Organic chemistry - (CCEA)

• Organic compounds contain carbon atoms. They can be sorted into groups called homologous series, such as: alkanes, alkenes, alcohols and carboxylic acids.

• Alkanes, alkenes, alcohols and carboxylic acids all have different functional groups, which affects the reactions they can undergo.

• Organic compounds are obtained from crude oil, which is separated into useful fractions by fractional distillation.

Organic chemistry is the study of carbon compounds.

Carbon can form four covalent bonds, allowing it to form a wide range of carbon compounds.

It is easier to study these compounds by putting them into groups known as homologous series.

For organic compounds in a homologous series:

• they have the same general formula,

• their molecular formulae differ by CH₂,

• they show a gradual variation in physical properties, such as their boiling points; and

• they have similar chemical properties.

For GCSE you will study 4 different homologous series:

• alkanes

• alkenes

• alcohols

• carboxylic acids

The alkanes are saturated hydrocarbons.

Hydrocarbons are compounds which contain onlyhydrogen and carbon.

Alkanes are described as saturated because their carbon atoms are joined by C–C single bonds.

They do not contain C=C bonds.

The general formula for the alkanes is C_nH_2n+2.

Example

Decane is an alkane.

A molecule of decane contains 10 carbon atoms.

How to predict the molecular formula of decane

n = 10

∴ 2n + 2 = (2 × 10) + 2 = 22

Formula: C₁₀H₂₂

What are the first four alkanes?

The table below shows the first four alkanes.

Alkanes are obtained from crude oil, which is drilled up from under the Earth’s crust.

Crude oil is made of organisms that lived and died millions of years ago.

It is a complex mixture of hydrocarbons.

Crude oil is an important source of fuels and is used as a raw material (feedstock) for the petrochemical industry, which makes plastics, rubber, solvents, lubricants and detergents.

Fractional distillation separates crude oil into simpler, more useful mixtures, because different hydrocarbons have different boiling points.

Each simpler mixture is called a fraction.

During fractional distillation:

• Heated crude oil enters a tall fractioning column, which is hot at the bottom and cooler at the top.

• Vapours from the oil rise through the column.

• Each fraction condenses when it becomes cool enough, each at a different position in the column.

• Pipes lead each fraction away.

Small hydrocarbon molecules have weak intermolecular forces, so they have low boiling points.

They do not condense, but leave the top of the column as gases.

Long hydrocarbon molecules have stronger intermolecular forces, so they have higher boiling points.

They leave the bottom of the column as hot, liquid bitumen.

Cracking is a reaction in which larger saturated hydrocarbon molecules are broken down into smaller, more useful hydrocarbon molecules, some of which are unsaturated:

• the original starting hydrocarbons are alkanes; and

• the products of cracking include alkanes and alkenes, members of a different homologous series,

• For example, hexane can be cracked to form butane and ethene as shown by the equation below:

hexane → butane + ethene

C₆H₁₄ → C₄H₁₀ + C₂H₄.

The starting compound will always fit the rule for an alkane, C_nH_2n+2.

The first product will also follow this rule.

The second product will contain all the other C and H atoms and a C=C.

Combustion is the reaction of a fuel with oxygen to produce oxides and release heat.

Alkanes can undergo either complete combustion or incomplete combustion depending on the amount of oxygen available for combustion.

What is complete combustion?

Complete combustion occurs when there is a plentiful supply of oxygen.

The carbon and hydrogen atoms react with oxygen in an exothermic reaction.

• Carbon dioxide and water are produced.

• The maximum amount of energy is given out.

In general:

hydrocarbon + oxygen → carbon dioxide + water

For example, the equation for the complete combustion of propane is:

propane + oxygen → carbon dioxide + water

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

Balancing numbers do not need to be whole numbers as they refer to the moleratio of the chemicals, and numbers of moles do not have to be whole numbers.

The most straightforward order to balance a combustion equation is: carbon atoms first, hydrogen atoms second and oxygen atoms third.

Key fact

The products of complete combustion can be identified by chemical tests.

Carbon dioxide can be bubbled through limewater (turns from colourless to cloudy).

Water can be condensed on a cold surface and tested with anhydrous copper(II) sulfate (turns from white to blue).

What is incomplete combustion?

Incomplete combustion occurs when there is a limited supply of oxygen.

• Carbon (soot), carbon monoxide, and water are produced.

• Less energy is released, compared to complete combustion.

In equations for incomplete combustion, the products are carbon monoxide and water.

Soot is not included in the equation.

For example:

butane + oxygen → carbon monoxide + water

C₄H₁₀ + 4.5O₂ → 4CO + 5H₂O

What are the problems with combustion?

Combustion produces four major atmospheric pollutants: carbon dioxide, carbon monoxide, soot and sulfur dioxide.

• Carbon dioxide is a cause of the greenhouse effect that can cause climate change, rising sea levels and increased flooding of low-lying areas.

• Carbon monoxide is a toxic gas that binds to haemoglobin in red blood cells in the body and prevents them from carrying oxygen.

• Soot (carbon) can cause lung irritation if breathed in.

• Sulfur dioxide is produced by combustion of fuels that contain sulfur impurities. Sulfur dioxide dissolves in rainwater causing acid rain, which can destroy vegetation, damage buildings and kill fish in lakes and rivers.

The alkenes are unsaturated hydrocarbons.

Hydrocarbons – because they contain hydrogen and carbon only.

Unsaturated – because they contain at least one C=C.

Each alkene containing one C=C has two fewer hydrogen atoms than the corresponding alkane.

Alkenes have a functional group, C=C.

A functional group is a reactive group in a molecule.

Alkanes, in contrast, do not have a functional group and are less reactive as a result.

The general formula for the alkanes is C_nH_2n.

What are the first four alkenes?

The table below shows the first four alkenes:

Note: But-1-ene and but-2-ene have the same molecular formula.

The only difference between the two molecules is the position of the double bond.

The number within each name describes the position of the double bond in the carbon chain.

Alkenes combust in a similar way to alkanes.

What is complete combustion of alkenes?

Complete combustion of alkenes produces carbon dioxide and water, provided there is a plentiful supply of oxygen.

For example:

propene + oxygen → carbon dioxide + water

C₃H₆ + 4.5O₂ → 3CO₂ + 3H₂O

What is incomplete combustion of alkenes?

Incomplete combustion of alkenes occurs where oxygen is limited and produces water, carbon monoxide and carbon (soot).

For example:

propene + oxygen → carbon monoxide + water

C₃H₆ + 3O₂ → 3CO + 3H₂O

Addition reactions occur when two molecules combine to form one molecule.

In alkenes, the C=C bond breaks, which gives room for two new bonds to the carbon atoms.

What happens in an addition reaction using bromine?

Bromine (Br₂) can combine with alkenes in an addition reaction.

C₂H₄ + Br₂ → C₂H₄Br₂

How can bromine water be used to identify alkanes and alkenes?

You can use bromine water to distinguish between alkanes and alkenes.

Bromine water is added to two different test tubes, one containing an alkane and the other an alkene, and they are shaken.

The bromine water remains orange in the alkane, but changes from orange to colourless in the alkene.

What happens in an addition reaction using hydrogen?

Hydrogen gas can combine with alkenes in an addition reaction to form alkanes.

This process is called hydrogenation, and it requires a catalyst.

For example:

propene + hydrogen → propane

C₃H₆ + H₂ → C₃H₈

What happens in an addition reaction using steam?

Steam is the gas form of water.

Steam can combine with alkenes in an addition reaction to form alcohols.

This process is called hydration.

It requires a high temperature and a catalyst.

For example:

ethene + water (steam) → alcohol

C₂H₄ + H₂O → C₂H₅OH

What is addition polymerisation?

A polymer is a long chain molecule made up of small, repeating units.

These small molecules, called monomers join together in a long chain to make polymers.

For example: polyethene is a polymer made from a very large number of ethene molecules bonded together.

The reaction is called an addition polymerisation reaction.

Ethene is the monomer and polyethene is the polymer.

To avoid having to write out a large structure in the polymer, a short two-carbon segment is shown in brackets.

This is called the repeating unit.

Equations use repeating units to model addition polymerisation reactions.

The letter n stands for a large number.

Polychloroethene is commonly known by the initials of its old name, PVC (polyvinyl chloride).

Polychloroethene is made by polymerising chloroethene (an ethene molecule in which one of the hydrogens has been replaced with a chlorine).

How to dispose of polymers

Polymers are unreactive, so they are suitable for storing food and chemicals safely.

Unfortunately, this also makes them difficult to dispose off.

They are often buried in landfill sites or incinerated.

Landfill

Waste polymers are disposed in landfill sites, but this uses valuable land and suitable sites often fill up quickly.

Incineration

Polymers release a lot of energy when they burn, which can be used to heat homes or generate electricity.

But burning polymers produces carbon dioxide, adding to global warming, and can release toxic gases.

The alcohols form a homologous series.

The general formula for the alcohols is: C_nH_2n+1OH

The alcohols’ functional group is –OH. It is responsible for the alcohols’ typical reactions.

What are the first four alcohols?

The table shows the first four alcohols:

How is ethanol produced?

Ethanol, C₂H₅OH, is an useful alcohol which is found in alcoholic drinks, fuels and solvents.

It can be produced by the hydration of steam or the fermentation of sugar.

What is hydration of ethene?

Ethene can react with steam in an addition reaction to produce ethanol.

ethene + steam → ethanol

C₂H₄ + H₂O → C₂H₅OH

How is ethanol made by fermentation?

In fermentation, a sugar solution is mixed with yeast to produce ethanol and carbon dioxide.

It takes place under the following conditions:

• anaerobic conditions (no air can get in) and

• a warm (25°C – 35°C) temperature.

Alcohols completely combust in the presence of oxygen to form carbon dioxide and water.

For example:

ethanol + oxygen → carbon dioxide + water

C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O

They combust incompletely when oxygen is in limited supply, producing water and either carbon monoxide or carbon (soot).

Methanol, ethanol and propan-1-ol can be oxidised to produce carboxylic acids:

Propan-2-ol can also be oxidised but for GCSE you do not need to know the product of this reaction.

Oxidation of an alcohol can take place in two ways:

• Exposure to oxygen in the air – this is a slow process.

• Reaction with acidified potassium dichromate solution. This can be mixed with an alcohol in a test tube and warmed in a warm water bath.

Acidified potassium dichromate solution is an orange solution that changes green when the alcohol is oxidised.

The reaction of acidified potassium dichromate can be used as a test for an alcohol.

The carboxylic acids form a homologous series with the general formula C_nH_2n+1COOH.

The functional group in the carboxylic acids is the carboxyl group –COOH.

It is responsible for the carboxylic acids’ typical reactions.

They are weak acids as they are partially ionised in solution.

What are the first four carboxylic acids?

The first four carboxylic acids (higher tier only) are:

Carboxylic acids show the normal characteristic reactions of weak acids.

When carboxylic acids react with acids they form salts:

• Methanoic acid forms methanoate salts; the methanoate ion is HCOO^-

• Ethanoic acid forms ethanoate salts; the ethanoate ion is CH₃COO^-

• Propanoic acid forms propanoate salts; the propanoate ion is C₂H₅COO^-

• Butanoic acid forms butanoate salts; the butanoate ion is C₃H₇COO^-

Key fact

When writing the formulae of carboxylic acid salts, the symbol for the metal is normally written at the end of the formula.

For example, the formula of sodium ethanoate is CH₃COONa.

How do carboxylic acids react with metals?

acid + metal → salt + hydrogen

For example:

ethanoic acid + magnesium → magnesium ethanoate + hydrogen

2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂

Example observations: solid metal disappears, fizzing, heat produced, colourless solution produced.

How do carboxylic acids react with hydroxides?

acid + metal hydroxide → salt + water

For example:

methanoic acid + sodium hydroxide → sodium methanoate + water

HCOOH + NaOH → HCOONa + H₂O

Example observations: heat produced, colourless solution produced.

How do carboxylic acids react with carbonates?

acid + metal carbonate → salt + water + carbon dioxide

For example:

propanoic acid + calcium carbonate → calcium propanoate + water + carbon dioxide

2C₂H₅COOH + CaCO₃ → (C₂H₅COO)₂Ca + H₂O + CO₂

Example observations: solid disappears, fizzing, heat produced, colourless solution produced (unless copper carbonate is used – a blue solution would be produced).

Please use the link below to access the article on: Prescribed practical (C7) - Investigate the reactions of carboxylic acids

(https://www.bbc.co.uk/bitesize/articles/zdjknk7)

The table below shows the observations when a reaction occurs with organic compounds using certain tests.

These tests can be used to distinguish between different organic compounds.