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Chem 51A: Midterm > Chapter 4: Alkanes > Flashcards

Chapter 4: Alkanes Flashcards

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1
Q

Name the alkanes:

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

What are the two types of Alkanes?

A
  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
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3
Q

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

A

Four groups

sp3 hybridized, tetrahedral

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4
Q

What are isomers?

A

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
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5
Q

What are constitutional/structural isomers?

A

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

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6
Q

What are the 3 parts to naming Alkanes?

A
  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

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7
Q

What is an alkyl group?

A

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

Eg,

CH3CH3 = ethane 
CH3CH2- = ethyl
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8
Q

How do you name a Cycloalkane?

A
  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

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9
Q

Alkanes have low boiling points and melting points.

What factors increase or decrease these?

A

Bp and mp increase as surface area increases

Bp and mp decrease with branching because of reduced surface area

Mp increases with symmetry

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10
Q

True or false

Alkanes are soluble in water and insoluble in organic solvents

A

False

Alkanes are insoluble in water and soluble in organic solvents

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11
Q

What is stereochemistry?

A

The 3D structure of molecules

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12
Q

What are conformations?

A

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)

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13
Q

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

A

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

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14
Q

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

A

Rotate on carbon by 60°

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15
Q

What is a dihedral angle?

A

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

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16
Q

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

A

0°

  •
  |
 / \    •    •
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17
Q

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

A

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)

18
Q

True or false

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

A

True

19
Q

What is torsional strain?

A

An increase in energy caused by eclipsing interactions

20
Q

What does each eclipsed C-H bond result in?

A

An increase in energy of 4.0kJ/mol

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

21
Q

What is torsional energy?

A

The energy difference between the staggered and eclipsed conformations

22
Q

What is anti conformation and gauche conformation?

A

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

23
Q

Which is lower in energy?

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

Explain why

A

a) a staggered, anti conformation

Because in anti conformations they don’t experience steric strain

24
Q

What is steric strain?

A

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

25
Q

What is angle strain?

A

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

26
Q

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

A

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.

27
Q

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

Why is this?

A

Chair form

  • all C-C-C bond angles are 109.5° and the hydrogens on adjacent carbons are staggered, not eclipsed
28
Q

What are axial hydrogens?

A

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

29
Q

What are equatorial hydrogens?

A

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

30
Q

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

A

They flip

31
Q

What is boat form?

A

When one side of the chair form has flipped up

32
Q

True or false

Boat forms are 30kJ/mol more stable than chair forms

A

False

Chair forms are 30kJ/mol more stable

33
Q

What is a second chair confirmation?

A

When ring flipping has occurred

34
Q

What kind of strain exists in boat form?

A

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

35
Q

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

A

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

36
Q

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

A

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

37
Q

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.

A
  1. 8 kJ/mol

7. 6 kJ/mol

38
Q

What is a stereoisomer?

A

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

39
Q

What is a cis isomer?

A

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

40
Q

What is a trans isomer?

A

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

Chem 51A: Midterm (4 decks)

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