3 Isomerism Lect 3

8/3/2019 3 Isomerism Lect 3

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ISOMERISM

Isomers are molecules which have the samemolecular formula but different structures.

There are a number of different types of isomerism inorganic compounds, we will consider structural

isomerism and stereoisomerism


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Structural Isomerism Structural isomers are molecules which have the

same molecular formula but a differentarrangement of covalent bonds.

The different arrangement of covalent bonds can resultfrom:

1. The functional group being in different positions(positional isomerism)

2. A different arrangement of the carbon skeleton(chain isomerism)

3. A different functional group (functionalisomerism)


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Positional isomerism Positional isomers are molecules with the same

molecular formula but which have the functionalgroup on different positions in the molecule.

Alkanes do not show functional isomerism as they have no

functional group.

Alkenes with four or more carbon atoms show positionalisomerism:

Eg but-1-ene and but-2-ene

C

H

H

H

HC C

HH

C

H

H

C C C

H

H

HHH

H

C

H

H


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Positional isomerism Haloalkanes with three or more carbon atoms show

positional isomerism

Eg 1-chloromethylpropane and2-chloromethylpropane

True for most other functional groups

CH3

CHCH3

ClCH2

CH3

CH3

Cl

C CH3


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Chain isomerism The arrangement of carbon atoms in an organic

molecule is known as the carbon skeleton.

Carbon skeletons containing up to three carbon atomscan only be arranged in one way i.e. a straight chain

with no branching:

Carbon skeletons containing four carbon atoms can bearranged in two ways:

C C C

C C CC C C C

C


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Chain isomerism Carbon skeletons containing five carbon atoms can be

arranged in three ways:

Carbon skeletons containing six carbon atoms can bearranged in five ways:

All molecules containing four or more carbon atoms

can thus show chain isomerism:

C C C CC

C C C

C

C

C C C

C

C

C C C CCC

C C C

C

CC

C CC

C

CC

C C

C

C

CC C C

C

C

CC


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Chain isomerism Eg butane and methylpropane

Eg pent-1-ene and 2-methylbut-1-ene

H

C

H

H

H C

H

H

C

H

H

HC

H

C

H

H

HC

H

C

H

HH

C

H

H

H

CC

H

H

H

H

H

HC C

H

H

C

H

H

C

HH

C

H

H

H

H

HC C

H

C

H

H


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Functional isomerism Functional isomers are molecules with the same

molecular formula but different functionalgroups.

Alkanes which have a ring rather than a straight chainarrangement are known as cycloalkanes. They have thegeneral formula CnH2n, which is the same as alkenes.Cycloalkanes and alkenes can thus show functionalisomerism.

Eg cyclohexane and hex-1-ene

C C

CC

CC

H HH

H

H

H

HH

H

H

H

H

CC

H

H

H

H

H

C C

H

H

C

H

H

C

H

H

H


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Functional group isomerism Eg 1-propanol, 2-ppropanol, and methyl-ethyl ether


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Stereoisomerism Molecules with the same atoms, and functional

groups, with the same connectivity, but differentorientation in space.

Includes geometrical (cis/trans) isomerism, andoptical isomerism.


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Geometrical isomerism

In a double bound molecules are not free to rotate inspace

In a double bond, the second bond is a -bond. This iscaused by the side-on overlap of two p-orbitals:

The result is an overlap in two places. This means that

the bond cannot be twisted and as a result there isrestricted rotation about the -bond. This is whythe cis and trans isomers cannot be interconverted andare therefore different

CH2 CH2 + or


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Geometrical isomerism In a double bound molecules are not free to rotate in

space

Because of this substituents s are held in place on afixed side the double bond. When similar substituents

are on the same side of the double bond, the moleculesis in the cis- conformation, while if the are across fromeach other they are in the trans configuration

CH2 CH2+ or


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Geometrical isomerism

C C

OH

Cl

CH3

Cl

C C

Cl

Cl

CH3

H

Cis Trans


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QuestionsName the following compounds:

a) b)

c) d)

CH3

CH3

H C

H

H

C

H

H

HCC

H

H

C C

H

H

H

H

C

H

H

CBr

H

C C C

H

H

HHH

H

C

H

H

C

H

H

CC

H

H

H

Cl HC

H

H

C

HH

H


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QuestionsName the following compounds:

e) CH3CH(CH3)CH3

f) CH3CHBrCH2Br g) CH2=CHCH2CH3

h) CH3CH=C(CH3)CH3


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QuestionsDraw the structures of the following compounds:

a) methylbutane b) cyclohexane

c) but-1-ene

d) 3-ethyl,2-methylhex-1-ene e) 3-chlorobut-1-

ene

f) 1,1-dichloropropane g) 2,2,4-trimethylheptaneh) pent-2-ene


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Questions Draw the four structural isomers of C4H9Cl and name

them.

Draw the three isomers of C4H8 containing a C=C bond andname them.

Draw the five structural isomers with the formula C6H14

and name them. Then arrange them in order of increasingboiling point, starting with the lowest.


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Questions Given two molecular formulae C4H10 and C4H8;

Select a compound which could be:

an alkane a cycloalkane

an alkene

In each case, draw one possible structure to show howyour choice is correct.

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3 Isomerism Lect 3