Classification of Organic Compounds – Definition, Properties, Homologous Series

Classification of Organic Compounds: Almost everything that we come across in our daily life is made up of organic compounds. Be it the cooking gas, the egg, or the plastic bag; everything is made up of organic compounds. About \(90\% \) of all compounds are organic. Thus, to study such a vast number of compounds, it is necessary to classify them. Let us dive into this article to know more about the categories in which Organic Compounds are classified.

Introduction to Organic Compounds

Organic compounds are compounds that contain carbon and hydrogen atoms covalently bonded to each other. The large variety of organic compounds is the result of the high catenating power of carbon atoms. The covalently bonded carbon-hydrogen chains form the backbone of a large variety of organic compounds. This means that all organic compounds have in common the presence of carbon atoms and hydrogen atoms.

Classification of Organic Compounds

Organic compounds can be broadly classified based on two categories:

1. Classification based on the carbon skeleton or structure.
2. Classification based on the presence of Functional Groups.

Classification Based on Carbon Skeleton or Structure

Depending upon the arrangement of carbon atoms in their structure, organic compounds are broadly categorised into:

1. Open Chain or Acyclic or Aliphatic compounds
2. Closed Chain or Cyclic or Ring chain compounds

The flow chart below will give a clear idea about the classification of organic compounds.

Open Chain or Acyclic or Aliphatic Compounds

The organic compounds in which the carbon atoms are present in long open chains are called Open Chain or Acyclic or Aliphatic hydrocarbons. These compounds are called Acyclic because they lack the presence of a cyclic structure. The two ends of the chain do not meet. Hence, these are open.

These compounds are known as Aliphatic compounds because these compounds are derived from either animal or vegetable fats. The open-chain or acyclic, or aliphatic compounds can further be categorised into:

Saturated Compounds

The hydrocarbons that contain only carbon-carbon single bonds are called saturated compounds. These are the simplest class of hydrocarbons. These hydrocarbons are saturated because each carbon atom is bonded to as many hydrogen atoms as possible. In other words, the carbon atoms are saturated with hydrogen and contain the maximum number of hydrogen atoms. For example – Alkanes

The saturated hydrocarbons are further classified into two sub categories which are:

1. Straight Chain Compounds: The compounds in which the carbon skeleton is in the form of a straight chain are called Straight Chain saturated hydrocarbons. Examples:
   \(\rm{n}\)-butane \({\text{C}}{{\text{H}}_{\text{3}}}{\text{-C}}{{\text{H}}_{\text{2}}}{\text{-C}}{{\text{H}}_{\text{2}}}{\text{-C}}{{\text{H}}_{\text{3}}}\)
   Butene \({\text{C}}{{\text{H}}_{\text{2}}}{\text{=CH-C}}{{\text{H}}_{\text{2}}}{\text{-C}}{{\text{H}}_{\text{3}}}\)
2. Branched Chain Compounds: The compounds in which the carbon skeleton form a branched-chain are called Branched Chain saturated hydrocarbons.
   Examples: Neopentane

Unsaturated Compounds

The hydrocarbons that contain carbon-carbon double bond and carbon-carbon triple bond are unsaturated compounds. These hydrocarbons are unsaturated because more hydrogen atoms can be added to the hydrocarbon to make it saturated,i.e., a carbon-carbon single bond. For example- Alkenes, Alkynes

Closed Chain or Cyclic Compounds

The compounds which contain one or more closed chains or rings of atoms in their structure are called Closed Chain or Cyclic hydrocarbons. When both the ends of an aliphatic carbon chain meet, a closed chain is obtained. When only one ring of atoms is present in the molecule, it is known as monocyclic, whereas the compounds with more than one ring of atoms are termed polycyclic.

The Closed chain compounds are further classified into the following-

Alicyclic Compounds

These compounds resemble that of the aliphatic compounds. These are ring derivatives of aliphatic hydrocarbons. As at least three carbon atoms are required to form a ring, these compounds contain rings of three or more carbon atoms. For example- Cyclobutane, Tetrahydrofuran
Alicyclic compounds are further classified into two categories which are-

Homocyclic closed chain compounds –  The cyclic compounds that contain rings of only one type of atom are called homocyclic compounds. For example-

Heterocyclic closed chain compounds – The cyclic compounds that contain heteroatoms such as \(\rm{O}\), \(\rm{N}\), etc. in the carbon ring are called heterocyclic compounds. For example –

Aromatic Compounds

The cyclic unsaturated compounds that exhibit aromaticity are called aromatic compounds. Compounds exhibiting aromaticity should satisfy Huckel’s Rule. To be aromatic, a compound must obey the following rules-

1. The molecule must be co-planar
2. Complete delocalisation of \(\rm{π}\) electron in the ring
3. Presence of \(\rm{(4n+2)\,π}\) electrons in the ring where \(\rm{n}\) is an integer \(\rm{(n=0,1,2….)}\)

Based on the type of the ring, the aromatic compounds are further classified into the following-

Benzenoid aromatic compounds- The aromatic compounds which contain at least one benzene ring (a cyclic structure of six carbon atoms having alternate double bonds) in their structure are called Benzenoid aromatic compounds.
For example –

Non-benzenoid aromatic compounds-The highly unsaturated aromatic compounds that do not contain any benzene ring in their ring structure are called non-benzenoid aromatic compounds. These compounds have conjugated systems based on planar cyclic structure. For example –

Classification Based on the Presence of Functional Groups

Functional Groups

When the \(-\rm{H}\) atom in methane is replaced by some other group such as hydroxyl group \(\rm{-OH}\), then methane is converted into methanol, a type of alcohol with the formula \(\rm{CH}_3 \rm{OH}\), and forms a new class of organic compounds called alcohols. A functional group is an atom or a group of atoms which can replace Hydrogen from the organic compound and change the characteristic chemical properties of the compound thus formed. The site where functional group combine to the organic compound are chemically active site and becomes site for major organic reactions.

Organic compounds can be classified based on functional groups, which is as given below-

Representation of Organic Compounds

Although an organic compound has only one molecular formula, it can be represented in a number of ways which are as follows–

1. Complete Structural Formula
2. Condensed Structural Formula
3. Bond Line Structural Formulas
4. Three dimensional (3D) representation

Complete Structural Formula

The Lewis dot structure forms the basis of representing organic compounds through complete structural formula. In Lewis structure, the electrons are represented by dots and the shared pair of electrons which results in covalent bonding is denoted by a dash (―).

In Lewis Structure, every single bond, a double bond, and a triple bond is represented by one dash, double dash, and triple dash, respectively. A complete structural formula illustrates every single bond formed between every atom in the organic compound. For example:

Condensed Structural Formula

A condensed structural formula is a more compact way of representing structural formula of a molecule. The dashes/bonds are removed and the identical atoms or groups are represented by a number equal to its count written in subscript to that particular atom. In condensed structural formula, every carbon atom is represented individually. For example –

Bond Line Structural Formula

In bond line structural formula every bond in the molecule is represented as a line in a zigzag manner. If not specified, every terminal is assumed to be a methyl \(\left({{\text{-C}}{{\text{H}}_{\text{3}}}} \right)\) group. In this method, the carbon and hydrogen atoms are not shown. Only the bonds between the carbon atoms are shown as lines.

The ends and vertices represent the carbon atoms. The number of hydrogens must be guessed according to the valence rules. However, atoms other than carbon and hydrogen are shown.

Three Dimensional or 3-D Representation of Organic Compounds

The 3-D structure of an organic compound is represented by using wedge-dash method. In wedge-dash method of representation, the bond that protrudes out of the plane of paper towards the viewer is denoted by a solid wedge. The bond that projects away from the viewer or into the plane of the paper is denoted by a dashed wedge, and the bond that lies in the plane of the paper is represented by a line.

Homologous Series

Organic compounds are divided into different groups having the same general formula, properties, and related structures. Such a series of organic compounds are known as homologous series.

A homologous series is a series of organic compounds whose members can be denoted by a single general formula and differ from each other by a \({\text{-C}}{{\text{H}}_{\text{2}}}\) group. The individual members of a homologous series are called homologue. While transforming a compound into its next homologue, \({\text{-C}}{{\text{H}}_{\text{2}}}\) group gets added to the chain.

The most frequently used nomenclature worldwide is the one created and developed by the International Union of Pure and Applied Chemistry (IUPAC). The basename or the parent name of an organic compound depends on the total number of carbon atoms. This is shown as below-

Alkanes

The simplest example of homologous series is the alkane series.

The general formula of the homologous series of Alkanes is \({{\text{C}}_{\text{n}}}{{\text{H}}_{{\text{2n + 2}}}}\), where \( {\text{n}} = 1,\,2,\,3…..\)
The table above shows that each member differs from the other by a \( {\text{-C}} { {\text{H}}_ {\text{2}}}\) group which is equal to \(14\,\rm{amu}\).

Alkenes

The homologous series of alkenes include the presence of a carbon-carbon double bond.
The general formula of homologous series of alkenes is \({{\text{C}}_{\text{n}}}{{\text{H}}_{{\text{2n}}}}\), where \(n = 1,\,2,\,3……\)

Alkynes

The homologous series of alkynes include the .presence of a carbon-carbon triple bond.
The general formula of homologous series of alkynes is \({{\text{C}}_{\text{n}}}{{\text{H}}_{{\text{2n-2}}}}\), where \( {\text{n}} = 1,\,2,\,3……\).

Alkyl Halides

The alkyl halides are represented by \(\rm{R-X}\) where \(\rm{R}\) is some alkyl group such as methyl \({\text{-C}}{{\text{H}}_{\text{3}}}\) or ethyl \({\text{-}}{{\text{C}}_2}{{\text{H}}_5}\), etc., and \(\rm{X}\) is a halogen atom such as fluorine \(\rm{F}\), chlorine \(\rm{Cl}\), bromine \(\rm{Br}\), and iodine \(\rm{I}\).

They form a homologous series represented by the general formula \({{\text{C}}_{\text{n}}}{{\text{H}}_{{\text{2n + 1}}}}{\text{X}}\). According to the nature of carbon to which halogen is attached, alkyl halides are further classified as primary, secondary, or tertiary.

Alkyl halide	Carbon number	Formula	Structure
Methyl bromide	\(1\)	\({\text{C}}{{\text{H}}_{\text{3}}}{\text{Br}}\)
ethyl bromide	\(2\)	\({\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_2}{\text{Br}}\)
propyl bromide	\(3\)	\({\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_2}{\text{C}}{{\text{H}}_2}{\text{Br}}\)
butyl bromide	\(4\)	\({\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_2}{\text{C}}{{\text{H}}_2}{\text{C}}{{\text{H}}_2}{\text{Br}}\)

Alcohols

Alkanes when substituted by a functional group \(–\rm{OH}\), called the hydroxyl group results in a different class of compounds called alcohols. The most common alcohol is known as ethanol. It is used in alcoholic drinks, as fuel (gasoline), as a preservative for biological specimens, and as a solvent for paints and drugs.
The general formula for homologous series of alcohol is \({{\text{C}}_{\text{n}}}{{\text{H}}_{{\text{2n + 1}}}}{\text{OH}}\), where \( {\text{n}} = 1,\,2,\,3…..\)

Aldehydes and Ketones

The aldehydes and ketones contain a functional group called a carbonyl group. A carbonyl group has carbon atom double-bonded to an oxygen atom \(\rm{C}=O\). In an aldehyde, the carbonyl group is always present on the end carbon atom.

Aldehyde	Carbon number	Formula	Structure
Methanal	\(1\)	\( {\text{HCHO}}\)
Ethanal	\(2\)	\({\text{C}}{{\text{H}}_{\text{3}}}{\text{CHO}}\)
Propanal	\(3\)	\({\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_2}{\text{CHO}}\)

The general formula of the Aldehyde homologous series is \({{\text{C}}_{\text{n}}}{{\text{H}}_{{\text{2n}}}}{\text{O}}\), where \(n = 1,\,2,\,3….\)

In a ketone, the carbonyl group may occur anywhere in the compound but never on an end carbon. Acetone, for example, is a ketone, the main component in nail polish remover, while cumin aldehyde is an aldehyde present in cumin seeds.

The general formula of the homologous ketone series is \({{\text{C}}_{\text{n}}}{{\text{H}}_{{\text{2n}}}}{\text{O}}\), where \(n = 1,\,2,\,3….\)

Ketone	Carbon number	Formula	Structure
Propanone	\(1\)	\({\text{C}}{{\text{H}}_{\text{3}}}{\text{COC}}{{\text{H}}_{\text{3}}}\)
Butanone	\(2\)	\({\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_2}{\text{COC}}{{\text{H}}_{\text{3}}}\)
Pentanone	\(3\)	\({\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_2}{\text{C}}{{\text{H}}_2}{\text{COC}}{{\text{H}}_{\text{3}}}\)

Carboxylic Acids

The carboxylic acids also known as organic acid contain the carboxyl group. A carboxyl group has a carbon atom double-bonded to an oxygen atom \(\rm{C=O}\), and a hydroxyl group \(–\rm{OH}\), attached to a hydrogen atom or an alkyl group. Vinegar, chemical formula \(\rm{CH}_3 \rm{COOH}\), is a common carboxylic acid also known as ethanoic acid or acetic acid.

The general formula of the carboxylic acid homologous series is \({{\text{C}}_{\text{n}}}{{\text{H}}_{{\text{2n}}}}{\text{O}}_2\), where \(\rm{n} = 1,2,3…\)

Carboxylic acid	Carbon number	Formula	Structure
Methanoic acid	\(1\)	\( {\text{HCOOH}}\)
Ethanoic acid	\(2\)	\({\text{C}}{{\text{H}}_{\text{3}}}{\text{COOH}}\)
Propanoic acid	\(3\)	\({\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_2}{\text{COOH}}\)
Butanoic acid	\(4\)	\({\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_2}{\text{C}}{{\text{H}}_2}{\text{COOH}}\)

Esters

An ester is a chemical compound derived from an acid (organic or inorganic) in which at least one \(–\rm{OH}\) hydroxyl group is replaced by an \(–\rm{O}–\) alkyl (alkoxy) group, as in the substitution reaction of a carboxylic acid and an alcohol.

The general formula of the homologous ester series is \({{\text{C}}_{\text{n}}}{{\text{H}}_{{\text{2n}}}}{{\text{O}}_{\text{2}}}\), where \(\rm{n} = 1,2,3…\)

Ester	Formula	Structure
Methyl acetate	\({\text{C}}{{\text{H}}_{\text{3}}}{\text{COOC}}{{\text{H}}_3}\)
Ethyl acetate	\({\text{C}}{{\text{H}}_{\text{3}}}{\text{COOC}}{{\text{H}}_2}{\text{C}}{{\text{H}}_{\text{3}}}\)
Ethyl propionate	\({\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_2}{\text{COOC}}{{\text{H}}_2}{\text{C}}{{\text{H}}_{\text{3}}}\)

Amides

Amides are derived from carboxylic acids. A carboxylic acid contains the \(−\rm{COOH}\) functional group, and in an amide, the \(−\rm{OH}\) part of that group is replaced by an \(-\rm{NH}_2\) group. So, amides contain the \(-\rm{CONH}_2\) group.

The general formula of the homologous amide series is \({{\text{C}}_{\text{n}}}{{\text{H}}_{{\text{2n + 1}}}}{\text{NO}}\), where \(\rm{n} = 1,2,3…\)

Amides	Carbon number	Formula	Structure
Methanamide	\(1\)	\({\text{HCON}}{{\text{H}}_{\text{2}}}\)
Ethanamide	\(2\)	\({\text{C}}{{\text{H}}_3}{\text{CON}}{{\text{H}}_{\text{2}}}\)
Propanamide	\(3\)	\({\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{CON}}{{\text{H}}_{\text{2}}}\)

Nitroalkane

The organic compounds that contain one or more nitro \(\left({{\text{-N}}{{\text{O}}_{\text{2}}}}  \right)\) functional groups are called nitroalkanes.

The general formula of the nitroalkanes homologous series is \({{\text{C}}_{\text{n}}}{{\text{H}}_{{\text{2n + 1}}}}{\text{N}}{{\text{O}}_{\text{2}}}\), where \(\rm{n} = 1,2,3…\)

Nitroalkanes	Carbon number	Formula	Structure
Nitromethane	\(1\)	\({\text{C}}{{\text{H}}_{\text{3}}}{\text{N}}{{\text{O}}_{\text{2}}}\)
Nitroethane	\(2\)	\({\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_2}{\text{N}}{{\text{O}}_{\text{2}}}\)
Nitropropane	\(3\)	\({\left({{\text{C}}{{\text{H}}_{\text{3}}}} \right)_2}{\text{C}}{{\text{H}}_2}{\text{N}}{{\text{O}}_{\text{2}}}\)

Amines

Amines are formally derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. The general formula of the homologous amine series \({{\text{C}}_{\text{n}}}{{\text{H}}_{{\text{2n + 3}}}}{\text{N}}\), where \(\rm{n} = 1,2,3…\)

Amines	Carbon number	Formula	Structure
Methenamine	\(1\)	\({\text{C}}{{\text{H}}_{\text{3}}}{\text{N}}{{\text{H}}_{\text{2}}}\)
Ethanamine	\(2\)	\({\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_2}{\text{N}}{{\text{H}}_{\text{2}}}\)
Propanamine	\(3\)	\({\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_2}{\text{C}}{{\text{H}}_2}{\text{N}}{{\text{H}}_{\text{2}}}\)

Properties of Organic Compounds

The general characteristics of Organic Compounds include:

1. Organic compounds can be isolated as well as prepared in laboratory.
2. Organic compounds possess highly complex structures and molecular weights.
3. The presence of functional groups determines the properties of organic compounds.
4. They are mostly insoluble in water but soluble in organic solvents.
5. Organic compounds easily undergo combustion reaction.
6. Chemical reactions involving organic compounds proceed at slower rates.
7. Organic compounds exhibit low melting points and boiling points in comparison to the inorganic compounds.
8. Organic acids and bases are less stronger and thus they have a limited dissociation in an aqueous medium.
9. They exhibit the phenomenon of isomerism in which a single molecular formula represents several organic compounds differing in physical and chemical properties.
10. They are volatile in nature.

Summary

Organic compounds consist of a vast range of compounds. In order to have a detailed knowledge of the individual compounds, it is essential that we categorise them. Classifying them into different groups will help us analyse the various compounds in a systematic manner.

FAQs on Classification of Organic Compounds

Q.1. What are the uses of organic compounds?
Ans. Some of the uses of Organic compounds are-
1. Alkanes are used extensively as fuels for things like automobile gasoline and home heating/cooking fuel.
2. Chloroform is used as an anaesthesia.
3. Acetylene is used as a welding gas.
4. Ethyl alcohol is extensively used as a sanitiser
5. Acetic acid finds its use as a food preservative.
6. Glucose is used as a sweetening agent.
7. Carbon tetrachloride is used in the form fire extinguisher.

Q.2. How do you know if a compound is organic?
Ans. A material is said to be organic if it contains carbon bound to other atoms by a covalent bond. The other atoms most commonly contain hydrogen, oxygen, and/or nitrogen. A few carbon compounds, including simple oxides (e.g. \({\text{C}}{{\text{O}}_{\text{2}}}\)) and cyanides (e.g. \(\rm{KCN})\), are arbitrarily omitted.

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