1. Chapter 17 in Green / Damjii
Food Chemistry
Chapter 17 in Green / Damjii

2. F.9: Texture Homework Read F9– Texture pp. 488-490 Do Qs 43-50
on p 494

3. F.9.1: Explain the three different conventions used for naming the different enantiomeric forms.

4. Types of Isomerism – II cont.
Stereoisomers – Enantiomers
(aka Optical Isomers)
Requires a molecule with a chiral center
An atom (usu C) with four different groups attached to it
aka an asymmetric carbon atom
Two forms are mirror images of one another that cannot be superimposed on each other (like gloves)

5. achiral
chiral
24.2

7. Types of Isomerism – II cont.
Stereoisomers – Enantiomers
Common example are amino acids
only glycine does not have an enantiomeric form

8. Types of Isomerism – II cont.
Stereoisomers – Enantiomers
Common example are amino acids
only glycine does not have an enantiomeric form - why?

10. Types of Isomerism – II cont.
Thalidomide
a drug once prescribed to counteract pregnancy-related morning sickness, is an effective sedative as the R enantiomer (left side), but the S enantiomer (right side) is a potent teratogen (it causes birth defects).
Source -

11. Types of Isomerism – II cont.
Stereoisomers – Enantiomers
have different effects on polarised light
said to be optically active in a polarimeter
if a mixture has both enantiomers it is said to be a racemic mixture and is not optically active

13. F.9.1: Explain the three different conventions used for naming the different enantiomeric forms.
dextrorotary [ + or (d) ] – clockwise rotating enantiomer; positive specific rotation value
laevorotatory [ - or (l) ] – counter clockwise rotating enantiomer; negative specific rotation value
Does differentiate between the 2 enantiomers… but does NOT indicate absolute configuration (spatial arrangement).

14. F.9.1: Explain the three different conventions used for naming the different enantiomeric forms.
CONVENTION 2: D, L (no relation to d and l )
older convention… used mostly for sugars and amino acids
Most naturally occurring sugars are in the D-form ! (and taste sweet)
Most amino acids are in the L-form ! (and are tasteless)
Enantiomers that don’t occur naturally are typically NOT metabolized by our bodies.

15. F.9.1: Explain the three different conventions used for naming the different enantiomeric forms.
CONVENTION 2: D, L (no relation to d and l )
For sugars… absolute configuration related to glyceraldehyde
Locate chiral center
Orient aldehyde group away from you
If OH is on right, the sugar is D-
If OH is on left, the sugar is L-
Most naturally occurring sugars are in the D-form ! (and taste sweet)

16. F.9.1: Explain the three different conventions used for naming the different enantiomeric forms.
CONVENTION 2: D, L (no relation to d and l )
For amino acids… use “CORN” rule
Locate chiral center and orient C-H bond away from you
If the groups COOH, R, NH2 are arranged clockwise around the chiral carbon the amino acid is the D-form
If the groups COOH, R, NH2 are arranged counterclockwise around the chiral carbon the amino acid is the L-form
Most amino acids are in the L-form ! (and are tasteless)

17. F.9.1: Explain the three different conventions used for naming the different enantiomeric forms.
CONVENTION 3: R, S
Newer convention… used for most other compounds
aka CIP system – named after originators: Cahn, Ingold, and Prelog

18. F.9.1: Explain the three different conventions used for naming the different enantiomeric forms.
SYSTEM 3: R,S
Locate chiral center
Rank atoms bonded to the chiral carbon in order of increasing atomic number. (One through four, one being the group of highest priority… highest atomic number).
H < C < N < O < F < Cl < Br
If there are two or more atoms with same atomic number the second atoms are used to rank the substituents… then the third… etc.
Orient the molecule so that the lowest ranking (4) substituent points away from you.
If the other three substituents decrease in a clockwise direction, it is the R-enantiomer
If the other three substituents decrease in a counterclockwise direction, it is the S-enantiomer

19. F.9.1: Explain the three different conventions used for naming the different enantiomeric forms.
Orient the molecule so that the lowest ranking substituent points away from you.
If the other three substituents decrease in a clockwise direction, it is the R-enantiomer
If the other three substituents decrease in a counterclockwise direction, it is the S-enantiomer

20. F.9.1: Explain the three different conventions used for naming the different enantiomeric forms.2 2 4 4 3 3 1 1
Orient the molecule so that the lowest ranking substituent points away from you.
If the other three substituents decrease in a clockwise direction, it is the R-enantiomer
If the other three substituents decrease in a counterclockwise direction, it is the S-enantiomer

21. F.9.1: Explain the three different conventions used for naming the different enantiomeric forms.
(S)-(+)-lactic acid (left) and (R)-(–)-lactic acid (right)

22. F.9.1: Explain the three different conventions used for naming the different enantiomeric forms.
Enantiomers of citalopram (anti-depressant).
The top is (S)-citalopram
The bottom is (R)-citalopram 2 4 1 1 4

23. F.9.1: Explain the three different conventions used for naming the different enantiomeric forms.3
This molecule (on left side)
needs to be re-oriented so the C-H bond is facing away from us 2 1 1 4 4 2 3
Enantiomers of mecoprop, 2-(4-chloro-2-methylphenoxy)propanoic acid – a herbicide

24. F.9.2: Distinguish between the properties of the different enantiomeric forms of stereoisomers found in food.
Natural flavors tend to be pure enatiomers – biosynthesis is stereospecific
Synthetic flavors tend to be racemic mixtures
Alpha-ionone (found in raspberries)
Natural = R-alpha-ionone

25. Stereoisomers – Enantiomers
Usually have similar physical properties
except when they interact with other optically active substances – which happens often in the human body
Usually have similar chemical properties
general, only one enantiomer of a drug, agrochemical (herbicide, pesticide), flavoring agent, or other molecule (when asymmetric) has the desired biological effect, while the other enantiomer has very different effects or, at least, places a metabolic burden on the body.

26. Types of Isomerism – II cont.
amino acid asparagine - taste
one form tastes bitter, the other tastes sweet
each of the two enantiomers binds differently to chemoreceptors in the tongue.
Source -

27. Carvone forms two mirror image forms or enantiomers:
R-(–)-carvone smells like spearmint
(happens to be laevorotary)
S-(+)-carvone, smells like caraway seeds.
each of the two enantiomers binds differently to chemoreceptors in the nose

28. Limone forms two mirror image forms or enantiomers:
R + (d) enantiomer - smells like orange
S – (l) enantiomer - smells like lemon

29. (R)-(+)-(E)-alpha-ionone
violet-like
fruity
raspberry-like
flowery
( Yamamoto et al., 2009)
(S)-(-)-(E)-alpha-ionone –
woody
cedar wood like
fresh juicy greenish flavor(aroma)
( Yamamoto et al., 2009)