What are Organic Compounds - Introduction to Organic Chemistry

What are Organic compounds?

Organic compounds are a large group of chemical compounds that mainly contain carbon. Most commonly it contains elements like hydrogen, oxygen, nitrogen, halogens, sulfur, phosphorous, etc. Organic chemistry is a branch of chemistry that studies the structures, properties, and reactions of organic compounds.

Just because a compound contains carbon does not make that compound an organic compound.

• Eg: Compounds such as CO₂, H₂CO₃, and NaHCO₃ are not considered organic compounds even if they contain carbon

To become an organic compound, there should be at least one covalent bond between carbon-carbon or carbon-hydrogen/other element.

The characteristics and properties of an organic molecule depend on multiple factors such as,

• Length of the carbon chain
• Branching of the molecule
• Type of the covalent bonds between carbon atoms
• Functional groups of the molecule

What are functional groups?

All organic compounds are built on a backbone of carbon and hydrogen. In most of these molecules, there are specific groups of atoms that determine the functionality, characteristics, reactivity, and properties (solubility, boiling point, melting point, molecular weight, acidity, polarity ...). These groups of specific atoms are known as functional groups.

Classification of organic compounds

Organic compounds can be classified in multiple ways based on multiple factors such as their structure, functional groups they contain, and the origin of the compound.

i. Based on the structure

First, they can basically categorized into two categories based on the structure of the compound.

• Cyclic Compounds (Closed-chain compounds)
  • Homocyclic compound - The ring structure is made out of atoms of the same element
    • Examples - Benzene, Cyclohexane, Naphthalene
  • Heterocyclic compounds - The ring structure is made out of atoms of more than one element
    • Examples - Furan (Contains carbon and oxygen), Thiophene (Contains sulfur and carbon)
• Acyclic compounds (Open-chain compounds)
  • Straight chained compounds
  • Branched compounds

ii. Based on the origin

When considering the origin of organic compounds, it can be divided into two categories.

• Natural organic compounds
• Synthetic organic compounds

There are a large number of organic compounds that can be either natural or synthetic. Carbohydrates, proteins, and lipids are some natural organic compounds that can be found in living organisms.

PVC, polyethylene, polystyrene, and polyester are some synthetic organic compounds.

iii. Based on the functional groups/ elements it contains

This classification is based on the elements and functional groups that are contained in an organic compound.

• Hydrocarbons
  • Acyclic Hydrocarbons
    • Alkanes (RH)
    • Alkenes (RC=CR')
    • Alkynes (RC≡CR')
  • Cyclic hydrocarbons
    • Aromatic hydrocarbons
      • Examples: Benzene
    • Aliphatic hydrocarbons
      • Examples: Cyclohexane
• Halogen-containing compounds
  • Alkyl halides (RX)
  • Aryl halides (ArX)
• Oxygen-containing compounds
  • Alcohols (ROH)
  • Phenols (ArOH)
  • Carboxylic acids (RCOOH)
  • Aldehydes (RCHO)
  • Ketones (RR'C=O)
  • Ethers (ROR')
  • Carboxylic acid derivatives
    • Esters (RCO₂R')
    • Amides (RCONHR')
• Nitrogen-containing compounds
  • Amines (RNH₂, RNHR', RNR'R'')
  • Nitriles (RC≡N)
  • Nitro compounds (ArNO₂)

• "Ar" means aryl group (aromatic ring group)
• "X" means halogens
• "R" means alkyl group

How could carbon make a large number of compounds?

Carbon is in the 14^th group of the periodic table. So, it has four valence electrons. Therefore, carbon could form four covalent bonds between carbon atoms as well as other functional groups.

When compared to silicon which is also in group 14, carbon-carbon bond energy (C - C), and carbon-hydrogen (C - H) is high. That means carbon could form more stable and strong bonds compared to silicon. Also, carbon forms double and triple bonds between carbon atoms.

Not only hydrogen but also carbon forms strong covalent bonds with atoms like Oxygen, Nitrogen, Sulfur, Chlorine, Bromine, Iodine, etc. Therefore carbon could form a wide range of compounds. It is also common for carbon to form long chains of carbon atoms which is known as carbon backbone. Sometimes it could be thousands of carbon atoms in a carbon backbone.