CH 3
*
1
2
3
4
Priority determination in 2-hexanol
(b) When a priority cannot be assigned on the basis of the atomic numbers
of the atoms that are directly attached to the chiral carbon, the next set
of atoms in the unassigned groups is examined. This process is
continued to the first point of difference.
In 2-hexanol, there are two carbon atoms directly attached to the
chiral carbon; one is of the methyl group, and the other is of the propyl
3.2 ISOMERISM
47
with sugars and amino acids (with amino acids, the –NH 2 configuration is
compared with the –OH of glyceraldehyde). This system is particularly used
to designate various carbohydrate or sugar molecules, e.g. D-glucose, Lrhamnose and L-alanine.
(R) and (S) system (Cahn–Ingold–Prelog system) Three chemists, R. S.
Cahn (UK), C. K. Ingold (UK) and V. Prelog (Switzerland), devised a
system of nomenclature that can describe the configuration of enantiomers
more precisely. This system is called the (R) and (S) system, or the Cahn–
Ingold–Prelog system.
According to this system, one enantiomer of 2-hexanol should be
designated (R)-2-hexanol, and the other (S)-2-hexanol. R and S came from
the Latin words rectus and sinister, meaning right and left, respectively.
CH 3
H
O
H
CH 2 CH 2 CH 3
CH 3
OH
H
CH 2 CH 2 CH 3
*
*
1
2
3
4
5
Enantiomers of 2-hexanol
1
2
3
4
5
The following rules or steps are applied for designating any enantiomer as R
or S.
(a) Each of the four groups attached to the chiral carbon is assigned 1–4
(or a–d) in terms of order of priority or preference, 1 being the highest
and 4 being the lowest priority. Priority is first assigned on the basis of
the atomic number of the atom that is directly attached to the chiral
carbon.
Higher atomic number gets higher priority. This can be shown by the
structure of 2-hexanol.
CH 3
H
O
H
CH 2 CH 2 CH 3
*
1
2
3
4
Priority determination in 2-hexanol
(b) When a priority cannot be assigned on the basis of the atomic numbers
of the atoms that are directly attached to the chiral carbon, the next set
of atoms in the unassigned groups is examined. This process is
continued to the first point of difference.
In 2-hexanol, there are two carbon atoms directly attached to the
chiral carbon; one is of the methyl group, and the other is of the propyl
3.2 ISOMERISM
47