alkanes & isomersg 1

8/6/2019 Alkanes & IsomersG 1

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Physical Properties of Organic

Compounds Alkanes

Melting and boiling points increase with increasingmolecular 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


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Compounds


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Compounds


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Compounds Alkanes

Boiling points decrease with chain branching.

Compoundbp

(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


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Compounds

Alkanes

Solubility Like dissolves like

Alkanes are nonpolar, hydrophobicThey are soluble in nonpolar solvents

and insoluble in water.


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Compounds

CH3

CH3

Compound MW p

(oC)

H2O

solubilit

6. 6 insolubl

ClCl . 6 3 0. %

CH3CH3

O

O

86.09 88 20%

x ne

1,2-di loroet ne

2,3-but nedione


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Compounds

CH3O

OCH3

HOOH

H2NNH2

CH3N

NCH3

H

H

Compound MWbp

(oC)

H2O

solubilit

90.12 83

90.12 230

88.15 158

88.15 119

dimet ox et ne

1, -but nediol

putres ine

N,N'-dimet let lenedi mine

g

g

g

g


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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.


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Compounds


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Compounds


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Isomerism

Isomerism The phenomenonwhereby certain chemical compoundshave structures that are different

although the compounds possess thesame elemental composition.

Isomers Two or more chemicalsubstances having the same elementary

composition and molecular weight butdiffering in structure.


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Consider C4H10

H

CH C

H

H

C

H

H

C

H

H

H

HC C

C

C

H

H

H HH

H

H H

H H

Normal Butane Isobutane

Isomerism


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ISOMERS

CONFIGURATIONALOPTICALSTRUCTURAL CONFORMATIONAL

SkeletalFunctionalPositionalTautomers

Compounds w/ samemolecular formula butw/ structures that aremirror image of theother

Rotate plane ofpolarized light equallybut in opposite direction

Enantiomers chiral,non-superimposablemirror images

Diastereomers non-superimposable, non-mirror imagesAnomersEpimers - different ingeometry

Samestereochemicalconfiguration butdiffering in 3Dconformation


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ConstitutionalIsomers-skeletal

C4H10

C5H12


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C6H14 ?


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H

O

H

O

OHOH

O

O


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ConstitutionalIsomers-positional

OH

OH

HO OHHO

OH

O

O


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ConstitutionalIsomers-functional

OH O

H

O O


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ConformationalIsomers


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=

H

H H

H H

H

St ggered

U = 60o

U

=

H

H H

H

H

H

Ecli

U


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U

Erel

180o

0 kcal/mol

120o

3.4 kc al/mol

60o

0.9 kcal/mol

0o

6.1 kcal/mol


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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.

alkanes & isomersg 1

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