Knights content lecture 6-10

Question 1

Define strain

Answer 1

strain is the distortion of bond lengths and angles from their ideal values

Question 2

What is the conformations of cyclic compounds determined by

Answer 2

the strain present in the structure

Question 3

What can the total strain energy be written as

Answer 3

Etot = Er + Eθ + EΦ + Es

Question 4

What is Er

Answer 4

strain due to bond stretching/compression. This strain produces only very small structural changes because even small distortions in bond lengths result in a large increase in energy

Question 5

What is Eθ

Answer 5

angle strain due to bond bending. A distortion of +/- 10 degrees from the tetrahedrally angle of 109 degrees causes an energy increase of about 4 KJmol-1

Question 6

What is EΦ

Answer 6

torsional strain (resistance to bond twisting). This occurs whenever two groups are force to eclipse one another

Question 7

What is Es

Answer 7

steric strain, this is caused whenever two atoms are forced closer together than the sum of their van der walls radii

Question 8

Describe how the total strain present in cycloalkanes can be determined

Answer 8

can be determined by measuring its heat of combustion. The lowest energy system, cyclohexane is arbitrarily assigned a strain energy of 0 and all other cycloalkenes are measured relative to this

Question 9

Describe the strain in 3 and 4 membered rings

Answer 9

3&4 membered rings are referred to as small rings and suffer high angle strain (bond angles < 109 degrees) and torsional strain (eclipsing)

Question 10

Describe the strain of 5-7 membered rings

Answer 10

5-7 membered rings are referred to as normal rings and are relatively untrained since the bond angles are equal to 109 degrees. 5 and 7 membered rings suffer torsional eclipsing strain

Question 11

Describe the strain of 8-13 membered rings

Answer 11

8-13 membered rings are referred to as medium rings and suffer torsional strain and steric strain (transangular interactions worst for 9 and 10 membered rings)

Question 12

Describe the strain of 14+ membered rings

Answer 12

14+ membered rings and larger are referred to as large, and are unstrained since a ring of this size is flexible enough to avoid angle, torsional or steric strain

Question 13

What does the rate of cyclisation reactions depend on

Answer 13

the activation energy

Question 14

What two components can the activation energy be split into

Answer 14

ΔH≠ the enthalpy of activation - which will be strongly dependent on the total strain energy of the cyclic product.
ΔS≠ the entropy of activation - which will be related to the likelihood of the two reacting atoms meeting each other

Question 15

Describe the rate of cyclisation reactions for 3 and 4 membered rings

Answer 15

small rings are highly strained the enthalpy of activation is large and cyclisation is slow

Question 16

Describe the rate of cyclisation reactions for 10 membered rings

Answer 16

formation of the 10 membered ring is particularly slow since this medium size rings suffers from both a large enthalpy of activation and a very large entropy of activation

Question 17

Describe the conformations of cyclopropane

Answer 17

Is flat and has a triangular shape with all carbon atoms in the same plane. The bond angles are 60 degrees therefor it has high angle strain and all of the CH2 groups eclipse on another so torsional strain is also large

Question 18

Describe the conformations of cyclobutane

Answer 18

Cyclobutane exists in a puckered form. Puckering actually reduces the bond angles to <90 degrees which increases angle strain. Puckering reduces the eclipsing interactions reducing the torsional strain

Question 19

describe a envelope

Answer 19

4 carbon atoms are coplanar, lowest energy conformation of cyclopentane

Question 20

Describe the conformations of cyclopentane

Answer 20

planar structure is too high in energy. Angle strain ~ 0 but has a very high torsional eclipsing strain, to reduce the eclipsing interactions cyclopentant can be drawn as an envelope or a half chair

Question 21

Describe a half chair

Answer 21

three carbons in the same plane and the remaining two, alternately above and below the plane. No torsional strain

Question 22

What are envelope and half-chairs both

Answer 22

they are both fluxional i.e the non-coplanar and coplanar atoms are constantly interchanging

Question 23

Describe planar 6 membered rings

Answer 23

a planar 6 membered ring would have bond angles of 120 degrees so would have large angle and torsional strain due to eclipsing interactions

Question 24

Describe chair conformation for cyclohexane

Answer 24

angle strain ~ 0 all c-c-c bond angles 109 degrees.
Torsional strain ~ 0 all adjacent CH2 groups are staggered (synclinal) to one another
Steric strain ~ 0 there are no close interactions

Question 25

Describe the strain for substituted chair conformations

Answer 25

axial substituents suffer steric strain (Es) called a 1,3-diaxial interaction. They also suffer torsional strain by being synclinal to two ring bonds

Question 26

Describe how we can calculate axial/equatorial ratio

Answer 26

gibbs free energy can be measured or calculated and is called the A value for the group R. If we know the delta G then we can calculate the equilibrium constant for axial to equatorial and hence the ratio of isomers at equilibrium

Question 27

Describe the general rule for preference of substituent to be equatorial

Answer 27

the preference for the substituent to be equatorial increases as its sizes increases

Question 28

What is a decalin and what are the two forms It can exist as

Answer 28

bicyclodecane is known as decalin. It can exsist in two forms depending on whether the hydrogen atoms at the ring junction are cis or trans

Question 29

What type of isomers are cis and trans decalin

Answer 29

Cis and trans decalin are Diastereoisomer and are not interconvertable

Question 30

Describe trans-decalin

Answer 30

trans-decalin has an extended structure and the top and bottom faces are identical. It is impossible for either 6 membered rings to flip to the other chair. This is a true conformational anchor

Question 31

Describe cis-decalin

Answer 31

cis-decalin has a bowl-shaped structure with an accessible outer face and a hindered inner face, one hydrogen atom at the ring junction is axial and the other is equatorial. It is possible for both six membered rings to flip to the other chair.

Question 32

Why is it important to understand the conformations of cyclic compounds

Answer 32

because the conformation of a molecule can have significant effect on its reactivity

Question 33

Define stereoelectronic effects

Answer 33

most molecular orbitals are directional. The conformation and reactivity of a compound can be influenced by orbital overlap - this is called steroelectronics

Question 34

Describe SN2 reactions in terms of stereoelectronic effects

Answer 34

SN2 proceeds with inversion due to attack of the nucleophile on σ*. The filled lP orbital on the nucleophile attacks the empty σ* of the electrophile. Optimal overlap occurs when the nucleophile attacks at 180 degrees to the leaving group