General Properties, Occurrence, and Preparation
1.2
61
are said to be optically active. It was also recognized that the optical rotation was dependent on
several factors: (1) the structure of the substance; (2) the length of the cell; (3) the concentration of the substance; (4) the wavelength of the plane polarized light; and (5) the temperature.
The following relationship was derived to encompass these variables:
α obs = [α]
t
λ lc
where α obs = the observed optical rotation in degrees, l = the length of the cell holding the
compound in dm (decimeter), c = the concentration of the sample in g mL −1 , usually in water,
[α] λ
t = the specific optical rotation constant of the substance at temperature, t, and wavelength,
λ. Most polarimetric measurements are made with the D-line from a sodium lamp and each
carbohydrate has a characteristic [α] t
D , although the optical rotation can also be measured
continuously as a as a function of the wavelength (i. e., optical rotatory dispersion, ORD).
Carbohydrate molecules with two-fold symmetry about a central point or plane do not rotate
plane polarized light and are said to have a meso-structure.
3 The Structures of Carbohydrates
3.1 The Simplest Carbohydrates
There are three carbohydrates that are the simplest carbohydrates that fulfill the definition
given above. They are the following:
⊡ Scheme 1
The simplest carbohydrates
There are two forms for glyceraldehyde that are distinct and cannot be superimposed onto
each other. Prof. Fischer defined the one with the chiral hydroxyl group to the right as D-glyceraldehyde, where the “D” indicates that the hydroxyl group is to the right or dextro and the
one with the chiral hydroxyl group to the left as L-glyceraldehyde where the “L” indicates that
the hydroxyl group is to the left or levo. It just so happened that for D-glyceraldehyde, plane
polarized light was rotated to the right and L-glyceraldehyde rotated plane polarized light to
the left. This is not always the case. Some carbohydrates with the D-configuration rotate plane
polarized light to the left and some carbohydrates with the L-configuration rotate plane polarized light to the right.
A large majority of the carbohydrates found on the earth belong to the D-family of structural
isomers. In the course of evolution, the reason that the D-family of structural isomers was
1.2
61
are said to be optically active. It was also recognized that the optical rotation was dependent on
several factors: (1) the structure of the substance; (2) the length of the cell; (3) the concentration of the substance; (4) the wavelength of the plane polarized light; and (5) the temperature.
The following relationship was derived to encompass these variables:
α obs = [α]
t
λ lc
where α obs = the observed optical rotation in degrees, l = the length of the cell holding the
compound in dm (decimeter), c = the concentration of the sample in g mL −1 , usually in water,
[α] λ
t = the specific optical rotation constant of the substance at temperature, t, and wavelength,
λ. Most polarimetric measurements are made with the D-line from a sodium lamp and each
carbohydrate has a characteristic [α] t
D , although the optical rotation can also be measured
continuously as a as a function of the wavelength (i. e., optical rotatory dispersion, ORD).
Carbohydrate molecules with two-fold symmetry about a central point or plane do not rotate
plane polarized light and are said to have a meso-structure.
3 The Structures of Carbohydrates
3.1 The Simplest Carbohydrates
There are three carbohydrates that are the simplest carbohydrates that fulfill the definition
given above. They are the following:
⊡ Scheme 1
The simplest carbohydrates
There are two forms for glyceraldehyde that are distinct and cannot be superimposed onto
each other. Prof. Fischer defined the one with the chiral hydroxyl group to the right as D-glyceraldehyde, where the “D” indicates that the hydroxyl group is to the right or dextro and the
one with the chiral hydroxyl group to the left as L-glyceraldehyde where the “L” indicates that
the hydroxyl group is to the left or levo. It just so happened that for D-glyceraldehyde, plane
polarized light was rotated to the right and L-glyceraldehyde rotated plane polarized light to
the left. This is not always the case. Some carbohydrates with the D-configuration rotate plane
polarized light to the left and some carbohydrates with the L-configuration rotate plane polarized light to the right.
A large majority of the carbohydrates found on the earth belong to the D-family of structural
isomers. In the course of evolution, the reason that the D-family of structural isomers was