Organic Chemistry
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Crude oil
Crude oil is a finite resource found in rocks. It is a complex mixture of hydrocarbons — compounds made of carbon and hydrogen atoms only.
Because it is a mixture, crude oil can be separated by physical methods — no chemical reaction needed.
Crude oil — Key Knowledge
- Crude oil mixture of hydrocarbons
- Hydrocarbon compound of carbon and hydrogen only
- Finite resource will run out, non-renewable
Fractional distillation
Crude oil is separated into fractions by fractional distillation. The column is hot at the bottom and cool at the top — hydrocarbons condense at different heights based on their boiling points.
Short-chain hydrocarbons rise highest before condensing; long-chain hydrocarbons condense near the bottom.
Fractional distillation — Key Knowledge
- Fractional distillation separates crude oil into fractions by boiling point
- Fractions groups of hydrocarbons with similar chain lengths
- Physical process no new substances formed
Properties and chain length
The properties of hydrocarbon fractions change with chain length. Longer chains have stronger intermolecular forces between molecules.
Shorter-chain fractions are in higher demand because they ignite easily and burn more cleanly.
Properties and chain length — Key Knowledge
- Longer chains higher boiling point, higher viscosity, less flammable, less volatile
- Shorter chains lower boiling point, runnier, more flammable, more useful as fuels
Cracking
Cracking breaks long-chain hydrocarbons into shorter, more useful molecules. It is a chemical change — covalent bonds are broken.
Alkenes produced by cracking are used to make polymers.
Cracking — Key Knowledge
- Thermal cracking high temperature and high pressure
- Catalytic cracking high temperature with a zeolite catalyst
- Products shorter alkanes + alkenes
Alkenes
Alkenes are unsaturated hydrocarbons containing a carbon-carbon double bond (C=C). Their general formula is CnH2n.
CnH2n
general formula for alkenes
The double bond makes alkenes more reactive than alkanes — it opens up during addition reactions.
Alkenes — Key Knowledge
- Unsaturated contains a C=C double bond
- General formula CnH2n, Bromine water test decolourises from orange to colourless — confirms a double bond
Addition reactions of alkenes
Alkenes undergo addition reactions where atoms add across the C=C double bond, which opens up to form a single bond.
These reactions all follow the same pattern — the double bond opens and new atoms attach.
Addition reactions of alkenes — Key Knowledge
- Hydrogenation add hydrogen — makes an alkane
- Hydration add water/steam — makes an alcohol
- Halogenation add a halogen, e.g. bromine
Addition polymerisation (4.7.2 / 4.7.3)
Many small alkene monomers join together to form a long-chain polymer. The C=C double bonds open up and link — no other product is formed.
Poly(ethene) is made from ethene monomers; poly(propene) from propene monomers.
Addition polymerisation (4.7.2 / 4.7.3) — Key Knowledge
- Monomer small molecule that joins to form a polymer
- Polymer long chain of repeating units
- Addition polymerisation double bonds open, monomers link, nothing else produced
Alcohols
Alcohols contain the -OH functional group. The first four are methanol, ethanol, propanol and butanol.
Ethanol can be oxidised to form ethanoic acid — a carboxylic acid.
Alcohols — Key Knowledge
- -OH functional group hydroxyl group
- Dissolve in water, React with sodium produce hydrogen gas
- Burn in air combustion
- Used as fuels and solvents
Carboxylic acids
Carboxylic acids contain the -COOH functional group. They are weak acids — they only partially ionise in solution.
Ethanoic acid is the carboxylic acid in vinegar.
Carboxylic acids — Key Knowledge
- -COOH functional group carboxyl group
- Weak acid partially ionises
- React with carbonates produce carbon dioxide
Condensation polymerisation (4.7.3) (HT)
Monomers with two functional groups join together, releasing a small molecule (usually water) each time a bond forms.
Natural proteins are formed by condensation polymerisation of amino acid monomers.
Condensation polymerisation (4.7.3) (HT) — Key Knowledge
- Two functional groups per monomer, Small molecule released usually water
- Different from addition polymerisation produces a by-product
Natural polymers
Many polymers occur naturally — not all polymers are synthetic plastics.
Amino acids are the monomers for proteins; nucleotides are the monomers for DNA.
Natural polymers — Key Knowledge
- Starch natural polymer of glucose
- Cellulose structural polymer in plant cell walls
- Proteins polymers of amino acids
- DNA polymer of nucleotide monomers