Unit 1 – Classification, Nomenclature & Isomerism in Organic Chemistry Notes (B. Pharmacy)

Organic chemistry is the study of carbon-containing compounds, which form the backbone of life as well as countless industrial and pharmaceutical products. To study these compounds systematically, chemists classify them, give them unique names through nomenclature systems, and understand how their structures can differ yet share the same molecular formula (isomerism).

This unit introduces three important aspects: classification of organic compounds, rules of nomenclature (common and IUPAC systems), and structural isomerism. Together, these concepts provide the foundation for all further studies in organic chemistry.

Classification of Organic Compounds

Organic compounds are a vast class of molecules primarily made of carbon and hydrogen atoms, often with other elements like oxygen, nitrogen, sulfur, and halogens. They are classified based on their structure, the type of bonds they contain, and the presence of specific functional groups.

Based on the Carbon Skeleton

This is the broadest way to classify organic compounds based on the arrangement of carbon atoms.

1. Acyclic or Open-Chain Compounds: The carbon atoms are linked to form a straight or branched chain. They are also known as aliphatic compounds.
   • Examples: Methane (CH4​), Ethane (C3​H8​).
2. Cyclic or Closed-Chain Compounds: The carbon atoms are arranged in a ring structure. These are further divided into two types:
   • Alicyclic Compounds: The ring is made of carbon atoms only. They behave similarly to aliphatic compounds.
     • Examples: Cyclopropane, Cyclohexane.
   • Aromatic Compounds: The ring contains a special type of stability due to delocalized pi electrons. Benzene is the most common example.
     • Examples: Benzene, Naphthalene, Phenol.

Based on Functional Groups

The functional group is the reactive site in an organic molecule that determines its properties and reactions.

• Alcohols (–OH), Aldehydes (–CHO), Ketones (–CO–), Carboxylic acids (–COOH), Amines (–NH₂), etc.

Why is classification important?
It helps in organizing thousands of organic compounds, predicting their chemical behavior, and providing a logical basis for naming.

Nomenclature of Organic Compounds

The nomenclature of organic compounds is the system for naming them based on their chemical structure. The most widely accepted system is the IUPAC (International Union of Pure and Applied Chemistry) system. The goal of this system is to ensure that every unique organic compound has a unique and unambiguous name, and that a chemical name can be used to derive a compound’s structure.

The Core Principles of IUPAC Nomenclature

The name of an organic compound is made up of three main parts: a prefix, a parent (or base) name, and a suffix.

1. Prefix: Indicates the identity and location of any substituents (branches or functional groups) attached to the main carbon chain.
2. Parent Name: Indicates the number of carbon atoms in the longest continuous carbon chain or ring.
3. Suffix: Indicates the principal functional group present in the compound.

Isomerism in Organic Compounds

Isomerism is a phenomenon in which two or more organic compounds have the same chemical formula but different structural or spatial arrangements of atoms. These compounds, called isomers, have different physical and chemical properties.

Types of Isomerism

There are two main types of isomerism in organic compounds: structural isomerism and stereoisomerism.

1. Structural Isomerism

Structural isomers have the same chemical formula but differ in the way their atoms are connected. There are several subtypes:

• Chain Isomerism: The carbon skeleton is arranged differently (straight chain vs. branched chain).
  • Example: Butane (C4​H10​) has a straight chain, while isobutane (C4​H10​) has a branched chain.
• Positional Isomerism: The position of a functional group or a multiple bond (double or triple) is different.
  • Example: 1-Butene (CH2​=CHCH2​CH3​) and 2-Butene (CH3​CH=CHCH3​).
• Functional Isomerism: The compounds have different functional groups.
  • Example: Ethanol (C2​H5​OH) and dimethyl ether (CH3​OCH3​). Both have the formula C2​H6​O.

2. Stereoisomerism

Stereoisomers have the same chemical formula and the same connectivity of atoms, but their atoms are arranged differently in three-dimensional space.

• Geometric Isomerism (cis-trans): This occurs in compounds with a double bond or a ring structure, where the groups attached to the carbons of the double bond are fixed in space.
  • cis-isomer: The similar groups are on the same side of the double bond.
  • trans-isomer: The similar groups are on opposite sides of the double bond.
• Optical Isomerism (Enantiomers): These are non-superimposable mirror images of each other. They contain a chiral center (a carbon atom bonded to four different groups). Optical isomers rotate plane-polarized light in opposite directions.