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

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 : C 6 H 14 CH 3 CH 2 CH 2 CH 2 CH 2 CH 3 Hexane → CH 3 CHCHCH 3 │ CH 3 │ → 2,3 - dimethylbutane

Example Pentane, C 5 H 12 Expanded Condensed

Positional Positional Isomers – Isomers that differ in the location of a noncarbon group or a double or triple bonds. Example : CH 3 – CH ═ CH – CH → 2 - butene CH 2 ═ CH – CH 2 – CH →1 - butene

Example C 3 H 7 Br

Functional Functional Isomers – Isomers with structural differences that place them in different classes of organic compouds. Example : C 2 H 6 O CH 3 – CH 2 – OH → ALCOHOL CH3 – O – CH → ETHER

Example C 3 H 6 O

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 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 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 eclipsedstaggered

“BOAT” CONFORMATION

“CHAIR” CONFORMATION

optical Isomers that are identical in structure except where they differ as mirror images.

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.

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