Week 5 (Carbohydrate Monomers)

Question 1

Why are carbohydrates important?

Answer 1

• distributed widely in nature
• key intermediates of metabolism (sugars)
• prove a huge range of cellular markers:

Polysaccharides are centrally involved in cell-cell recognition (how one type of cell distinguishes itself from another)

Small polysaccharide chains, covalently bound by glycosidic links to hydroxyl groups on proteins (glycoproteins) act as biochemical markers on cell surfaces, determining things such as blood type

Question 2

What is an oligosaccharide?

What is a polysaccharides?

Answer 2

They are made up of several units

They are made up of many units

Question 3

What is an Aldose?

Answer 3

Contains an aldehyde

Question 4

What is a ketose?

Answer 4

A carbohydrate containing a ketone

Question 5

What is a chiral centre?

Answer 5

A carbon atom with 4 different groups attached

Question 6

What are the root names of sugars with varying numbers of carbon atoms?

Answer 6

3 carbons= triose

4 carbons= tetrose

5 carbons= pentouse

6 carbons= hexose

7 carbons= heptose

Question 7

What are enantiomers?

Answer 7

Mirror images

Question 8

Transposing any 2 of the groups on a chiral carbon results in a different compound with different physical properties: How do you work out the number of isomers?

Answer 8

2ⁿ Where n=the number of asymmetric carbons

Question 9

What does ‘D’ (dextrorotatory) mean?

Answer 9

The plane has been rotated to the right

Question 10

Do enantiomers have identical physical properties (e.g boiling point)?

Answer 10

Yes

Question 11

What are epimers?

Answer 11

Sugars differing at a single asymmetric centre

Question 12

What are enantiomers?

Answer 12

Sugars differing at all of their asymmetric (chiral) centres are mirror images

Question 13

What are diasterioisomers?

Answer 13

Sugars differing at one or more chiral centre and which are not enantiomers (mirror images)

Question 14

Do Diasterioisomers have the same physical properties?

Answer 14

No

Question 15

What does an aldehyde and a alcohol react to produce?

Answer 15

A hemiacetal (There is an equilibrium set up)

Question 16

What does a ketone and an alcohol react to produce?

Answer 16

A hemiketal (There is an equilibrium set up)

Question 17

What is the 5 membered ring called?

Answer 17

Furan (Furanses)

Question 18

Why are most sugars optically active?

Answer 18

They can rotate the plane of a beam of polarised light

Question 19

What does ‘L’ (Levorotatary) mean?

Answer 19

The plane has been rotated to the left

Question 20

Why does ‘D’ or ‘L’ not define the absolute stereochemistry at the chiral centre?

Answer 20

It is wavelength dependent; material could be dextrorotatory at a certain angle at one wavelegnth but something completely different (other direction or different angle) at a different wavelength

Identify if the sterocenter is ‘R’ or ‘S’ (to define the absolute stereochemistry)

Question 21

When do D and L-glyceraldehyde (enantiomers) not have identical chemical properties?

Answer 21

When exposed to a chiral environment i.e. the optical rotation instrument/ polarimeter (e.g. they have equal but opposite specific rotations)

Question 22

Do enantiomers have the same physical properties?

Answer 22

Yes

Question 23

How are molecules in simplified Fisher projections depicted?

Answer 23

• The oxidised group (CHO- aldehyde/ketone) is at the top
• Vertical substiutents away
• Horizontal substiutuents towards
• CH₂OH group at the bottom

Question 24

The enantiomers for each molecule in the D series are in which series?

Answer 24

The L series

Question 25

Aldehyde + alcohol →

Answer 25

Hemiacetal

Question 26

Ketone + alcohol →

Answer 26

Hemiketal

Question 27

How can carbohydrates form the equivalent of a Hemiketal or a hemiacetal?

Answer 27

Carbohydrates that have aldehyde or ketone functional groups and lots of hydroxy groups can form a hemiketal or a hemiacetal if the ring size is correct because one of the hydroxy group can bite round onto the aldehyde or ketone to produce the cyclic compound.

Question 28

What the the 6 membered ring called?

Answer 28

Pyran (Pyranoses)

Question 29

How are hemiacetal rings created?

Answer 29

By intermolecular nucleophilic addition reactions (Linear to cyclic form)

Question 30

Which hemiacetal rings are the most stable?

Answer 30

Five and six membered rings

(Three, four, seven and eight membered rings provide too much strain)

Question 31

What does the formation of the cyclic hemiacetal create?

Answer 31

An additional chiral centre giving two diasterioisomeric forms designated alpha and beta ( these diastereomers are called anomers)

Question 32

If it’s the alpha anomer which direction does the OH (hydroxyl group) point?

Answer 32

Down (New hydroxyl is below the plane of the ring)

Question 33

If it’s the beta anomer which direction does the OH (hydroxyl group) point?

Answer 33

Up (New hydroxyl above the plane of the ring)

Question 34

Where is the intramolecular chiral centre formed in a hemiketal?

Descibe the change in shape that occurs

Answer 34

It’s formed at what was the C=O

(From sp2 Hybridized trigonal planar to sp3 hybridized tetrahedral)

Question 35

What is mutarotation?

Answer 35

Mutarotation is occurs when the hydroxyl group at the anomeric position (C-1) switches configuration between alpha and beta

In water alpha and beta D- glucose interconvert via the open chains form to give an equilibrium mixture

Originally detected by changes in optical rotation

(Cyclisation of Glucose to Glucopyranose and mutarotation of alpha- and beta-D-glucose)

Question 36

What and where are carbohydrates joined to alcohols and amines?

Answer 36

At the anomeric centre by glycosidic bonds

Question 37

What kind of glycosidic bonds are in DNA?

Answer 37

N- glycosidic bonds

This is when an anomeric carbon of a sugar is linked to the nitrogen of an amine by an N-glycosidic bond e.g. adenosine

Question 38

What is sucrose made from?

Answer 38

Sucrose (table sugar): disaccharide from two monosaccharides (glucose linked to fructose)

Question 39

Describe Fischer Projections

Answer 39

Question 40

Explain L and D configurations

Answer 40

The configuration is either D- or L depending upon the position of the hydroxyl (left or right) on the penultimate carbon.

The central carbon is asymmetric.

Question 41

Why are many sugars described as opticaly active?

Answer 41

can rotate the plane of a beam of polarised light

Question 42

What is Specific alpha rotation?

What needs to be defined?

Answer 42

Specific rotation, [α], is a fundamental property of chiral substances that is expressed as the angle to which the material causes polarized light to rotate at a particular temperature, wavelength, and concentration

rotation of a beam of polarised light by 100 g of solute in 100 mL of solvent with a path length of 10 cm.

(Need to define solvent, wavelength and temperature.)

Question 43

D- and L-glyceraldehyde have identical physical properties except when:

Answer 43

exposed to a chiral environment eg they have equal but opposite specific rotations

Question 44

4 asymmetric centers: How many sterioisomers

Answer 44

Question 45

What do Pentoses and hexoses cyclise to form?

Answer 45

Furanose and pyranose rings

Question 46

Describe the formation of Hemiacetala and Hemiketal

Answer 46

Question 47

Describe the cyclisation of fructose to fructofuranose

Answer 47

Question 48

What are the 4 compounds arising fro cyclisation of D-Fructose

Answer 48

This molecule can form α or β, 5 or 6 membered rings

Question 49

What kind of bonds are Sugars linked to each other by?

Answer 49

Sugars can be linked to each other by O-glycosidic bonds to form disaccharides and polysaccharides eg sucrose, maltose & lactose.