CH. 4

Question 1

Saturated Hydrocarbon

Answer 1

Hydrocarbon that lacks pi bond (alkane)

Question 2

First Step in Naming an Alkane

Answer 2

Identify Parent Chain (longest chain)

Question 3

If there are two parent chains of equal length what do you do? (First step in naming an Alkane)

Answer 3

Choose the chain with the greater number of substituents (groups attached to the carbons)

Question 4

How many constitutional isomers does C3H8 have?

Answer 4

1

Question 5

Cyclopropane

Answer 5

Alkane ring structure with 3 carbons

Question 6

Cyclobutane

Answer 6

Alkane ring structure with 4 carbons

Question 7

Cyclopentane

Answer 7

Alkane ring structure with 5 carbons

Question 8

How many constitutional isomers does C₄H₁₀ have?

Answer 8

2

Question 9

How many constitutional isomers does C₅H₁₂ have?

Answer 9

3

Question 10

How many constitutional isomers does C₆H₁₄ have?

Answer 10

5

Question 11

How many constitutional isomers does C₇H₁₆ have?

Answer 11

9

Question 12

Does the number of constitutional isomers increase or decrease with size?

Answer 12

Increase

Question 13

What method do you use to avoid drawing two of the same constitutional isomers?

Answer 13

Use IUPAC rules to name each compound.

Question 14

What process is used to compare stability of isomeric alkanes?

Answer 14

Looking at the heat of combustion (heat given off) during combustion reaction, measuring the potential energy the isomer had before combustion.

Alkane + O₂ –> CO₂ + H₂O

Question 15

Are straight-chain or branch-chain alkanes lower in energy (more stable) during combustion rxns?

Answer 15

branch-chain alkanes are lower in energy and therefore more stable

Question 16

What is the main source of alkanes?

Answer 16

Petroleum

Question 17

What is refining (in reference to petroleum)?

Answer 17

Seperating crude oil (petroleum) into commercial products

Question 18

What are the two processes used to increase yield of gasoline from petroleum?

Answer 18

Cracking and Reforming

Question 19

What is Cracking?

Answer 19

C–C bonds of larger alkanes are broken into smaller alkanes (suitable for gasoline but not preferred)

Question 20

What is Reforming?

Answer 20

Converting straight chain alkanes into branched hydrocarbons and aromatic compounds (better for gasoline)

Question 21

What is a conformation?

Answer 21

3-D shape of a compound as a result of rotation of single bonds

Question 22

What is dihedral angle?

Answer 22

Angle in which two groups are separated in Newman projection (also called torsional angle)

Question 23

What is highest/lowest energy: Staggered Conformation or Eclipsed Conformation

Answer 23

Staggered Conformation = Lowest Energy

Eclipsed Conformation = Highest Energy

Question 24

What is Torsional Strain?

Answer 24

Difference in energy between staggered and eclipsed conformations

Question 25

What is an Anti Conformation?

Answer 25

Conformation where the dihedral angle between two groups is 180 degrees (lowest energy conformation of butane)

Question 26

What is a Gauche Interaction?

Answer 26

When two groups in a Newman projection are separated by a dihedral angle of 60 degrees

Question 27

Give Interaction, Type of Strain, and Energy Cost of the following:

Answer 27

Interaction: **H/H Eclipsed** Type of Strain: **Torsional Strain** Energy Cost: **4 kj/mol**

Question 28

Give Interaction, Type of Strain, and Energy Cost of the following:

Answer 28

Interaction: **CH₃/H Eclipsed** Type of Strain: **Torsional Strain** Energy Cost: **6 kj/mol**

Question 29

Give Interaction, Type of Strain, and Energy Cost of the following:

Answer 29

Interaction: **CH₃/CH₃ Eclipsed** Type of Strain: **Torsional Strain +** **Steric Interaction** Energy Cost: **11 kj/mol**

Question 30

Give Interaction, Type of Strain, and Energy Cost of the following:

Answer 30

Interaction: **CH₃/CH₃ Gauche** Type of Strain: **Steric Interaction** Energy Cost: **3.8 kj/mol**

Question 31

How are rings (cycloalkanes) of different sizes compared?

Answer 31

Dividing heat of combustion by number of CH₂ groups, giving heat of combustion per CH₂ group.

Question 32

Does heat of combustion per CH₂ group increase or decrease with the addition of CH₂ groups?

Answer 32

Generally decreases

Question 33

The angle strain of Cyclopropane is severe, how is it alleviated?

Answer 33

The orbitals making up the bonds bend outwards (does not completely alleviate angle strain =\> inefficient overlap of orbitals)

Question 34

What two factors contribute to Cyclopropane's high energy?

Answer 34

1) Angle strain from small bond angles 2) Torional strain from locked eclipsed formation

Question 35

Does Cyclobutane have more/less **angle/torsional strain** than Cyclopropane?

Answer 35

Cyclobutane has: 1) Less Angle strain than Cyclopropane 2) More torsional strain due to 4 sets of eclipsed H's

Question 36

How does Cyclobutane alleviate torsional strain?

Answer 36

Adopting slightly puckered conformation

Question 37

Does Cyclopentane have more/less angle strain that Cyclobutane & Cyclopropane?

Answer 37

Less Angle Strain and reduces much of its torsional strain by adopting the following conformation

Question 38

What two conformations of Cyclohexane have the smallest angle strain and how are they different?

Answer 38

1) Chair and Boat Conformations 2) Both have bond angles near 109.5 = little angle strain 3) Chair conformation has all H's staggered = 0 Torsional strain 4) Boat Conformation has two H's eclipsed resulting in some torsional strain

Question 39

Cyclohexane chair formation

Answer 39

All Bond Angle near 109.5 Degrees = 0 Angle Strain All H's staggered = 0 Torsional Strain

Question 40

Cyclohexane in Boat Formation

Answer 40

All Bond Angles near 109.5 = 0 Angle Strain Two H's Eclipsed = Some Torsional Strain H's on either side of the ring experience Flagpole interactions

Question 41

How does Cyclohexane in Boat formation alleviate strain from flagpole interactions?

Answer 41

Twist Boat Formation

Question 42

Chair Conformation (Cyclohexane) Axial Position

Answer 42

Substituent of Cyclohexane that is parallel to imaginary vertical axis

Question 43

Chair Conformation (Cyclohexane) Equatorial position

Answer 43

Substituent of Cyclohexane that is approximately along equator of ring

Question 44

Draw all axial/equatorial positions on a chair conformation of Cyclohexane

Answer 44

Question 45

Draw both Chair Conformations of Bromocyclohexane

Answer 45

Question 46

When two conformations are in equilibrium, will the higher or lower energy conformation be favored?

Answer 46

The lower energy. For example, methycyclohexane

Question 47

Which is the lower energy chair conformation with a methyl group attached, axial or equatorial position?

Answer 47

The substituent in the equatorial position is the lower energy conformation. A steric interaction occurs with the axial position. (1,3-Diaxial Interaction)

Question 48

What is a 1, 3-diaxial interaction?

Answer 48

Steric interaction that occurs between axian substituents in chair conformation. Really just a gauche interaction.

Question 49

Is there a gauche interaction present in a chair conformation when its substituents are in equatorial position?

Answer 49

No

Question 50

What are the two chair conformations for the following compound?

Answer 50

Chlorine was on a wedge so it must be up Methyl Group was on a dash so it must be down

Question 51

What are the two chair conformations of this compound and which is more stable?

Answer 51

The first compound is more stable (and favored more) because its substituents are equatorial

Question 52

What are the two chair conformations of this compound and which is more stable?

Answer 52

The second conformation is more stable because the energy is less for Cl than it is for Ehtyl Group (has greater preference for equatorial) Cl \< OH \< CH₃ \< CH₂CH₃ \< CH(CH₃)₂ \< C(CH₃)₃

Question 53

What are the two chair conformations for the below stereoisomers and which is more stable?

Answer 53

trans-1,2-Dimethycyclohexane is more stable because it can adopt a chair conformation with both methyl gtoups in equatorial positions.