Alkanes Isomers

Roldan M. de Guia Department of Biochemistry Faculty of Pharmacy University of Santo Tomas

Physical Properties of Organic Compounds • Alkanes

– Melting and boiling points increase with increasing molecular weight within a homologous series. Compound

Formula

MW (g/mol)

mp (ºC)

bp (ºC)

Methane

CH4

16

–182

–164

Pentane

CH3(CH2)3CH3

72

–130

36

Decane

CH3(CH2)8CH3

142

–30

174

Pentadecane

CH3(CH2)13CH3

212

10

271

Eicosane

CH3(CH2)18CH3

282

37

343

Physical Properties of Organic Compounds

Physical Properties of Organic Compounds

Physical Properties of Organic Compounds • Alkanes

– Boiling points decrease with chain branching.

Compound CH3

CH3

octane

CH3 CH3

4-methylheptane

CH3 CH3

CH3

CH3

mp (oC)

bp (oC)

−57

127

−121

118

−107

99

CH3 CH3

2,2,4-trimethylpentane

Physical Properties of Organic Compounds • Alkanes – Solubility – “Like dissolves like” – Alkanes are nonpolar, hydrophobic – They are soluble in nonpolar solvents and insoluble in water.

Physical Properties of Organic Compounds Compound CH3

CH3

MW

bp (oC)

H2O solubility

86.18

69

insoluble

98.96

83

0.8%

86.09

88

20%

hexane

Cl

Cl

1,2-dichloroethane

O CH3

CH3 O

2,3-butanedione

Physical Properties of Organic Compounds bp (oC)

H2O solubility

83

∞

90.12

230

∞

NH2 88.15

158

∞

CH3 88.15 N H

119

∞

Compound CH3 HO H2N CH3

O

MW

CH3 90.12 O

dimethoxyethane

OH 1,4-butanediol

putrescine

H N

N,N'-dimethylethylenediamine

Physical Properties of Organic Compounds Which of the following has the highest boiling point? CH3CH2 CH CH3

1.

CH3

4.

CH3CH2 CH CH3

2.

CH3CH2 CH CH3 Cl

NH2

5.

CH3CH2 CH CH3

3.

OH

CH3CH2 CH CH3 CH2CH3

Physical Properties of Organic Compounds

Isomerism • Isomerism – The phenomenon whereby certain chemical compounds have structures that are different although the compounds possess the same elemental composition. • Isomers – Two or more chemical substances having the same elementary composition and molecular weight but differing in structure.

Isomerism • Consider C4H10 H H H H H C C C C H H H H H Normal Butane

H

H

H C

H

H

H C C C H H H H Isobutane

ISOMERS

STRUCTURAL

Skeletal Functional Positional Tautomers

OPTICAL

CONFIGURATIONAL

Compounds w/ same molecular formula but w/ structures that are mirror image of the other Rotate plane of polarized light equally but in opposite direction

Enantiomers – chiral, non-superimposable mirror images Diastereomers – nonsuperimposable, nonmirror images Anomers Epimers - different in geometry

CONFORMATIONAL

Same stereochemical configuration but differing in 3D conformation

Constitutional Isomers-skeletal C 4H10

C 5H12

Constitutional Isomers-skeletal C 6H14

?

Constitutional Isomers-skeletal

Constitutional Isomers-skeletal O

O H

OH

O

H

OH

O

Constitutional Isomerspositional OH OH

HO

OH

HO

OH

O

O

Constitutional Isomersfunctional OH

O

O

O H

Conformational Isomers

Conformational Isomers H θ

H

=

H

H H

Staggered θ = 60o

H

H H

= HH

H H

Eclipsed θ = 0o

Conformational Isomers

Conformational Isomers

θ Erel

o

60

0.9 kcal/mol

o

120

o

180

3.4 kcal/mol 0 kcal/mol

o

0

6.1 kcal/mol

Which is the most stable conformation for 1chloropropane about the C1-C2 bond? 1.

H H

Cl H

CH3

CH3

CH3 H

2.

H

H

H Cl

H

CH3 H

H

H

H

H

Cl CH3

H

4.

3.

Cl

5. Cl

H

H

H

H

H

H

Ring Conformers

Ring Conformers

Ring Conformers

H

H

H

H

H

H

H

H H

H H

H

Cyclohexane

Cyclohexane Ring Conformers

Cyclohexane Ring Conformers BOAT

H

H H

H H H

H H

H

H

H

H

Cyclohexane Ring Conformers BOAT

H

H H

H H H

H H

H

H

H

H

Steric interactions

Cyclohexane Ring Conformers BOAT

H

H H

H H H

H H

H

H

H

H

Eclipsing

Cyclohexane Ring Conformers CHAIR

H H

H H H

H

H

H H H

H H

Cyclohexane Ring Conformers CHAIR

H H

H H H

H

H

H H H

H H

Equatorial hydrogens

Cyclohexane Ring Conformers CHAIR

H H

H H H

H

H

H H H

H H

Axial hydrogens

Cyclohexane Ring Conformers

Cyclohexane Ring Conformers

Cyclohexane Ring Conformers

Cyclohexane Ring Conformers tert-Butylcyclohexane

severe steric interactions

much more stable

•

Geometric Isomerisms in Cycloalkanes Geometric isomers have the same molecular

formula and the same order of attachment but a different orientation in space that cannot be overcome by rotation around a s bond. 1,2-Dimethylcyclopentane:

H3C

CH3

1,2-dimethylcyclopentane

H H3C

H CH3

cis-1,2-dimethylcyclopentane

H H3C

CH3 H

trans-1,2-dimethylcyclopentane

Geometric Isomerisms in Cycloalkanes CH3

H C

H C

C

H

H

H3C

trans-1,2-dimethylcyclopropane

H3C

H

H

H

H

H

H3C

CH3 C CH3

H

H

H H

H

cis-1-tert-butyl-4-methylcyclohexane

Geometric Isomerisms in Cycloalkanes CH 3

H H3C

CH3

H CH3

H3C H

H CH3

Geometric Isomerisms in Cycloalkanes

Geometric Isomerisms in Cycloalkanes