Carbon Chemistry: Hydrocarbons and Organic Compounds — CXC CSEC Integrated Science Revision Notes

What you'll learn

This revision guide covers the essential carbon chemistry concepts tested in CXC CSEC Integrated Science examinations. You will understand the structure and properties of hydrocarbons, distinguish between saturated and unsaturated compounds, identify functional groups in organic molecules, and recognize the importance of carbon compounds in Caribbean industries including petroleum refining and alcohol production.

Key terms and definitions

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

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

Unsaturated hydrocarbon — A hydrocarbon containing at least one double or triple bond between carbon atoms (alkenes and alkynes).

Functional group — A specific group of atoms within a molecule that determines the chemical properties and reactions of that compound.

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

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

Fractional distillation — The process of separating crude oil into different fractions based on their boiling points.

Fermentation — The anaerobic breakdown of sugars by microorganisms (yeast) to produce ethanol and carbon dioxide.

Core concepts

Carbon and its unique bonding properties

Carbon forms the basis of all organic compounds due to its exceptional bonding capacity. Each carbon atom has four valence electrons, allowing it to form four covalent bonds with other atoms. This tetravalency enables carbon to:

Form long chains and branched structures
Create single, double, and triple bonds with other carbon atoms
Bond with hydrogen, oxygen, nitrogen, and other elements
Produce millions of different organic compounds

Carbon atoms can bond together to form straight chains, branched chains, or ring structures. This versatility explains why carbon chemistry forms a separate branch of chemistry and why carbon-based molecules are essential to life.

The alkane homologous series

Alkanes are saturated hydrocarbons with the general formula CₙH₂ₙ₊₂. They contain only single C-C and C-H bonds.

First ten members:

Methane (CH₄)
Ethane (C₂H₆)
Propane (C₃H₈)
Butane (C₄H₁₀)
Pentane (C₅H₁₂)
Hexane (C₆H₁₄)
Heptane (C₇H₁₆)
Octane (C₈H₁₈)
Nonane (C₉H₂₀)
Decane (C₁₀H₂₂)

Properties of alkanes:

Relatively unreactive (saturated bonds are stable)
Undergo combustion reactions readily
Boiling points increase as chain length increases
Used as fuels (natural gas, LPG, petrol, diesel)

In Trinidad and Tobago's petrochemical industry, methane extracted from natural gas serves as feedstock for ammonia and methanol production. Propane and butane are sold as liquified petroleum gas (LPG) for cooking throughout the Caribbean.

The alkene homologous series

Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond (C=C). Their general formula is CₙH₂ₙ.

First five members:

Ethene (C₂H₄)
Propene (C₃H₆)
Butene (C₄H₈)
Pentene (C₅H₁₀)
Hexene (C₆H₁₂)

Properties of alkenes:

More reactive than alkanes due to the double bond
Undergo addition reactions
Decolorize bromine water (test for unsaturation)
Used to manufacture plastics and polymers

Test for unsaturation: When bromine water (orange-brown solution) is added to an alkene, the bromine adds across the double bond, causing the solution to become colorless. Alkanes do not react with bromine water under normal conditions.

Crude oil and fractional distillation

Crude oil is a complex mixture of hydrocarbons formed from the remains of marine organisms over millions of years. Caribbean countries with petroleum reserves include Trinidad and Tobago, Barbados, and Cuba.

Fractional distillation separates crude oil into useful fractions based on boiling point differences:

Fraction (boiling range) → Uses

Refinery gas (below 40°C) → LPG for cooking
Gasoline/petrol (40-180°C) → Vehicle fuel
Kerosene (180-250°C) → Aircraft fuel, heating
Diesel oil (250-350°C) → Diesel engines, heating
Fuel oil (above 350°C) → Ships, power stations
Bitumen (residue) → Road surfacing, waterproofing

The process occurs in a fractionating column where:

Crude oil is heated and vaporized
Vapors rise up the column (cooler at the top)
Different fractions condense at different temperatures
Smaller molecules (lower boiling points) collected near the top
Larger molecules (higher boiling points) collected near the bottom

The Point Lisas Industrial Estate in Trinidad processes crude oil and natural gas, producing fuels and petrochemicals for regional and international markets.

Alcohols and their properties

Alcohols form a homologous series with the general formula CₙH₂ₙ₊₁OH. The hydroxyl group (-OH) is the functional group that characterizes alcohols.

First three members:

Methanol (CH₃OH)
Ethanol (C₂H₅OH)
Propanol (C₃H₇OH)

Properties of ethanol:

Colorless liquid at room temperature
Characteristic smell
Dissolves in water (forms hydrogen bonds)
Flammable (burns with a clean blue flame)
Used in alcoholic beverages, as a solvent, and as fuel

Production of ethanol:

Method 1: Fermentation Sugarcane molasses, abundant in Caribbean countries like Jamaica, Barbados, and Guyana, can be fermented to produce ethanol:

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

Process:

Yeast enzymes break down glucose
Temperature maintained at 25-37°C
Anaerobic conditions (no oxygen)
Produces ethanol concentration up to 15%
Distillation increases concentration

Caribbean rum production relies on this fermentation process, making it economically significant to regional economies.

Method 2: Hydration of ethene Industrial production uses ethene from petroleum:

C₂H₄ + H₂O → C₂H₅OH

Process:

Ethene gas passed over phosphoric acid catalyst
Temperature around 300°C
High pressure (60-70 atmospheres)
Produces pure ethanol directly
Faster than fermentation

Structural formulas and isomerism

Understanding how to represent organic molecules is essential for CSEC examinations.

Types of formulas:

Molecular formula: Shows the actual number of each type of atom (e.g., C₄H₁₀)

Structural formula: Shows how atoms are arranged and bonded (e.g., CH₃CH₂CH₂CH₃)

Displayed formula: Shows all bonds between all atoms

Isomerism: Compounds with the same molecular formula but different structural arrangements are called isomers.

Example: Butane (C₄H₁₀) has two isomers:

n-butane: CH₃CH₂CH₂CH₃ (straight chain)
methylpropane: CH₃CH(CH₃)CH₃ (branched chain)

These isomers have different physical properties (boiling points, melting points) but similar chemical properties.

Combustion of hydrocarbons

Hydrocarbons burn in oxygen to release energy, making them valuable fuels.

Complete combustion (sufficient oxygen): Hydrocarbon + Oxygen → Carbon dioxide + Water + Energy

Example: CH₄ + 2O₂ → CO₂ + 2H₂O

Incomplete combustion (limited oxygen): Hydrocarbon + Oxygen → Carbon monoxide + Carbon + Water + Energy

Example: 2CH₄ + 3O₂ → 2CO + 4H₂O

Incomplete combustion is dangerous because:

Carbon monoxide is a toxic gas (binds to hemoglobin)
Produces soot (carbon particles) causing pollution
Releases less energy than complete combustion

In Caribbean households using gas stoves, proper ventilation ensures complete combustion and prevents carbon monoxide accumulation.

Worked examples

Example 1: Identifying homologous series members

Question: A hydrocarbon has the molecular formula C₅H₁₀. (a) State whether this compound is an alkane or alkene. Give a reason for your answer. [2 marks] (b) Name this hydrocarbon. [1 mark]

Solution: (a) This compound is an alkene [1 mark]. The general formula for alkenes is CₙH₂ₙ. When n=5, the formula is C₅H₁₀, which matches [1 mark].

(b) Pentene [1 mark]

Mark scheme notes: Students must identify the compound type AND provide the correct formula justification. Simply stating "it's an alkene" without showing why earns only 1 mark.

Example 2: Fractional distillation application

Question: Crude oil is separated into useful products at the Petrotrin refinery in Trinidad. (a) Name the process used to separate crude oil. [1 mark] (b) Explain why this process can separate crude oil into different fractions. [2 marks] (c) State which fraction has the lowest boiling point. [1 mark]

Solution: (a) Fractional distillation [1 mark]

(b) Different hydrocarbons in crude oil have different boiling points [1 mark]. Smaller molecules have lower boiling points and rise higher in the fractionating column before condensing, while larger molecules condense lower down [1 mark].

Example 3: Testing for unsaturation

Question: A student has two unlabeled test tubes, one containing hexane (C₆H₁₄) and one containing hexene (C₆H₁₂). (a) Describe a chemical test to identify which test tube contains hexene. [3 marks] (b) State one use of hexene. [1 mark]

Solution: (a) Add bromine water (orange-brown solution) to each test tube [1 mark]. The test tube where the bromine water becomes colorless contains hexene [1 mark]. This occurs because hexene contains a C=C double bond that reacts with bromine, while hexane does not react [1 mark].

(b) Manufacturing plastics/polymers OR as a chemical feedstock [1 mark]

Common mistakes and how to avoid them

Confusing alkanes and alkenes: Remember that alkanes END in "-ane" and are saturated (CₙH₂ₙ₊₂), while alkenes END in "-ene" and are unsaturated (CₙH₂ₙ). The extra H₂ in alkanes comes from having only single bonds.
Incorrect combustion equations: Always check that carbon dioxide (CO₂) and water (H₂O) are the products for complete combustion. Incomplete combustion produces CO and/or C, not CO₂ alone.
Misidentifying functional groups: The -OH group in alcohols must be bonded to a carbon atom. Simply having hydrogen and oxygen doesn't make a compound an alcohol (water is H₂OH but it's not an alcohol).
Wrong boiling point trends: Larger hydrocarbon molecules have HIGHER boiling points, not lower. They have stronger intermolecular forces requiring more energy to separate.
Forgetting conditions for fermentation: Fermentation requires yeast, warm temperature (25-37°C), absence of oxygen, and a sugar source. Missing any of these conditions in your answer loses marks.
Incomplete test descriptions: When describing the bromine water test, state the initial color (orange-brown), final color (colorless), and explain WHY it occurs (addition reaction across the double bond).

Exam technique for "Carbon Chemistry: Hydrocarbons and Organic Compounds"

Command word precision: "State" requires a simple answer (1 mark); "Explain" requires reasoning with connecting words like "because" or "therefore" (typically 2-3 marks); "Describe" requires a sequence of observations or steps.
Chemical equations earn marks: If the question involves a reaction, writing the balanced equation often secures marks even if your written explanation is incomplete. Practice writing combustion and fermentation equations from memory.
Use correct chemical terminology: Write "carbon dioxide" not "CO₂ gas" when answering in sentences. Use terms like "saturated," "unsaturated," "functional group," and "homologous series" appropriately to demonstrate understanding.
Caribbean context questions: Be prepared to apply your knowledge to regional industries (Trinidad's petroleum, Caribbean rum production, LPG for cooking). These contexts test the same concepts but reward candidates who connect chemistry to local economies.

Quick revision summary

Carbon forms four bonds, creating diverse organic compounds. Alkanes (CₙH₂ₙ₊₂) are saturated hydrocarbons with single bonds; alkenes (CₙH₂ₙ) are unsaturated with C=C double bonds tested using bromine water. Fractional distillation separates crude oil by boiling point differences. Alcohols contain the -OH functional group, with ethanol produced by fermenting sugarcane or hydrating ethene. Complete combustion produces CO₂ and H₂O; incomplete combustion produces dangerous CO. Isomers share molecular formulas but differ structurally.