Organic Families

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Notes

Fossil Fuels & Fractional Distillation

Fossil fuels (coal, natural gas, petroleum) release heat energy when burned.
**Petroleum (crude oil)** is a mixture of hydrocarbons separated by **fractional distillation**.
Fractional distillation separates fractions by **boiling point** in a column with a temperature gradient (hot at bottom, cool at top).
Fractions include: **refinery gas** (heating/cooking), **gasoline** (car fuel), **naphtha** (chemicals), **diesel** (diesel engines), **bitumen** (road surfacing).
As chain length increases down the column, boiling point increases due to stronger intermolecular forces.

Alkanes

**Alkanes** are saturated hydrocarbons with only single C–C bonds and general formula **CₙH₂ₙ₊₂**.
They are colourless, generally unreactive due to strong single covalent bonds.
Complete combustion of alkanes produces CO₂ and H₂O (e.g., CH₄ + 2O₂ → CO₂ + 2H₂O).
First five members: methane (CH₄), ethane (C₂H₆), propane (C₃H₈), butane (C₄H₁₀), pentane (C₅H₁₂).

Alkenes & Cracking

**Alkenes** are unsaturated hydrocarbons with at least one $C=C$ double bond and general formula **CₙH₂ₙ**.
The $C=C$ bond makes alkenes more reactive than alkanes; it can open to form new bonds.
**Cracking** breaks long-chain alkanes into shorter alkanes, alkenes, and H₂ using high temperature (600–700 °C) and a catalyst (Al₂O₃ or SiO₂).
Example: decane → octane + ethene (C₁₀H₂₂ → C₈H₁₈ + C₂H₄).
Cracking increases supply of useful fuels (e.g., petrol) and produces alkenes for polymers.

Test for Alkenes

Alkenes are identified by adding **aqueous bromine (bromine water)**, which is orange.
Alkane: no colour change (remains orange).
Alkene: **decolourises** (orange → colourless) because bromine adds across the $C=C$ bond.

Addition Reactions (Extended)

**Addition reactions** involve atoms adding across the $C=C$ double bond, forming a single saturated product.
Alkene + bromine → dibromoalkane (e.g., ethene + Br₂ → 1,2-dibromoethane).
Alkene + hydrogen (with Ni catalyst) → alkane (e.g., ethene + H₂ → ethane).
Alkene + steam (with acid catalyst) → alcohol (hydration; e.g., ethene + H₂O → ethanol).

Alcohols

**Alcohols** contain the hydroxyl (–OH) functional group and have general formula **CₙH₂ₙ₊₁OH**.
First three: methanol (CH₃OH), ethanol (C₂H₅OH), propanol (C₃H₇OH).
**Ethanol** is used in alcoholic drinks, as a fuel, and as a solvent.
Ethanol undergoes complete combustion: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O.

General structure of an amino acid (for reference; not directly in topic but illustrates functional groups).

Atomic structure of carbon, showing 2 electrons in inner shell and 4 in outer shell.

Particle arrangement in solid, liquid, and gas states (for understanding physical properties).

Practice questions

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1.What is the main constituent of natural gas?
Easy
- Aethane
- Bpropane
- Cmethane
- Dbutane
2.State the general formula of the alkanes.
Easy
3.Alkenes are saturated hydrocarbons.
Easy
True or false?
4.Complete the sentence.
Medium
The process of breaking down long-chain hydrocarbons into shorter, more useful molecules is called ____.
5.Match each fraction from petroleum fractional distillation to its correct use.
Medium
- Refinery gas
- Gasoline
- Bitumen
- fuel for cars
- surfacing roads
- heating and cooking
6.How many carbon atoms are in a molecule of butane?
Medium
- A3
- B5
- C4
- D6
7.Which of the following is the product of the complete combustion of methane?
Medium
- Acarbon monoxide and water
- Bcarbon dioxide and water
- Ccarbon and hydrogen
- Dcarbon dioxide and hydrogen
8.What colour change is observed when bromine water is added to an alkene?
Medium

Slides

Organic Families Overview

Organic compounds contain carbon and are found in fossil fuels.
The main families are alkanes, alkenes, and alcohols.
Each family has a characteristic functional group and general formula.

Fossil Fuels & Fractional Distillation

Fossil fuels include coal, natural gas, and petroleum (crude oil).
Petroleum is a mixture of hydrocarbons separated by fractional distillation.
Fractions have different boiling points and uses (e.g., petrol for cars, bitumen for roads).

What is the main constituent of natural gas?

Methane
Ethane
Propane
Butane

Alkanes: Saturated Hydrocarbons

Alkanes are saturated hydrocarbons with only single C–C bonds.
General formula: CₙH₂ₙ₊₂.
They are generally unreactive but undergo complete combustion to give CO₂ and H₂O.

Which is the correct general formula for alkanes?

CₙH₂ₙ
CₙH₂ₙ₊₂
CₙH₂ₙ₋₂
CₙH₂ₙ₊₁OH

Alkenes: Unsaturated Hydrocarbons

Alkenes contain at least one $C=C$ double bond and are unsaturated.
General formula: CₙH₂ₙ.
They are more reactive than alkanes because the double bond can open up.

What is the general formula of alkenes?

CₙH₂ₙ₊₂
CₙH₂ₙ
CₙH₂ₙ₋₂
CₙH₂ₙ₊₁OH

Cracking: Producing Alkenes

Cracking breaks long-chain alkanes into shorter alkanes and alkenes.
Conditions: high temperature (600–700 °C) and a catalyst (aluminium oxide).
Example: decane → octane + ethene.

What is the purpose of cracking?

To separate crude oil into fractions
To produce shorter-chain hydrocarbons and alkenes
To convert alkenes into alkanes
To remove impurities from petroleum

Test for Alkenes: Bromine Water

Bromine water (orange) is used to distinguish alkenes from alkanes.
Alkene: orange → colourless (decolourises bromine).
Alkane: no change, remains orange.

What colour change occurs when bromine water is added to an alkene?

Orange to blue
Orange to colourless
Colourless to orange
Blue to colourless

Addition Reactions of Alkenes

Alkenes undergo addition reactions across the $C=C$ bond.
With bromine: forms a dibromo compound (test for unsaturation).
With hydrogen (Ni catalyst): forms an alkane.
With steam (acid catalyst): forms an alcohol (hydration).

Ethene reacts with steam to form ____.

Alcohols: Functional Group –OH

Alcohols contain the hydroxyl (–OH) functional group.
General formula: CₙH₂ₙ₊₁OH.
First three: methanol (CH₃OH), ethanol (C₂H₅OH), propanol (C₃H₇OH).

What is the functional group of alcohols?

–COOH
–OH
–NH₂
$C=C$

Uses and Combustion of Ethanol

Ethanol is found in alcoholic drinks, used as a fuel and solvent.
Complete combustion: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O.

Which of these are true about ethanol?

It is used as a fuel.
It has the formula CH₃OH.
It undergoes complete combustion to give CO₂ and H₂O.
It is an alkane.

Summary of Organic Families

Alkanes: saturated, CₙH₂ₙ₊₂, unreactive, complete combustion.
Alkenes: unsaturated, CₙH₂ₙ, reactive, addition reactions, decolourise bromine.
Alcohols: contain –OH, CₙH₂ₙ₊₁OH, e.g., ethanol used as fuel.

Flashcards

What is a hydrocarbon?

A compound made only from hydrogen and carbon atoms.

What is the main constituent of natural gas?

Methane (CH₄).

What is petroleum (crude oil)?

A thick, sticky, black liquid mixture of hydrocarbons found under porous rock.

How are the fractions of petroleum separated?

By fractional distillation, which separates them according to their boiling points.

Describe the temperature gradient in a fractionating column.

It is very hot at the bottom and cooler at the top.

State one use of refinery gas.

Heating and cooking.

State one use of bitumen.

For surfacing roads.

What is the trend in boiling point as chain length increases in petroleum fractions?

Boiling point increases as chain length increases.

What is an alkane?

A saturated hydrocarbon with only single carbon-carbon bonds.

What is the general formula of alkanes?

CₙH₂ₙ₊₂.

Write the balanced equation for the complete combustion of methane.

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l).

What is an alkene?

An unsaturated hydrocarbon containing at least one carbon-carbon double bond $(C=C)$ .

What is the general formula of alkenes?

CₙH₂ₙ.

Why are alkenes more reactive than alkanes?

The $C=C$ double bond can open up and form new bonds, whereas alkanes have only strong single bonds.

What is cracking?

The thermal decomposition of long-chain alkanes into shorter-chain alkanes, alkenes, and hydrogen.

State the conditions required for cracking.

A high temperature (600–700 °C) and a catalyst (aluminium oxide or silicon dioxide).

Write an equation for the cracking of decane.

C₁₀H₂₂ → C₈H₁₈ + C₂H₄ (decane → octane + ethene).

How can you distinguish an alkane from an alkene using aqueous bromine?

Alkane: no colour change (orange remains). Alkene: orange to colourless (decolourises).

What type of reaction occurs when bromine adds across $a C=C$ bond?

An addition reaction.

What is formed when ethene reacts with hydrogen?

Ethane (C₂H₆).

What is formed when ethene reacts with steam?

Ethanol (C₂H₅OH).

What functional group do alcohols contain?

The hydroxyl group (-OH).

What is the general formula of alcohols?

CₙH₂ₙ₊₁OH.

State two uses of ethanol.

As a fuel for cars and as a solvent.

Write the balanced equation for the complete combustion of ethanol.

C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l).

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