General Concepts of Organic Chemistry for the ESAT

Hydrocarbons and Crude Oil

Hydrocarbons are organic compounds that consist of carbon and hydrogen atoms only. Common examples include methane ($CH_4$), butane ($C_4H_{10}$), octane ($C_8H_{18}$), and ethene ($C_2H_4$).

The primary source of these compounds is crude oil. This is a dark, viscous liquid that formed over millions of years from the decaying remains of ancient organisms. In its raw state, crude oil is a complex mixture of many different hydrocarbon molecules, making it largely unusable until it is refined through fractional distillation.

Fractional Distillation

Fractional distillation separates the mixture into useful parts called fractions. The process occurs in a fractionating column, which is hot at the base and progressively cooler towards the top.

1. The crude oil is heated until it vaporises.
2. The vapour enters the bottom of the column and rises.
3. Different molecules condense into liquids at different heights based on their specific boiling points.
4. The separated liquids are then collected.

Each fraction contains hydrocarbons with a similar range of boiling points. As the carbon chain length increases, the following physical properties change:

• Boiling Point: Increases with chain length.
• Viscosity: Increases with chain length (the liquid becomes thicker and flows less easily).
• Flammability: Decreases with chain length (larger molecules are harder to ignite).

Exercise 101 The table below shows the boiling points of straight-chain alkanes. State and explain the trend.

Hydrocarbon	Formula	Boiling Point / °C
Methane	$CH_4$	-164
Ethane	$C_2H_6$	-88
Propane	$C_3H_8$	-42
Butane	$C_4H_{10}$	0
Pentane	$C_5H_{12}$	36
Octane	$C_8H_{18}$	126

Answer: The boiling point increases as the carbon chain length increases. This is because larger molecules have greater intermolecular forces, requiring more energy to separate the molecules into the gas phase.

Catalytic Cracking

Smaller hydrocarbon molecules are often in higher demand than longer-chain ones. Catalytic cracking is the process of breaking down long-chain alkanes into more useful shorter-chain alkanes and alkenes. This is achieved by passing the long molecules over a heated catalyst.

Cracking is a random process. For instance, a long alkane might break into a shorter alkane and an alkene. Equations must always be balanced for the number of carbon and hydrogen atoms.

Exercise 102 When a molecule of dodecane ($C_{12}H_{26}$) is cracked, it produces two molecules of ethene ($C_2H_4$), one molecule of pentane ($C_5H_{12}$), and one other molecule. Identify the formula of the final molecule.

Working: Total C: $12 - (2 \times 2) - 5 = 3$ Total H: $26 - (2 \times 4) - 12 = 6$ The remaining molecule is $C_3H_6$.

Representing Organic Molecules

There are three main ways to write the formula of an organic compound:

1. Molecular Formula: Shows the total number of atoms of each element, such as $C_4H_{10}$ for butane.
2. Full Structural Formula (Displayed Structure): Shows every atom and every bond in the molecule.

1. Condensed Structural Formula: Shows the arrangement of atoms without drawing every bond, such as $CH_3CH_2CH_2CH_3$ or $CH_3(CH_2)_2CH_3$ for butane.

Exercise 103 Write condensed formulae for: a. Ethane ($C_2H_6$): $CH_3CH_3$ b. Hexane ($C_6H_{14}$): $CH_3(CH_2)_4CH_3$ c. Ethene ($C_2H_4$): $CH_2CH_2$

Structural Isomerism

Structural isomers are compounds that share the same molecular formula but have different structural formulae. This happens when atoms are sequenced in different ways.

Butane ($C_4H_{10}$)

2-methylpropane ($C_4H_{10}$)

Exercise 104 Give the structural formulae of the five structural isomers of $C_6H_{14}$. Answer: Hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, and 2,3-dimethylbutane.

Combustion

Combustion involves reacting a hydrocarbon with oxygen, releasing energy.

Complete Combustion: Occurs in an excess of oxygen. The only products are carbon dioxide and water. To balance the equation:

1. Carbon dioxide molecules = number of Carbon atoms.
2. Water molecules = half the number of Hydrogen atoms.
3. Balance Oxygen atoms last.

Example for pentane: $C_5H_{12} + 8O_2 \rightarrow 5CO_2 + 6H_2O$

Exercise 105 Write a balanced equation for the complete combustion of octane, $C_8H_{18}$. Answer: $C_8H_{18} + 12.5O_2 \rightarrow 8CO_2 + 9H_2O$ (or $2C_8H_{18} + 25O_2 \rightarrow 16CO_2 + 18H_2O$)

Incomplete Combustion: Occurs when oxygen is limited. Products include carbon monoxide ($CO$) or carbon soot ($C$) along with water. Example for butane: $C_4H_{10} + 5.5O_2 \rightarrow 2CO + 2CO_2 + 5H_2O$

Exercise 106 Write the equation for the incomplete combustion of pentane where $CO$ is the only carbon product. Answer: $C_5H_{12} + 5.5O_2 \rightarrow 5CO + 6H_2O$

IUPAC Nomenclature

The International Union of Pure and Applied Chemistry (IUPAC) provides rules for naming compounds. The prefix indicates the number of carbons in the longest chain:

• 1: meth-
• 2: eth-
• 3: prop-
• 4: but-
• 5: pent-
• 6: hex-
• 7: hept-
• 8: oct-

The suffix indicates the series: -ane for alkanes and -ene for alkenes.

Propane (3 carbons, alkane)

Propene (3 carbons, alkene)

Naming Branches:

1. Identify the longest straight carbon chain.
2. Number the chain so the branches have the lowest possible numbers.
3. Name branches as alkyl groups: methyl ($CH_3$), ethyl ($C_2H_5$).
4. List branches alphabetically (ethyl before methyl) and use prefixes like di- or tri- for multiples.

Examples:

• 2-methylpentane: A 5-carbon chain with a methyl group on the second carbon.
• 2,3-dimethylhexane: A 6-carbon chain with methyl groups on carbons 2 and 3.
• 4-ethylheptane: A 7-carbon chain with an ethyl group on carbon 4.
• 4-ethyl-2-methylheptane: Alphabetical order applies.

Exercise 107 Name the following: a. (3-methylhexane) b. (2,2-dimethylbutane) c. (3-ethyl-2-methylpentane) d. (2,2,4-trimethylpentane)

Homologous Series and Alkanes

A homologous series is a family of compounds with the same general formula and similar chemical properties because they contain the same functional group. Alkanes are saturated hydrocarbons (containing only single bonds) with the general formula $C_nH_{2n+2}$.

Exercise 108 Calculate molecular formulae for: a. 10 carbons: $C_{10}H_{22}$ b. 26 total atoms: $n + (2n+2) = 26 \rightarrow 3n = 24 \rightarrow n=8$. Result: $C_8H_{18}$. c. 60 hydrogens: $2n+2 = 60 \rightarrow 2n = 58 \rightarrow n=29$. Result: $C_{29}H_{60}$.

Alkenes

Alkenes are unsaturated hydrocarbons containing at least one $C=C$ double bond. Their general formula is $C_nH_{2n}$. The position of the double bond is indicated by a number (the lower of the two carbons involved).

Pent-1,2-diene (Note the use of -diene for two double bonds).

Exercise 109 Name the heptene isomers: a. (Hept-1-ene) b. (Hept-2-ene) c. (Hept-3-ene)

Reactions of Alkenes

Alkenes undergo addition reactions where the double bond opens, allowing new atoms to bond to the carbons.

1. Hydrogenation: Addition of $H_2$ to form an alkane.
2. Halogenation: Addition of halogens (e.g., $Br_2$) to form a dihaloalkane.
3. Hydrogen Halides: Addition of $HCl$ or $HBr$.
4. Hydration: Addition of steam ($H_2O$) to form an alcohol ($H$ goes to one carbon, $OH$ to the other).

Test for Unsaturation: Bromine water is used to distinguish alkanes from alkenes. When added to an alkene, the orange-brown bromine water is decolourised (becomes colourless) because an addition reaction occurs. Alkanes do not react with bromine water in the dark, so the solution remains orange-brown.