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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
• 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.
Strength
Dispersion Forces and Molecular Structure
Branching decreases surface area, reduces dispersion forces and, thus, boiling point.
Molecular Structure and Heat of Combustion
8CO2 + 9H2O
-5470.6 -5451.8
Difference in heats of combustion indicates a greater stability of branched structures.
18.8 kJ
Isomerism and Naming
• Hexane
CH3CH2CH2CH2CH2CH3
2-methylpentane
CH3CH2CH2CHCH3
CH3
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
Cl
6-chlorobicyclo[3.1.1]heptane 2,7-dimethylbicyclo[4.2.2]decane
1
2
5 7
Conformations
• Rotations about single bonds produce different conformations.
Staggered Conformation. Eclipsed Conformation.
60
Newman Projections
Eclipsed Conformation.Staggered Conformation.
More stable!Less stable.
CH3CH2CH2CHCH3
CH3
Rotational Profile of ethane
What are the forces in a molecular structure?
Bond angle strain: when a bond angle, A-B-C, diverges from the ideal (180, 120, 109)
Torsional strain: Strain between groups on adjacent atoms.
A-B-C-D. Worst when eclipsed; best when staggered.
Rotation about C2 – C3 in butane
H
CH3
H
H
CH3
H
Anti conformation Methyls 180 deg, lower energy
120 deg.
Gauche conformation, Methyls closer, 60 deg, more repulsion, higher energy
H3C
H
H
H
CH3
H
Gauche!!
View from here yields view below.
View from here yields view below.
Anti!!
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.
Me
H
Me
H
Me
H
First, staggered structures.
Me
H
Me
Me
H
H
Me
H
Me
H
H
Me
Rotate the front Me group.
Relative energies….
18060 300
Now, eclipsed….
Me
H
Me
H
Me
H
This was the high energy staggered structure,180 deg. Shown for reference only.
Me
H
Me
H
HMe
Me
H
Me
H
MeH
120 240180
Me
H
Me
Me
HH
Me
H
Me
Me
HH
0 360 = 0
Now relative energies…..
Now put on diagram…
Me
H
Me
H
MeHMe
H
Me
H
HMe
Me
H
Me
Me
HH Me
H
Me
Me
HH
Me
H
Me
H
Me
H
Me
H
Me
Me
H
H
Me
H
Me
H
H
Me
0 180 36060 120 240 300
staggered
eclipsed
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
Planar: eclipsing, torsional strain and bond angles of 90o
Folded, bent: less torsional strain but increased bond angle strain
Fold on diagonal
Cyclobutane molecular dynamics
Cyclopentane
Cyclohexane
planar: bond angle 120, eclipsed.
Chair conformation
Boat conformation
Ideal solution: Everything staggered and all angles tetrahedral.
Chair Conformation
Axial:
Equatorial:
Axial and EquatorialAxial Up/Equatorial
Down: (A/E)
Equatorial Up/Axial Down: (E/A)
E/A
E/A
E/A
A/E
A/E
A/E
Ring Flips
Chair
Boat or
Twisted Boat
Chair
A becomes E
E becomes A
Up stays Up
Down stays Down
Substituents: Axial vs Equatorial
Substituent, R 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
R
R
equatorialsubstituent
axial substituent
REach repulsion is still about 3.6 kJ. Note that the gauche interaction in butane is about 3.8.
Substituent InteractionsRH
H
1,3 diaxial repulsions
Destabilizes axial substituent. Each repulsion is about 7.28/2 kJ = 3.6 kJ
Alternative description:
gauche interactions
Newman Projection of methylcyclohexane
CH3
H
H
H
ring
ring
Axial methyl group Equatorial methyl group
H
CH3
H
H
ring
ring
gauche anti
0.0 kJ equatorial
7.3 kJ (axial)7.3 kJ (axial)
0.0 kJ equatorial
Disubstituted cyclohexanes1,2 dimethylcyclohexane
3.6 kJ (gauche)
H
CH3
CH3
Hring
ringH3C
H
H
CH3
ring
ring
7.3 + 3.6 = 10.9 kJ 7.3 + 3.6 = 10.9 kJ
interactions
3.6 kJ (gauche)
CH3
H3C
CH3
CH3
7.3 kJ (axial)0.0 kJ equatorial
0.0 kJ
equato
rial
7.3 kJ (axial)
diequatorial diaxial
H
CH3
H
CH3ring
ringH3C
H
CH3
H
ring
ring
3.6 kJ (gauche)
0.0 kJ + 3.6 kJ = 3.6 kJ 14.6 kJ + 0.0 kJ = 14.6 kJ
When does the gauche interaction occur?
Translate ring planar structure into 3D
E/A
E/A
E/A
C(CH3)3
A/E
A/E
A/E
C(CH3)3Energy accounting
No axial substituents
One 1,2 gauche interaction between methyl groups, 3.6 kJ/mol
Total: 3.6 kJ
Assume the tert-butyl group is equatorial.
Problem: Which has a higher heat of combustion per mole, A or B?
t-Bu t-Bu
A B
E/A
E/AE/A
E/A
E/A E/A
A/E A/E
3.6 3.6 3.67.3
7.3
7.2 18.2
More repulsion, higher heat of combustion by 11.0 kJ/mol
Trans and Cis Decalin
Trans decalin
Locked, no ring flipping Cis decalin, can ring flip
decahydronaphthalenedecalinbicyclo [4.4.0] decane
Build trans decalin starting from cyclohexane, one linkage up, one down
Now build cis decalin, both same side.
Trans sites used on the left ringTrans sites used on the right ring Cis sites used on left ring.
Cis sites used on right ring.
Trans fusions determine geometry
H
H
H3C
HO
What is the geometry of the OH and CH3?
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.
A/EA/E
A/E
E/A
E/A E/A
OH is equatorial and CH3 is axial