Organic Chemistry

What you'll learn

Organic Chemistry forms a substantial component of the CXC CSEC Chemistry syllabus, focusing on carbon-based compounds and their properties. This section examines the structure, nomenclature, reactions, and uses of hydrocarbons and their derivatives, with particular emphasis on homologous series, functional groups, and isomerism. Questions on this topic appear consistently in Paper 01 (multiple choice), Paper 02 (structured questions), and Paper 03/2 (alternative to School-Based Assessment).

Key terms and definitions

Organic compound — A compound containing carbon chemically bonded to hydrogen, and often to other elements such as oxygen, nitrogen, or halogens.

Hydrocarbon — An organic compound containing only carbon and hydrogen atoms.

Homologous series — A family of organic compounds with the same general formula, similar chemical properties, and successive members differing by CH₂.

Functional group — A specific atom or group of atoms responsible for the characteristic chemical reactions of a compound.

Isomer — Compounds with the same molecular formula but different structural arrangements of atoms.

Saturated compound — A hydrocarbon containing only single covalent bonds between carbon atoms (alkanes).

Unsaturated compound — A hydrocarbon containing at least one double or triple carbon-carbon bond (alkenes, alkynes).

Alkyl group — A hydrocarbon fragment formed by removing one hydrogen atom from an alkane (e.g., methyl CH₃—, ethyl C₂H₅—).

Core concepts

Homologous series of hydrocarbons

The three major hydrocarbon homologous series tested at CSEC level are alkanes, alkenes, and alkynes.

Alkanes have the general formula CₙH₂ₙ₊₂ and contain only single C—C bonds. The first four members are:

• Methane (CH₄)
• Ethane (C₂H₆)
• Propane (C₃H₈)
• Butane (C₄H₁₀)

Alkanes are saturated hydrocarbons obtained from petroleum (crude oil), which is extensively processed at the Petrotrin refinery in Trinidad and Tobago. They undergo substitution reactions with halogens in the presence of ultraviolet light:

CH₄ + Cl₂ → CH₃Cl + HCl

Alkenes have the general formula CₙH₂ₙ and contain at least one C=C double bond. The first three members are:

• Ethene (C₂H₄)
• Propene (C₃H₆)
• Butene (C₄H₈)

Alkenes are unsaturated and undergo addition reactions. The characteristic test for unsaturation involves adding bromine water (brown/orange solution), which decolorizes when an alkene is present:

C₂H₄ + Br₂ → C₂H₄Br₂

Alkynes have the general formula CₙH₂ₙ₋₂ and contain at least one C≡C triple bond. Ethyne (C₂H₂), also called acetylene, is the simplest alkyne used in oxyacetylene welding torches throughout Caribbean construction industries.

Nomenclature and structural formulae

CXC CSEC Chemistry requires students to name organic compounds systematically and draw their structural formulae.

For straight-chain hydrocarbons, the prefix indicates the number of carbon atoms:

• Meth- = 1 carbon
• Eth- = 2 carbons
• Prop- = 3 carbons
• But- = 4 carbons
• Pent- = 5 carbons
• Hex- = 6 carbons

The suffix indicates the homologous series: -ane (alkane), -ene (alkene), -yne (alkyne).

Structural formulae show how atoms are bonded. For butane:

CH₃—CH₂—CH₂—CH₃

Condensed structural formula: CH₃(CH₂)₂CH₃

Displayed (full structural) formula shows all bonds explicitly.

Isomerism

Structural isomerism occurs when compounds have the same molecular formula but different structural arrangements. This is a critical concept for Paper 02 questions.

Butane (C₄H₁₀) has two isomers:

1. n-butane (normal butane): CH₃—CH₂—CH₂—CH₃ (straight chain)
2. 2-methylpropane (isobutane): CH₃—CH(CH₃)—CH₃ (branched chain)

Both have identical molecular formulae but different physical properties (boiling points, melting points) due to different structural arrangements. Branched isomers generally have lower boiling points than their straight-chain counterparts because of reduced surface contact and weaker van der Waals forces.

Pentane (C₅H₁₂) has three isomers, and students must be able to draw all three structures when asked.

Functional groups and derivatives

When hydrogen atoms in hydrocarbons are replaced by other atoms or groups, derivatives are formed. The replacing group is the functional group.

Alcohols contain the hydroxyl group (—OH) and have the general formula CₙH₂ₙ₊₁OH. Examples include:

• Methanol: CH₃OH
• Ethanol: C₂H₅OH (produced industrially in Trinidad for rum production and as biofuel)
• Propanol: C₃H₇OH

Alcohols undergo oxidation reactions. Ethanol oxidizes to ethanoic acid when exposed to air and bacterial action:

C₂H₅OH + O₂ → CH₃COOH + H₂O

This reaction explains why opened bottles of rum or wine turn sour over time.

Carboxylic acids contain the carboxyl group (—COOH) and have the general formula CₙH₂ₙ₊₁COOH. Examples include:

• Methanoic acid: HCOOH (formic acid, found in ant stings)
• Ethanoic acid: CH₃COOH (acetic acid, the active component in vinegar)
• Propanoic acid: C₂H₅COOH

Carboxylic acids are weak acids that react with metals, bases, and carbonates:

CH₃COOH + NaOH → CH₃COONa + H₂O

Esters are formed when alcohols react with carboxylic acids in the presence of concentrated sulfuric acid (catalyst) through esterification:

CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O

Esters have pleasant fruity odours and are used in the food flavouring industry across the Caribbean. Ethyl ethanoate smells like pears, while pentyl ethanoate smells like bananas.

Halogenoalkanes (alkyl halides) contain halogen atoms (F, Cl, Br, I) replacing hydrogen. Chloroethane (C₂H₅Cl) and bromoethane (C₂H₅Br) are examples. These compounds undergo substitution reactions with aqueous sodium hydroxide to form alcohols:

C₂H₅Br + NaOH → C₂H₅OH + NaBr

Polymerization and macromolecules

Polymerization is the process where many small molecules (monomers) join together to form large molecules (polymers). Alkenes undergo addition polymerization.

Ethene polymerizes to form polyethene (polythene):

nC₂H₄ → (C₂H₄)ₙ

Propene forms polypropene, and chloroethene (vinyl chloride) forms polyvinyl chloride (PVC), widely used for water pipes in Caribbean plumbing systems.

Polymers are generally unreactive, waterproof, and non-biodegradable, leading to environmental concerns about plastic waste in Caribbean waters and beaches.

Fractional distillation of petroleum

Petroleum (crude oil) is a mixture of hydrocarbons separated by fractional distillation based on different boiling points. The process involves:

1. Heating crude oil to approximately 350°C in a furnace
2. Vapours rise through a fractionating column (cooler at the top)
3. Different fractions condense at different heights
4. Fractions are collected at various levels

Common fractions from lowest to highest boiling point:

• Refinery gas (C₁-C₄): used as LPG (liquefied petroleum gas) for cooking throughout the Caribbean
• Gasoline/Petrol (C₅-C₁₀): vehicle fuel
• Naphtha (C₇-C₁₄): chemical feedstock
• Kerosene (C₁₀-C₁₆): jet fuel, lighting
• Diesel oil (C₁₅-C₂₀): diesel engines, heating
• Lubricating oil (C₂₀-C₃₀): lubrication
• Bitumen (>C₃₀): road surfacing, widely used on Caribbean highways

Smaller molecules have lower boiling points and are collected near the top of the column. Larger molecules have higher boiling points and are collected near the bottom.

Cracking is the process of breaking down large hydrocarbon molecules into smaller, more useful molecules. Catalytic cracking uses heat (450-500°C) and a catalyst (aluminium oxide or silicon dioxide):

C₁₀H₂₂ → C₈H₁₈ + C₂H₄

This produces alkenes (used for polymer production) and shorter alkanes (used as fuels).

Worked examples

Example 1: Identifying homologous series

Question: A hydrocarbon has the molecular formula C₅H₁₀.

(a) State the homologous series to which this compound belongs. [1 mark]

(b) Write the general formula for this homologous series. [1 mark]

(c) Name one member of this series used to ripen bananas and other fruits in Jamaica. [1 mark]

Solution:

(a) Alkene [1 mark]

(b) CₙH₂ₙ [1 mark]

(c) Ethene (C₂H₄) [1 mark]

Examiner note: The formula fits CₙH₂ₙ where n=5. Students often confuse alkenes with alkynes; check the hydrogen count carefully.

Example 2: Isomerism

Question: Butene has the molecular formula C₄H₈.

(a) Draw the displayed structural formulae for two isomers of butene. [4 marks]

(b) Describe a chemical test to distinguish butene from butane. [3 marks]

Solution:

(a) Isomer 1: But-1-ene

H H H H
| | | |
C=C-C-C-H
| |
H H

Isomer 2: But-2-ene

H H H
| | |
H-C-C=C-C-H
| |
H H

[2 marks each for correct structures]

(b) Add bromine water to both samples [1 mark]. Butene will decolorize the brown/orange bromine water [1 mark], while butane will show no change because it is saturated [1 mark].

Example 3: Esterification

Question: Ethanoic acid reacts with propanol to form an ester.

(a) Write a balanced equation for this reaction. [2 marks]

(b) Name the ester formed. [1 mark]

(c) State the name and formula of the catalyst used. [2 marks]

Solution:

(a) CH₃COOH + C₃H₇OH ⇌ CH₃COOC₃H₇ + H₂O [2 marks — 1 for correct products, 1 for balanced equation with water]

(b) Propyl ethanoate [1 mark]

(c) Concentrated sulfuric acid [1 mark], H₂SO₄ [1 mark]

Common mistakes and how to avoid them

• Mistake: Confusing the general formulae of alkanes, alkenes, and alkynes. Memorize: alkanes CₙH₂ₙ₊₂ (most hydrogen), alkenes CₙH₂ₙ (double bond uses two H), alkynes CₙH₂ₙ₋₂ (triple bond uses four H). The hydrogen count decreases with increasing unsaturation.

• Mistake: Drawing isomers that are actually the same molecule rotated or flipped. Check that the carbon skeleton arrangement is genuinely different. CH₃CH₂CH₂CH₃ flipped around is still the same straight chain, not an isomer. Only different branching patterns count.

• Mistake: Writing "turns colourless" instead of "decolorizes" for the bromine water test. Use precise terminology: bromine water decolorizes (loses its brown/orange colour) in the presence of unsaturated compounds. Colourless implies the solution was already clear.

• Mistake: Forgetting water as a product in esterification reactions. Esterification is a condensation reaction where a small molecule (water) is eliminated. Always include H₂O on the product side of the equation.

• Mistake: Claiming all polymers are plastics or that polymerization only involves alkenes. While addition polymerization uses alkenes, condensation polymerization (not required in detail at CSEC) produces polymers like nylon and polyester. State specifically "addition polymerization" when discussing alkene reactions.

• Mistake: Using molecular formulae instead of structural formulae when specifically asked for structure. C₂H₆ is a molecular formula; CH₃—CH₃ is a structural formula. Read the question carefully and provide the format requested for full marks.

Exam technique for Organic Chemistry

• Command word "Name" requires the IUPAC systematic name (ethanoic acid, not acetic acid; ethanol, not alcohol). Command word "State" requires brief answers without explanation. Command word "Describe a test" requires the reagent, observation, and result (positive/negative).

• When drawing structural formulae, show all bonds clearly. Use straight lines for single bonds, double lines for double bonds. Ensure correct valency (carbon forms four bonds, hydrogen one, oxygen two). Examiners deduct marks for incorrect bonding.

• For isomer questions worth 4+ marks, expect to draw at least two distinct structures. Label them clearly as Isomer 1, Isomer 2, or use their systematic names. Check that molecular formulae are identical before submitting.

• Organic Chemistry questions frequently combine knowledge areas: a single question might test nomenclature, isomerism, and chemical tests. Allocate time proportionally to marks available (roughly 1 minute per mark on Paper 02).

Quick revision summary

Organic compounds contain carbon bonded to hydrogen and other elements. Alkanes (CₙH₂ₙ₊₂) are saturated; alkenes (CₙH₂ₙ) and alkynes (CₙH₂ₙ₋₂) are unsaturated. Isomers share molecular formulae but differ structurally. Functional groups (—OH, —COOH, halogens) create derivatives with distinct properties. Alkenes undergo addition reactions and decolorize bromine water. Petroleum fractional distillation separates hydrocarbons by boiling point. Alcohols oxidize to carboxylic acids; acids react with alcohols to form esters. Alkenes polymerize through addition polymerization to form plastics. Master nomenclature, structural formulae, and characteristic tests for exam success.