ISOMERISM

A property in which two or more compounds

having the same molecular formula

but different arrangements of atoms

in a structure…………….

Isomers – are compounds with the

same molecular formula but having different

structural orientations. Note: isomers do not necessary share similar properties unless

they belong to one family.

CLASSIFICATIONS

ISOMERISM

STRUCTURAL

STEREO

OPTICAL

GEOMETRICAL

CONFORMATIONAL

FUNCTIONAL

POSITIONAL

CHAIN/SKELETAL

Structural Isomerism

or sometimes called constitutional isomers is defined as isomers that

vary in the bonding attachments of atoms.

3 TYPES

Skeletal • Skeletal Isomers – Isomers

that differ in the arrangement of the carbon chain. Example : C6H14

CH3CH2CH2CH2CH2CH3Hexane

-------------→

CH3CHCHCH3

│CH3

│CH3

-------------→ 2,3 - dimethylbutane

Example Pentane, C5H12

Expanded

Condensed

Positional• Positional Isomers – Isomers

that differ in the location of a noncarbon group or a double or triple bonds. Example :CH3 – CH ═ CH – CH3 ---------→ 2 - butene

CH2 ═ CH – CH2 – CH3 -------------→ 1 - butene

Example C3H7Br

Functional •Functional Isomers –

Isomers with structural differences that place them in different classes of organic compouds. Example : C2H6O

CH3 – CH2 – OH -----------→ ALCOHOL

CH3 – O – CH3 -----------→ ETHER

Example C3H6O

Stereoisomerism

Isomers with the same bonding attachments of

atoms but different spatial orientations.

3 TYPES

Geometric• Or sometimes called cis and trans isomers – a type of stereoisomerism in

which atoms or groups display orientation

differences around a double bond or ring.

Example

Cis - Isomer• Geometric isomer in which

groups are on the same side of the ring or double bond.

Example : 1,2 – dichloroethane

C ═ C

Cl Cl

H H

Trans – Isomer

• Geometric Isomer in which groups are on the opposite sides of ring or double bond.

Example : 1,2 - dichloroethane

C ═ C

Cl

Cl

H

H

Conformational•Isomers that differ

as a result of the degree of rotation around a carbon – carbon single bond.

2 Kinds

“ECLIPSED”• The highest energy conformation

corresponds to a structure in which the hydrogen atoms are "eclipsed.“

• Geometric isomer in which conformation around carbon – carbon single bonding which attached atoms are as close together as possible.

“ECLIPSED” Ethane

“STAGGERED”• The lowest energy

conformation is a structure in which the hydrogen atoms are "staggered“

• Conformation around a carbon – carbon single bond in which attached atoms are as far apart as possible.

“STAGGERED” Ethane

Difference

eclipsed staggered

“BOAT” CONFORMATION

“CHAIR” CONFORMATION

optical• Isomers that are

identical in structure except where they

differ as mirror images.

Example

Example

Example

• Isomerism was first noticed in 1827, when Friedrich Wohler prepared cyanic acid and noted that although its elemental composition was identical to fulminic acid (prepared by Justus von Liebig the previous year), its properties were quite different.

• This finding challenged the prevailing chemical understanding of the time, which held that chemical compounds could be different only when they had different elemental compositions.

• After additional discoveries of the same sort were made, such as Wohler's 1828 discovery that urea had the same atomic composition as the chemically distinct ammonium cyanate, Jöns Jakob Berzelius introduced the term isomerism to describe the phenomenon.

• In 1849, Louis Pasteur separated tiny crystals of tartaric acid into their two mirror-image forms. The individual molecules of each were the left and right optical Stereoisomer, solutions of which rotate the plane of polarized light in opposite directions.

FIN