Cyclic Carbohydrates

Question 1

What structure is formed when an aldehyde reacts with an alcohol?

Answer 1

Hemiacetal

(An aldehyde (C1) and an OH (C5) on the same molecule join together, creating a cyclic structure.)

Question 2

What structure is formed when a ketone reacts with an alcohol?

Answer 2

Hemiketal

(A ketone (C2) and an OH (C5) on the same molecule join together, creating a cyclic structure.)

Question 3

Why is the cyclic form of carbohydrates more stable than the open chain form of carbohydrates?

Answer 3

Because it is lower in energy.

Question 4

What is a pyranose? Furanose?

Answer 4

pyranose: six-membered ring

furanose: five-membered ring

Question 5

Which projections are used to describe the stereochemistry of open chain carbohydrates?

Answer 5

Fischer projections

Question 6

Which projections are used to describe the stereochemistry of cyclic carbohydrates?

Answer 6

Haworth projections

[“Downright”, “Uplefting”. Substituents on the right side of a Fischer projection will point down and those on the left side will point up when converted into a Haworth projection.]

Question 7

In six sugar rings (pyranoses) which anomer predominates, the α-anomer or β-anomer?

Answer 7

β-anomer

Because all of its groups lie along the less sterically hindered equatorial position.

Question 8

What are anomers?

Answer 8

Two molecules that differ in configuration on carbon-1 (C-1) of a cyclic carbohydrate.

(C-1 is the carbon next to the O in the ring.)

Question 9

What are two types of anomers?

Answer 9

1. α-anomer
2. β-anomer

Question 10

How do you determine if an anomer is an α-anomer? β-anomer?

Answer 10

Question 11

[Mnemonic]

“bow down to alpha”

“beta beat me up”

(how to remember the direction of the alpha and beta anomers)

Answer 11

“bow down to alpha”

“beta beat me up”