Lecture 3- Isomers 1

What is the definition of isomers?

Isomers are molecules that have the same molecular formula but different structures, constitution, conformation (rotational structure), or configuration (arrangement of atoms in space).

What are the main types of structural isomers mentioned in the text?

The main types of structural isomers mentioned are

• skeletal isomers,

• positional or regio isomers, and

• isomers with different functional groups.

Explain what skeletal isomers are. Provide an example from the text.

Skeletal isomers have the same molecular formula but differ in the structure of their hydrocarbon skeleton or backbone.

For example, molecules with the formula C5H12 can be pentane or 2-methylbutane, which have different carbon backbones. Similarly, for C6H14, possible skeletal isomers are hexane, 2-methylpentane, and 2,3-dimethylbutane.

What are positional or regio isomers? Provide an example from the text.

Positional or regio isomers have the same molecular formula and the same functional groups, but the functional group is located at a different position along the carbon backbone.

For example, two isomers with the formula C3H8O can both be alcohols, but with the -OH group at a different carbon. Another example is xylenes (C8H10), where the methyl groups can be in the ortho (1,2), meta (1,3), or para (1,4) positions on the benzene ring.

Give an example of isomers with different functional groups from the text.

Isomers with different functional groups have the same molecular formula but different functional groups.

For example, molecules with the formula C2H6O can be an ether or an alcohol.

For C5H10O, examples include molecules with different arrangements of double bonds and hydroxyl groups.

Define tautomerism. What is required for this interconversion?

Tautomerism is a phenomenon where certain structural isomers are readily interconverted. This interconversion typically occurs in the presence of traces of acids or bases. The interconverting isomers are called tautomers.

What is prototropy?

Prototropy is a more precise term for tautomerism when the group R in the generalized tautomer representation is hydrogen, involving a proton shift.

Describe keto-enol tautomerism and provide an example.

Keto-enol tautomerism is a common type of tautomerism involving the interconversion between a keto form (containing a C=O bond) and an enol form (containing a C=C double bond and a hydroxyl group, -OH, attached to one of the carbons of the double bond).

Why is tautomerism important in biological systems?

Tautomerism is very important in several biological processes.

For example, the nitrogen bases cytosine, uracil, and thymine undergo amide-iminol tautomerism.

Also, the conversion of glucose–6–phosphate into fructose–6–phosphate in carbohydrate metabolism involves keto-enol tautomerism.

What is the X-Y-Z triad in the generalized representation of tautomerism? What are the two tautomers capable of?

The X, Y, Z triad in the generalized representation of tautomerism can be any sensible permutation of carbons, nitrogens, and oxygens. The two tautomers are each capable of stable independent existence.

Give an example of tautomers of uracil shown in the text.

The text shows uracil existing in at least two tautomeric forms, differing in the

• position of a hydrogen atom and

• a double bond within the ring.