Alkanes

1
Alkanes

• Acyclic CnH2n2
• Cyclic (one ring) CnH2n
• Bicyclic (two rings) CnH2n-2

Only single bonds, sp3 hybridization, close to
tetrahedral bond angles
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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
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Intermolecular Forces

• Ionic Forces
• Hydrogen Bonding
• Dipole Dipole Forces
• Dispersion Forces

Dispersion Forces due to fluctuating motion of
the electrons in a molecule. Motion in one
molecule is correlated with that in the other
molecule.
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Dispersion Forces and Molecular Structure
Branching decreases surface area, reduces
dispersion forces and, thus, boiling point.
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Molecular Structure and Heat of Combustion
Difference in heats of combustion indicates a
greater stability of branched structures.
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Isomerism and Naming

• Hexane

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2-methylpentane
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CycloAlkanes
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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.
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Examples of numbering
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Conformations

• Rotations about single bonds produce different
  conformations.

Staggered Conformation.
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Newman Projections
Eclipsed Conformation.
Staggered Conformation.
Less stable.
More stable!
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Rotational Profile of ethane
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What are the forces in a molecular structure?
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Rotation about C2 C3 in butane
View from here yields view below.
View from here yields view below.
Gauche conformation, Methyls closer, 60 deg, more
repulsion, higher energy
Anti conformation Methyls 180 deg, lower energy
Anti!!
Gauche!!
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Energy Profile for Rotation in Butane
Three hills (eclipsed) 120 apart.
Three valleys (staggered forms) 120 apart
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Problem Rotational profile of 2-methylbutane
about C2-C3.
First, staggered structures.
300
60
180
Rotate the front Me group.
Relative energies.
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Now, eclipsed.
180
240
120
360 0
0
This was the high energy staggered structure,180
deg. Shown for reference only.
Now relative energies..
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Now put on diagram
eclipsed
staggered
120
360
60
180
300
0
240
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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.
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Conformation of cyclobutane
Fold on diagonal
Planar eclipsing, torsional strain and bond
angles of 90o
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Cyclobutane molecular dynamics
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Cyclopentane
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Cyclohexane
Ideal solution Everything staggered and all
angles tetrahedral.
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Chair Conformation
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Axial and Equatorial
Axial Up/Equatorial Down (A/E)
Equatorial Up/Axial Down (E/A)
E/A
A/E
E/A
A/E
E/A
A/E
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Ring Flips
Chair
Boat or Twisted Boat
A becomes E E becomes A Up stays Up Down stays
Down
Chair
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Substituents Axial vs Equatorial
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
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Substituent Interactions
Destabilizes axial substituent. Each repulsion is
about 7.28/2 kJ 3.6 kJ
1,3 diaxial repulsions
Alternative description
gauche interactions
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Newman Projection of methylcyclohexane
Axial methyl group
Equatorial methyl group
gauche
anti
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Disubstituted cyclohexanes
1,2 dimethylcyclohexane
3.6 kJ (gauche)
3.6 kJ (gauche)
interactions
0.0 kJ equatorial
0.0 kJ equatorial
7.3 3.6 10.9 kJ
7.3 3.6 10.9 kJ
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diequatorial
diaxial
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When does the gauche interaction occur?
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Translate ring planar structure into 3D
E/A
A/E
A/E
Assume the tert-butyl group is equatorial.
E/A
E/A
A/E
Energy accounting
No axial substituents
One 1,2 gauche interaction between methyl groups,
3.6 kJ/mol
Total 3.6 kJ
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Problem Which has a higher heat of combustion
per mole, A or B?
7.3
3.6
3.6
3.6
7.3
18.2
7.2
More repulsion, higher heat of combustion by 11.0
kJ/mol
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Trans and Cis Decalin
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
Cis sites used on left ring.
Trans sites used on the right ring
Cis sites used on right ring.
Trans decalin Locked, no ring flipping
Cis decalin, can ring flip
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Trans fusions determine geometry
What is the geometry of the OH and CH3?
E/A
A/E
A/E
E/A
E/A
A/E
Trans fusions, rings must use equatorial position
for fusion. Rings are locked.
The Hs must both be axial
Work out axial / equatorial for the OH and CH3.
OH is equatorial and CH3 is axial