Organic Compounds: Definition, Classification, Examples

The substances which are obtained directly or indirectly from living organisms such as plants and animals are called organic compounds. These compounds are essential for the existence and maintenance of life on earth. They include complex molecules like DNA (deoxyribonucleic acid), which transmit genetic information from one generation to the other as well as proteins, vitamins, fats, etc. which are vital components of our blood, muscle and skin.

Most of the materials which we use in daily life such as clothing, fuel, polymers, dyes, and medicines are composed of organic compounds. All these compounds are studied under a separate branch of chemistry known as organic chemistry.

All organic compounds are chemical compounds primarily made of the element carbon. Most of them also contain hydrogen, and many include other elements such as oxygen, nitrogen, sulfur, phosphorus, and halogens. The defining feature of an organic compound is the presence of carbon–carbon (C–C) or carbon–hydrogen (C–H) bonds.

Vital Force Theory

Until the early 19th century, it was believed that organic compounds could not be prepared in the laboratory but could only be obtained from plants and animals. Based on this belief, Berzelius, a leading Swedish chemist in 1815, proposed a Vital Force Theory.

According to this theory, organic compounds were produced only under the influence of some mysterious force present in the living organisms. Such a mysterious force was named vital force. Since this mysterious force could not be created artificially, it is impossible to synthesize organic compounds in the laboratory. This theory reigned dominant for a number of years.

In 1828, a German chemist Friedrich Wohler made an interesting discovery. He accidentally synthesized urea in the laboratory, a well known organic compound present in the urine of humans and other mammals. He obtained this compound by evaporating an aqueous solution of ammonium cyanate (NH₄CNO). It is a typical inorganic compound formed by the double decomposition of ammonium chloride (NH₄Cl) and potassium cyanate (KCNO). The balanced chemical reaction is as:

NH₄Cl + KCNO → NH₄CNO + KCl
NH₄CNO → (heat) → NH₂CONH₂ (Urea)

This experiment disproved the Vital Force Theory and clearly demonstrated that no mysterious force was required for the formation of organic compounds in the laboratory. It marked the beginning of modern organic chemistry.

Modern Definition of Organic Compounds

With the downfall of the Vital Force Theory, the original meaning of the term organic (pertaining to life) lost its significance. However, an analysis of these compounds revealed that all of them, whether natural or synthetic, essentially contain carbon and hydrogen, and occasionally, a few other elements such as oxygen, nitrogen, Sulphur, halogens, phosphorus, etc.

These compounds are considered as the derivatives of hydrocarbons because they can be formed by replacing one or more of their hydrogen atoms with other atoms or functional groups. Thus, the modern definition of organic compounds is now defined as:

Organic compounds are the hydrocarbons and their derivatives. The branch of chemistry which deals with the study of these compounds is known as organic chemistry.

However, there are certain exceptions to this definition. Many carbon compounds obtained from mineral sources are not included in organic compounds. These include:

• Carbonates like Na₂CO₃
• Bicarbonates like NaHCO₃
• Carbides like CaC₂, Al₄C₃
• Cyanides like KCN
• Compounds of carbon like CO and CO₂

Such compounds containing carbon are studied under the inorganic chemistry.

Reasons for Existence of Large Number of Organic Compounds

So far, more than 35 million organic compounds have been identified. This number is significantly greater than the number of compounds formed by all the other elements combined. The existence of such a vast number of organic compounds is due to the following reasons:

1. Catenation:

Catenation is the property of atoms of an element to link with one another, forming long straight chains, balanced chains, or rings of different sizes made up of identical atoms. A carbon exhibits the maximum catenation property. Due to this property of carbon, a carbon atom can combine with other carbon atoms by single, double, or triple bonds and can make various types of straight chain, branched chain, and cyclic structures. As a result, a carbon forms a large variety of compounds than any other elements.

2. Isomerism:

Another reason for the large number of organic compounds is the phenomenon of isomerism, which is very common in organic compounds. When two or more compounds have the same molecular formula but different arrangements of atoms, they are called isomers, and the phenomenon is known as isomerism. For example, ethanol and methoxymethane both have the same molecular formula (C₂H₆O):

• Ethanol: CH₃CH₂OH
• Methoxymethane: CH₃OCH₃

Thus, both ethanol and methoxymethane are isomers of each other.

3. Electronegativity:

The electronegativity of carbon is 2.5, which is neither very low nor very high. Therefore, carbon can form strong covalent bonds with hydrogen, oxygen, nitrogen, chlorine, phosphorus, and other carbon atoms. This also adds to the large number of carbon compounds.

4. Formation of Multiple Bonds:

Since the carbon atom is very small in size and has a valency of four, it can form multiple bonds, such as double and triple bonds with other carbon atoms to satisfy its tetravalency. This ability further contributes to the formation of a large variety of compounds. For example:

• CH₃-CH₂-CH₃ (Propane)
• CH₃-CH=CH₂ (Propene)
• CH3-C≡CH (Propyne)

Classification of Organic Compounds

Organic compounds occur in large numbers. Therefore, in order to study them systematically, we need to classify them. By classifying organic compounds into various categories, we can easily study the physical properties of compounds, their structures, and their reactions. Organic compounds are classified based on the types of chains present in their molecules. They are:

• Open-chain, acyclic or aliphatic compounds
  • Straight-chain compounds
  • Branched-chain compounds
• Closed-chain, cyclic or ring compounds
  • Homocyclic or carbocyclic compounds
    • Alicyclic compounds
    • Aromatic compounds
      • Benzenoid compounds
      • Non-benzenoid compounds
  • Heterocyclic compounds

Open Chain, Acyclic or Aliphatic Compounds

Organic compounds in which all the carbon atoms are linked together to form open carbon chains are called aliphatic compounds. They are also known as open-chain or acyclic compounds. The open chain of carbon atoms may be either straight or branched. For example:

Closed or Cyclic Compounds

The organic compounds which have at least one ring of atoms or closed chain of atoms in their structure are called cyclic compounds. They are also known as ring compounds. Cyclic compounds are further sub-divided into two categories:

• Homocyclic compounds
• Heterocyclic compounds

a) Homocyclic or Carbocyclic Compounds

The compounds which contain rings consisting of only one type of atoms are called homocyclic compounds. If all the atoms in the ring are carbon, they are called carbocyclic compounds. These compounds are further divided into two types:

• Alicyclic compounds
• Aromatic compounds

Alicyclic Compounds

Alicyclic compounds are those carbocyclic compounds in which all the corners of the rings are occupied by carbon atoms. These compounds contain rings of three or more carbon atoms. Alicyclic compounds are similar to aliphatic hydrocarbons in chemical properties. For example:

Aromatic Compounds

Carbocyclic compounds that contain one or more isolated or fused benzene rings are called aromatic compounds. These are also known as benzenoid compounds. Aromatic compounds are characterized by an alternating system of single and double bonds in a six-membered ring, which provides extra stability due to resonance. Some examples of aromatic compounds are as:

Some aromatic compounds do not have benzenoid rings, but have unsaturated ring/s. These compounds are called non-benzenoid aromatic compounds. Tropolone is an example of non-benzenoid aromatic compound.

b) Heterocyclic Compounds

Organic compounds that contain one or more heteroatoms such as oxygen (O), nitrogen (N), or sulfur (S) other than carbon atoms within the ring structure are called heterocyclic compounds. These are of two types:

• Alicyclic heterocyclic compounds
• Aromatic heterocyclic compounds

Alicyclic Heterocyclic Compounds

Alicyclic compounds are cyclic organic compounds that resemble aliphatic compounds in their properties. If such cyclic compounds contain one or more heteroatoms (such as O, N, or S) in the ring, they are called heterocyclic alicyclic compounds. For example,

• Tetrahydrofuran (THF) contains an oxygen atom in a five-membered ring.
• Pyrrolidine contains a nitrogen atom in a saturated ring.

Aromatic Heterocyclic Compounds

Compounds that resemble benzene and other aromatic compounds in most of their properties and contain one or more heteroatoms in the ring are called aromatic heterocyclic compounds. For example,

• Pyridine (C₅H₅N) contains a six-membered aromatic ring with one nitrogen atom.
• Furan (C₄H₄O) contains a five-membered aromatic ring with one oxygen atom.

These kinds of compounds follow the rules of aromaticity (like Huckel’s Rule). They are widely found in biological systems and pharmaceuticals.

We can also classify organic compounds according to their functional groups into families or homologous series. We will learn about this topic in the next organic chapter.

Characteristics of Organic Compounds

Organic compounds generally exhibit the following characteristics:

1. Low melting and boiling points – Most organic compounds have relatively low melting and boiling points compared to inorganic compounds.
2. Low solubility in water – Due to their nonpolar nature, many organic compounds do not dissolve well in water.
3. High solubility in nonpolar solvents – They are more soluble in nonpolar solvents and weak in polar organic solvents such as ether, benzene, or chloroform.
4. Poor electrical conductivity – Organic substances typically do not conduct electricity because they do not produce ions in solution.
5. Flammability – Most organic compounds are combustible and burn in the presence of oxygen.
6. Covalent bonding – Organic compounds are mainly composed of covalently bonded atoms.
7. Isomerism – They often exhibit isomerism, where compounds have the same molecular formula but different structural arrangements.
8. Slow reaction rates – Chemical reactions involving organic compounds are generally slower and need specific conditions such as catalysts, temperature, solvents.

Importance of Organic Compounds

Organic compounds are extremely useful in our daily lives. The food we eat is primarily organic in nature, and the changes it undergoes in our bodies involve organic chemical reactions. The soaps and shampoos we use while bathing contain organic compounds. Products like powders, perfumes, and lipsticks also consist of organic substances. Even the clothes and shoes we wear are made from materials that contain organic compounds.

In addition, dyes, drugs, paints, fuels, lubricants, and various materials used in motor vehicles are all composed of organic compounds. These substances have made our lives more convenient and comfortable. The number of known organic compounds is very large and continues to grow every day. We must thank to organic chemists who conduct researches around the world in order to synthesize new and useful compounds.

Important Examples of Organic Compounds in Daily Life

Below is the list of a large number of organic compounds that we use in everyday life:

• Food: Carbohydrates, proteins, fats, vitamins, amino acids, and food additives (e.g., preservatives, flavor enhancers)
• Clothing and Textiles: Cotton, silk, wool, nylon, rayon, polyester, acrylic fibers
• Shelter and Furnishings: Wood, paints, varnishes, adhesives, foams, polishes, synthetic flooring
• Power and Transportation: Natural gas (methane), petroleum products (diesel, petrol, kerosene), coal, biodiesel, ethanol (biofuel), lubricants
• Medicines and Pharmaceuticals: Penicillin, streptomycin, paracetamol, ibuprofen, aspirin, morphine, LSD, antacids, antibiotics, vaccines
• Agriculture:
  • Insecticides: DDT, malathion
  • Herbicides: 2,4-D, Treflan
  • Fertilizers: Urea, ammonium carbamate
  • Plant hormones: Auxins, gibberellins
• Hormones and Steroids: Estrogen, testosterone, insulin, cortisone, progesterone
• Vitamins and Enzymes: Vitamin A, B-complex, C, D, E, K; digestive enzymes like amylase and pepsin (protein-based)
• Healthcare Products: Antiseptics (iodine tincture, Dettol), anesthetics (chloroform, ether), disinfectants (phenol derivatives)
• Cosmetics and Personal Care: Perfumes, deodorants, lipsticks, creams, lotions, shampoos, soaps, detergents, hair dyes
• Printing and Office Supplies: Paper, ink, toners, adhesives, correction fluid
• Household Items: Plastics (PVC, PET), rubbers, resins, refrigerants (CFC substitutes), cleaning agents
• Electronics and Technology: Semiconducting organic materials, organic LEDs (OLEDs), printed circuit boards (resins)

Classification of Organic Compounds based on Source

We can also classify organic compounds into two types based on their origin:

• Natural organic compounds
• Synthetic organic compounds

Natural Organic Compounds

Organic substances that are extracted directly from natural sources, such as plants, animals, and microorganisms, are called natural organic compounds. They are produced by living organisms through biological processes such as photosynthesis, respiration, fermentation, and digestion.

Sources of Natural Organic Compounds:

A. Biological Sources:

• Plants: Many important organic compounds such as sugars, starch, cellulose, glucose, certain proteins, and various medicinal drugs are obtained from plants.
• Animals: Compounds such as urea, proteins, fats, lipids, enzymes, and hormones are obtained from animals.
• Microorganisms such as bacteria and fungi produce a variety of organic compounds. For example,
  • Antibiotics – Penicillin, streptomycin
  • Enzymes – Amylase, cellulase
  • Vitamins – Vitamin B₁₂)
  • Organic acids – Citric acid, lactic acid through fermentation and metabolic processes.

B. Fossil-Based Sources:

• Coal: Coal is the major source of organic compounds, which produce coke and coal-tar through destructive distillation. More than 200 organic compounds have been directly obtained from coal-tar. Aromatic compounds such as benzene, toluene, naphthalene, phenol, pyridine, dyes, drugs, and perfumes are examples of products derived from coal-tar.
• Natural Gas and Petroleum: Natural gas and petroleum are fossil fuels that are another important natural source of organic compounds. A large number of organic compounds, such as saturated and unsaturated hydrocarbons as well as compounds used in the production of fuels, plastics, synthetic fibers, and chemicals, are derived from them.
• Wood: Destructive distillation of wood involves heating wood in the absence of air, which breaks it down into simpler chemical substances. This process yields products like methanol, acetone, acetic acid, tar, and gases such as methane and hydrogen.

Synthetic Organic Compounds

Organic substances that are man made in laboratories, or industries by chemical synthesis are called synthetic organic compounds. These compounds are not directly obtained from natural sources but are created using chemical reactions by scientists and chemists to meet specific needs. They are used in almost every industry, such as medicine, agriculture, plastic, cosmetics, etc.

Examples of Synthetic Organic Compounds:

• Plastics
• Dyes
• Detergents
• Pharmaceutical Drugs
• Synthetic Fibers

Multiple Choice Questions on Organic Compounds

A. Which element is present in all organic compounds?

1. Carbon
2. Nitrogen
3. Oxygen
4. Phosphorus

Answer: Carbon

B. Which of the following is general characteristics of an organic compound?

1. High melting point
2. Low melting point
3. Soluble in polar solvents
4. Insoluble in nonpolar solvents

Answer: Low melting point

C. Which of the following statement describes why carbon element forms so many compounds?

1. Carbon atoms combine readily with oxygen.
2. Carbon atoms have very high electronegativity.
3. Carbon readily forms ionic bonds with other carbon atoms.
4. Carbon readily forms covalent bonds with other carbon atoms.

Answer: Carbon readily forms covalent bonds with other carbon atoms.

D. Which of the following is a major source of organic compounds?

1. Natural gas
2. Fermentation
3. Sea water
4. Atmorphere

Answer: Natural gas