Ch 4 - Alkanes Flashcards
(105 cards)
1
Q
a flexible molecule is one that can
A
adopt many different shapes, or conformations
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
3
Q
conformational analysis
A
the study of three dimensional shapes of molecules
4
Q
alkane(saturated hydrocarbons)
A
hydrocarbon which lacks pie bonds(all single bonds)
- the name usually ends in “ane” - propane, butane, pentane
5
Q
nomenclature
A
the system for naming chemical compounds
6
Q
IUPAC
A
international union of pure and applied chemistry
- set up the Geneva rules in 1892 to standardize organic nomenclature
7
Q
Systemic names
A
names produced by IUPAC rules
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
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
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
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
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)
13
Q
Naming parent chain
meth
A
1 carbon – methane
14
Q
Naming parent chain
eth
A
2 carbon – ethane
15
Q
Naming parent chain
prop
A
3 carbon – propane
16
Q
Naming parent chain
but
A
4 carbon – butane
17
Q
Naming parent chain
pent
A
5 carbon – pentane
18
Q
Naming parent chain
hex
A
6 carbon – hexane
19
Q
Naming parent chain
hept
A
7 carbon – heptane
20
Q
Naming parent chain
oct
A
8 carbon – octane
21
Q
Naming parent chain
non
A
9 carbon – nonane
22
Q
Naming parent chain
dec
A
10 carbon – decane
23
Q
cycloalkanes
A
“cyclo” is used to indicate the presence of a ring in the structure of an alkane
24
Q
Naming Substituents
methyl
A
1 carbon
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Naming Substituents
ethyl
2 carbon
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Naming Substituents
propyl
3 carbon
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Naming Substituents
butyl
4 carbon
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Naming Substituents
pentyl
5 carbon
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Naming Substituents
hexyl
6 carbon
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Naming Substituents
heptyl
7 carbon
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Naming Substituents
octyl
8 carbon
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Naming Substituents
nonyl
9 carbon
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Naming Substituents
decyl
10 carbon
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Naming Alkanes recap:
- 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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bicyclic
compounds containing two fused rings
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bridgehead
the two point which fuse two rings of carbon together
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start at one bridgehead and number the longest path, then the next longest, then the shortest path
- if there is a sub group anywhere number it in such a way that it is the lowest number possible
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try to look at molecules from the IUPAC point of view(parent chain and groups on chain)
helps identify when two isomers may be drawn a little different but are actually the same
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use the heat liberated from the combustion with oxygen
to produce CO2 and water
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the deltaH standard is the change in enthalpy associated with
the complete combustion of 1 mol of the alkane in the presence of oxygen
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heat of combustion
the negative deltaH standard
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branched alkanes are lower in energy(more stable) than
straight chain alkanes
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heats of combustion are an important way to
determine the relative stability of compounds
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where do alkanes come from naturally?
crude oil in the earth
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cracking
C-C bonds of larger Alkanes are broken producing alkanes suitable for gasoline
- tend to be straight chains which increase knocking in the engine
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reforming
the goal is to convert straight chain alkanes into branched and aromatic hydrocarbons
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conformation
rotation of a C-C single bond allows a compound to adopt a variety of possible 3D shapes
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Newman projection
drawing type designed to show the conformation of a molecule
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sawhorse
Newman projection drawing after 45degrees of rotation
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newman projections represent a snapshot
after 90 degrees of rotation where one carbon in directly in front of another
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dihedral angle(torsional angle)
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
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there are an infinite number of conformations since
dihedral angles can be forever changing
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staggered conformation
where atoms in a Newman projection are as far apart as possible
- lowest in energy
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eclipsed conformation
where atoms in a Newman projection are as close as possible
| - highest in energy
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degenerate
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
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torsional strain
the difference in energy between staggered and eclipsed conformations of Ethane
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the torsional strain in ethane is
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)
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the three eclipsed orbitals are not degenerate(the same)for butane
one has higher energy
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the three staggered orbitals are not degenerate(the same) for butane
one is lower than the other two
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anti conformation
dihedral staggered conformation at 180 degrees for butane
- represents the lowest energy conformation of butane
- 3.8kJ/mol lower than the other two orbitals
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two types of interactions for staggered conformations
- anti
| - gauche
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Anti conformation type
Methyl(CH3) groups are farthest apart
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Gauche conformation type
methyl groups experience a gauche interaction
| - electron clouds get close together and repel each other causing a need for more energy to keep together
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gauche interactions
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
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H/H = 4kJ/mol
4kJ/mol
- torsional strain
- eclipsed conformation
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H/CH3 = 6kJ/mol
6kJ/mol
- torsional strain
- eclipsed conformation
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CH3/CH3 = 11kJ/mol
11kJ/mol
- torsional strain + steric interaction
- eclipsed conformation
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CH3/CH3 = 3.8kJ/mol
- steric interaction
| - staggered conformation
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angle strain
the increase in energy associated with a bond angle that has deviated from the preferred 109.5 degrees
- proposed by Adolph von Baeyer
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cyclopropane has high energy
angle strain(small bond angles) and torsional strain(eclipsing H’s)
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cyclobutane has less
angle strain than cyclopropane but has more torsional strain
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cyclopentane has much less
angle and torsional strain than cyclobutane or cyclopropane
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2 often used cyclohexane formations
- chair conformation
| - boat conformation
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both chair and boat conformations have bond angles close to 109.5 degrees and
possess very little angle strain
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significant difference between chair and boat conformations:
- Chair conformation has not torsional strain
| - boat conformation has two sources of torsional strain
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to alleviate torsional strain boat conformation can
twist into a twist boat
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flagpole interactions
steric interactions experienced by H’s on either side of a cyclohexane ring
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the most important cyclohexane conformation is
the chair conformation
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the lowest energy conformations are the
chair(and mirrored chair) conformations
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chair is
3 sets of 2 parallel lines
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each carbon atom in a cyclohexane ring can bear two substituents
- axial position
| - equatorial position
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axial position
group parallel to a vertical axis passing through the center of the ring(cyclohexane)
- 6(3 up and 3 down) lines from the carbons
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equatorial position
group positioned approximately along the equator of the ring
| - 6(2 right,2 left,1 forward,1backward) lines from the carbons
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only one substituent can be in either an axial position or equatorial
possibilities are in equilibrium with each other
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ring flip
a conformational change accomplished only through a rotation of all C-C single bonds
- the axial should become equatorial
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when two chair conformations are in equilibrium the
lower energy conformation will be favored
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1,3-diaxial interactions
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
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the chair conformation will generally favor
the conformation with the equatorial substituent
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a wedge line is
UP
90
a dashed line in
DOWN
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if the two groups compete with each other then the one with less 1,3-diaxial interactions is better as its lower energy
- 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
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1,30diaxial interactions
Cl
2 kJ/mol
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1,30diaxial interactions
OH
4.2 kJ/mol
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1,30diaxial interactions
CH(CH3)2
7.6 kJ/mol
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1,30diaxial interactions
CH3
8.0 kJ/mol
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1,30diaxial interactions
CH2CH3
9.2 kJ/mol
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1,30diaxial interactions
C(CH3)3
22.8 kJ/mol
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cis and trans are used to signify the
relative spatial relationship of similar substituents
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cis
two groups are on the same face of the ring
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trans
two groups are on opposite faces of the ring from each other
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Haworth projections
planar representations and do not represent conformations
| - dark bolded area to the front and groups above and below
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Stereoisomers
different compounds with different physical properties, and they cannot be interconverted via a conformation change
- cis-1,2-Dimethylcyclohexane and trans-1,2-dimethylcyclohexane
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a stereoisomer will be more stable if
all groups can be in equilateral positions
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norborane
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
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many naturally occurring compounds are polycyclic systems
steroids(4 fused rings)
| - 3 six membered rings and one five membered ring
