Organic chemistry and Biological chemistry for Health Sciences
59-191
Lecture 14
Synthesis of amides:
Amides can be made from amines by acyl group transfer reaction from acid derivatives.
Either acid chloride or acid anhydride reacts smoothly with amonia or amines to give
amides.
In the body, other kinds of acyl group carrier molecules serve instead of ordinary acid
chlorides and anhydrides as sources of the acyl group. When proteins are made from
amino acids, for example, the acyl portion of amino acids-they are called aminoacyl
units-are held by carrier molecules.
When a cell makes an amide bond, it transfers an aminoacyl group from its carrier
molecule to the nitrogen atom of the amino group. The carrier molecule is released to be
reused.
CARBOHYDRATES:
Monosaccharides:
Monosaccharides or simple sugars are carbohydrates that cannot be hydrolyzed. The
names of monosaccharides end in –ose. The monosaccharides, which contain aldehyde
group, are called aldose and those containing ketone groups are called ketose.
Monosaccharides (aldose or ketose) with three carbons are called triose, those with four
carbons tetraose and so on. Two trioses, glyceraldehyde and dihydroxyacetone, occur in
metalbolism. Two pentoses, ribose and deoxyribose, are needed by nucleic acids.
The nutritionally important monosaccharides are all hexoses. Glucose is a hexose with an
aldehyde group, called aldohexose. Galactose is a stereoisomer of glucose is also an
aldohexose. Fructose is a ketohexose and a constitutional isomer of glucose and
galactose.
Disaccharides are carbohydrates that can be hydrolyzed to two monosaccharides.
F.example sucrose (table sugar), maltose, and lactose .
Polysaccharides can be hydrolyzed in water to give hundreds of monosaccharides. F.exp.
starch, cellulose.
Carbohydrates that give positive Tollen’s test and positive Benedict’s test are called
reducing sugars. All monosaccharides and nearly all disaccharides are reducing sugars.
But sucrose is not a reducing sugar and neither the polysaccharides.
All aldohexoses, including glucose, have 2,3,4,5,6-pentahydroxyhexanal structure.
Carbons 2,3,4, and 5 in the glucose chain are all tetrahedral stereocenters. Each center has
unique set of four different groups. Those carbons are also called chiral carbon.
Compounds having chiral carbon are optically active. They can rotate plane polarized
light. The degree of rotation of the plane of plane-polarized light caused by an optically
active solution is called optical rotation. It is usually expressed by using the symbol .
The number of stereoisomers of a compound whose molecules have n differnent
tetrahedral stereocenters is 2n. In 2,3,4,5,6-pentahydroxyhexanal, n equals four, so there
must be 24 or 16 stereoisomers, occuring as eight pairs of enantiomers. Glucose is one of
those 16; galactose is another.
Enantiomers are stereoisomers whose molecules are related as an object is related to its
mirror image but that cannot be superimposed. Stereoisomers whose molecules are not
related this way are called diestereomers.
Absolute configurations of the enantiomers of glyceraldehydes are used to devise
configurational or optical families for the rest of the carbohydrates. Any compound that
has a configuration like that of (+) glyceraldehyde (D-glyceraldehyde) is said to be in Dfamily.
When the molecules of a compound are the mirror images of an enantiomer in the Dfamily, the compound is in the L-family.
The letters D and L are only family names. They have nothing to do with actual
signs of the values of their specific optical rotation.
When a molecule has many stereocenters, it becomes quite complicated to make a
perspective, three dimensional drawing of an absolute configuration. Emil Fisher, a
chemist devised an easy way to project three-dimensional configuration of each
stereocenter in a molecule onto a plane surface. His structural representations are called
Fisher projection formulas.
The following rules are used to write the fisher projection formulas:

Visualize the molecule with its main carbon chain vertical and with the bonds that
hold the chain together projecting to the rear at each stereocenter. Carbon 1 is at the
top.

Mentallly flatten the structure, stereocenter by stereocenter, onto a plain surface.

In the projected structure, represent each stereocenter either as the intersection of two
lines or conventionally as C.

The horizontal lines at a stereocenter actually represent bonds that project forward,
out of the plane of the paper and the vertical lines represent bonds that project
rearward, behind the plane
A fisher projection formula can have more than one intersection of lines, each
representing a stereocenter. At each stereocenter, a horizontal line is a bond coming
toward you and a vertical line is a bond going away from you.