B: Stereochemistry HL

Question 1

Polysaccharides - Starch

Answer 1

• Polymer of a-glucose sugars -(condensation)
• mixture of a-amylose and amylopectin
  
  • a-amylose - a-glucose with a-1,4,-glycosidic links
  • amylopectin - branched polymer of a-glucose with a-1,4,- and a-1,6-glycosidic links
  • more space between molecules –> not as strong IMF
• Helical
• not fibrou, not strong
• soluble in hot water

Question 2

Polysaccharides - Cellulose

Answer 2

• polymer of b-glucose sugars -(condensation)
• herbivores survive on rich cellulose diets such as grass
• provides structure and strength to plants
• b-1,4-glycosidic links
  
  • linear chains, no branching –> strong IMF between the chains, OH groups form H bonds
    
    • insoluble
• Humans cannot digest cellulose as we don’t possess the cellulase enzyme
  
  • major component of dietary fibre
    
    • prevents constipation, hemorrhoids

Question 3

Hydrogenated Fats

Answer 3

• cis C=C are much more common naturally than trans C=C
  
  • cis C=C produce kinks reducing IMFs and are liquids at room temp

C=C + H₂ —^{_{Ni 150degrees}}–> -C-C- ^{_{hydrogenated fat}}

• hydrogenated fats have higher melting points because of a higher degree of saturation (Mr increases)
  
  • fats like margarine are produced

Question 4

Advantages - Hydrogenated Fats

Answer 4

• cheaper than animal fats
• increased hardness of products
• fewer C=C –> reduction in oxidative and hydrolytic rancidity

Question 5

Disadvantages - Hydrogenated Fats

Answer 5

• Saturated fats are more damaging to the heart and circulatory system
• cis fatty acids can isomerise into the trans form - implicated in cardiovascular diseases

Question 6

Vision Chemistry

Answer 6

• in the retina there are rod cells (light intensity) and cone cells (color determination)
• these cells have a large no. of protein rhodopsin on the surface
  
  • consists of the protein opsin bonded to a moldecule of cis-retinal

Question 7

cis/trans-retinal

Answer 7

• conjugated system –> π electrons in P_z orbitals delocalized –> absorbs light
• retinal changes to the trans form –> changes the conformation of the protein which triggers a series of events that result in a signal being sent tot he brain
• once the signal has been sent, the trans-retinal molecules converted back to cis-retinal by an enzyme

Question 8

a and B sugars

Answer 8

• when a sugar cyclises, an extra chiral center is formed the O on carbon 5 can either attack the C=O group from above or below the plane of the group
  
  • alpha - if the OH is on the opposite side to carbon 6/ below the ring
  • beta - if the OH is on the same side as carbon 6 /above the ring

Question 9

D and L Optical Isomerism - Amino Acids

Answer 9

• All amino acids are chiral except glycine (only has 3 different groups)
• only the L-configuration is found in proteins

_C.O.R.N.

If moving from the COOH to R to NH₂ is

• clockwise it is dextrorotatory
• anticlockwise it is lavorotatory

Question 10

D and L Optical Isomerism - Carbohydrates

Answer 10

• If the OH group on the chiral carbon furthest away from the carbonyl group is on the right, the molecule is dextrarotatory
• if the OH group is on the left, it is lavorotatory
• D sugars are much more common htan L sugars in nature