NIOS Lesson 23 - NOMENCLATURE AND GENERAL PRINCIPLES
Organic compounds are all around us in several forms. They are present in a vast range of substances like fuels, foods, polymers and plastics, textiles, dyes, drugs, medicines, explosives, cosmetics, paints and pesticides.
The word organic is derived from the word organism because the body of living things is composed mainly of organic compounds. In addition to the organic compounds of animals and plants origin, a large number of them have been synthesized in the laboratory.
All organic compounds are known to contain carbon. The carbon atoms have a unique property called ‘catenation’ which is the ability to form long chains, rings and networks of carbon atoms resulting into the formation of large number of carbon compounds.
The basic organic compounds are hydrocarbons (compounds of carbon and hydrogen) which can be converted to different types of organic compounds by performing different reactions.
The organic chemistry is the branch of chemistry which deals with the study of compounds of carbon. Some compounds containing carbon are not studied in this branch of chemistry such as oxides of carbon, metal carbides, metal cyanides, and metal carbonates and these come under ‘Inorganic Chemistry’.
This lesson describes various rules for naming of organic compounds based upon IUPAC system. A distinction between different types of bond fission in organic compounds is also explained. Various types of reactions and electronic effects are discussed with examples. This lesson also covers different types of isomerism.
OBJECTIVES
After reading this lesson, you will be able to:
name various types of organic compounds according to IUPAC system;
distinguish between different types of bond fission;
explain different types of reactions: substitution, addition, elimination and molecular rearrangements;
identify nuclophiles and electrophiles;
explain electronic effects in a covalent bond such as inductive effect;
electromeric effect, resonance, hyperconjugation and steric hindrance;
explain structural isomerism and stereoisomerism.
define absolute configuration;
assign absolute configuration (R-S and D-L) to a chiral centre; and
qualitative and quantitative analysis of organic compound.
23.1 CLASSIFICATION OF HYDROCARBONS
All organic compounds may be divided into two broad classes based upon the pattern of chain of carbon atoms. Let us now understand these classes of compounds.
1. Open-chain or Aliphatic compounds: This class includes all hydrocarbons (saturated and unsaturated) and their derivatives which have open-chain structures. Saturated hydrocarbons are those which contain single bonds between all carbon atoms such as
2. Closed-chain or cyclic compounds: These compounds have atleast one ring (cyclic) system. These are further divided into two sub-classes: homocyclic and heterocyclic based on the atoms present in the ring. They are called homocyclic or carbocyclic when the ring is formed by carbon atoms only.
Homocyclic (carbocyclic) compounds may again be divided into two groups namely alicyclic and aromatic compounds.
(i) Alicyclic compounds: This group includes saturated and unsaturated cyclic hydrocarbons which resemble with the aliphatic hydrocarbons in properties.
Some examples are given below:
The above compounds can be represented in the form of condensed structures Compounds as shown below where each corner represents a –CH2–group.
(ii) Aromatic compounds: The group of homocyclic compounds having special set of properties are called aromatic compounds which will be discussed in Lesson 24. They also have characteristic smell or aroma and hence called aromatic. These include aromatic hydrocarbons and their derivatives are examples of such compounds are as follows:
23.2 NOMENCLATURE OF ORGANIC COMPOUNDS
In the beginning, the organic compounds were named after the source from which
they were obtained e.g. methane was named as marsh gas as well as damp fire
because it was obtained from marshy places. Similarly, formic was named so
because it was obtained from red ants (Latin name formica). These names of
organic compounds are called common names or trivial names. There was no
systematic basis for naming them and it was very difficult task to remember the
names of so many organic compounds. Even the same compound was known by
different names. In order to bring uniformity and rationality in naming the organic
compounds throughout the world, International Union of Chemistry (in 1958)
came out with a system of nomenclature later known as IUPAC (International
Union of Pure and Applied Chemistry) system. Before explaining IUPAC system
of nomenclature, we shall discuss about homologous series.
Homologous Series: A series of compounds in which the molecular formula of a
compound differs from those of its neighbouring compounds by the CH2 group,
is known as a homologous series. Each of such homologous series is given a
general name. For example, homologous series of open chain saturated
hydrocarbons is known as alkanes and open chain unsaturated hydrocarbons
form two series of compounds namely alkenes and alkynes, which contain carbon
- carbon double bond and triple bond, respectively. Some members of homologous
series of aliphatic hydrocarbons are listed in the Table 23.1.
23.2.1 IUPAC Nomenclature of Acyclic Hydrocarbons
Acyclic hydrocarbons include straight chain as well as branched chain compounds.
(a) Straight chain Hydrocarbons: The names of these hydrocarbons consist of Compounds two parts. The first one is word root and second one is suffix. The word root designates the number of carbon atoms in the chain. Special word roots (Meth-, Eth-, Prop-, But-, etc.) are used for chains containing one to four carbon atoms but for chains of five and more carbon atoms, Greek number roots such as Pent-, Hex - etc. are used the in IUPAC word roots for a few carbon chains are given below in Table 23.2.
The general word root for any carbon chain is alk.
In order to write the IUPAC name, a suffix is added to the word root to indicate saturation or unsaturation in the hydrocarbons. These suffixes are listed below in the Table 23.3.
b) Branched chain Hydrocarbons
In branched chain hydrocarbons, one or more alkyl groups are present as side
chain attached to the main straight chain of carbon atoms. The carbon atoms of
the side chain constitute alkyl groups. These alkyl groups are written as prefixes
in the IUPAC name. An alkyl group is obtained from an alkane by removing one
hydrogen atom. Since the general formula of alkane is CnH2n+2, the general formula
of alkyl group is CnH2n+1. The alkyl groups are generally represented by R– and named by replacing the suffix ane of the corresponding alkane by yl. Let us see
some examples of the alkyl groups given in the Table 23.4.
Branched chain hydrocarbons are named using the following rules in IUPAC
system.
Rule 1. Longest chain Rule: According to this rule, the longest possible chain of carbon atoms is considered and the compound is named as the derivative of the corresponding alkane. If some multiple bond is present, the selected chain must contain the carbon atoms of the multiple bond. The number of carbon atoms in the selected chain determines the word root and the saturation or unsaturation will determine the suffix.Let us consider the following example:
Since it has a main chain of six carbon atoms; hence, it will be named as a derivative of hexane.
In the given structure, we find that the longest chain consists of five carbon
atoms and the substituent is methyl group at position number 3. The word root is
Pent and suffix is ane. Hence, the name is 3-methylpentane.
Rule 5 : Naming the same alkyl groups at different positions or more than
one alkyl groups
If the compound contains more than one identical alkyl groups, their positions
are indicated separately and the prefixes di (for two), tri (for three) etc. are attached
to the name of the substituents. The positions of the substituents are separated by
commas (,). In the following structure, two methyl groups are attached to the
main chain of five carbon atoms.
You can see that they are attached to the positions 2 and 3 of the main chain.
Hence, the name of the compound is 2,3-dimethylpentane.
Rule 6: Naming different alkyl substituents
If there are different alkyl substituents present in the compound, their names are written in the alphabetical order. However, the prefixes di, tri, etc. are not considered in deciding the alphabetical order.
For example, in the compound shown below the longest chain consists of five carbon atoms; hence, the parent hydrocarbon is pentane. The main chain has two methyl groups at C2 and C3 and one ethyl group at C3 as substituents. The names of these alkyl
23.2.2 Nomenclature of Cyclic Hydrocarbons
We already know that cyclic hydrocarbons can be divided into alicyclic and
aromatic compounds. Now let us learn the nomenclature of these compounds.
a) Alicyclic Compounds
As we have already discussed (in Section 23.3) that alicyclic compounds have closed chain i.e. cyclic structures, hence their names are derived by putting prefix ‘cyclo’ before the word root. The suffix ane, ene or yne are written according to the saturation or unsaturation in the ring structure. Given below are some examples of alicyclic compounds.
WHAT YOU HAVE LEARNT
Organic compounds are classified into aliphatic (open-chain), homocyclic (closed ring) or carbocyclic (alicyclic and aromatic) hydrocarbons and their derivatives, and heterocyclic compounds (contain at least one heteroatom i.e., N,S,O in the ring).
Homologous series of organic copmounds and IUPAC naming of different classes.
The substitution, elimination, additions reactions and molecular rearrangements.
Homolytic fission of a covalent bond produces free radicals because each of the parting away atoms takes away its shared electron.
Heterolytic fission of a covalent bond produces ions because one of the atoms takes away both the shared electrons.
Electrophiles are positively charged or electron deficient species.
Nucleophiles are negatively charged or electron rich species.
Benzene ring undergoes aromatic substitution reactions.
A functional group is an atom or group responsible for specific properties of a compound.
The compounds which have the same molecular formula but different structure are called structural isomers.
Isomerism is classified into structural isomerism and stereoisomerism.
A carbon atom attached to four different groups is called chiral atom or asymmetric carbon atom.
The non-superimposable mirror image isomers of a compound are called enantiomers. They are optically active and rotate the plain of plain polarized light in opposite directions.
The absolute configuration of a chirol carbon can be specified as R or S.
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