Alkanes

Report
Alkanes
Acyclic: CnH2n+2
Cyclic (one ring): CnH2n
Bicyclic (two rings) : CnH2n-2
Only single bonds, sp3 hybridization, close to
tetrahedral bond angles
Physical properties
• Boiling points
– Lower than other organic molecules of same
size.
– Lower attractive forces between molecules
than in alcohols.
methane
-164 oC
water
100 oC
hexane
68.7 oC
1-pentanol 137 oC
Intermolecular Forces
• Ionic Forces
• Hydrogen
Bonding
Dispersion
Forces:
due to fluctuating motionStrength
of the electrons
in a molecule. Motion in one molecule is correlated with that in
• other
Dipole
Dipole Forces
the
molecule.
• Dispersion Forces
Dispersion Forces and Molecular
Structure
Branching decreases surface area, reduces dispersion forces and, thus,
boiling point.
Molecular Structure and Heat of
Combustion
Difference in heats of
combustion indicates a
greater stability of branched
structures.
18.8 kJ
-5470.6
-5451.8
8CO2 + 9H2O
Isomerism and Naming
• Hexane
CH3CH2CH2CH2CH2CH3
2-methylpentane
CH3
CH3CH2CH2CHCH3
CycloAlkanes
Cl
1-chloro-3-methylcyclohexane
1,2-diisopropylcyclobutane
1-methyl-2-propylcyclopropane
Bicycloalkanes
Parent name: name of alkane with same number of carbons.
Number from bridgehead along largest bridge. If substituent choose
bridgehead to assign low number to substituent.
Size of bridges indicated by number of carbons in bridge.
Examples of numbering
1
Cl
2
5
6-chlorobicyclo[3.1.1]heptane
7
2,7-dimethylbicyclo[4.2.2]decane
Conformations
• Rotations about single bonds produce
different conformations.
60
Staggered Conformation.
Eclipsed Conformation.
Newman Projections
Staggered Conformation.
More stable!
Eclipsed Conformation.
Less stable.
Rotational Profile of ethane
CH3
CH3CH2CH2CHCH3
What are the forces in a molecular
structure?
Torsional strain: Strain between
groups on adjacent atoms.
A-B-C-D. Worst when eclipsed;
best when staggered.
Bond angle strain: when a bond
angle, A-B-C, diverges from the
ideal (180, 120, 109)
View from here
yields view below.
View from here
yields view below.
Rotation about C2 – C3 in butane
120
deg.
Anti conformation Methyls
180 deg, lower energy
Gauche conformation, Methyls closer, 60
deg, more repulsion, higher energy
H
CH3
H
H
H
H
CH3
Anti!!
Gauche!!
H
H
H3C
H
CH3
Energy Profile for Rotation in
Butane
Three valleys (staggered forms) 120 apart;
Three hills (eclipsed) 120 apart.
Problem: Rotational profile of
2-methylbutane about C2-C3.
First, staggered structures.
60
180
300
H
H
H
Me
H
Me
Me
H
H
H
Me
Me
Me
Rotate the front Me group.
Relative energies….
H
Me
Me
Me
H
Now, eclipsed….
H
Me
H
Me
Me
240
H
HH
HMe
H
Me
180
120
0
Me
H
HH
HMe
H
Me
Me
H
Me
Me
This was the high
energy staggered
structure,180 deg.
Shown for reference
only.
Now relative energies…..
360 = 0
Me
Me
H
Me
Me
H
Now put on diagram…
HH
HH
Me
Me
HMe
H
Me
H
Me
Me
H
Me
Me
H
Me
Me
H
H
H
H
Me
Me
H
Me
Me
60
120
eclipsed
Me
H
H
H
Me
Me
H
Me
Me
Me
staggered
H
H
0
HMe
180
240
300
360
Conformations of cycloalkanes:
cyclopropane
Planar ring (three points define a plane); sp3 hybrization: 109o.
Hydrogens eclipsing. Torsional
angle strain.
Bond angle strain. Should be 109
but angle is 60o.
Cyclopropane exhibits unusual reactivity for an alkane.
Conformation of cyclobutane
Fold on
diagonal
Planar: eclipsing, torsional strain
and bond angles of 90o
Folded, bent: less torsional strain
but increased bond angle strain
Cyclobutane molecular dynamics
Cyclopentane
Cyclohexane
Boat conformation
planar: bond angle 120, eclipsed.
Ideal solution: Everything
staggered and all angles
tetrahedral.
Chair conformation
Chair Conformation
Axial:
Equatorial:
Axial and Equatorial
Axial Up/Equatorial
Down: (A/E)
Equatorial Up/Axial
Down: (E/A)
A/E
E/A
E/A
A/E
A/E
E/A
Ring Flips
Chair
Boat or
A becomes E
Twisted
Boat
E becomes A
Up stays Up
Down stays Down
Chair
Substituents: Axial vs Equatorial
R
R
axial
substituent
equatorial
substituent
Substituent, R
D G Preference for
Equatorial
K at 25 deg
-CH3, methyl
7.28 kJ/mol
18.9
-CH2CH3, ethyl
7.3
19.
-CH(CH3)2, iso propyl
9.0
38.
-C(CH3)3, tert butyl
21.0
4.8 x 103
Substituent Interactions
R
H
H
Destabilizes axial
substituent. Each
repulsion is about
7.28/2 kJ = 3.6 kJ
1,3 diaxial repulsions
Alternative description:
R
gauche interactions
Each repulsion is still
about 3.6 kJ. Note
that the gauche
interaction in butane
is about 3.8.
Newman Projection of
methylcyclohexane
Axial methyl group
Equatorial methyl group
gauche
anti
H
CH3
ring
H
ring
H
ring
ring
CH3
H
H
H
Disubstituted cyclohexanes
1,2 dimethylcyclohexane
trans
trans
cis
cis
7.3 kJ (axial)
3.6 kJ (gauche) 3.6 kJ (gauche)
CH3
CH3
CH3
0.0 kJ equatorial
ring
ring
CH
CH3
3
CH
CH3
3
ring
ring
7.3 kJ (axial)
H
H
H
H
7.3 + 3.6 = 10.9 kJ
H3C
interactions
0.0 kJ equatorial
ring
ring
H
C
H3
3C
CH
CH3
3
H
H
ring
ring
H
H
7.3 + 3.6 = 10.9 kJ
trans
cis
7.3 kJ (axial)
CH3
CH3
H3C
3.6 kJ (gauche)
0.0 kJ equatorial
CH3 7.3 kJ (axial)
diequatorial
diaxial
H
ring
H 3C
CH3
ring
CH3
H
0.0 kJ + 3.6 kJ = 3.6 kJ
ring
H
H
ring
CH3
14.6 kJ + 0.0 kJ = 14.6 kJ
When does the gauche interaction
occur?
carbons
adjacentcarbons
onadjacent
beon
mustbe
groupsmust
alkylgroups
alkyl
axial
/ equatorial
axial/ equatorial
equatorial
equatorial
equatorial/ /equatorial
axial
axial
axial/ /axial
Translate ring planar structure into
E/A
3D
A/E
A/E
C(CH3)3
Assume the tert-butyl
group is equatorial.
E/A
E/A
A/E
C(CH3)3
Energy accounting
No axial substituents
One 1,2 gauche interaction between methyl groups, 3.6 kJ/mol
Total: 3.6 kJ
Problem: Which has a higher heat
of combustion per mole, A or B?
A
B
t-Bu
t-Bu
E/A
E/A
E/A
E/A
E/A
E/A
A/E
A/E
3.6
3.6
7.2
3.6
7.3
7.3
18.2
More repulsion, higher
heat of combustion by
11.0 kJ/mol
Trans and Cis Decalin
decahydronaphthalene
decalin
bicyclo [4.4.0] decane
Now build cis decalin, both same side.
Build trans decalin starting from
cyclohexane, one linkage up, one down
Trans sites used on the left ring
Trans sites used on the right ring
Trans decalin
Locked, no ring flipping
Cis sites used on left ring.
Cis sites used on right ring.
Cis decalin, can ring flip
Trans fusions determine geometry
A/E
H3C
E/A
HO
E/A
H
A/E
What is the geometry of the OH and
CH3?
E/A
A/E
H
Trans fusions, rings must use equatorial position for fusion. Rings are locked.
The H’s must both be axial
Work out axial / equatorial for the OH and CH3.
OH is equatorial and CH3 is axial

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