alkanes & isomersg

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 (C)

bp (C)

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


Compounds


Compounds Alkanes

Boiling points decrease with chain branching.

Compound bp

(oC)

mp

(oC)

CH3CH3

CH3 CH3

CH3

CH3 CH3

CH3 CH3

CH3

octane

4-methylheptane

2,2,4-trimethylpentane

57

121

107

127

118

99


Compounds

Alkanes

Solubility Like dissolves like

Alkanes are nonpolar, hydrophobic

They are soluble in nonpolar solvents and insoluble in water.


Compounds

CH3CH3

Compound MW bp

(oC)

H2O

solubility

86.18 69 insoluble

ClCl 98.96 83 0.8%

CH3CH3

O

O

86.09 88 20%

hexane

1,2-dichloroethane

2,3-butanedione


Compounds

CH3O

OCH3

HOOH

H2NNH2

CH3N

NCH3

H

H

Compound MW bp

(oC)

H2O

solubility

90.12 83

90.12 230

88.15 158

88.15 119

dimethoxyethane

1,4-butanediol

putrescine

N,N'-dimethylethylenediamine


Compounds

Which of the following has the highest

boiling point?

CH3CH2 CH

CH3

CH3 CH3CH2 CH

NH2

CH3 CH3CH2 CH

OH

CH3

CH3CH2 CH

Cl

CH3 CH3CH2 CH

CH2CH3

CH3

1. 2. 3.

4. 5.


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.

Consider C4H10

H

CH C

H

H

C

H

H

C

H

H

H

HC C

C

C

H

H

H HH

H

H HH H

Normal Butane Isobutane

Isomerism

ISOMERS

CONFIGURATIONAL OPTICAL STRUCTURAL CONFORMATIONAL

Skeletal Functional Positional Tautomers

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 non-superimposable, non-mirror images Anomers Epimers - different in geometry

Same stereochemical configuration but differing in 3D conformation

Constitutional Isomers-skeletal

C4H10

C5H12


C6H14 ?


H

O

H

O

OH OH

O O

Constitutional Isomers-positional

OH

OH

HO OH HO

OH

O

O

Constitutional Isomers-functional

OH O

H

O O

Conformational Isomers


=

H

H H

H H

H

Staggered = 60o

=

H

H H

H

HH

Eclipsed = 0o


E r e l

1 8 0 o

0 k c a l / m o l

1 2 0 o

3 . 4 k c a l / m o l

6 0 o

0 . 9 k c a l / m o l

0 o

6 . 1 k c a l / m o l

Which is the most stable conformation for 1-

chloropropane about the C1-C2 bond?

ClH

H

CH3

H H

HH

Cl

CH3

H H

HCl

H

CH3

H H

Cl

H H

CH3

H H

H

H Cl

CH3

H H

1. 2. 3.

4. 5.

Ring Conformers

Ring Conformers

H H

H

H

H

H

H H

H

H

H

H

Cyclohexane

Cyclohexane Ring Conformers


HH

H

H

H H

H

H H

H

HH

BOAT


HH

H

H

H H

H

H H

H

HH

Steric

interactions

BOAT


HH

H

H

H H

H

H H

H

HH

Eclipsing

BOAT


H

H

H

H

H

H

H

H

H

H

H

H

CHAIR


Equatorial

hydrogens H

H

H

H

H

H

H

H

H

H

H

H

CHAIR


Axial

hydrogens H

H

H

H

H

H

H

H

H

H

H

H

CHAIR


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

H3C CH3

H H

H3C H

H CH3

1,2-dimethylcyclopentane

cis-1,2-dimethylcyclopentane trans-1,2-dimethylcyclopentane

Geometric Isomerisms in Cycloalkanes

cis-1-tert-butyl-4-methylcyclohexane

trans-1,2-dimethylcyclopropane

C

C C H

H 3 C H

C H 3 H

H

H 3 C

H H

C C H 3

C H 3 H 3 C

H H

H H

H

H H

H

Geometric Isomerisms in Cycloalkanes CH3

CH3

H

CH3

H

H3C

H

CH3

H3C

H

Geometric Isomerisms in Cycloalkanes

alkanes & isomersg

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