L3 - Structural Isomerism

objectives

• Structural isomers are compounds with the same molecular formula but different arrangements of atoms.
• Saturated compounds contain single bonds only and unsaturated compounds contain double or triple bonds.

Structural Isomerism - Definition and Types

'COMPOUNDS HAVE THE SAME MOLECULAR FORMULA BUT DIFFERENT STRUCTURAL FORMULA'

Chain Isomerism

• different arrangements of the carbon skeleton
• similar chemical properties
• slightly different physical properties
• more branching = lower boiling point

Chain Isomerism - Example - Butane

• There are two structural isomers of C₄H₁₀.
• One is a straight chain molecule where all the carbon atoms are in a single row.
• The other is a branched molecule where three carbon atoms are in a row and one carbon atom sticks out of the main chain.

Differences between Chain Isomers

Chemical

• Isomers show similar chemical properties because the same functional group is present.

Physical

• Properties such as density and boiling point show trends according to the of the degree of branching

Boiling Point

• “straight” chain isomers have higher values than branched ones
• the greater the degree of branching the lower the boiling point
• branching decreases the effectiveness of intermolecular forces so less energy has to be put in to separate the molecules

Positional Isomerism

• same carbon skeleton
• same functional group
• functional group is in a different position
• similar chemical properties - slightly different physical properties

Example - Position of Double Bond in an Alkene

There are no other isomers with five C’s in the longest chain but there are three other structural isomers with a chain of four carbons plus one in a branch.

Example 2 - Position of a Halogen on a Halogenoalkane

Moving the chlorine along the chain makes new isomers; the position is measured from the end nearest the functional group... the third example is 2- NOT 3-chlorobutane.

There are 2 more structural isomers of C₄H₉Cl but they have a longest chain of 3

Example 3 - Positions on a Benzene Ring

Functional Group Isomerism

• different functional group
• different chemical properties
• different physical properties

Example - Aldehydes and Ketones

• Sometimes more than one type of isomerism occurs in the same molecule.

• The more carbon atoms there are, the greater the number of possible isomers

Summary Videos