Carbohydrates

Question 1

A sugar with a carbonyl at one end of the molecule is called a(n) ______; a sugar with a carbonyl on one of the middle carbons is called a(n) ______.

Answer 1

Aldose, ketose

Question 2

How would you number the following Fischer projection?

Answer 2

Question 3

Describe the general process for cyclization

Answer 3

Cyclization of monosacharides occurs via a nuclephilic addition reaction. The 4th or 5th hydroxyl carbon attacks the carbonyl. This forms a hemiacetal or hemiketal.

Question 4

Hemiacetal

Answer 4

Question 5

Hemiketal

Answer 5

Question 6

Furanose

Answer 6

Five membered sugar ring. Furious Five.

Question 7

Pyranose

Answer 7

Six membered sugar ring. Perilous six.

Question 8

Which of the numbered carbons (1-6) is the site of the hemiacetal?

Answer 8

Carbon 1

Question 9

Alpha or Beta? Pyranose or Furanose?

Answer 9

This molecule is a pyranose, because it is made up of a six-membered ring. The anomeric hydroxyl points down, trans to the CH2OH, making it an alpha-pyranose. This sugar is alpha-galactopyranose.

Question 10

Pyranose or Furanose? Hexose or Pentose? Aldose or Ketose?

Answer 10

Furanose, Ketose, Hexose

Question 11

Tollens’ test typically involves exposure of a carbohydrate to a solution of CuO in ammonia (NH3). Glucose yields a positive Tollens’ test, but sucrose does not. Which of the following best explains this fact?

Answer 11

.Glucose contains a hemiacetal group, while sucrose does not; this classifies glucose as a reducing sugar.

Tollens’ test is intended to identify “reducing sugars,” or sugars with the capacity to serve as reducing agents. Specifically, sugars with hemiacetal groups can undergo mutarotation, allowing them to be oxidized by CuO. The process of mutarotation requires ring opening, which occurs at a hemiacetal group. Thus, sugars with hemiacetal groups can be oxidized and can thus function as reducing sugars. We do not need to have the structures of glucose and sucrose memorized to answer this question. Instead, simply note that the question stem tells us that glucose yields a positive Tollens’ test. Glucose must therefore contain a hemiacetal group, whereas sucrose (which gives a negative test result) must not. The hemiacetal group on glucose, shown below, marks it as a classic reducing sugar. Remember, a hemiacetal consists of a carbon atom directly attached to one -OR and one -OH group. The same carbon atom is also attached to a hydrogen atom and an R group.

Question 12

Is this the alpha or beta anomer of glucose?

Answer 12

The anomeric carbon for D-glucose is C1 (the C4 carbon has significance in this passage only because it is the site that has inverted stereochemistry between glucose and galactose). The β-anomer has the hydroxyl in the equatorial position, while the α-anomer has the hydroxyl in the axial position. Here, the hydroxyl group is in the axial position (α-anomer).

Question 13

What is the empirical formula for carbs?

Answer 13

Cm(H2O)n

Question 14

Sugar whose bottomost hydroxyl group is pointing to the right

Answer 14

D-Isomer

Question 15

Identify the following:

Answer 15

Ribose which is a five carbon aldose, otherwise known as an aldopentose. D-Ribose is an easy and useful structure to memorize and be able to recognize on the MCAT, because all of its hydroxyl groups point to the right.

Question 16

At which position do deoxyribose and ribose differ?

Answer 16

C2

Question 17

Identify the following:

Answer 17

Deoxyribose differs from ribose in that it is missing a hydroxyl group at the carbon 2 position.

Question 18

ID the following:

Answer 18

D- glucose

Question 19

ID the following

Answer 19

L-glucose

Question 20

What are the components of sucrose?

Answer 20

Glucose and Fructose

Question 21

What bond links glucose and fructose together in sucrose?

Answer 21

Glycosidic bond

Question 22

During the formation of a glycosidic bond, what happens to the hemiacetal or hemiketal?

Answer 22

It forms an acetal or ketal

Question 23

True or False: glycosidic bonds require at least one anomeric carbon

Answer 23

true

Question 24

True or false: the anomeric carbon of a monosaccharide is always located at the C-1 position.

Answer 24

This statement is false. The anomeric carbon of a monosaccharide is the carbon at which a carbonyl functional group is located. On some sugars, such as aldoses like glucose, this is at the C-1 position. On other sugars, such as ketoses like fructose, this is at the C-2 position.