2. INTRODUCTION
• Chiral carbon atoms - carbon atom that is
bonded to four different groups
– which bring about two distinct spatial
arrangement of molecule, called
enantiomers.
Similarity
• Enantiomers are mirror image of each other,
have identical physical and chemical
properties
3. Differences
One of them will rotate the plane of polarization of
plane polarized light in a clockwise direction
This enantiomer is said to be dextrorotatory (+ or d)
and has a positive specific rotation value.
Another enantiomer will rotate the plane of
polarization of the plane polarized light in an
anticlockwise direction.
It is said to be laevorotatory (- or l) where it has
negative specific rotation value.
4. F9.1: Explain the three different
conventions used for naming the different
enantiometric forms
1) D, L notation
• An older convention used for sugar and amino
acids.
• The absolute configuration of other sugars about
each chiral centre is then named by analogy:
o If the C=O pointed away, the –OH group on the right then
it is a D-isomer.
o And if the C=O pointed towards you , the –OH group on
the left then it is a L-isomer
5. • The D,L system is also applied to amino acids.
• Using the “CORN” rule, the “COOH”, “R-side
chain group”, “NH2” are around the Carbon atom.
• CORN rule:
o Molecule is viewed with the C-H bond pointed away from
the observer (same as in the R,S system)
o Made up of COOH, R and NH2 arrange around the carbon
atom.
If it is arranged clockwise – D-form
If anticlockwise – L-form
6. • Almost all naturally occurring amino acids are the
L-form and are usually tasteless.
• Synthetic D-amino acids taste sweet and most
naturally occurring sugars are D-form.
• Enantiomers that does do not occur naturally cannot
be metabolized by our body.
• SUGARS contain an aldehyde or ketone, and all
carbons which do not have it is attached to an
alcohol.
• Amino acids contain carboxylic acid (COOH),
Hydrocarbon chain (R) and amino group (NH2)
7. 1) R,S system (CIP system) Cahn, Ingold & Prelog
• Used for other groups of compounds.
• Involve 3 steps:
i. Atom bonded to the chiral carbon are ranked in order of
increasing atomic number – most common would be
(H<C<N<O<F<Cl<Br)
ii. When 2 @ more atoms have the same atomic no. (eg: -CH3
& -COOH) , the 2nd atoms are used to rank the substituents
(so –COOH >CH3, since atomic no of 3xH < O) . And if the
2nd atom is also the same, then move on the next atom and
so on. (remember that double bond count as double -
COOH = 3O, since one of O have double bonds.
iii. View the double molecule with the lowest ranking is
pointing away from you, other three substituent must either
decrease in order in a clockwise direction (R-enantiomer)
@ in an anti clockwise direction (S-enantiomer)
8. F9.2: DISTINGUISH BETWEEN THE
PROPERTIES OF ENANTIOMERIC FORMS
OF STEREOISOMERS FOUND IN FOOD
9. • Enantiomers may be similar but different enantiomeric
forms of molecule have different taste smells and
toxicity.
• This is because biological systems are much more
sensitive to the shape of molecule
• E.g.:
Carvone:
R-form= Laevorotatory= has smell and flavour of spearmint.
S-form= Dextrarotatory= has the smell and flavour of
caraway seeds.
Limonene:
+(d) = Smells of oranges
-(l)= Smells of lemons
10. • Sometimes, a natural flavour is a pure enantiomer,
because it is biosynthesis tends to stereo specific.
• Synthetic equivalence is often a racemix mixture
mixture of equal amounts of the enantiomers .
(easier to synthesis)
• E.g.: alphaionone in raspberries.
• As for toxicity it can varies tremendously
between the different enantiomer, as become
tragically apparent in the Thalidomide disaster.
11.
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