Comprehensive Guide to Organic Chemistry and Petrochemicals

Core Principles of Organic Chemistry and Carbon Bonding

• Definition of Organic Chemistry: The field of chemistry dedicated to the study of carbon-containing compounds. Carbon is chemically unique due to the immense variety of molecules it can form.
• Special Features of Carbon Bonding: There are three primary characteristics of covalent bonding involving carbon that allow for this diversity:
  • Chain Formation: Carbon atoms possess the ability to bond to each other to create long, stable chains. Other elements can subsequently attach to these chains as side groups.
  • Bond Versatility: Carbon atoms in a chain can be linked by single, double, or triple covalent bonds.
  • Ring Structures: Carbon atoms can arrange themselves into circular or ring molecules, such as glucose ($C_6H_{12}O_6$).
• Valency of Carbon: Carbon can form four covalent bonds. In alkanes, where only hydrogen is attached to the side positions, the valency of 4 ensures that any bond not used to link the carbon chain is saturated with hydrogen atoms.

The Alkane Homologous Series

• Definition of Hydrocarbons: Compounds that contain carbon and hydrogen only. Alkanes are one of the simplest types of organic compounds.
• Saturated Nature: Alkanes are considered saturated hydrocarbons because they contain only single covalent bonds between carbon atoms. Because the carbon atoms have a valency of 4 and all bonds are filled, no further atoms can be added to the molecule.
• Homologous Series Characteristics: A homologous series is a family of organic compounds with similar chemical properties. Alkanes follow this pattern:
  • General Formula: Alkanes fit the general formula $C_nH_{2n+2}$.
  • Successive Change: Each member in the series increases by a $-CH_2-$ group as the chain length grows.
  • Displayed Formula: This type of formula shows all atoms and every bond between them. In alkanes, the displayed formula emphasizes that all carbon-carbon bonds are single bonds.
• Naming and Prefixes: The names of alkanes all end in the suffix -ane. The prefix indicates the number of carbon atoms:
  • Meth-: 1 carbon ($CH_4$, Methane)
  • Eth-: 2 carbons ($C_2H_6$, Ethane)
  • Prop-: 3 carbons ($C_3H_8$, Propane)
  • But-: 4 carbons ($C_4H_{10}$, Butane)
  • Pent-: 5 carbons ($C_5H_{12}$, Pentane)
  • Hex-: 6 carbons ($C_6H_{14}$, Hexane)
• Nomenclature Mnemonic: To remember the prefixes in order (Meth, Eth, Prop, But, Pent, Hex, Hept, Oct, Non, Dec): "My Elder sister Pamela Buys Pretty Hexagonal Hot Orange New Donuts."

Chemical Reactivity and Reactions of Alkanes

• General Reactivity: Alkanes are generally unreactive. They are unaffected by acids or alkalis. Their primary chemical behaviors involve combustion and substitution.
• Alkanes as Fuels: Alkanes are used as fuels because they burn very exothermically. Methane is the major component of natural gas. Butane is used in home heating, camping stoves, and gas lighters. Gas is kept liquid under pressure for storage.
  • Safety Note (Butane): Pure butane is odorless and colorless. A chemical odorant called mercaptan is added to give it a "rotten egg" or sulfur smell for leak detection.
• Complete Combustion: In a good supply of air, alkanes produce carbon dioxide and water vapor.
  • Methane: $CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g)$
  • Ethane: $2C_2H_6(g) + 7O_2(g) \rightarrow 4CO_2(g) + 6H_2O(g)$
  • Butane: $2C_4H_{10}(g) + 13O_2(g) \rightarrow 8CO_2(g) + 10H_2O(g)$
• Substitution Reactions with Chlorine: This is a photochemical reaction requiring ultraviolet (UV) light or sunlight to provide activation energy ($E_a$).
  • Definition: A reaction where an atom (or atoms) of a molecule is replaced by different atoms without changing the general structure.
  • Mechanism: The UV light splits chlorine molecules into energized atoms. One chlorine atom replaces a hydrogen atom.
  • Reactants/Products: $Methane + Chlorine \xrightarrow{\text{sunlight}} Chloromethane + Hydrogen\,Chloride$
  • Equation: $CH_4(g) + Cl_2(g) \rightarrow CH_3Cl(g) + HCl(g)$
  • Detection: Hydrogen chloride gas turns moist blue litmus paper red.
  • Isomers: For longer chains like propane or butane, multiple monosubstituted products (isomers) can form (e.g., 1-chlorobutane and 2-chlorobutane). In naming, the substituent position number is kept as low as possible.

The Alkene Homologous Series and Unsaturation

• Definition of Alkenes: A homologous series of unsaturated hydrocarbons containing at least one carbon-carbon double bond ($C=C$).
• Unsaturated Compound: A molecule in which one or more carbon-carbon bonds are not single bonds.
• General Formula: Alkenes follow the formula $C_nH_{2n}$. They have two fewer hydrogen atoms than the corresponding alkane due to the double bond.
• Simplest Member: Ethene ($C_2H_4$), which requires at least two carbons to form the double bond.
• Nomenclature: Names end in -ene.
  • Ethene: 2 carbons ($C_2H_4$)
  • Propene: 3 carbons ($C_3H_6$)
  • Butene: 4 carbons ($C_4H_8$)
  • Pentene: 5 carbons ($C_5H_{10}$)
  • Hexene: 6 carbons ($C_6H_{12}$)
• Bromine Water Test for Unsaturation: A diagnostic test to distinguish between alkanes and alkenes.
  • Alkene Reaction: When an alkene is shaken with orange-brown bromine water, it decolourises to colourless. This is an addition reaction.
  • Alkane Reaction: No reaction occurs; the solution remains orange-brown.

Industrial Source of Alkenes: Catalytic Cracking

• Process Definition: The decomposition of long-chain alkanes into smaller, more useful alkanes and alkenes of lower relative molecular mass.
• Motivation: Fractional distillation produces a surplus of heavier fractions (bitumen, fuel oil) and insufficient lighter fractions (petrol/gasoline). Cracking converts the low-demand heavy molecules into high-demand light ones.
• Conditions:
  • Temperature: Approximately $500\,^\circ C$.
  • Catalyst: Powdered minerals such as silica ($SiO_2$), alumina ($Al_2O_3$), or zeolites.
• Representative Reaction: $Decane \xrightarrow{\text{heat/catalyst}} Octane + Ethene$ ($C_{10}H_{22} \rightarrow C_8H_{18} + C_2H_4$).
• Byproducts: Cracking also produces hydrogen gas ($H_2$), which can be used in the Haber process for ammonia synthesis.

Addition Reactions of Alkenes

• Bromination: Addition of bromine ($Br_2$) across the double bond.
  • Example: $Ethene + Bromine \rightarrow 1,2\text{-dibromoethane}$ ($CH_2=CH_2 + Br_2 \rightarrow CH_2BrCH_2Br$).
  • With Bromine Water: The product is 2-bromoethanol ($CH_2BrCH_2OH$) because an $-OH$ group from the water also adds to the chain.
• Hydrogenation: Addition of hydrogen ($H_2$) to convert an alkene into its corresponding alkane.
  • Conditions: Temperature between $150\,^\circ C$ and $300\,^\circ C$ with a nickel catalyst.
  • Equation: $Ethene + Hydrogen \rightarrow Ethane$ ($C_2H_4 + H_2 \rightarrow C_2H_6$).
• Hydration (Catalytic Addition of Steam): Industrial manufacture of ethanol.
  • Conditions: Temperature of $300\,^\circ C$, pressure of $6000\,kPa$, and a phosphoric(V) acid catalyst immobilised on silica pellets.
  • Equation: $C_2H_4(g) + H_2O(g) \rightarrow C_2H_5OH(g)$.

Alcohols and Ethanol Chemistry

• Functional Group: The hydroxyl group ($-OH$).
• General Formula: Alcohols follow $C_nH_{2n+1}OH$.
• Naming: Ending in -ol (e.g., Methanol, Ethanol).
• Methods of Ethanol Production:
  1. Fermentation:
     • Mechanism: Yeast enzymes catalyse the anaerobic respiration of glucose ($C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2$).
     • Conditions: No oxygen (anaerobic), temperature between $25\,^\circ C$ and $35\,^\circ C$. It is self-limiting (yeast dies at 14% ethanol concentration).
     • Nature: Renewable, batch process, slow, produce impure ethanol requiring distillation.
  2. Catalytic Hydration of Ethene:
     • Nature: Non-renewable (from petroleum), continuous process, fast, produces high-purity ethanol.
• Uses of Ethanol: Solvent for paints, glues, and perfumes; as a fuel (burns with a clear flame: $C_2H_5OH + 3O_2 \rightarrow 2CO_2 + 3H_2O$).

Carboxylic Acids and Esters

• Functional Group: The carboxyl group ($-COOH$).
• General Formula: $C_nH_{2n+1}COOH$. (Note: In methanoic acid, $n=0$; in ethanoic acid, $n=1$).
• Ethanoic Acid ($CH_3COOH$): Found in vinegar. Produced by the biochemical oxidation of ethanol by Acetobacter bacteria or by powerful oxidising agents like acidified potassium manganate(VII).
• Acidic Properties: Carboxylic acids are weak acids, only partially dissociating into ions in equilibrium: $CH_3COOH(aq) \rightleftharpoons CH_3COO^-(aq) + H^+(aq)$.
• Reactions of Ethanoic Acid:
  • With Metals: $2CH_3COOH + Mg \rightarrow (CH_3COO)_2Mg + H_2$ (Magnesium ethanoate + hydrogen).
  • With Bases: $CH_3COOH + NaOH \rightarrow CH_3COONa + H_2O$ (Sodium ethanoate + water).
  • With Carbonates: $2CH_3COOH + CaCO_3 \rightarrow (CH_3COO)_2Ca + H_2O + CO_2$ (Calcium ethanoate + water + carbon dioxide).
• Esterification: Alcohols react with carboxylic acids to form sweet-smelling, oily esters and water.
  • Catalyst: Concentrated sulfuric acid ($H_2SO_4$).
  • Example: $Ethanoic\,Acid + Ethanol \rightleftharpoons Ethyl\,Ethanoate + Water$ ($CH_3COOH + C_2H_5OH \rightarrow CH_3COOC_2H_5 + H_2O$).

Petroleum and Fractional Distillation

• Formation: Result of prehistoric marine creatures decaying under high pressure, high temperature, and bacterial action over 400 million years.
• Petroleum Fractions (from top to bottom of the column):
  1. Refinery Gas ($C_1-C_4$): Below $25\,^\circ C$. Bottled gas for cooking/heating.
  2. Petrol (Gasoline) ($C_4-C_{12}$): $40\text{--}100\,^\circ C$. Fuel for cars.
  3. Naphtha ($C_7-C_{14}$): $90\text{--}150\,^\circ C$. Chemical feedstock.
  4. Kerosene (Paraffin) ($C_9-C_{16}$): $150\text{--}240\,^\circ C$. Jet engine fuel, heating oil.
  5. Diesel Oil (Gas Oil) ($C_{14}-C_{18}$): $220\text{--}300\,^\circ C$. Diesel engine fuel.
  6. Fuel Oil ($C_{19}-C_{25}$): $250\text{--}320\,^\circ C$. Fuel for ships and home heating.
  7. Lubricating Fraction ($C_{20}-C_{40}$): $300\text{--}350\,^\circ C$. Waxes, polishes, lubrication.
  8. Bitumen ($>C_{70}$): Above $350\,^\circ C$. Surfacing roads.
• Trends in Properties (Bottom to Top):
  • Decreasing chain length.
  • Increasing volatility (ease of evaporation).
  • Lowering boiling points.
  • Lowering viscosity (flows more easily).

Polymers: Addition and Condensation

• Definitions: Polymers are large molecules made of small repeating units called monomers joined by polymerisation.
• Addition Polymerisation: monomers contain a $C=C$ bond.
  • Mechanism: The double bond opens to allow monomers to link into a chain of single bonds. Only one product is formed.
  • Examples: Poly(ethene), Poly(propene), Poly(chloroethene) [PVC], Poly(tetrafluoroethene) [PTFE].
• Condensation Polymerisation: Reaction between two different monomers (copolymers) that eliminates a small molecule (usually water).
  • Polyamides (Nylon): Formed from a diamine and a dicarboxylic acid. Contains an amide link ($-CONH-$).
  • Polyesters (PET): Formed from a diol and a dicarboxylic acid. Contains an ester link ($-COO-$).
• Natural Polymers (Proteins): Built from amino acid monomers. Amino acids contain both an amine group ($-NH_2$) and a carboxylic acid group ($-COOH$). They join via amide links (peptide links).
• Comparison:
  • Addition: Single monomer, unsaturated, resistant to acids, non-biodegradable.
  • Condensation: Two monomers, functional groups at ends, two products (polymer + water), can be biodegradable or hydrolysed by acids/alkalis.
• Environmental Challenges: Disposal in landfills, accumulation in oceans, and formation of toxic gases during incineration.

Questions & Discussion

• Q: Write down the names and molecular formulae of the first four alkanes.
  • A: Methane ($CH_4$), Ethane ($C_2H_6$), Propane ($C_3H_8$), Butane ($C_4H_{10}$).
• Q: What do you observe if ethene is bubbled through bromine water?
  • A: The orange-brown bromine water is decolourised to colourless. This signifies the presence of a $C=C$ double bond (unsaturation).
• Q: How do observations differ if ethane is bubbled through bromine water?
  • A: Ethane would produce no change; the solution would stay orange-brown because ethane is saturated.
• Q: What is the main constituent of natural gas?
  • A: Methane.
• Q: Can we use petroleum directly after extracting it?
  • A: No, it is a mixture and must be separated into useful fractions via fractional distillation.
• Q: What are the products of the complete combustion of ethanol?
  • A: Carbon dioxide and water vapor.