Chapter 3

Question 1

Boiling Point of Alkanes

Answer 1

As the number of carbons in an alkane increases, the boiling point will increase due to the larger surface area and the increased Van der Waal attractions

Question 2

Melting points of Alkanes

Answer 2

• Melting points increase as the carbon chain increases
• Alkanes with an even number of carbons have higher melting points than those with an odd number of carbons
• Branched alkanes have higher melting points than unbranched alkanes

Question 3

Methane Representation

Answer 3

• Tetrahedral
• sp³ hybrid carbon with angles of 109.5 degrees

Question 4

Ethane Representations

Answer 4

• Two sp³ hybrid carbons
• Rotation about the C-C sigma bond

Question 5

Conformations definition

Answer 5

Conformations are different arrangements of atoms caused by rotation about a single bond

Question 6

Conformations of Ethane

Answer 6

Pure confomers cannot be isolated in most cases, because the molecules are constantly rotating through all the possible conformations

Question 7

ethane eclipsed conformation

Answer 7

Question 8

ethane staggered conformation

Answer 8

Question 9

Ethane Conformations Energy Diagram

Answer 9

The torsional energy of ethane is lowest in the staggered conformation

Question 10

Propane Conformations

Answer 10

Question 11

propane eclipsed conformation

Answer 11

Question 12

propane staggered conformation

Answer 12

Question 13

Propane conformation energy diagram

Answer 13

The staggered conformations of propane are lower in energy than the eclipsed conformations. Since the methyl group occupies more space than a hydrogen, there is a greater torsional strain for propane than for ethane.

Question 14

butane totally eclipsed conformation

Answer 14

Question 15

butane gauche (staggered) conformation

Answer 15

Question 16

butane eclipsed conformation

Answer 16

Question 17

butane anti (staggered) conformation

Answer 17

Question 18

Steric Strain in Butane

Answer 18

• The totally eclipsed conformation is higher in energy because it forces the two end methyl groups so close together that their electron clouds experience a strong repulsion
• This kind of interference between two bulky groups is called steric strain

Question 19

Butane conformation energy diagram

Answer 19

Question 20

Geometric Isomers

Answer 20

Question 21

Torsional Strain

Answer 21

• Torsional strain reflects barrier to rotating about the C-C single bond.
• For butane, high torsional energy for the totally eclipsed conformer is due to steric strain (two groups trying to occupy the same space).

Question 22

Angle Strain in Cycloalkanes

Answer 22

• When a cycloalkane has an angle other than 109.5º, there will not be optimum overlap and the compound will have angle strain.
• This is because all the C atoms are sp³ hybridized

Question 23

Torsional Strain in Cycloalkanes

Answer 23

• Torsional strain arises when all the bonds are eclipsed

Question 24

Ring Strain in Cycloalkanes

Answer 24

• Ring strain reflects contributions of angle strain and torsional strain.

Question 25

Cyclopropane

Answer 25

- The bond angles are compressed to 60° from the usual 109.5° bond angle of sp3 hybridized carbon atoms - This severe angle strain leads to nonlinear overlap of the sp3 orbitals and “bent bonds” (sometimes called banana bonds) - Leads to interesting reactivity.

Question 26

Torsional strain in cyclopropane

Answer 26

- All the C—C bonds are eclipsed, generating torsional strain that contributes to the total ring strain.

Question 27

Planar Cyclobutane

Answer 27

In a planar conformation: - Angle strain from the compressing of the bond angles to 90° - Torsional strain from eclipsing of the bonds

Question 28

Non-planar Cyclobutane

Answer 28

- Cyclic compound with four carbons or more adopt non-planar conformations to relieve torsional strain. - Cyclobutane adopts the folded conformation (“envelope”) to decrease the torsional strain caused by eclipsing hydrogens.

Question 29

Cyclopentane

Answer 29

- The conformation of cyclopentane is slightly folded, like the shape of an envelope. This puckered conformation reduces the eclipsing of adjacent methylene (CH2) groups.

Question 30

Cyclohexane Chair Conformation

Answer 30

Question 31

Cyclohexane Boat Conformation

Answer 31

Question 32

Conformational Energy Diagram of Cyclohexane

Answer 32

Question 33

Axial and Equitorial Positions

Answer 33

Question 34

Chair-Chair Interconversion

Answer 34

- Chair conversion results in switching all the axial substituents into the equatorial positions, and vice versa.

Question 35

Equitorial Methyl Group

Answer 35

Question 36

Axial Methyl in Methylcyclohexane

Answer 36

Question 37

Tert-butylcyclohexane

Answer 37

- Very large substituents (like t-butyl) will ONLY occupy equatorial positions - Chair will be locked in this conformation

Question 38

Cis-1,3-dimethylcyclohexane

Answer 38

- Cis-1,3-dimethylcyclohexane can have both methyl groups in axial positions or both in equatorial positions. - The conformation with both methyl groups equatorial is more stable.

Question 39

Tert-butylcyclohexane

Answer 39

- Very large substituents (like t-butyl) will ONLY occupy equatorial positions - Chair will be locked in this conformation

Question 40

Cis-1,4-ditertbutylcyclohexane

Answer 40

The most stable conformation of cis-1,4-di-tertbutylcyclohexane is the twist boat. Both chair conformations require one of the bulky t-butyl groups to occupy an axial position.

Question 41

cis - 1,2-dimethylhexane positions

Answer 41

Question 42

trans - 1,2-dimethylhexane positions

Answer 42

Question 43

cis - 1,3-dimethylhexane positions

Answer 43

Question 44

trans - 1,3-dimethylhexane positions

Answer 44

Question 45

cis - 1,4-dimethylhexane positions

Answer 45

Question 46

trans - 1,4-dimethylhexane positions

Answer 46