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Orgo 1 > Ch 4 - Alkanes > Flashcards

Ch 4 - Alkanes Flashcards

1
Q

a flexible molecule is one that can

A

adopt many different shapes, or conformations

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2
Q

alkanes and cycloalkanes

A

lack a functional group which allow them to change their three dimensional shape as a result of rotating C-C bonds

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3
Q

conformational analysis

A

the study of three dimensional shapes of molecules

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4
Q

alkane(saturated hydrocarbons)

A

hydrocarbon which lacks pie bonds(all single bonds)

	- the name usually ends in “ane”
		- propane, butane, pentane
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5
Q

nomenclature

A

the system for naming chemical compounds

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6
Q

IUPAC

A

international union of pure and applied chemistry

- set up the Geneva rules in 1892 to standardize organic nomenclature

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7
Q

Systemic names

A

names produced by IUPAC rules

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8
Q

4 steps to naming Alkanes

A
  • identify the parent chain
    - identify and name the substituents
    - number the parent chain and assign a locant to each substituent
    - Arrange the substituents alphabetically
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9
Q

4 steps to naming Alkanes

Step 1: Select the parent chain

A
  • identify the longest chain
    - if 2 chains equal then the one with more substituents is chosen
  • substituent – groups connected to the parent chain
    - meth – 1 carbon – methane
    - eth – 2 carbon – ethane
    - prop – 3 carbon – propane
    - but – 4 carbon – butane
    - pent – 5 carbon – pentane
    - hex – 6carbon – hexane
    - hept – 7 carbon – heptane
    - oct – 8 carbon – octane
    - non – 9 carbon – nonane
    - dec – 10 carbon – decane
    - cycloalkanes – “cyclo” is used to indicate the presence of a ring in the structure of an alkane
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10
Q

4 steps to naming Alkanes

Step 2: Naming Substituents

A

same naming as above except with “yl” group

alkyl group – the above smaller chained groups attached to the parent chain
- when an alkyl group is next to a ring the ring is the parent as long as the ring has more carbons than the alkyl group

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11
Q

4 steps to naming Alkanes

Step 3: Naming Complex Substituents(parent and assigning locants to substituents)

A
  • When a substituent has a branch in it find the longest part and number each carbon going away from the parent chain
    - This becomes a miniparent chain
    - (2-methylbutyl) is a butyl group with a methyl group coming off the 2nd carbon
    - must be in parentheses
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12
Q

4 steps to naming Alkanes

Step 4: Assembling the Systemic Name of an Alkane

A
  • number the atoms of the parent chain
    - locant – the location of a group off the parent chain identified by a number by a carbon atom along the parent chain
    - Rules:
    - if one substituent is present – assign the lowest number possible
    - when multiple substituents present – assign so the lowest number is assigned first
    - if tied then use the second substituent as lowest
    - if still tied assign alphabetically by other atoms(Br then Cl etc)
    - all above rules apply to cycloalkanes
    - when a substituent appears more than once then a prefix is used to identify how many times
    - 1,1,3-trimethylcyclohexane
    - di = 2
    - tri = 3
    - tetra = 4
    - penta = 5
    - hexa = 6
    - after all substituents are assigned to proper locants the name can be arranged alphabetically(excluding prefixes for alphabetizing)
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13
Q

Naming parent chain

meth

A

1 carbon – methane

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14
Q

Naming parent chain

eth

A

2 carbon – ethane

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15
Q

Naming parent chain

prop

A

3 carbon – propane

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16
Q

Naming parent chain

but

A

4 carbon – butane

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17
Q

Naming parent chain

pent

A

5 carbon – pentane

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18
Q

Naming parent chain

hex

A

6 carbon – hexane

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19
Q

Naming parent chain

hept

A

7 carbon – heptane

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20
Q

Naming parent chain

oct

A

8 carbon – octane

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21
Q

Naming parent chain

non

A

9 carbon – nonane

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22
Q

Naming parent chain

dec

A

10 carbon – decane

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23
Q

cycloalkanes

A

“cyclo” is used to indicate the presence of a ring in the structure of an alkane

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24
Q

Naming Substituents

methyl

A

1 carbon

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25
Q

Naming Substituents

ethyl

A

2 carbon

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26
Q

Naming Substituents

propyl

A

3 carbon

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27
Q

Naming Substituents

butyl

A

4 carbon

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28
Q

Naming Substituents

pentyl

A

5 carbon

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29
Q

Naming Substituents

hexyl

A

6 carbon

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30
Q

Naming Substituents

heptyl

A

7 carbon

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31
Q

Naming Substituents

octyl

A

8 carbon

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32
Q

Naming Substituents

nonyl

A

9 carbon

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33
Q

Naming Substituents

decyl

A

10 carbon

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34
Q

Naming Alkanes recap:

A
  • identify the parent chain
    - identify and name the substituents
    - number the parent chain and assign a locant to each substituent
    - arrange the substituents alphabetically
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35
Q

bicyclic

A

compounds containing two fused rings

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36
Q

bridgehead

A

the two point which fuse two rings of carbon together

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37
Q

start at one bridgehead and number the longest path, then the next longest, then the shortest path

A
  • if there is a sub group anywhere number it in such a way that it is the lowest number possible
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38
Q

try to look at molecules from the IUPAC point of view(parent chain and groups on chain)

A

helps identify when two isomers may be drawn a little different but are actually the same

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39
Q

use the heat liberated from the combustion with oxygen

A

to produce CO2 and water

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40
Q

the deltaH standard is the change in enthalpy associated with

A

the complete combustion of 1 mol of the alkane in the presence of oxygen

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41
Q

heat of combustion

A

the negative deltaH standard

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42
Q

branched alkanes are lower in energy(more stable) than

A

straight chain alkanes

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43
Q

heats of combustion are an important way to

A

determine the relative stability of compounds

44
Q

where do alkanes come from naturally?

A

crude oil in the earth

45
Q

cracking

A

C-C bonds of larger Alkanes are broken producing alkanes suitable for gasoline
- tend to be straight chains which increase knocking in the engine

46
Q

reforming

A

the goal is to convert straight chain alkanes into branched and aromatic hydrocarbons

47
Q

conformation

A

rotation of a C-C single bond allows a compound to adopt a variety of possible 3D shapes

48
Q

Newman projection

A

drawing type designed to show the conformation of a molecule

49
Q

sawhorse

A

Newman projection drawing after 45degrees of rotation

50
Q

newman projections represent a snapshot

A

after 90 degrees of rotation where one carbon in directly in front of another

51
Q

dihedral angle(torsional angle)

A

the angle of separation of two atoms in a Newman projection

- the value of a dihedral changes as the C-C bond rotates
	- can be any value between 0 and 180 degrees
52
Q

there are an infinite number of conformations since

A

dihedral angles can be forever changing

53
Q

staggered conformation

A

where atoms in a Newman projection are as far apart as possible
- lowest in energy

54
Q

eclipsed conformation

A

where atoms in a Newman projection are as close as possible

- highest in energy

55
Q

degenerate

A

equivalent energy of all conformations of the same type in Ethane
– all staggered conformations are the same amount of energy
- All eclipsed conformation are the same amount of energy

56
Q

torsional strain

A

the difference in energy between staggered and eclipsed conformations of Ethane

57
Q

the torsional strain in ethane is

A

12kJ/mol

  • 4kJ/mol for each eclipsed H/H which means we can use this as a baseline for other torsional strains
    - propane has a torsional strain of 14kJ/mol or 4(H/H),4(H/H), and 6(H/CH3)
58
Q

the three eclipsed orbitals are not degenerate(the same)for butane

A

one has higher energy

59
Q

the three staggered orbitals are not degenerate(the same) for butane

A

one is lower than the other two

60
Q

anti conformation

A

dihedral staggered conformation at 180 degrees for butane

	- represents the lowest energy conformation of butane
	- 3.8kJ/mol lower than the other two orbitals
61
Q

two types of interactions for staggered conformations

A
  • anti

- gauche

62
Q

Anti conformation type

A

Methyl(CH3) groups are farthest apart

63
Q

Gauche conformation type

A

methyl groups experience a gauche interaction

- electron clouds get close together and repel each other causing a need for more energy to keep together

64
Q

gauche interactions

A

type of steric interaction

	- different than torsional strain
	- when methyl groups are closer than 180 degree together and their electron clouds repel each other(trying to occupy the same region of space) creating an unfavorable interaction requiring more energy
65
Q

H/H = 4kJ/mol

A

4kJ/mol

	- torsional strain
	- eclipsed conformation
66
Q

H/CH3 = 6kJ/mol

A

6kJ/mol

	- torsional strain
	- eclipsed conformation
67
Q

CH3/CH3 = 11kJ/mol

A

11kJ/mol

	- torsional strain + steric interaction
	- eclipsed conformation
68
Q

CH3/CH3 = 3.8kJ/mol

A
  • steric interaction

- staggered conformation

69
Q

angle strain

A

the increase in energy associated with a bond angle that has deviated from the preferred 109.5 degrees
- proposed by Adolph von Baeyer

70
Q

cyclopropane has high energy

A

angle strain(small bond angles) and torsional strain(eclipsing H’s)

71
Q

cyclobutane has less

A

angle strain than cyclopropane but has more torsional strain

72
Q

cyclopentane has much less

A

angle and torsional strain than cyclobutane or cyclopropane

73
Q

2 often used cyclohexane formations

A
  • chair conformation

- boat conformation

74
Q

both chair and boat conformations have bond angles close to 109.5 degrees and

A

possess very little angle strain

75
Q

significant difference between chair and boat conformations:

A
  • Chair conformation has not torsional strain

- boat conformation has two sources of torsional strain

76
Q

to alleviate torsional strain boat conformation can

A

twist into a twist boat

77
Q

flagpole interactions

A

steric interactions experienced by H’s on either side of a cyclohexane ring

78
Q

the most important cyclohexane conformation is

A

the chair conformation

79
Q

the lowest energy conformations are the

A

chair(and mirrored chair) conformations

80
Q

chair is

A

3 sets of 2 parallel lines

81
Q

each carbon atom in a cyclohexane ring can bear two substituents

A
  • axial position

- equatorial position

82
Q

axial position

A

group parallel to a vertical axis passing through the center of the ring(cyclohexane)
- 6(3 up and 3 down) lines from the carbons

83
Q

equatorial position

A

group positioned approximately along the equator of the ring

- 6(2 right,2 left,1 forward,1backward) lines from the carbons

84
Q

only one substituent can be in either an axial position or equatorial

A

possibilities are in equilibrium with each other

85
Q

ring flip

A

a conformational change accomplished only through a rotation of all C-C single bonds
- the axial should become equatorial

86
Q

when two chair conformations are in equilibrium the

A

lower energy conformation will be favored

87
Q

1,3-diaxial interactions

A

the substituents electron cloud is trying to occupy the same region of space as the H’s causing steric interactions

	- 1,3 describes the distance between the substituent and each H
	- most 1,3 interactions are gauche interactions
88
Q

the chair conformation will generally favor

A

the conformation with the equatorial substituent

89
Q

a wedge line is

A

UP

90
Q

a dashed line in

A

DOWN

91
Q

if the two groups compete with each other then the one with less 1,3-diaxial interactions is better as its lower energy

A
  • 1,3-diaxial interactions
    - Cl 2kJ/mol
    - OH 4.2 kJ/mol
    - CH3 7.6 kJ/mol
    - CH2CH3 8.0kJ/mol
    - CH(CH3)2 9.2kJ/mol
    - C(CH3)3 22.8kJ/mol
92
Q

1,30diaxial interactions

Cl

A

2 kJ/mol

93
Q

1,30diaxial interactions

OH

A

4.2 kJ/mol

94
Q

1,30diaxial interactions

CH(CH3)2

A

7.6 kJ/mol

95
Q

1,30diaxial interactions

CH3

A

8.0 kJ/mol

96
Q

1,30diaxial interactions

CH2CH3

A

9.2 kJ/mol

97
Q

1,30diaxial interactions

C(CH3)3

A

22.8 kJ/mol

98
Q

cis and trans are used to signify the

A

relative spatial relationship of similar substituents

99
Q

cis

A

two groups are on the same face of the ring

100
Q

trans

A

two groups are on opposite faces of the ring from each other

101
Q

Haworth projections

A

planar representations and do not represent conformations

- dark bolded area to the front and groups above and below

102
Q

Stereoisomers

A

different compounds with different physical properties, and they cannot be interconverted via a conformation change
- cis-1,2-Dimethylcyclohexane and trans-1,2-dimethylcyclohexane

103
Q

a stereoisomer will be more stable if

A

all groups can be in equilateral positions

104
Q

norborane

A

bicycle[2.2.1]heptane

	- commonly encountered in bicyclic systems
	- six membered ring locked into a boat conformation by a CH2 group that serves as a bridge
105
Q

many naturally occurring compounds are polycyclic systems

A

steroids(4 fused rings)

- 3 six membered rings and one five membered ring

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