12. Basic Concepts of Organic Chemistry

(Differences between organic and inorganic compounds.)

Living things are made up of atoms covalently bonded to form molecules of organic compounds. All these molecules are carbon-based compounds. The complexity of life requires a great variety of compounds. The great variety of compounds is possible because every carbon atom can bond with other atoms to form chains and rings. These chains and rings are often found bonded to atoms of other elements, such as hydrogen, oxygen, and nitrogen. This explains the existence of millions of organic compounds.

• Carbon has 4 electrons in the last shell; it’s neither too electropositive nor too electronegative, forming mostly covalent bonds with other nonmetals and itself (i.e., other carbon atoms). Hence, it has valency four.
• Another special property of carbon that results in the existence of such a wide variety of organic compounds is the catenation property of carbon. It is the property by which carbon gets bonded with another carbon atom, forming a wide variety of possible chains. E.g.,

Due to moderate electronegativity and 4 electrons in the valence shell, the carbon-carbon covalent bond is very strong. This is why catenation is very effective in carbon.

• The organic compounds made up of hydrogen and carbon atoms only are called hydrocarbons.
• These are considered to be the parent organic compounds, and all other organic compounds are considered to be derived from them by the replacement of one or more hydrogen atoms by other substituents.
• With the development of organic chemistry, the number of organic compounds became very large.
• Therefore, to simplify and systematize the study of organic chemistry, all the known organic compounds are broadly divided into two main classes, viz., hydrocarbons and derivatives of hydrocarbons.
• On the basis of the nature of the carbon skeleton, the hydrocarbons can be classified into cyclic and acyclic.

1. Acyclic or Open Chain Compounds

These are hydrocarbons that contain carbon atoms linked together in an open-chain form. The carbon chain may be a straight chain or branched. E.g.,

The open-chain compounds can further be classified as alkanes, alkenes, and alkynes.

[A] Alkanes (CₙH₂ₙ₊₂)

These are the simplest hydrocarbons, which contain carbon-carbon single bonds. These are also called saturated hydrocarbons as they cannot add any more hydrogen. e.g.

• Methane (CH₄) — 16 amu
• Ethane (CH₃CH₃) — 30 amu
• Propane (CH₃CH₂CH₃) — 44 amu
• Butane (CH₃CH₂CH₂CH₃) C₄H₁₀ — 58 amu
• 2-methyl propane (CH₃)₂CHCH₃ C₄H₁₀ etc.

[B] Alkenes (CₙH₂ₙ)

These are unsaturated hydrocarbons that contain at least one carbon-carbon double bond. E.g.,

• Ethene (CH₂=CH₂) — 12×2+1×4 = 28 amu
• Propene (CH₂=CHCH₃) = 42 amu
• But-1-ene (CH₂=CHCH₂CH₃) = 56 amu C₄H₈
• But-2-ene (CH₃CH=CHCH₃) = 56 amu C₄H₈
• Buta-1,3-diene (CH₂=CHCH=CH₂) etc.

[C] Alkynes

These are also unsaturated hydrocarbons that contain at least one carbon-carbon triple bond. E.g.,

2. Cyclic or Closed Chain (Ring) Compounds

The hydrocarbons that contain one or more rings are called cyclic compounds. These are further divided into the following two categories.

[A] Homocyclic (Carbocyclic) Compounds

These are closed or ring compounds where the ring is made up of the same atom, i.e., carbon. These are further classified into:

a. Aliphatic Cyclic Hydrocarbons (Alicyclic)

These are closed-chain hydrocarbons but resemble open-chain hydrocarbons in many of their properties. These could be further classified into cycloalkanes, cycloalkenes, and cycloalkynes. e.g.,

b. Aromatic Hydrocarbons

The term aromatic was derived from the Greek word aroma, meaning sweet smell. So, the organic compounds having a sweet smell were originally called aromatic hydrocarbons. Later, it was found that planar cyclic compounds having (4n+2) π electrons in a conjugated system (i.e., alternate single and double bonds) are aromatic compounds. e.g.,

[B] Heterocyclic Compounds

These are cyclic or ring compounds where, apart from carbon, the ring consists of at least one hetero atom (Oxygen, Sulphur, Nitrogen, etc.). For example,

Organic compounds except hydrocarbons (i.e., derivatives of hydrocarbons) are generally made up of 2 parts: a skeleton hydrocarbon and another part that determines most of the chemical and physical characteristics of that compound. The latter is called the functional group, and the former one is generally called the alkyl group (-R). e.g.

Alkyl groups (-R)

Alkyl groups can be defined as the groups obtained by removing one hydrogen atom from an Alkane.

Functional groups

The functional group can be defined as the atom or group of atoms that is present in an organic compound and largely determines the properties of the compound.

E.g., the properties of Ethanol are entirely different than that of aminoethane, though both have the same alkyl group (ethyl).

On the other hand, most of the properties of Methanol and Ethanol or Aminoethane and Aminomethane are the same.

There are many functional groups present among organic compounds. A few important ones are listed below.

Homologous series

It is a family or class of compounds obtained when compounds having the same functional group are arranged in increasing order of their molecular weight. It is defined as a series of organic compounds having the same functional group differing by a -CH₂- unit or 14 amu weight among successive members. The members of the homologous series are called homologs, and the phenomenon is called homology.

The main characteristics of the homologous series are:

• The members of a homologous series have the same functional group, if any.
• They can be represented by a general formula.
• They can be prepared by the same general methods of preparation.
• They have similar chemical properties.
• Each successive member differs by 14 amu mass or -CH₂- unit.
• There is regular gradation in the physical properties.
• The first member of the homologous series is generally different and special.

Different representations can be used to indicate organic compounds. The most widely used among them are the structural formula, condensed/contracted formula, and bond line structural formula.

The structural formula shows each atom in the molecule and the nature of the bonds between the different atoms, so it is the clearest one.

The condensed formula is somewhat shorter and uses carbon atoms and hydrogen atoms attached to it as a unit.

The bond line structural formula is the least work to represent an organic molecule; here, a line indicates a single bond between carbon atoms, generally, and the end of the line or corners in the geometrical shape indicate a carbon atom along with hydrogen atoms directly attached to it. Atoms other than these should be written as their symbols.

An example of different representations of molecules of the alkane homologous series, along with names, is presented in the following table.

1. Pyrolysis or Cracking

Coal, petroleum, and natural gases are common sources of hydrocarbons. Generally, long-chain hydrocarbons are abundantly found in these sources. But short-chain alkanes and alkenes find more commercial value. Therefore, the long-chain hydrocarbons should be changed into short ones, which is effectively done by pyrolysis or cracking.

Long chain alkanes, when heated in the absence of air to a temperature 500-600°C, decompose into lower alkanes and alkenes. A mixture of lower alkanes and alkenes is obtained by this process.

Reforming (Aromatisation)

When alkanes having 6-8 carbon atoms are heated at a temperature of about 500°C in the presence of Pt, aromatic hydrocarbons are obtained. These reactions are aromatisation or reforming. This is done to increase the octane number of gasoline as well as fulfil the demand for aromatic compounds.

Quality of Gasoline

In a petrol engine, a mixture of petrol and air is compressed in the cylinder. The fuel burns just before the completion of the compression. But if the quality of fuel is not good, combustion takes place before the spark plug ignites, and the piston is pushed suddenly, producing a metallic noise which is called knocking. Knocking reduces efficiency and the life span of the engine.

Octane Number

It is a number assigned to grade the quality of gasoline (petrol). Octane number of a fuel (gasoline) is defined as the percentage of iso-octane in a mixture of iso-octane and n-heptane which has the same knocking property as the given fuel. E.g., a fuel has an octane number of 70 means that it has the same anti-knocking properties as a mixture of 70% iso-octane and 30% n-heptane.

The bond line structures of iso-octane and n-heptane are:

Generally, straight-chain hydrocarbons have a lower octane number, and branched-chain hydrocarbons have a higher octane number.

Cetane Number

Similar to the octane number of gasoline, the cetane number of diesel is defined as the % of n-cetane in the mixture of n-cetane and α-methylnaphthalene, having the same time lag for ignition as the fuel (diesel) under consideration.

n-cetane is Hexadecane (C₁₆H₃₄) a straight chain saturated hydrocarbon (alkane).

Gasoline Additives (Anti-knocking Agents)

• The compounds added to gasoline to reduce the fuel’s knocking tendency are called gasoline additives or anti-knock agents.
• Tetraethyl lead (TEL) was one of the most common anti-knock agents.
• But due to its pollution and adverse effects on human health, the use of TEL has been phased out worldwide (2021).

Modern alternatives (oxygenates and other hydrocarbons):

• MTBE (methyl tert-butyl ether) — good octane booster but causes groundwater pollution (banned in many countries).
• Ethanol — widely used, renewable, high octane, but lowers fuel energy density.
• Aromatics (toluene, xylene) — boost octane but increase soot and benzene emissions.