Stereochemistry

1. Stereochemistry

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4. I)-atoms e.g., n- and isobutStructural isomerism:Is due to the difference in structure or constitution in three different ways.Chainor skeletal isomerism:Is exhibited by compounds which differ in the arrangement of the carbon atoms.

5. 2- Position isomerismIs exhibited by compounds having the same carbon skeleton but differing in the position occupied by a substituent group, e.g.,n- and isopropyl alcohols; ortho-, meta- and para-nitrophenols.

6. 3- Functional group isomerism Is exhibited by compounds having different functional groups; i.e., compounds with the same molecular formula but belonging to different homologous series, e.g., ethanol and dimethyl ether; acetone and propionaldehyde.

7. Keto-Enol system:Aldehydes and ketones with α- hydrogen atom are in equilibrium with the enol forms. The tautomeric shift is catalyzed by acids, bases or solvents. The sum of the bond energies of the keto form (361 K.cal/mol) is more than that of the enol form (343 K.cal/mol) by 18 K.cal/mol. Hence in simple aldehydes and ketones, the ketoform is stronger than the enol form. Conjugations and chelation stabilize the enol form thus the enol form in acetyl acetone and benzoylacetone is 76% and 90% respectively.

8. 2- Valence tautomerismIt involves change in electronic distribution with changes in bonds lengths and angles e.g., cycloocta-1,3,5-triene.

9. 3- Ring chain tautomerism Molecules containing a nucleophilic and a multiple bond group, and capable of forming a 5- or 6- membered ring, can exist in two tautomers i.e., the chain and the ring tautomer.