ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES

 ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES

CLASSIFICATION OF ORGANIC COMPOUNDS

I.  Acyclic or open-chain compounds

These compounds are also called as aliphatic compounds and consist of straight or branched chain compounds, for example:

II  Cyclic or closed chain or ring compounds

(a) Alicyclic compounds

Alicyclic (aliphatic cyclic) compounds contain carbon atoms joined in the form of a ring (homocyclic).

Sometimes atoms other than carbon are also present in the ring (heterocyclic). Tetrahydrofuran given below is an example of this type of compound:

(b) Aromatic compounds

Aromatic compounds are special types of compounds. These include benzene and other related ring compounds (benzenoid).  

Benzene has a homocyclic hexagonal ring of six carbon atoms with three double bonds in the alternate positions. 

Like alicyclic compounds, aromatic comounds may also have hetero atom in the ring. Such compounds are called heterocyclic aromatic compounds. Some of the examples of various types of aromatic compounds are:

Benzenoid aromatic compounds

Non-benzenoid compound

Heterocyclic aromatic compounds

Functional Group

The functional group is an atom or a group of atoms joined to the carbon chain which is responsible for the characteristic chemical properties of organic compounds. 

 Examples are:

 hydroxyl group (–OH), aldehyde group (–CHO) and carboxylic acid group (–COOH), etc.

 

Homologous Series

A group or a series of organic compounds each containing a characteristic functional group forms a homologous series

👉 Members of the series are called homologues. 

👉The successive members differ from each other in molecular formula by a `–CH_2` unit.

Some of these are alkanes, alkenes, alkynes, haloalkanes, alkanols, alkanals, alkanones, alkanoic acids, amines etc.

Hydrocarbons

Compounds containing carbon and hydrogen only are called hydrocarbons.

Saturated Hydrocarbon 

A hydrocarbon is termed saturated if it contains only carbon-carbon single bonds. 
The IUPAC name for a homologous series of such compounds is alkane.

Unsaturated Hydrocarbons 

Unsaturated hydrocarbons are those, which contain at least one carbon-carbon double or triple bond.

Trivial or common names

These names are traditional and are considered as trivial or common names. 

👉 Organic compounds were assigned names based on their origin or certain properties.

Examples:

Citric acid is named so because it is found in citrus fruits.

Acid found in red ant is named formic acid since the Latin word for ant is formica.

The IUPAC System of Nomenclature

IUPAC (International Union of Pure and Applied Chemistry) 

In this systematic nomenclature, the names are correlated with the structure such that the reader or listener can deduce the structure from the name.

👉 A systematic name of an organic compound is generally derived by identifying the parent hydrocarbon, alkyl groups, and the functional groups attached to it. See the example given below.

IUPAC Nomenclature of Alkanes

👉 Straight chain hydrocarbons

👉 Branched chain alkanes

👉 Cyclic Compounds

👉 Straight chain hydrocarbons

The names of such compounds are based on their chain structure, and end with suffix ‘-ane’ and carry a prefix indicating the number of carbon atoms present in the chain (except from `CH_4` to `C_4H_10`, where the prefixes are derived from trivial names). 

 

Examples:

Alkyl Group

An alkyl group is derived from a saturated hydrocarbon by removing a hydrogen atom from carbon.

Thus, `CH_4` becomes `-CH_3` and is called methyl group. An alkyl group is named by substituting ‘yl’ for ‘ane’ in the corresponding alkane. 

 

👉 Branched-chain hydrocarbons

In a branched chain compound, small chains of carbon atoms are attached at one or more carbon atoms of the parent chain. The small carbon chains (branches) are called alkyl groups. For example:

The rules for naming them are given below

Rule 1

 The longest carbon chain in the molecule is identified. In the example (I) given below, the longest chain has nine carbons and it is considered as the parent or root chain. Selection of parent chain as shown in (II) is not correct because it has only eight carbons.

Rule 2

The carbon atoms of the parent chain are numbered to identify the parent alkane and to locate the positions of the carbon atoms at which branching takes place due to the substitution of alkyl group in place of hydrogen atoms. The numbering is done in such a way that the branched carbon atoms get the lowest possible numbers. Thus, the numbering in the above example should be from left to right (branching at carbon atoms 2 and 6) and not from right to left (giving numbers 4 and 8 to the carbon atoms at which branches are attached).

Rule 3

The names of alkyl groups attached as a branch are then prefixed to the name of the parent alkane and position of the substituents is indicated by the appropriate numbers. If different alkyl groups are present, they are listed in alphabetical order. Thus, name for the compound shown above is: 6-ethyl-2-methylnonane. [Note: the numbers are separated from the groups by hyphens and there is no break between methyl and nonane}]

Rule 4

If two or more identical substituent groups are present then the numbers are separated by commas. The names of identical substituents are not repeated, instead prefixes such as di (for 2), tri (for 3), tetra (for 4), penta (for 5), hexa (for 6) etc. are used. While writing the name of the substituents in alphabetical order, these prefixes, however, are not considered. Thus, the following compounds are named as: