Ch.19 Hybridization, Isomerism, and Nomenclature Review Points

syllabus

Hybridisation of carbon;
Sigma and pi-bonds;
Shapes of molecules;
Structural and geometrical isomerism;
Optical isomerism of compounds containing up to two asymmetric centers, (R,S and E,Z nomenclature excluded);
Conformations of ethane and butane (Newman projections);
IUPAC nomenclature of simple organic compounds (only hydrocarbons, mono-functional and bi-functional compounds);
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Hybridization of carbon
sp - in alkynes - triple bond
sp2 - in alkenes - double bond
sp3 - in alkanes

sigma and pi bonds
alkanes - all sigma bonds with hydrogen atoms or single bonds with carbon atoms
alkenes - one sigma and one pi bond in double bond, and all sigma bonds with hydrogen atoms or single bonds with carbon atoms
alkynes - one sigma and two pi bonds in triple bond, and all sigma bonds with hydrogen atoms or single bonds with carbon atoms

Isomerism

The existence of two or more compounds with same molecular formula but different properties (physical, chemical or both) is known as isomerism; and the compounds themselves are called isomers.

Isomerism types:

i) Chain, nuclear or skeleton isomerism

This type of isomerism is due to the difference in the nature of the carbon chain (i.e. straight or branched) which forms the nucleus of the molecule,

ii) Position isomerism

It is due to the difference in the position of the substituent atom or group or an unsaturated linkage in the same carbon chain.

iii) Functional isomerism



This type of isomerism is due to difference in the nature of functional group present in the isomers,


iv) Metamerism

It is due to the difference in nature of alkyl groups attached to the same functional group. This type of isomerism is shown by compounds of the same homologous series.

v) Tautomerism

Tautomerism may be defined as the phenomenon in which a single compound exists in two readily interconvertible structures that differ markedly in the relative position of at least one atomic nucleus, generally hydrogen. The two different structures are known as tautomers of each other.

Stereo isomerism

When isomers have the same structural formula but differ in relative arrangement of atoms or groups in space within the molecule, these are known as stereoisomers and the phenomenon as stereoisomerism. The spatial arrangement of atoms or groups is also referred to as configuration of the molecule and thus we can say that the stereoisomers have the same structural formula but different configuration. Stereoisomerism is of two types.

(i) Geometrical isomerism

The isomers which possess the same structural formula but differ in the spatial arrangement of the groups around the double bond are known as geometrical isomers and the phenomenon is known as geometrical isomerism.

ii) Optical isomerism


This type of isomerism arises from different arrangements of atoms or groups in three dimensional space resulting in two isomers which are mirror image of each other. Optical isomers contain an asymmetric (chiral) carbon atom ( a carbon atom attached to four different atoms or groups) in their molecules.

Nomenclature

The longest possible chain is numbered from one side to the other by Arabic numerals, the direction being so chosen as to given the lowest numbers possible to the side chains. When series of locants containing the same number of terms are compared term by term, that series is “lowest” which contains the lowest number on the occasion of the first difference (Lowest sum rule). This rule is applied irrespective of the nature of the side chains.

Univalent branched radicals derived from hydrocarbon are named by prefixing the designation of the side chains to the name of the unbranched alkyl radical containing the LPCC starting from the carbon atom with the free valence, this atom being numbered as 1.

If two or more side chains of different nature are present, they are cited in alphabetical order and decided as follows

(i) The names of simple radicals are first alphabetized and the multiplying prefixes are then inserted.
Ethyl is cited before methyl, thus 4-Ethyl-3, 3-dimethylheptane

ii) The name of a complex radical is considered to start with the first letter of its complete name.

(iii) In cases where names of complex radicals are composed of identical words, preference for citation is given to that radical which contains the lowest locant at the first cited point of difference in the radical.