Ch. 4 - Alkanes Flashcards
(102 cards)
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
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
3
Q
What is an alkane?
A
aliphatic hydrocarbon with only C-C and C-H bonds (sigma bonds)
4
Q
What are acyclic alkanes and what is their formula?
A
saturated hydrocarbons, chains; CnH2n+2
5
Q
What is a saturated hydrocarbon?
A
has max number of hydrogens (check!!)
6
Q
How are degrees of unsaturation calculated?
A
degrees of unsaturation = (2C + 2 + N – H – X) / 2
7
Q
What are cycloalkanes and what is their formula?
A
rings; CnH2n
8
Q
What are isomers?
A
2 different compounds with same molecular formula
9
Q
What are constitutional isomers?
A
differ in connectivity
10
Q
What are stereoisomers?
A
differ in space
11
Q
What is a normal alkane?
A
a straight-chain (n-alkane)
12
Q
Does bending of a molecule chain change its identity?
A
no
13
Q
How does the number of constitutional isomers change as the number of carbons changes?
A
As # of carbons increase, # const. isomers increase
14
Q
What is a methylene group?
A
CH2; creates every consecutive acyclic alkane
15
Q
What is a homologous series?
A
group of compounds that differ only by a CH2 group
16
Q
1 carbon
A
methane
17
Q
2 carbons
A
ethane
18
Q
3 carbons
A
propane
19
Q
4 carbons
A
butane
20
Q
5 carbons
A
pentane
21
Q
6 carbons
A
hexane
22
Q
7 carbons
A
heptane
23
Q
8 carbons
A
octane
24
Q
9 carbons
A
nonane
25
10 carbons
decane
26
11 carbons
unedecane
27
12 carbons
dodecane
28
13 carbons
tridecane
29
14 carbons
tetradecane
30
15 carbons
pentadecane
31
20 carbons
icosane
32
What is the systematic name?
IUPAC name, general structure
33
What is the generic name?
official and approved
34
What is the trade name?
assigned by a company
35
What makes up an alkane name?
prefix + parent + suffix
36
What does the parent name respresent?
number of carbons in the longest continuous chain
37
What does the suffix represent?
functional group (alkane)
38
What does the prefix represent?
identity, location, number of substituents
39
What are alkyl groups? How are they formed?
carbon substituents bonded to long carbon chain
| formed by removing a hydrogen from alkane
40
How are alkyl groups named?
change -ane to -yl
41
How many alkyl groups are there for propanes?
2; propyl and isopropyl
42
How many alkyl groups are there for butanes?
4; butyl, sec-butyl, isobutyl, tert-butyl
43
What is 2-methylpropane?
forms butyl substituents (isobutyl or tert-butyl)
44
What is the parent of the chain?
longest continuous carbon chain
45
What do you do if there are two carbon chains of equal length (parent)?
pick chain with more substituents
46
What are the steps for naming IUPAC compounds (simple)?
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
(numbering carbon atoms) If the first substituent is the same number from both sides of chain, what should be done?
number the chain to give the second substituent the lower number
48
When substituents are numbered the same from either end, how should they be numbered?
number the chain so the lowest number is given alphabetically
49
T/F: A carbon can belong to both a parent chain and a substituent.
False. It can only belong to one or the other
50
How are the substituents names placed in the fully combined name?
alphabetically, ignoring all prefixes except for -iso
51
What is the structure for naming a cycloalkane?
prefix + cyclo + parent + suffix
52
T/F: No number is needed to indicate a single substituent on a cycloalkane.
True
53
How should substituents be numbered in a ring?
assign lower numbers to substituents alphabetically
54
What do you do if an alkane has both a chain and a ring?
if the number of carbons in the ring >/= to the number of carbons in longest chain, it is a cycloalkane
55
What is natural gas composed of?
methane
56
Do alkanes have nonpolar bonds?
yes, C-C and C-H are nonpolar
57
What types of intermolecular forces do alkanes have?
van der waals (weak) only
58
Are alkanes water soluble?
no, nonpolar
59
What does is the trend for the boiling/melting point of alkanes?
as the number of carbons increases, so does the surface area and so does the BP and MP (IMFs increase)
60
How does branching affect the BP of isomers?
decreases with increased branching (decreases surface area)
61
How does the symmetry affect the melting point of isomers?
symmetry increases melting point
62
What are alkanes soluble in? Water or organic solvents?
organic solvents
63
What is stereochemistry?
3D structure of molecules
64
What are conformations?
different arrangements of atoms that are interconverted by rotation around single sigma bonds (either eclipsed or staggered)
still the same compound!!
65
What is the eclipsed conformation?
C-H bonds of both carbons are directly aligned (on top of each other looking down)
66
What is the staggered conformation?
C-H bonds on one carbon bisect the H-C-H bond angle on adjacent carbon (don't overlap when looking down)
67
What is the degree of rotation between the staggered and eclipsed conformations?
60º
68
What is a dihedral angle?
angle that separates a bond on one atoms from a bond on another (0º or 60º)
69
What is a Newman projection?
the graphic that shows the three groups bonded to each carbon in a particular C-C bon and their dihedral angles
70
T/F: Staggered and eclipsed conformations interconvert at room temp but are not equally stable.
true
71
Which is the more stable conformation, staggered or eclipsed?
staggered
72
Why is the staggered conformation more stable than eclipsed?
less e- – e- repulsion
73
How much does each C-H bond eclipsed increase the energy by?
4 kJ/mol
74
What is the energy difference between eclipsed and staggered conformations?
12 kJ/mol
75
What is torsional energy?
energy difference between staggered and eclipsed conformations
76
What is torsional strain?
increased in E caused by eclipsing interactions within a molecule
77
Is staggered anti or staggered gauche higher in energy?
staggered gauche, electron density is closer together
78
What do relative energies of conformations depend on?
steric strain
79
What is steric strain?
increase in energy that results from atoms being forced too close to one another
80
How do larger groups that are closer in eclipsed conformation affect steric strain?
increase eclipsed energy
81
What is the barrier to rotation?
energy difference between the lowest and highest E conformations
82
Does the zig zag of alkanes assume eclipsed or staggered conformation?
staggered
83
What is angle strain?
increase in E when a tetrahedral bond angle deviates from optimum angle (109.5º); only affects cycloalkanes
84
What is the Baeyer strain theory?
rings with bond angles different that tetrahedral bond angles are strained and reactive; therefore carbon rings cannot be flat
85
What is cycloalkane puckering?
contracted cycloalkanes to reduce angle strain (smaller the angle, less stable)
86
What is the most common ring in nature?
cyclohexane
87
In reality, what is the conformation of cyclohexanes?
chair conformation
88
What is the chair conformation?
puckered cyclohexane conformation that eliminate angle strain and torsional strain and is therefore more stable
all hydrogens are staggered as well
89
How many up Cs and down Cs are there in puckered ring?
3 ups, 3 downs
90
What are axial groups?
located above or below the ring in perpendicular axis
| up carbon, up axial; down carbon, down axial
91
What are equatorial groups?
located in the plane of the ring (up carbon, down equatorial; down carbon, up equatorial)
92
T/F: cyclohexane does not remain in a single conformation
true
93
What is ring flipping?
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
How much more stable are chair forms compared to boat, why?
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
What are flagpole carbons?
up carbons on either end of boat conformation; forced close together causing steric strain
96
Does the equatorial or axial position better accommodate large substituents?
equatorial position, has more space
97
T/F: substituents move above or below the ring when a chair conformation intercoverts
false, substituents always stay above or below the ring and never switch
98
How does the presence of a large substituent affect the percentages of certain chair conformations?
the % of the conformation with large sub on equatorial will be larger because it is more stable
99
How many chairs does a cis/trans cyclohexane have?
2 chairs
100
How are cis and trans chair conformations drawn?
cis: same subs on same side (up/down) of ring
trans: same subs on different side of ring
101
What are the two types of chair conformations that can form for a trans hexane? Which is more stable?
diaxial or diequatorial
| diequatorial is more stable
102
What are two types of chair conformations that can form for cis hexane? Which is more stable?
one of axial, one on equatorial and vice versa
| equally stable, 50%
