cyclic conformations

Question 1

Baeyer strain

Answer 1

Strain due to a distorted bond angle by amount: theta

Question 2

Possible conformations of cyclobutane

Answer 2

Could be both bent or planar. Increased Baeyer strain in the bent conformation with a bond angle of 87 degrees but decreased Pitzer strain

Question 3

Pitzer strain

Answer 3

Strain due to sub-optimal dihedral angles

Question 4

Conformers of cyclpentane

Answer 4

The bond angle has almost no Baeyer strain but the planar structure has maximum Pitzer strain. Envelope and twisted envelope conformers exist

Question 5

Why is the chair conformation of cyclohexane special

Answer 5

Unlike other cyclic structures, this conformer has almost no strain at all

Question 6

Origins of instability of the boat conformation

Answer 6

Pitzer strain. Substituents of the ‘base’ C-C bond are almost completely eclipsed

Question 7

Twist-boat conformation

Answer 7

local energy minima of cyclohexane. There are 10,000 chair conformers for every twist-boat

Question 8

Which position on a cyclohexane ring do substituents prefer

Answer 8

Equatorial for almost all X

Question 9

What can increase the effective bulk of a substituent and hence increase axial interactions

Answer 9

Heavy solvation. Ionic groups in polar solvents are heavily solvated which adds to the bulk of the substituent

Question 10

Conformational locks

Answer 10

A substituent is so highly favoured in one conformation that the structure is effectively locked, even if other substituents are added

Question 11

What is a decalin

Answer 11

Two cyclohexane rings that share a C-C bond

Question 12

Locking of decalin structures

Answer 12

Assuming the hydrogen are axial, in order to twist the trans structure, a huge C-C bond exists in the transition state which is not feasible. This trans-decalin structure is therefore locked. The cis structure however can interconvert between two chair conformers

Question 13

Why is the structure of glucose unique?

Answer 13

The all-trans structure of glucose has all substituents in equatorial sites. This is the only carbohydrate in which this exists

Question 14

What are the pair of glucose structures called and which is which and at what ratio do they exist in water

Answer 14

Anomers. the beta anomer is the all equatorial species and the alpha has a single OH in the equatorial position. They exist in a 2:1 ratio of beta:alpha

Question 15

The anomeric effect

Answer 15

The axial bond at the anomeric centre has an empty sigma* orbital. This overlaps well filled p orbital on oxygen (its lone pair) which stabilises the axial position over the equatorial

Question 16

When is the anomeric effect the most pronounced

Answer 16

when the sigma* orbital is low in energy e.g. when the C-X bond is to an electronegative X

Question 17

Why is the beta glucose structure the most stable in water?

Answer 17

Solvation can overcome the anomeric effect. This means that in polar solvents such as water, the equatorial site is still favoured

Question 18

Why are the conformations of steroids locked

Answer 18

They contain a decalin structure