Revision of Organic Chemistry Review of hydrocarbons, functional groups, reactions; practical activities and past questions.

CHEMISTRY S S 3: Revision of Organic Chemistry

Welcome, SS3 students! Organic chemistry is a vital branch of chemistry, dealing with carbon-containing compounds. This revision note will help you consolidate your understanding of hydrocarbons, functional groups, common reactions, and prepare for practical activities and examinations using past questions.

Core Concepts

Hydrocarbons

Hydrocarbons are organic compounds composed solely of hydrogen and carbon atoms. They are classified based on the types of bonds between carbon atoms.

• Alkanes: These are saturated hydrocarbons, meaning they contain only carbon-carbon single bonds. They have the general formula CnH2n+2. Alkanes like methane (CH4) and ethane (C2H6) are relatively unreactive due to their strong C-C and C-H bonds, undergoing mainly substitution reactions and combustion.

• Alkenes: These are unsaturated hydrocarbons containing at least one carbon-carbon double bond. Their general formula is CnH2n. Ethene (C2H4) is the simplest alkene. The double bond makes them more reactive than alkanes, readily undergoing addition reactions (e.g., with hydrogen, halogens, or water).

• Alkynes: These are also unsaturated hydrocarbons, characterized by at least one carbon-carbon triple bond. Their general formula is CnH2n-2. Ethyne (C2H2) is the simplest alkyne. Alkynes are the most reactive among the three, also undergoing addition reactions. Ethene (top) and Ethyne (bottom) showing double and triple bonds, respectively.

Functional Groups

A functional group is an atom or group of atoms within a molecule that is responsible for the characteristic chemical reactions of that molecule. The presence of a functional group dictates the chemical properties of an organic compound.

• Common Functional Groups:
  • Hydroxyl group (-OH): Present in alcohols (e.g., ethanol).
  • Carboxyl group (-COOH): Present in carboxylic acids (e.g., ethanoic acid).
  • Carbonyl group (C=O): Present in aldehydes (-CHO) and ketones (C=O within a chain).
  • Ester group (-COO-): Formed from carboxylic acids and alcohols.
  • Amino group (-NH2): Present in amines.
  • Haloalkanes (R-X, where X is a halogen): Contain a halogen atom. A table illustrating various common functional groups.

Reactions of Organic Compounds

Organic compounds undergo various types of reactions:

• Substitution Reactions: An atom or group of atoms in a molecule is replaced by another atom or group. Characteristic of alkanes (e.g., halogenation of methane in UV light).
• Addition Reactions: Atoms are added across a double or triple bond, converting an unsaturated compound into a saturated one. Characteristic of alkenes and alkynes (e.g., hydrogenation of ethene to ethane).
• Esterification: The reaction between an alcohol and a carboxylic acid to form an ester and water, typically catalyzed by a strong acid.
• Polymerization: The process where many small molecules (monomers) link together to form a large molecule (polymer). Can be addition polymerization (e.g., ethene to polyethene) or condensation polymerization.
• Combustion: The rapid reaction of an organic compound with oxygen, producing carbon dioxide and water, releasing heat (e.g., burning of methane).

Practical Activities and Past Questions

Practical activities are crucial for understanding organic chemistry. They often involve identifying functional groups (e.g., using bromine water or acidified KMnO4 for unsaturation, litmus for acids) and understanding reaction conditions. Regularly attempting past questions from WAEC, NECO, and JAMB is essential. This helps you understand question patterns, manage your time, and identify areas needing further study. Focus on drawing structures, naming compounds, writing balanced equations, and explaining reaction mechanisms. A typical setup for an organic chemistry experiment in a lab.

Real-World Example & Home Practice

Ethanol, an alcohol (containing the -OH functional group), is a key ingredient in many hand sanitizers due to its ability to denature proteins in bacteria and viruses. For home practice, try to identify the functional groups present in common household products like vinegar (ethanoic acid) or cooking oil (esters/triglycerides), and explain their characteristic properties.

Practice/Reflection Question

Consider the reaction of ethene (an alkene) with bromine water.

1. Describe the observation you would make.
2. Write the balanced chemical equation for this reaction.
3. What type of reaction is this, and why is it important in organic chemistry?