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Food chemistry

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  • 1. Azhar & Aizat MD63 FOOD CHEMISTRYGuided Question (F9): Stereochemistry in Food1) Give definition of these terms: i. Stereochemistry ii. Enantiomers iii. Racemic Mixture iv. Dextrorotary v. Laevorotatory2) There are 3 different conventions for naming enantiomers, name all three of them, give examples for each of them and draw their structures.3) Explain the difference between the (+)/(-) or d-./l- system and the D and L notation?4) D and L system is used for amino acids and carbohydrates. Distinguish between these two.5) Define and explain the third convention for naming enantiomers which is the R and S notation.6) Explain the step involve in naming using the R and S sytem.7) Although enantiomers may be similar, they have different effect on the biological systems since it is more sensitive towards the shape of the molecule. Give three biological effects that these enantiomers have.8) Give the taste and smell that these type of enantiomers produced: i) R-form: ii) S-form: iii) +(d): iv) –(l):
  • 2. Azhar & Aizat MD63 FOOD CHEMISTRYMarkscheme (F9): Stereochemistry in Food 1) Definition: i. Stereochemistry The branch of chemistry that deals with the spatial arrangement of atoms in molecules and the effects of these arrangements on physical and the chemical properties. ii. Enantiomers A pair of mirror image molecules that are not superimposeable. iii. Racemic Mixture A mixture containing equal amount of the two enantiomers and has no rotation value iv. Dextrorotary {+ or (d)}: clockwise rotating enantiomers v. Laevorotatory {- or (-)}: anticlockwise rotating enantiomers 2) There are 3 different conventions for naming enantiomers, name all three of them, give examples for each of them and draw their structures. i. D and L notation ii. d-form and l-form iii. R and S system 3) Explain the difference between the (+)/(-) or d-./l- system and the D and L notation? The D and L system naming is completely unrelated to the lower case of d and l which represent dextrorotary and laevorotatory structures.
  • 3. Azhar & Aizat MD63 4) D and L system is used for amino acids and carbohydrates. Distinguish between these two. Carbohydrate/Sugar Amino acid - Contains and aldehyde or ketone - Contain carboxylic acid (COOH), carbon - All carbon which do not have an aldehyde chain (R) and amino group(NH2) or ketone have an alcohol attached - if –OH group is on the right, then it is a D- - if the groups COOH, NH2 and R chain are isomer arranged clockwise around the chiral - if –OH group is on the left, then it is a L- carbon, then it is a D-isomer. isomer - if the groups COOH,R chain and NH2 are arranged anti-clockwise around the chiral carbon, then it is the L-isomer. - Most naturally occurring sugar is D-isomer - Naturally occurring amino acid are the L- and it tastes sweet form and they are tasteless. Examples : Glyceraldehyde Examples: alanine 5) Define and explain the third convention for naming enantiomers which is the R and S notation. - R and S system can also be defined as CIP system which is after the name Cahn, Ingold and Prelog who proposed it. - The R and S terminology derives from the Latin “rectus” for right or clockwise and “sinister” for left or anticlockwise. 6) Explain the step involve in naming using the R and S sytem. Step 1: The atoms bonded to the chiral carbon are ranked in order of increasing atomic number. Step 2: When atoms have the same atomic number, the second atoms are used for rankings and if those are the same, the sums on the third are used. Step 3: When ranking, a double bond counts for double the atomic mass. 7) Although enantiomers may be similar, they have different effect on the biological systems since it is more sensitive towards the shape of the molecule. Give three biological effects that these enantiomers have. i. Smells ii. Toxicity iii. Tastes 8) Give the taste and smell that these type of enantiomers produced: i) R-form: smell and flavour of spearmint. ii) S-form: smell and flavour of caraway seeds. iii) +(d): smell of oranges iv) –(l): smell of lemons