ISOMERISM

Isomerism

Isomers are a group of compounds with the same molecular formula but different structural formula .
Isomers are into tow types ;

• structural isomers
• stereoisomerism

Structural Isomers

They have the same r formula but different molecular structure .This means that the isomers have the same number of atoms of the same elements but the atoms are arranged differently .There are three structural isomers and they are ;
1)Carbon-chain
2)Positional
3)Functional Group

• Carbon-chain

-Different arrangement of carbon skeleton
-same chemical properties
-More branching=lower boiling point
for example butane and methyl propane bothy have the same molecular formula C4H10 but butane has a chain of four atoms and methyl propane has 3 .
Illustration:

butane                                                                                                         Methyl propane

• Positional Isomer  

-Have the same functional group in different locations on the carbon framework

-Have the same similar chemical properties but different chemical properties

-Same carbon skeleton .

Example : Propan-1-ol and propan-2-ol

and pent-1-ene and pent-2-ene

pent-2-ene

                                                                                                    pent-1-ene

• Functional Group

-Different functional group

-Different chemical and physical properties

-Members of different homologous series with the same general formula

Example ;

Methyl methanoate and Ethanoic acid

methyl methanoate                       Ethanoic acid

STEREOISOMERISM

Same Molecular  formula but atoms are joined to each other in a different spatial arrangement .There are two types ,Geometric and optical

• Geometric Isomers

Isomers that differs only in the spatial arrangement of atoms in the planar part of the molecule or above or below a ring .

Example; but-2-ene exists as two geometric isomers .The double-bonded carbon atoms and the four atoms joined to them all lie in a plane and can take up different spatial positions

In the right -hand isomer ,the two -CH3 group are on the same side and its called cis isomer . On the left hand ,the two CH3 group are opposite at 180 degrees to each other. This type is called trans isomers .

Reasons for Geometric Isomerism

• Restricted rotation around the double bond
• Two different atoms or groups of atoms on each carbon of the double bond

Priorities has to be assigned to the atoms attached to each of the double bond carbon atoms . The higher the atomic number ,the higher the priority assigned to that atom . If two higher priority atoms are on the opposite side of the double bond ,the isomer is labelled E and if they are on the side ,the isomer is labelled Z

note; forget about the 2 in front for now