Topic 7 - Organic Chemistry


Hydrocarbons and Alkanes

Hydrocarbons are the simplest organic compounds.
They contain carbon and hydrogen atoms only.

  • Single C-C bonds
  • Simplest type of hydrocarbons
  • General formula of CnH2n+2
  • They are a homologous series, meaning that they react in a similar way
  • They are saturated compounds, meaning that there are only singular covalent bonds
  • Each carbon atom forms form single covalent bonds
  • The first four alkanes are:
    • Methane (CH4)
    • Ethane (C2H6)
    • Propane (C3H8)
    • Butane (C4H10)
      Ethane displayed formula:
      H H
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      H H
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Hydrocarbon Properties

A displayed formula is the drawing of all the atoms and bonds in a molecule.
As the length of the carbon chain changes, the properties of the hydrocarbon change.
Viscous is how ‘gloopy’ or thick a substance is.
The shorter the carbon chain, the less viscous (or more runny) the hydrocarbon is.
Volatile is how quickly a substance condenses or vaporises; with a high volatility being a lower boiling point and a low volatility being a higher one.
The shorter the carbon chain the more volatile (lower boiling point) the hydrocarbon is.
The shorter the carbon chain, the more flammable the hydrocarbon is.
The properties of hydrocarbons affect how they’re used for fuels, for example; a short-chain hydrocarbon with a lower boiling point would be used as a ‘bottled gas’ which is stored under pressure as a liquid in a bottle.

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Complete Combustion

Complete combustion occurs when there is plenty of oxygen.
The complete combustion of any hydrocarbon in oxygen releases lots of energy and the only waste products are carbon dioxide and water.
The equation for complete combustion is:
Hydrocarbon + Oxygen -> Carbon Dioxide + Water (+ Energy).
During combustion, both carbon and hydrogen from the hydrocarbon are oxidised (gain oxygen).
Hydrocarbons are used as fuels because of how much energy they release when they combust completely.
Balanced symbol equation for the complete combustion of methane (CH4):
1 - Complete combustion equation - Hydrocarbon + O2 -> CO2 + H2O
2 - Add in the given hydrocarbon - CH4 + O2 -> CO2 + H2O
3 - Balance the symbol equation - CH4 + 2O2 -> CO2 + 2H2O

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Crude Oil

Crude oil can be used to make many different things, such as fuels.
First, the different hydrocarbons have to be separated through fractional distillation.
Crude oil is a fossil fuel and is formed from the remains of plants and animals (mainly plankton), that died millions of years ago and were buried in mud.
Over millions of years, with the high temperature and pressure, the remains turn to crude oil, which can be drilled up from the rocks where it’s found.
Fossils fuels like coal, oil and gas are called non-renewable fuels as they take so long being made that they’re being used up much faster than they’re being formed. They’re finite resources meaning that one day they’ll run out.
Crude oil is a mixture of lots of different hydrocarbons, most of which are alkanes.
The different compounds/hydrocarbon fractions in crude oil are separated by fractional distillation.

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Fractional Distillation

Fractional Distillation Method:
1 - The crude oil is heated and vaporised until most of it has turned into gas. The gases enter a fractioning column (and the liquid part is drained off)
2 - In the column there’s a temperature gradient (it’s hot at the bottom and gets cooler as you go up)
3 - The longer hydrocarbons (40-50 chain) have higher boiling points and so condense back into liquids and drain out of the column earlier on, when they’re near the bottom. The shorter hydrocarbons (3-5 chain) have lower boiling points and so they condense and drain out much later on; near the top where it’s cooler
4 - You end up with the crude oil mixture separated out into different fractions. Each fraction contains a mixture of hydrocarbons that all contain a similar number of carbon atoms, so have similar boiling points.

Hottest - 40 to 50 chain - Heavy fuel oil
Second hottest - 20 to 40 chain - Diesel oil
Third hottest - 15 to 20 chain - Kerosene
Second coolest - 8 - 15 chain - Petrol
Coolest - 3 - 8 chain - LPG (Liquefied petroleum gas).

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Long and Short Chain Hydrocarbons

Crude oil has fuelled modern civilisation and oil provides fuel for most modern transport.
The petrochemical industry uses some of the hydrocarbons from crude oil as a feedstock to make new compounds for use in things like polymers, solvents, lubricants and detergents.
All the product you get from crude oil are examples of organic compounds (compounds containing carbon atoms) and the reason you get such a large variety of products is because carbon atoms can bond together to form different groups called homologous series. These groups contain similar compounds with many properties in common. Alkanes and alkenes are examples of homologous series.
Short-chain hydrocarbons are flammable and so make good fuels and are in high demand. However, long chain hydrocarbons form thick gloopy liquids like tar which aren’t very useful.
A lot of the longer chain alkane molecules produced from fractional distillation are turned into smaller, more useful ones through a process called cracking.
As well as alkanes, cracking also produces another type of hydrocarbon called alkenes which are used as a starting material when making lots of others compounds and can be used to make polymers.
Some of the products of cracking are useful as fuels (e.g. petrol for cars and paraffin for jet fuel).

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There are two different methods of cracking.
Cracking is a thermal decomposition reaction (it breaks down molecules by heating them).
The first step is to heat long chain hydrocarbons to vaporise them (turn into a gas).
Then the vapour can be passed over a hot aluminium oxide catalyst.
The long chain molecules spilt apart on the surface of the specks of catalyst - this is called catalytic cracking.
The other method is called steam cracking which involves vaporising hydrocarbons, mixing them with steam and then heating them up to a very high temperature.
Cracking of a long-chain alkane hydrocarbon molecule produces a shorter alkane molecule and an alkene.
You have to be able to balance the chemical equations for cracking, with there being the same number of hydrogen atoms on both sides of the equation.

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Alkenes are unsaturated because they have a double carbon-carbon bond.
They are hydrocarbons which have a double bond between two of the carbon atoms in their chain.
The C=C double bond means that alkenes have two fewer hydrogens compared with alkanes containing the same number of carbon atoms. This makes them unsaturated.
The C=C double bond can open up to make a single bond, allowing the two carbons to bond with other atoms. This makes alkenes reactive, and much more reactive than alkanes.
The first four alkenes are ethane (two carbon bonds), propane (three carbons), butene (four) and pentene (five).
Straight-chain alkenes have twice as many hydrogen atoms as carbon.
The general formula for alkenes is:
Example of an alkene structure - Butene: (there are two different structures for butene and pentane as the double bond can be in two different places).
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Incomplete Combustion

The double bond means that alkenes have reactions with lots of different compounds.
Alkenes burn with a smoky flame and in a large amount of oxygen, they combust completely to produce only water and carbon dioxide.
When you burn alkenes in air, however, they tend to undergo incomplete combustion, where carbon and water are still produced, but you can also get carbon monoxide.
Carbon monoxide (CO) is a poisonous gas.
Standard equation for incomplete combustion of alkenes:
Alkene + Oxygen ➡️ Carbon Monoxide + Carbon Dioxide + Water ( + Energy)
Incomplete combustion results in a smoky yellow flame, and less energy is released compared to complete combustion of the same compound.
Equation for incomplete combustion of butene:
C4H8 + 5O2 ➡️ 2CO + 2CO2 + 4H2O
Similarly, another equation could be:
C4H8 + 3O2 ➡️ 2C + 2CO + 4H2O
The products depend on how much oxygen is present. The only rule is that the equation always has to be balanced.

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Alkenes and Addition Reactions

Alkenes react via addition reactions.
All alkenes have the same functional group and is they react in similar ways.
Therefore, you can suggest the products of a reaction based on your knowledge of alkene reactions in general.
Most of the time, alkenes react via addition reactions and the carbon-carbon double bond opens up to leave a single bond and a new atom is added to each carbon.
For example:
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C=C + X-Y ➡️ R - C - C - H
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This is an example of an addition reaction in alkenes, with R being the carbon chain.
Addition reactions of hydrogen are known as hydrogenation.
Hydrogen can also react with double-bonded carbons to open up the double bond and form the equivalent. saturated, alkane.
The alkene is reacted with hydrogen in the presence of a catalyst.

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Alkenes and Halogens

Halogens can also react with alkenes.
Alkenes will also react in addition reactions with halogens such as bromine, chlorine and iodine to form saturated alkane molecules, with the C=C carbons each becoming bonded to a halogen atom.
For example, bromine and ethernet react together to for, dibromoethane, where ethene and bromine react together, the C=C double bond splits and a bromine is added to each carbon atom, and the molecule is now a saturated alkane with two bromine atoms, hence the name ‘dibromo’.
This reaction is the same for chlorine and iodine, and all halogens come in pairs.
Alkenes turn bromine water colourless and therefore it can be used to test for alkenes:

  • When orange bromine water is added to a saturated compound, like an alkane, no reaction will happen and it will stay bright orange.
  • However, if it’s added to an alkene the bromine water will add across the double bond, making a colourless dibromo-compound, so the bromine water is decolourised.
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Alkenes and Steam

Steam can react with alkenes to form alcohols.
When alkenes react with steam, water is added across the double bond and an alcohol is formed.
For example, ethanol can be made by mixing ethene with steam and passing it over a catalyst.
Each carbon atom in the C=C double bond gains a new hydrogen, and the singular oxygen atom is added to the end, next to an existing hydrogen.
The conversion of ethene to ethanol is one way of making ethene industrially. After the reaction has taken place, the reaction mixture is passed through the reactor into a condenser. Ethanol and water have a higher boiling point that ethene, so both condense whilst any unreacted ethene gas is recycled back into the reactor. The alcohol can then be purified from the mixture by fractional distillation.

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Addition Polymers

Polymers are made up of lots of the same molecule joined together in one long chain.
They’re what make up plastics and have many unique properties which make them very useful to the modern society.
Polymers are long molecules formed when lots of short molecules (monomers) join together.
This reaction is called polymerisation and it usually needs high pressure and a catalyst.
Plastics are made up of polymers and they’re usually carbon based with their monomers often being alkenes.
Addition polymers are made from unsaturated monomers with a double covalent bond (alkenes).
Lots of alkenes (unsaturated monomer molecules) open up their double bonds and join together to form polymer chains. This is called addition polymerisation.
Polymer can either be drawn as a long chain of carbon and hydrogen like this:

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Addition Polymers Continued

Or they can be drawn inside a bracket.
In the reactants, an ethene monomer is drawn inside a pair of brackets, with an n in front of the brackets, representing many monomers.
Then for the products, the same is drawn again buy the double bond has broken, and there are two bonds going nowhere coming out of each carbon atom.
Finally, an n is drawn on the outside of the bracket to represent the amount.
When ethene reacts to become a polymer, for example, it becomes poly(ethene).
When monomers react in addition polymerisation reactions, the only product is a polymer, so an addition polymer contains exactly the same type and number of atoms as the monomers that formed it.
The monomer that forms a plastic, for example, is made from, affects the properties of the plastic formed.

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Alcohols are another homologous series of organic compounds.
The name of an alcohol always ends in -ol and they have an -OH functional group.
The general formula for an alcohol is:
So therefore, an alcohol with two carbons has the formula C2H5OH.
The first four alcohols in the homologous series are methanol, ethanol, propanol and butanol with methanol being 1 carbon, ethanol being 2 and so on.
The formula of alcohols can be shown in two different ways, for example ethanol can be shown as C2H5OH or CH3CH2OH to represent the atoms bonded to each carbon in the molecule shown separately.
Alcohols are drawn like alkanes, but with an -OH on the right.
The first four alcohols have similar properties.
Alcohols are flammable and undergo complete combustion in air to produce carbon dioxide and water.
For example, the equation for the complete combustion of methanol is:
2CH3OH + 3O2 ➡️ 2CO2 + 4H2O
The equation must be balanced.

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Alcohol Properties

Methanol, ethanol, propanol and butanol are all soluble in water and their solutions have a neutral pH.
They also all react with sodium to produce hydrogen amongst other products.
Alcohols can also be oxidised by reacting with oxygen (e.g. from the air) to produce a carboxylic acid. This would cause another oxygen to add on in a double bond.
Different alcohols form different carboxylic acids, with the names being linked to the alcohol name. For example, methanol would be oxidised to make methanoic acid.
Alcohols are also used as solvents and fuels.
Especially methanol and ethanol, which are used as solvents in industry.
This is because they can dissolve most things water can dissolve, but they can also dissolve substances that water can’t dissolve such as hydrocarbons, oils and fats.
The first four alcohols are also used as fuels, for example ethanol is used as a fuel in spirit burners and it burns fairly cleanly and doesn’t smell.

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Fermentation and Ethanol

Ethanol can be made by fermentation.
This is the alcohol found in alcoholic drinks such as wine or beer and it’s usually made using fermentation.
Fermentation used an enzyme called yeast to convert sugars into ethanol. The reaction also produces carbon dioxide and occurs in a solution so the ethanol produced is aqueous.
The equation for this is:
Sugar ➡️ Ethanol + Carbon Dioxide
Fermentation happens fastest at a temperature of around 37 degrees celsius, in a slightly acidic solution and under anaerobic conditions (no oxygen).
Under these conditions, the enzyme in yeast work best to convert the sugar to alcohol.
If the conditions were different, for example a lower pH/higher temperature or a higher pH/lower temperature, the enzyme could be denatured or could work at a much slower rate.

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Carboxylic Acids

Carboxylic acids are another homologous series of compounds.
They have the functional group of -COOH.
Their names end in -anoic acid and start with the normal meth-, eth-, prop-, but-, etc.
The formulas for the first four alcohols are:
Methanoic acid - HCOOH
Ethanoic acid - CH3COOH
Propanoic acid - C2H5COOH
Butanoic acid - C3H7COOH
They have the same functional group as alcohols: CnH2n+1 but have a COOH on the end.
For example, you take the formula for methanol which is CH3OH, and drawn with a carbon in the middle, with a hydrogen coming out from either way, and on the right is the -OH functional group.
To draw this as an alcohol, you would take the carbon in the middle, take away all bonds from in apart from the one on the left, and add onto the right of it a single bond going to the -OH functional group (but not split apart) and a double bond going to an oxygen atom, so it still remains as 4 bonds from the carbon.
This then makes the formula HCOOH; we have taken away the carbon from the front to add into the functional group, we have lost two hydrogens and gained an -OOH.
The pattern is that the formula for alcohol loses one carbon and two hydrogen to make a carboxylic acid, because the carbon needs to join the functional group and the hydrogen are lost due to four bonds needing to remain, and this would be too many,

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Carboxylic Acid Reactions

Carboxylic acids react like any other acids.
They react with carbonates to produce a salt, water and carbon dioxide.
The salts formed in these reactions end in -anoate, for example methanoic acid will form methanoate, etc.
For example, equation for sodium carbonate reacting with ethanoic acid is:
Ethanoic acid + Sodium carbonate ➡️ Sodium ethanoate + Water + Carbon dioxide
Carboxylic acids can dissolve in water and ionise when they dissolve to release H+ ions, resulting in an acidic solution.
They don’t ionise completely and so they just form weak acidic solutions.
This means they have a higher pH (are less acidic) than aqueous solutions of strong acids with the same concentration.
Esters can be made from carboxylic acids.
These are another homologous series with the functional group -COO-.
Esters are formed from an alcohol and carboxylic acid.
An acid catalyst is usually added (e.g. concentrated sulfuric acid) and the equation is:
Alcohol + Carboxylic acid ➡️ Ester + Water
(Acid catalyst)

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Ethyl ethanoate can be made from ethanoic acid and ethanol with an acid catalyst:
Ethanoic acid + Ethanol ➡️ Ethyl ethanoate + Water
The ethanol adds to the end and the ethanoic acid adds to the front. The hydrogen is lost in both functional groups to make the water, as well as the oxygen from the ethanol, so as to create the -COO- ester functional group.
The displayed formula is drawn as ethanoic acid on the top, but with no -OH, and then this is linked with a single bond of oxygen to ethanol underneath but also without the
-OH functional group.
Water is just drawn as an oxygen with two single bonds of hydrogen coming from it.
ETHYL ETHANOATE is important to remember and is the product of ethanol and ethanoic acid.

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Condensation Polymers

The monomers for condensation polymerisation need to have two functional groups.
Condensation polymerisation involves monomers which contain different functional groups.
The monomers react together and bonds between them, making a polymer chain.
For each new bond that forms, a small molecule (e.g. water) is lost, which is why it has the name ‘condensation’.
The simplest type of condensation polymers contain two different types of monomer, each with two of the same functional groups.
For example, a polyester can be made by condensation polymerisation:
A diol + A dicarboxylic acid ➡️ A condensation polymer + Water
The diol and dicarboxylic acid are drawn with an n in font of them, and the the polymer is a mixture of these, with four hydrogens and two oxygens taken out to form the water, which is not a polymer.
The water also has an n in front of it, and a 2 in front of that, to balance the equation.

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Addition vs Condensation Polymerisation

Addition polymerisation:

  • Only one monomer containing a C=C bond
  • Only one product formed
  • Carbon-Carbon double bond functional group in monomer.

Condensation polymerisation:

  • Two monomer types each containing two of the same functional groups OR one monomer type with two different functional groups
  • Two types of product: a monomer and a small molecule (e.g. water)
  • Two reactive groups on each monomer.
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Amino Acids and Polymers

Amino acids have an amino group and a carboxyl group
An amino acid contains two different functional groups - a basic amino group (NH2) and an acidic carboxyl group (COOH)
An example of an amino acid is glycine - the smallest and simplest amino acid possible.
Proteins are polymers of amino acids and these polymers are called polypeptides and are made via condensation polymerisation.
The amino group of an amino acid can react with the acid group of another, and so on, to form a polymer chain. For each new bond that is formed, a molecule of water is lost.
The functional groups in this are what make up the water molecule.
One or more long-chains of polypeptides are known as proteins, which have loads of important uses in the human body, e.g. for enzymes which work as catalysts.
Polypeptides and proteins can also contain different amino acids in their polymer chains, and the order of the amino acids is what gives proteins their different properties and shapes.

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DNA, Sugar and Polymers

DNA is made of two polymer chains of monomers called ‘nucleotides’, and these nucleotides contain a small molecule known as a ‘base’, in which there are four of.
The structure of DNA is made up of nucleotide strands.
The bases in the different polymer chains pair up with each other and form cross links keeping the two strands of nucleotides together and giving the double helix structure.
The order of the bases acts as a code for an organism’s genes.

Simple sugars can also form polymers.
Sugars are small molecules that contain carbon, oxygen and hydrogen.
Sugars can react together through polymerisation reactions to form larger polymers, e.g. starch, which living things use to store energy, and cellulose, which is found in plant cell walls.

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