Unit 7 - Organic Chemistry



A hydrocarbon is any compound that is formed from carbon and hydrogen atoms only.  A homologous series are a group of organic compounds which react in a very similar way

Alkanes are saturated compounds so have a single compound. Each carbon atom forms four single covalent bonds. The general formula for alkanes is C H  .

Hydrocarbon properties change as the chain gets longer:

  • the shorter the carbon chain, the less viscous a hydrocarbon is 
  • the shorter the carbon chain, the more volatile it is
  • the shorter the carbon chain, the more flammable the hydrocarbon is 

The properties of hydrocarbons affect how they're used for fuels e.g. short chains of hydrocarbons with lower boiling points are used as 'bottled gases'.

The complete combustion of any hydrocarbon, both carbon and hydrogen are oxidised.

Hydrogen + Oxygen -----> Carbon Dioxide + Water

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Fractional Distillation

Crude Oil is a mixture of lots of different hydrocarbons. The different compounds in crude oil are seperated by fractional distillation.

  • The oil is heated until most of the it has turned into gas. The gases enter a fractionating column and the liquid bit is drained off.
  • In the column there's a temperature gradient, it's hotter at the bottom and coler at the top.
  • The longer hydrocarbons have high boiling points. They condense back into liquids and drain out of the column early on, when they're near the bottom.
  • The shorter hydrocarbons have lower boiling points and they condense and drain out much later on, near the top of the column where it's cooler.
  • You end up with the crude mixture oil seperated 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.

Fossil fuels like coal, oil and gas are called non-renewable fuels as they take so long to make that they're being used up faster than they're being formed. One day they will run out as they're finite resources.

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  • Oil provides the fuel for most modern transport e.g. cars, trains and planes.
  • 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 products you get from crude oil are examples of organic compounds.

Cracking involves splitting up long-chain hydrocarbons. Cracking is a themal decomposition reaction as molecules are broken down by heat. 

  • The first step is to heat long chain hydrocarbons to vapourise them.
  • Then the vapour is passed over a hot powdered aluminium oxide catalyst.
  • The long chain molecules split apart on the surface of the specks of the catalyst - this is catalytic cracking.
  • You can also crack hydrocarbons if you vapourise them, mix them with steam and then heat them to a very high temperature - this is steam cracking.
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Alkenes are hydrocrabons which have a double bond between the two of the carbon atoms in their chain. The C=C bonds means that alkenes have two fewer hydrogens compared with alkanes  containing the same number of the carbon atoms making them unsaturated. The C=C bond can be broken to make a single bond allowing two carbon atoms to bond with other atoms. This makes alkenes reactive.

In a large amount of oxygen, alkenes combust completely to produce only water and carbon dioxide. However, there isn't enough oxygen in the air for this, so when you burn them they tend to undergo incomplete combustion.

Incomplete combustion results ina yellow, smoky flame and less energy being released compared to complete combustion of the same compound.

Alkene + Oxygen -----> Carbon + Carbon Monoxide + Carbon Dioxide + Water

The general formula for alkenes = 

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Addition Polymers

Polymers are long molecules formed when lots of small molecules called monomers join together. This reaction is called polymerisation and it usually needs high pressure and a catalyst.

Addition polymers are made form unsaturated monomers:

  • The monomers that make up polymers have a double covalent bond.
  • Unsaturated monomer molecules (alkenes) can open up their double bonds and join togeteher to form polymer chains.
  • When monomers react in addition polymerisation reactions, the only product is the polymer so an addition polymer contains exactly the same type and number of atoms as the monomers that formed it.

Drawing the displyed formula of addition polymer by the displayed formula of its monomer:

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Alcohols have a functional group of OH-

Methanol, Ethanol, Propanol and Butanol are the first four alcohols and have similar properties:

  • flammable
  • soluble in water
  • react with sodium
  • alcohols can be oxidised by reacting with oxygen to form a carboxylic acids and different alcohols form different carboxylic acids

Fermentation uses an enzyme in yeast to convert sugars into ethanol. This process happens fastest at 37 degrees, in a slightly acidic solution and under anaerobic conditions.

The word equation for fermentation is:

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Carboxylic Acids

Carboxylic acids are a homologuous series of compounds that have a functional group of '-COOH'. They react with carbonates to produce a salt, water and carbon dioxide. The salts formed in these reactions all end in -anoate.

Carboxylic acids can dissolve in water. When they dissolve, they ionise and release H+ ions resulting in an acidic solution. But they don't ionise completely, they just form weak acidic solutions. This means that they have a higher pH than aqueous solutions of strong acids with the same concentration.

Esters can be made from Carboxylix Acids:

  • Esters have the functional group '-COO.'
  • Esters are formed from an alcohol and carboxylix acid.
  • An acid catalyst is usually used e.g. concentrated sulfuric acid
  • The word equation for the formation of esters is:
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Condensation Polymers

Condesation Poleymerisation involves monomers which contain different functional groups:

  • The monomers react together and bonds form between them, making polymer chains.
  • For each new bond that forms, a small molecule (e.g. water) is lost. 
  • The simplest types of condesation polymers contain two different types of monomer, each with two of the same functional groups

Addition and Condesation Polymerisation are different:

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Naturally Ocurring Polymers

An amino acid contains two different functional groups - a basic amino acid group and an acidic carboxyl group.

Proteins are polymers of amino acids. Amino acids can form polymers known as polypeptides via condesation polymerisation. One or more long-chains of polypeptides are known as proteins. Polypeptides and proteins can contain different amino acids in their polymer chains. The order of the amino acids is what gives proteins their different properties and shapes.

DNA is made of two polymer chains of monomers called 'nucleotides'. The nucleotides each contain a small molecule known as a base. The bases on the different polymer chains pair up with each other and form cross links keeping up 2 strands of nucleotides together and giving the the double helix structure.

Sugars are small molecules that contain carbon, oxygen and hydrogen. Sugars can react together through polynerisation reactions to form larger carbohydrate polymers.

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Reactions of Alkenes

Alkenes react via addition reactions:

  • A functional group is a group of atoms ina molecule that determine how that molecule typically reacts.
  • All alkenes have the functional group 'C=C', so they all react in similar ways. So you can suggest the products of a reaction based on your knowledge of how alkenes react in general. 
  • The carbon-carbon double bond will open up to leave a single bond and a new atom is added to each carbon.

Addition of Hydrogen is known as hydrogenation:

  • Hydrogen can react with the 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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