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