Chapter 4: Alkanes

Question 1

Name the alkanes:

1. CH4
2. C2H6
3. C3H8
4. C4H10
5. C5H12
6. C6H14
7. C7H16
8. C8H18
9. C9H20
10. C10H22

Answer 1

1. CH4 Methane
2. C2H6 Ethane
3. C3H8 Propane
4. C4H10 Butane
5. C5H12 Pentane
6. C6H14 Hexane
7. C7H16 Heptane
8. C8H18 Octane
9. C9H20 Nonane
10. C10H22 Decane

Question 2

What are the two types of Alkanes?

Answer 2

1. Acyclic Alkanes
   
   • CnH(2n+2)
   • saturated hydrocarbons (max #Hs per carbon)
   • only linear, chains of C
2. Cycloalkanes
   
   • CnH2n
   • carbons joined in one or two rings
   • fewer Hs than acyclic alkanes

Question 3

All C atoms in an alkane are surrounded by _____ groups. What is their geometry?

Answer 3

Four groups

sp3 hybridized, tetrahedral

Question 4

What are isomers?

Answer 4

Two different compounds with the same molecular compound

Eg, C4H10

Butane:
      H    H   H
       |     |     |
H - C - C - C - H
       |     |     | 
      H    H   H 

Isobutane:
               H
                |
         H - C - H 
        H     |    H
         |     |     |
   H- C - C - C - H
         |     |     | 
        H    H    H

Question 5

What are constitutional/structural isomers?

Answer 5

Isomers that differ in the way the atoms are connected to each other

Question 6

What are the 3 parts to naming Alkanes?

Answer 6

1. The parent name (number of carbons in the longest continuous chain)
2. The suffix (the functional group)
3. The prefix (# of substituents on carbon and their location)

Prefix + parent + suffix

Question 7

What is an alkyl group?

Answer 7

An alkyl group is formed by removing one H from an alkane

Eg,

CH3CH3 = ethane 
CH3CH2- = ethyl

Question 8

How do you name a Cycloalkane?

Answer 8

1. The parent name (number of carbons in the longest continuous chain)
2. The suffix (the functional group)
3. The prefix (# of substituents on carbon and their location)

Prefix + cyclo- + parent + suffix

Question 9

Alkanes have low boiling points and melting points.

What factors increase or decrease these?

Answer 9

Bp and mp increase as surface area increases

Bp and mp decrease with branching because of reduced surface area

Mp increases with symmetry

Question 10

True or false

Alkanes are soluble in water and insoluble in organic solvents

Answer 10

False

Alkanes are insoluble in water and soluble in organic solvents

Question 11

What is stereochemistry?

Answer 11

The 3D structure of molecules

Question 12

What are conformations?

Answer 12

Different arrangements of atoms that are interconverted by rotation about single bonds

(Basically, when you have a molecule that can be rotated around the sigma bond)

Question 13

What are the two different arrangements of conformation? Describe them.

Answer 13

Eclipsed conformation
- when the C-H bonds on one carbon are directly aligned with the C-H bonds on the adjacent carbon

Staggered conformation
- when the C-H bonds on one carbon bisect the H-C-H come angle on the adjacent carbon

Question 14

How do you convert an eclipsed conformation to a staggered conformation?

Answer 14

Rotate on carbon by 60°

Question 15

What is a dihedral angle?

Answer 15

The angle that separates a bond on one atom from a bond on an adjacent atom

Question 16

What is the dihedral angle on an eclipsed conformation of ethane?

Answer 16

0°

  •
  |
 / \    •    •

Question 17

What is the dihedral angle on a staggered conformation of ethane?

Answer 17

60°

• • •
\ | /
/ | \
• • •

(Think of it like the eclipsed conformation, but you rotated one of the carbons 60° and now from the side it looks like a star)

Question 18

True or false

The staggered conformations are more stable (lower in energy) than the eclipsed conformations

Answer 18

True

Question 19

What is torsional strain?

Answer 19

An increase in energy caused by eclipsing interactions

Question 20

What does each eclipsed C-H bond result in?

Answer 20

An increase in energy of 4.0kJ/mol

Which is a total of 12kJ/mol for one entire C with its 3 Hs rotating

Question 21

What is torsional energy?

Answer 21

The energy difference between the staggered and eclipsed conformations

Question 22

What is anti conformation and gauche conformation?

Answer 22

Anti conformation is when you have a molecule that is in a staggered conformation, and there are two larger groups (such as CH3 in butane) that are directly opposite each other at a 180° angle

Gauche conformation is when you have a molecule that is in a staggered conformation, and there are two larger groups (such as CH3 in butane) that are side by side each other at a 60° angle

Question 23

Which is lower in energy?

a) a staggered, anti conformation
b) a staggered, gauche conformation

Explain why

Answer 23

a) a staggered, anti conformation

Because in anti conformations they don’t experience steric strain

Question 24

What is steric strain?

Answer 24

An increase in energy when two similar atoms are forced too close together

Question 25

What is angle strain?

Answer 25

An increase in energy when tetrahedral bond angles deviate from the optimum angle of 109.5°

Question 26

Cyclohexanes experience angle strains and torsional strains, explain why this is.

Answer 26

For a cyclohexane (hexagon ring of carbons), their internal bond angles are 120°, which is bigger than the tetrahedral bond angle of 109.5°, so it has an angle strain. All of the hydrogens would be eclipsed, meaning they are closer to each other and experience a torsional strain.

Question 27

What form can a cyclohexane take to eliminate angle and torsional strain? Why is this?

Answer 27

Chair form - all C-C-C bond angles are 109.5° and the hydrogens on adjacent carbons are staggered, not eclipsed

Question 28

What are axial hydrogens?

Answer 28

Hydrogens located above and below the ring (along the perpendicular axis)

Question 29

What are equatorial hydrogens?

Answer 29

Hydrogens locates in the plane of the ring (around the equator)

Question 30

During a ring flip, what happens to the axial and equatorial hydrogens?

Answer 30

They flip

Question 31

What is boat form?

Answer 31

When one side of the chair form has flipped up

Question 32

True or false Boat forms are 30kJ/mol more stable than chair forms

Answer 32

False Chair forms are 30kJ/mol more stable

Question 33

What is a second chair confirmation?

Answer 33

When ring flipping has occurred

Question 34

What kind of strain exists in boat form?

Answer 34

Torsional strain because there are 4 carbons in the eclipsed plain Steric strain because two Hs at the ends (flagpole hydrogens) are forced close to each other

Question 35

Is a cyclohexane with its larger group CH3 in the equatorial position more or less stable? Why?

Answer 35

It is more stable because the equatorial position has more room than the axial position

Question 36

What happens when CH3 in a cyclohexane is in the axial position?

Answer 36

The CH3 is very close to two other axial H atoms, creating two destabilizing interactions called 1,3-diaxial interactions.

Question 37

Each unfavorable H,CH3 interaction destabilized the conformation by _____ kJ/mol. This happens for two H atoms, so the axial conformation is _____ kJ/mol less stable than the equatorial conformation.

Answer 37

3. 8 kJ/mol | 7. 6 kJ/mol

Question 38

What is a stereoisomer?

Answer 38

An isomer that differs only in the way that the atoms are orientated in space

Question 39

What is a cis isomer?

Answer 39

An isomer that has two groups on the same side of the ring

Question 40

What is a trans isomer?

Answer 40

An isomer that has two groups on opposite sides of the ring