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Organic Chemistry > Ch. 4 - Alkanes > Flashcards

Ch. 4 - Alkanes Flashcards

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1
Q

What type of interaction(s) destabilize a cyclohexane chair conformation that has a bulky substituent in an axial position?

A

steric interactions, 1,3-diaxial interactions

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2
Q

True or false: Alkanes undergo few reactions since they are the only family of organic molecules that do not contain a functional group.

A

True

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3
Q

What is an alkane?

A

aliphatic hydrocarbon with only C-C and C-H bonds (sigma bonds)

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4
Q

What are acyclic alkanes and what is their formula?

A

saturated hydrocarbons, chains; CnH2n+2

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5
Q

What is a saturated hydrocarbon?

A

has max number of hydrogens (check!!)

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6
Q

How are degrees of unsaturation calculated?

A

degrees of unsaturation = (2C + 2 + N – H – X) / 2

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7
Q

What are cycloalkanes and what is their formula?

A

rings; CnH2n

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8
Q

What are isomers?

A

2 different compounds with same molecular formula

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9
Q

What are constitutional isomers?

A

differ in connectivity

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10
Q

What are stereoisomers?

A

differ in space

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11
Q

What is a normal alkane?

A

a straight-chain (n-alkane)

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12
Q

Does bending of a molecule chain change its identity?

A

no

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13
Q

How does the number of constitutional isomers change as the number of carbons changes?

A

As # of carbons increase, # const. isomers increase

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14
Q

What is a methylene group?

A

CH2; creates every consecutive acyclic alkane

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15
Q

What is a homologous series?

A

group of compounds that differ only by a CH2 group

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16
Q

1 carbon

A

methane

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17
Q

2 carbons

A

ethane

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18
Q

3 carbons

A

propane

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19
Q

4 carbons

A

butane

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20
Q

5 carbons

A

pentane

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21
Q

6 carbons

A

hexane

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22
Q

7 carbons

A

heptane

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23
Q

8 carbons

A

octane

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24
Q

9 carbons

A

nonane

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25
Q

10 carbons

A

decane

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26
Q

11 carbons

A

unedecane

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27
Q

12 carbons

A

dodecane

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28
Q

13 carbons

A

tridecane

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29
Q

14 carbons

A

tetradecane

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30
Q

15 carbons

A

pentadecane

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31
Q

20 carbons

A

icosane

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32
Q

What is the systematic name?

A

IUPAC name, general structure

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33
Q

What is the generic name?

A

official and approved

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34
Q

What is the trade name?

A

assigned by a company

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35
Q

What makes up an alkane name?

A

prefix + parent + suffix

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36
Q

What does the parent name respresent?

A

number of carbons in the longest continuous chain

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37
Q

What does the suffix represent?

A

functional group (alkane)

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38
Q

What does the prefix represent?

A

identity, location, number of substituents

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39
Q

What are alkyl groups? How are they formed?

A

carbon substituents bonded to long carbon chain

formed by removing a hydrogen from alkane

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40
Q

How are alkyl groups named?

A

change -ane to -yl

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41
Q

How many alkyl groups are there for propanes?

A

2; propyl and isopropyl

42
Q

How many alkyl groups are there for butanes?

A

4; butyl, sec-butyl, isobutyl, tert-butyl

43
Q

What is 2-methylpropane?

A

forms butyl substituents (isobutyl or tert-butyl)

44
Q

What is the parent of the chain?

A

longest continuous carbon chain

45
Q

What do you do if there are two carbon chains of equal length (parent)?

A

pick chain with more substituents

46
Q

What are the steps for naming IUPAC compounds (simple)?

A

1) Find parent chain and add suffix -ane
2) Number atoms in carbon chain giving first substituent the lowest number
3) Name and number the substituents as (-yl) groups, denote di, tri, tetra, etc if there is more than one of that type of substituent
4) Combine the substituent names and numbers based on alphabetical order (ignoring prefixes except for iso) _ parent + suffix

47
Q

(numbering carbon atoms) If the first substituent is the same number from both sides of chain, what should be done?

A

number the chain to give the second substituent the lower number

48
Q

When substituents are numbered the same from either end, how should they be numbered?

A

number the chain so the lowest number is given alphabetically

49
Q

T/F: A carbon can belong to both a parent chain and a substituent.

A

False. It can only belong to one or the other

50
Q

How are the substituents names placed in the fully combined name?

A

alphabetically, ignoring all prefixes except for -iso

51
Q

What is the structure for naming a cycloalkane?

A

prefix + cyclo + parent + suffix

52
Q

T/F: No number is needed to indicate a single substituent on a cycloalkane.

A

True

53
Q

How should substituents be numbered in a ring?

A

assign lower numbers to substituents alphabetically

54
Q

What do you do if an alkane has both a chain and a ring?

A

if the number of carbons in the ring >/= to the number of carbons in longest chain, it is a cycloalkane

55
Q

What is natural gas composed of?

A

methane

56
Q

Do alkanes have nonpolar bonds?

A

yes, C-C and C-H are nonpolar

57
Q

What types of intermolecular forces do alkanes have?

A

van der waals (weak) only

58
Q

Are alkanes water soluble?

A

no, nonpolar

59
Q

What does is the trend for the boiling/melting point of alkanes?

A

as the number of carbons increases, so does the surface area and so does the BP and MP (IMFs increase)

60
Q

How does branching affect the BP of isomers?

A

decreases with increased branching (decreases surface area)

61
Q

How does the symmetry affect the melting point of isomers?

A

symmetry increases melting point

62
Q

What are alkanes soluble in? Water or organic solvents?

A

organic solvents

63
Q

What is stereochemistry?

A

3D structure of molecules

64
Q

What are conformations?

A

different arrangements of atoms that are interconverted by rotation around single sigma bonds (either eclipsed or staggered)

still the same compound!!

65
Q

What is the eclipsed conformation?

A

C-H bonds of both carbons are directly aligned (on top of each other looking down)

66
Q

What is the staggered conformation?

A

C-H bonds on one carbon bisect the H-C-H bond angle on adjacent carbon (don’t overlap when looking down)

67
Q

What is the degree of rotation between the staggered and eclipsed conformations?

A

60º

68
Q

What is a dihedral angle?

A

angle that separates a bond on one atoms from a bond on another (0º or 60º)

69
Q

What is a Newman projection?

A

the graphic that shows the three groups bonded to each carbon in a particular C-C bon and their dihedral angles

70
Q

T/F: Staggered and eclipsed conformations interconvert at room temp but are not equally stable.

A

true

71
Q

Which is the more stable conformation, staggered or eclipsed?

A

staggered

72
Q

Why is the staggered conformation more stable than eclipsed?

A

less e- – e- repulsion

73
Q

How much does each C-H bond eclipsed increase the energy by?

A

4 kJ/mol

74
Q

What is the energy difference between eclipsed and staggered conformations?

A

12 kJ/mol

75
Q

What is torsional energy?

A

energy difference between staggered and eclipsed conformations

76
Q

What is torsional strain?

A

increased in E caused by eclipsing interactions within a molecule

77
Q

Is staggered anti or staggered gauche higher in energy?

A

staggered gauche, electron density is closer together

78
Q

What do relative energies of conformations depend on?

A

steric strain

79
Q

What is steric strain?

A

increase in energy that results from atoms being forced too close to one another

80
Q

How do larger groups that are closer in eclipsed conformation affect steric strain?

A

increase eclipsed energy

81
Q

What is the barrier to rotation?

A

energy difference between the lowest and highest E conformations

82
Q

Does the zig zag of alkanes assume eclipsed or staggered conformation?

A

staggered

83
Q

What is angle strain?

A

increase in E when a tetrahedral bond angle deviates from optimum angle (109.5º); only affects cycloalkanes

84
Q

What is the Baeyer strain theory?

A

rings with bond angles different that tetrahedral bond angles are strained and reactive; therefore carbon rings cannot be flat

85
Q

What is cycloalkane puckering?

A

contracted cycloalkanes to reduce angle strain (smaller the angle, less stable)

86
Q

What is the most common ring in nature?

A

cyclohexane

87
Q

In reality, what is the conformation of cyclohexanes?

A

chair conformation

88
Q

What is the chair conformation?

A

puckered cyclohexane conformation that eliminate angle strain and torsional strain and is therefore more stable

all hydrogens are staggered as well

89
Q

How many up Cs and down Cs are there in puckered ring?

A

3 ups, 3 downs

90
Q

What are axial groups?

A

located above or below the ring in perpendicular axis

up carbon, up axial; down carbon, down axial

91
Q

What are equatorial groups?

A

located in the plane of the ring (up carbon, down equatorial; down carbon, up equatorial)

92
Q

T/F: cyclohexane does not remain in a single conformation

A

true

93
Q

What is ring flipping?

A

conformational change of chair to boat to chair where up carbons become down and down carbons become up (axials become equatorial and equatorials become axial)

94
Q

How much more stable are chair forms compared to boat, why?

A

chair forms are 30 kJ/mol more stable

boat has torsional strain because carbons are eclipsed, steric strain because 2 hydrogens are at either end of the boat

95
Q

What are flagpole carbons?

A

up carbons on either end of boat conformation; forced close together causing steric strain

96
Q

Does the equatorial or axial position better accommodate large substituents?

A

equatorial position, has more space

97
Q

T/F: substituents move above or below the ring when a chair conformation intercoverts

A

false, substituents always stay above or below the ring and never switch

98
Q

How does the presence of a large substituent affect the percentages of certain chair conformations?

A

the % of the conformation with large sub on equatorial will be larger because it is more stable

99
Q

How many chairs does a cis/trans cyclohexane have?

A

2 chairs

100
Q

How are cis and trans chair conformations drawn?

A

cis: same subs on same side (up/down) of ring
trans: same subs on different side of ring

101
Q

What are the two types of chair conformations that can form for a trans hexane? Which is more stable?

A

diaxial or diequatorial

diequatorial is more stable

102
Q

What are two types of chair conformations that can form for cis hexane? Which is more stable?

A

one of axial, one on equatorial and vice versa

equally stable, 50%

Organic Chemistry (26 decks)

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