Cycloalkanes and Their Stereochemistry

Question 1

What are cycloalkanes?

Answer 1

Saturated cyclic hydrocarbons

Question 2

What is another term for cycloalkanes?

Answer 2

Acyclic compounds

Question 3

What is the general formula for a cycloalkane?

Answer 3

CnH2n

Question 4

When is the parent chain the cycloalkane?

Answer 4

If the number of C atoms in the ring is equal to or greater than the substitute

Question 5

Which is more flexible, cycloalkanes or open-chain alkanes?

Answer 5

Open-chain

Question 6

Which rings sizes are the most restricted?

Answer 6

C3-C7

Question 7

Define stereoisomers?

Answer 7

Compounds which have their atoms connected in the same order but differ in 3D orientation

Question 8

What are cis-trans isomers?

Answer 8

Stereoisomers that differ in their stereochemistry about a ring or double bond.

Question 9

What strain is added to cycloalkanes?

Answer 9

Angle strain

Question 10

What is angle strain?

Answer 10

Induced in a molecule when bond angles are forced to deviate from the ideal 109.5 degree tetrahedral value

Question 11

How can cyclic molecules minimize angle strain and torsional strain?

Answer 11

Ring-puckering allows them to assume nonplanar conformations.

Question 12

What ring is the most strained?

Answer 12

Cyclopropane

Question 13

Why is cyclopropane the most strained of all rings?

Answer 13

Angle strain caused by its C-C-C bond angles of 60 degrees

Question 14

Besides angle strain, what is another considerable type of strain in cyclopropane? Why?

Answer 14

Torsional strain due to eclipsed C-H bonds

Question 15

What is the overall strain energy of cyclopropane?

Answer 15

115 kJ/mol

Question 16

Which has more angle strain, cyclobutane or cyclopropane?

Answer 16

Cyclobutane

Question 17

What is are the bond angles in cyclobutane?

Answer 17

88 degrees

Question 18

Which strain is greater in cyclobutane, torsional or angle?

Answer 18

Torsional

Question 19

Why is torsional strain greater than angle strain in cyclobutane?

Answer 19

Larger number of ring hydrogens and their proximity

Question 20

Explain the conformation of cyclobutane

Answer 20

Slightly bent out of plane with one C atom ~25 degrees above the plane

Question 21

What does bending one C atom out of plane in cyclobutane do to the strain?

Answer 21

Increases angle strain (88 degrees vs 90 degrees if it were planar) but decreases torsional strain

Question 22

What is the overall strain energy of cyclobutane?

Answer 22

110 kJ/mol

Question 23

What is the bond angle value of pentane?

Answer 23

108 degrees

Question 24

Why does cyclopentane adopt a non-planar conformation?

Answer 24

To strike a balance between increase angle strain and decreased torsional strain.

Question 25

What is the overall strain energy of cyclopentane?

Answer 25

26 kJ/mol

Question 26

What conformation does cyclohexane adopt?

Answer 26

Chair conformation

Question 27

What is chair conformation?

Answer 27

Strain-free, 3D shape

Question 28

What is the overall strain energy of cyclohexane?

Answer 28

0 kJ/mol

Question 29

What are the bond angle values of cyclohexane?

Answer 29

~109 degrees

Question 30

What are the two type of chair conformation positions for substituents on the cyclohexane ring?

Answer 30

Axial positions
Equatorial positions

Question 31

How are the hydrogens arranged in cyclohexane in terms of positions?

Answer 31

6 axial hydrogens perpendicular to the ring
6 Equatorial hydrogens near the plane of the ring
Each carbon atom has one axial and one equatorial
Each face of the ring has three axial and three equatorial hydrogens in an alternating arrangement

Question 32

What is meant by ring-flip?

Answer 32

Interconversion of chair conformations, resulting in the exchange of axial and equatorial positions.

Question 33

What happens to cyclohexane at room temp?

Answer 33

Rapidly flips between chair conformations

Question 34

Are two conformations of monosubstituted cyclohexane equally stable?

Answer 34

No

Question 35

Which conformer of methylcyclohexane is more stable?

Answer 35

Equatorial

Question 36

How much more stable is the equatorial conformer of methylcyclohexane than the axial?

Answer 36

7.6 kJ/mol

Question 37

What causes the difference between axial and equatorial conformers?

Answer 37

Steric strain

Question 38

Explain the 7.6 kJ/mol of steric strain in methylcyclohexane.

Answer 38

The axial methyl group hydrogens on C1 are too close to the axial hydrogens three carbons away on C3 and C5

Question 39

What is the percentage of equatorial methylcyclohexane at equilibrium?

Answer 39

95%

Question 40

Explain the relationship of methylcyclohexane to Gauche butane.

Answer 40

Gauche butane is less stable than anti butane by 3.8 kJ/mol due to steric interference between H atoms on the two methyl groups.
4-carbon fragment of axial methylcyclohexane and gauche butane have the same steric interaction
Equatorial methylcyclohexane has no such interactions and is thus more stable.

Question 41

Where are the methyl groups on the cis isomer of 1,2-dimethylcyclohexane?

Answer 41

Both on the same face of the ring

Question 42

How many chair conformations are possible for cis-1,2-dimethylcyclohexane?

Answer 42

2

Question 43

How many trans conformations are there of 1,2-dimethylcyclohexane?

Answer 43

2

Question 44

Differentiate between the two trans conformations of 1,2-dimethylcyclohexane.

Answer 44

One has both methyl groups equatorial and only a gauche butane interaction between methyls.
Ring-flipped conformations has both methyl groups axial with four 1,3-diaxial interactions.

Question 45

What conformation does the trans isomer of disubstituted cyclohexane mainly exist in?

Answer 45

Diequatorial (>98%)