Organic chemistry

Organic chemistry is the chemistry of compounds containing carbon. Carbon is a unique element because its atoms can bond with each other to form stable chains, rings, and complex three-dimensional shapes. From the fuels that power our vehicles to the DNA that forms the blueprint of life, organic compounds are everywhere.

What you'll learn:

• How crude oil is separated into useful fractions using fractional distillation.
• How to recognize, draw, and name alkanes, alkenes, alcohols, and carboxylic acids.
• How small monomer molecules join together to form addition and condensation polymers.
• How chemical cells and hydrogen fuel cells generate potential differences.

1. Crude Oil and Fractional Distillation

Crude oil is a dark, viscous liquid found trapped in the Earth's crust. It is a finite resource, meaning it is being used up much faster than it is naturally formed, and will eventually run out. It is also an essential feedstock (a raw material used to provide reactants for an industrial process) for the petrochemical industry.

Crude oil is not useful on its own because it is a complex mixture of hundreds of different hydrocarbons. Most of these are alkanes—saturated hydrocarbons with the general formula $C_n{H}_{2n+2}$. To make them useful, we must separate them into simpler mixtures of similar-sized molecules called fractions.

How Fractional Distillation Works

The separation of crude oil relies on fractional distillation, which separates hydrocarbons based on their different boiling points. This physical property depends directly on molecular size and intermolecular forces:

1. Boiling Point and Molecular Size: As the carbon chain length (molecular size) increases, the molecules have more surface contact area.
2. Intermolecular Forces: Larger molecules have stronger intermolecular forces (forces between the molecules) holding them together.
3. Energy Required: More thermal energy is required to overcome these stronger forces, meaning larger molecules have much higher boiling points.

During the industrial process:

• Crude oil is heated and vaporised before entering the bottom of the fractionating column.
• The column has a temperature gradient: it is very hot at the bottom (approx. 350 °C) and cooler at the top (approx. 20 °C).
• The vapours rise up the column. When a substance reaches a tray that is cooler than its boiling point, it condenses back into a liquid and is piped off.
• Very small hydrocarbons have extremely low boiling points and remain as gases, leaving from the top of the column.
• Very large hydrocarbons have extremely high boiling points and condense immediately at the bottom, leaving as a thick residue.

The fractions obtained from the top to the bottom of the column are:

• LPG (Liquefied Petroleum Gases): Used for domestic heating and cooking.
• Petrol (Gasoline): Fuel for cars.
• Kerosene: Fuel for aircraft.
• Diesel oil: Fuel for trains, lorries, and some cars.
• Heavy fuel oil: Fuel for ships and power stations.
• Bitumen: Used for surfacing roads and roofs.

2. The Alkanes

Alkanes are the simplest family of hydrocarbons. They are saturated, meaning they contain only single covalent carbon-to-carbon bonds ($C-C$). Each carbon atom forms four single covalent bonds to achieve a stable outer shell of electrons.

Because we represent three-dimensional molecules on flat two-dimensional pages, we use fully displayed formulae to show every single atom and bond clearly.

The First Four Alkanes

The prefix of the organic name tells you the number of carbon atoms present:

• Meth- = 1 carbon
• Eth- = 2 carbons
• Prop- = 3 carbons
• But- = 4 carbons

  H
  |
H-C-H
  |
  H

  H H
  | |
H-C-C-H
  | |
  H H

  H H H
  | | |
H-C-C-C-H
  | | |
  H H H

  H H H H
  | | | |
H-C-C-C-C-H
  | | | |
  H H H H

3. Cracking

The fractional distillation of crude oil often produces a surplus of long-chain hydrocarbons (like heavy fuel oil) and a shortage of highly in-demand short-chain hydrocarbons (like petrol). To balance this supply and demand, the chemical industry uses a process called cracking.

Conditions for Cracking

Cracking is carried out in two main ways:

1. Catalytic cracking: The long-chain hydrocarbon is heated to vaporise it (approx. 550 °C) and passed over a hot catalyst (such as alumina or silica).
2. Steam cracking: The hydrocarbon vapour is mixed with steam and heated to a very high temperature (above 800 °C) without a catalyst.

The Products of Cracking

Cracking always breaks a single covalent bond in the carbon backbone, producing at least one alkane and one alkene:

For example:

The short-chain alkanes are used directly as fuels, while the alkenes are highly reactive and are used as chemical feedstocks to make polymers and plastics.

4. Alkenes, Alcohols, and Carboxylic Acids

In organic chemistry, the chemical behavior of a compound is determined by its functional group. Compounds with the same functional group belong to the same homologous series and undergo the same types of chemical reactions.

A. Alkenes (Functional Group: $C=C$ double bond)

Alkenes are unsaturated hydrocarbons. They contain a reactive carbon-to-carbon double covalent bond ($C=C$).

Their general formula is $C_n{H}_{2n}$. Note that there is no "methene" because you need at least two carbon atoms to form a double bond.

• Ethene ($C_2{H}_4$):

    H   H
     \ /
      C=C
     / \
    H   H

• Propene ($C_3{H}_6$):

    H   H H
     \ /  |
      C=C-C-H
     /    |
    H     H

• Butene ($C_4{H}_8$): There are different structural arrangements for butene depending on where the double bond is located (e.g., but-1-ene or but-2-ene).

Reactions of Alkenes

Because of the unstable $C=C$ double bond, alkenes are much more reactive than alkanes. They undergo addition reactions where the double bond opens up to accept new atoms:

1. Reaction with Bromine Water (Bromination): Alkenes rapidly decolourise orange bromine water because the bromine atoms add across the double bond to form a colourless dibromoalkane. This is the diagnostic test for unsaturation.
2. Reaction with Hydrogen (Hydrogenation): At around 150 °C in the presence of a nickel catalyst, alkenes react with hydrogen ($H_2$) to form the corresponding saturated alkane.
3. Reaction with Steam (Hydration): At high temperature and pressure, in the presence of a phosphoric acid catalyst, alkenes react with steam ($H_{2}O$) to produce alcohols.

B. Alcohols (Functional Group: $-OH$ hydroxyl group)

Alcohols contain the oxygen-hydrogen functional group ($-OH$). Their names always end in -ol.

The first four members are:

• Methanol (${CH}_{3}OH$)
• Ethanol ($C_2{H}_{5}OH$)
• Propanol ($C_3{H}_{7}OH$)
• Butanol ($C_4{H}_{9}OH$)

Reactions of Alcohols

• Combustion: Alcohols burn cleanly in oxygen to produce carbon dioxide and water.
• Reaction with Sodium: Alcohols react with sodium metal to produce sodium alkoxides and hydrogen gas (a much slower, gentler fizzing than sodium reacting with water).
• Oxidation: Alcohols can be oxidised by chemical oxidising agents (like acidified potassium dichromate) to form carboxylic acids.

C. Carboxylic Acids (Functional Group: $-COOH$ carboxyl group)

Carboxylic acids contain a carbon atom double-bonded to an oxygen atom, and single-bonded to an $-OH$ group ($-COOH$). They are weak acids and their names end in -anoic acid.

The first four members are:

• Methanoic acid ($HCOOH$)
• Ethanoic acid (${CH}_{3}COOH$) — the main acid component of vinegar.
• Propanoic acid ($C_2{H}_{5}COOH$)
• Butanoic acid ($C_3{H}_{7}COOH$)

5. Polymerisation

Polymers are very large, long-chain molecules made of many repeating units joined together by covalent bonds. The small reactive molecules that join to form a polymer are called monomers.

There are two main types of polymerisation: addition and condensation.

A. Addition Polymerisation

In addition polymerisation, unsaturated monomer molecules (alkenes) join together. The carbon-to-carbon double bonds open up, allowing thousands of monomers to link up in a long chain. No other molecules are formed during this reaction.

To deduce the structure of an addition polymer from its monomer:

1. Draw the monomer showing only the $C=C$ double bond in the centre, with its four groups pointing away at angles.
2. Change the double bond to a single bond ($C-C$).
3. Draw single bonds extending outwards from both carbon atoms.
4. Put large square brackets around this repeating unit.
5. Place a small subscript '$n$' on the bottom right corner to show that the unit repeats $n$ times.

B. Condensation Polymerisation (Higher Tier Only)

Unlike addition polymerisation, condensation polymerisation involves monomer molecules that contain at least two different functional groups (one on each end of the molecule).

When these monomers react to form covalent bonds, they link up and simultaneously lose a small, stable molecule (such as water, $H_{2}O$, or hydrogen chloride, $HCl$) as a by-product.

We often represent the carbon backbones of these monomers using simplified block diagrams (boxes) so we can focus entirely on the reacting functional groups at the ends.

Making a Polyester

A polyester is formed by reacting a dicarboxylic acid (a monomer with a $-COOH$ group at each end) and a diol (a monomer with an $-OH$ group at each end):

1. The $-OH$ group is removed from the carboxylic acid monomer.
2. The $-H$ atom is removed from the alcohol monomer.
3. These combine to form water ($H_{2}O$).
4. The remaining carbon on the acid links directly to the remaining oxygen on the diol, forming an ester link ($-COO-$).

Because both monomers have functional groups at both ends, this process repeats thousands of times to form a long polyester chain.

Practical Techniques: Making a Condensation Polymer

A classic practical demonstration of condensation polymerisation is the nylon rope trick (making a polyamide):

• Solutions of a diamine dissolved in water and a dicyclic acid chloride dissolved in an organic solvent are carefully layered in a beaker.
• A polyamide (nylon) layer forms immediately at the interface of the two immiscible liquids.
• Using tweezers, the nylon film at the boundary can be pulled up as a continuous, strong thread and wound around a glass rod.

C. Natural Polymers

You must recall that many biologically vital materials are naturally-occurring polymers:

1. DNA (Deoxyribonucleic acid): A polymer essential for life, which codes genetic instructions. It is made of two long polymer strands wound in a double helix. The monomers of DNA are called nucleotides, and there are four different nucleotides (represented by the bases A, T, C, and G).
2. Proteins: Polymers made from monomer molecules called amino acids. Amino acids contain two functional groups: a basic amine group ($-{NH}_2$) and an acidic carboxylic acid group ($-COOH$).
3. Starch and Cellulose: Complex carbohydrate polymers made from simple sugar monomers (like glucose).

6. Chemical Cells and Fuel Cells

Chemical Cells

A simple chemical cell consists of two different metal electrodes immersed in an electrolyte solution.

• Because different metals have different tendencies to lose electrons (reactivity), a redox reaction occurs.
• The more reactive metal loses electrons more easily, creating a flow of electrons (electricity) through an external circuit to the less reactive metal.
• This creates a potential difference (voltage).
• The cell will continue to produce a potential difference only until one of the reactant chemicals is completely used up.

Hydrogen-Oxygen Fuel Cells

A fuel cell is a special type of chemical cell that is supplied with an external source of fuel (hydrogen) and an oxidant (oxygen). Unlike traditional batteries, fuel cells do not run down or need recharging; they will produce electricity continuously as long as fuel and oxygen are supplied.

Inside the fuel cell, hydrogen and oxygen react electrochemically in a redox reaction to produce water and electricity:

Electrode Chemistry (Acidic Electrolyte)

1. At the Anode (Oxidation): Hydrogen gas is fed to the negative electrode (anode), where it loses electrons to form hydrogen ions:

2. At the Cathode (Reduction): Oxygen gas is fed to the positive electrode (cathode). It gains electrons and reacts with the hydrogen ions that have migrated through the electrolyte membrane to form water:

Evaluating Fuel Cells vs Rechargeable Batteries

You must be able to evaluate the advantages and disadvantages of using hydrogen fuel cells compared to rechargeable lithium-ion batteries (such as those used in modern electric vehicles).