s&r topic 2

Question 1

give the angle of each cycloalkane

• cyclopropane
• cyclobutane
• cyclopentane
• cyclohexane

Answer 1

• 60°
• 90°
• 108°
• 120°

Question 2

define angle strain

Answer 2

increase in potential energy of a molecule due to bond angles deviating away from the ideal angle

Question 3

why is there angle strain in cycloalkanes

Answer 3

carbons are sp3 hybridised = expected to have 109.5° angles

cycloalkane angles are different so causes angle strain

Question 4

how is angle strain measured in cycloalkanes

what correlation is observed between strain and angle deviation

Answer 4

strain is measured by working out bond enthalpy (of combustion) because lowest energy enthalpy is the most stable
compare combustion enthalpy to deviation from ideal angle and measure the correlation

no apparent correlation between angle strain and angle deviation

the most stable/least strained = cyclohexane
we expect the most stable to be pentane because it has the least angle deviation

Question 5

describe the shape and strain of cyclopropane

Answer 5

planar, three points define a plane

high angle strain, 60° far from ideal

high torsional strain because carbon-carbon bonds are all eclipsed

Question 6

describe the shape and strain of cyclobutane

Answer 6

corners of cyclobutane pucker away from planar shape like a butterfly
two shapes, opposite corners pucker up and down and there is a equilibrium between the two forms

this increases angle strain a little = 88°

but reduces torsional strain from eclipsing hydrogens

cyclopentane behaves in a similar manner

Question 7

in the chair conformation of cyclohexane, what types of hydrogens are there

Answer 7

axial H’s - point above and below the ring, all aligned to an axis

equatorial H’s - point out at an angle from the centre of the ring, point into the equator

see topic2 video2 for images of cyclohexane conformers

Question 8

in the boat conformation of cyclohexane, where does the torsional strain come from

what kind of conformation is the boat considered because of this

Answer 8

all the hydrogens on their adjacent carbons are eclipsing

the carbons that point ‘up’ (to make the outer points of the boat) have flagpole hydrogens that point into the middle of the ring that experience steric strain

this is a very high energy conformation, energy is at a maximum, it is transition state

Question 9

in the chair conformation of cyclohexane, how many hydrogen environments can you expect on the NMR spectrum

• at -100°C
• at room temp

Answer 9

• at -100°C, there are two peaks/environments because axial and equatorial are the two distinct environments and at that temp they are distinguishable
• at room temp, enough energy to constantly and quickly flip between two environments
• leads to half chairs and twist boats

see topic2 vid2 for energy diagram of conformations and how to draw chair conformation

Question 10

axial R-group on cyclohexane

• what kind of conformation is it
• what strain is experienced

Answer 10

• R group is synclinal/gauche to the hydrogen
• 1,3-diaxial interaction - R group interacts with the space of the other axial H’s
• this interaction is unfavourable
• steric strain = destabilising

Question 11

equatorial R-group on cyclohexane

• what kind of conformation is it
• how stable is it compared to the axial R-group cyclohexane

Answer 11

• antiperiplanar

- more stable than the axial R-group because there is no 1,3-axial interactions

Question 12

as the R-group in cyclohexane increases in size, what 3 trends are observed

Answer 12

as R-group on cyclohexane gets bigger…
- energy difference between axial and equatorial conformations increases

• equilibrium constant increases
• percentage of molecules with R-group in an equatorial position get bigger

Question 13

in disubstituted cyclohexane, describe the positions of the substitutions in the cis-isomer

Answer 13

both R groups have positions pointing in the same direction (either towards the upper or lower side of the ring)

• both groups occupy positions below the ring or above the ring
• one could be equatorial or one could be axial if its 1,4 substitution
• if its 1,3 substitution, both are axial or both are equatorial

Question 14

in 1,4-disubstituted cyclohexane, describe the positions of substitutions in the trans-isomer

Answer 14

one R group points up one points down

• diaxial, one up one down
• diequatorial = more stable = more favourable

see topic2 vid3 for OH substitution on cyclohexane

Question 15

when making a more than one substitution on cyclohexane, which group takes priority on the axial position

Answer 15

the smaller group takes the axial position to minimise the 1,3-diaxial strain