Fiche de révision : Organic Hydrocarbon Fundamentals

📋 Course Outline

1. History of Organic Chemistry
2. Structural Representations
3. Hydrocarbons and Classification
4. Alkanes and Homologous Series
5. Physical Properties of Alkanes
6. Alkane Nomenclature
7. Benzene and Aromatic Compounds
8. Natural Sources of Hydrocarbons
9. Petroleum Fractions and Octane Rating
10. Cracking and Coal Distillation

📖 1. History of Organic Chemistry

🔑 Key Concepts & Definitions

• Vitalism theory : Vitalism was the nineteenth-century idea that only living organisms could produce organic compounds using a vital force.
• Wöhler urea synthesis : Wöhler’s 1828 result showed that urea could be made in a lab from an inorganic starting material, ammonium cyanate.
• Catenation : Catenation is the ability of atoms of the same element to bond to each other, forming chains and rings.
• Organic chemistry definition : Organic chemistry is the study of carbon compounds except the oxides of carbon, carbonates, hydrogen carbonates, cyanides, and cyanates.

📝 Essential Points

• Vitalism proposed that compounds from living things were organic while those from non-living sources were inorganic.
• Friedrich Wöhler synthesized urea in 1828 by heating an aqueous solution of ammonium cyanate, disproving vitalism’s main claim.
• More than 50 million organic compounds are known from both natural isolation and laboratory synthesis.
• Organic compounds all contain carbon as an element, and their diversity is largely linked to carbon catenation and its strong covalent bonding to elements like H, N, O, and S.

💡 Memory Hook

Vitalism failed in 1828: living-force idea collapsed when urea (from urine) was made from ammonium cyanate.

📖 2. Structural Representations

🔑 Key Concepts & Definitions

• Lewis structure : A Lewis structure represents covalent bonding with shared electron pairs shown as dot or line pairs between atoms and lone pairs shown on individual atoms.
• Structural formula : A structural formula shows every covalent bond between atoms using dashes to represent bond order.
• Condensed structural formula : A condensed structural formula abbreviates a structural formula by omitting some or all bond dashes and sometimes uses parentheses with subscripts for repeats.
• Bond-line representation : A bond-line representation uses zig-zag lines for carbon-carbon bonds while carbon and hydrogen atoms are implied by valency.
• Polygon formula : A polygon formula represents cyclic compounds using polygon bond-lines where corners are carbon atoms and sides are carbon-carbon bonds.

📝 Essential Points

• In a structural formula, a single dash denotes a single bond, a double dash denotes a double bond, and a triple dash denotes a triple bond.
• In a condensed structural formula, identical repetitive units can be grouped in parentheses and their repetition number is written with a subscript.
• In a bond-line representation, carbon and hydrogen are not written; only heteroatoms are indicated explicitly.
• In bond-line structures, terminal positions represent CH3 groups unless a functional group changes the terminal composition.
• In a polygon formula, polygon corners represent carbon atoms and polygon sides represent carbon-carbon bonds, with any attached non-hydrogen groups shown as labels.
• In cyclic examples like cyclopentane or cyclohexane, carbon and hydrogen are omitted from the drawing because the ring geometry encodes the carbon-carbon connectivity.

💡 Memory Hook

Dash-order cue: 1 dash single, 2 dashes double, 3 dashes triple.

📖 3. Hydrocarbons and Classification

🔑 Key Concepts & Definitions

• Hydrocarbons : Hydrocarbons are compounds that contain only carbon and hydrogen atoms.
• Aliphatic hydrocarbons : Aliphatic hydrocarbons are hydrocarbon types that are not classified as aromatic and include alkanes, alkenes, alkynes, and derivatives.
• Aromatic hydrocarbons : Aromatic hydrocarbons are hydrocarbons with a low hydrogen-to-carbon ratio compared with typical aliphatic hydrocarbons.
• Saturated compounds : Saturated compounds are hydrocarbons whose molecules contain only single bonds, giving the maximum possible hydrogens per carbon.
• Unsaturated compounds : Unsaturated compounds are hydrocarbons with multiple bonds that usually have fewer than the maximum possible hydrogens per carbon and can add hydrogen under suitable conditions.

📝 Essential Points

• Alkenes have a carbon–carbon double bond as their functional group, and alkynes have a carbon–carbon triple bond as their functional group.
• Alkanes have no functional group because their molecules contain only carbon–carbon and carbon–hydrogen single bonds.
• Aromatic hydrocarbons have a hydrogen-to-carbon ratio lower than typical, with benzene C6H6 having a ratio of 1.
• Compounds are saturated if they contain only single bonds and therefore contain the maximum possible hydrogens per carbon atom.
• Compounds are unsaturated if they contain multiple bonds (like alkenes, alkynes, or benzene) and can react with hydrogen under proper conditions.

💡 Memory Hook

Double bond = unsaturated (add H); triple bond = unsaturated too (add H); only single bonds = saturated (no extra H to add).

📖 4. Alkanes and Homologous Series

🔑 Key Concepts & Definitions

• Homologous series : A homologous series is a family of compounds with the same functional group and a repeating difference in molecular structure and $\mathrm{C}$–$\mathrm{H}$ units.
• Paraffins : Paraffins are alkanes described as having little chemical affinity, so they are chemically inert to most common reagents and conditions.
• Structural isomers : Structural isomers are compounds with the same molecular formula but different connectivity of atoms, giving different structures and properties.
• n-, iso- and neo- prefixes : The prefixes n-, iso-, and neo- describe how carbon atoms in branched alkanes are arranged to form the continuous chain, the near-end branching, or a four-carbon central branching pattern.

📝 Essential Points

• Alkanes are saturated hydrocarbons with general formula $C_nH_{2n+2}$.
• In n- alkanes, the carbon atoms form a continuous chain, in iso- alkanes branching occurs next to the last carbon, and in neo- alkanes the central carbon is bonded to four other carbons.
• Combustion of alkanes with oxygen produces carbon dioxide and water while releasing a large amount of heat.
• For alkanes with 4–7 carbon atoms, the number of isomers is given by $2n-4+1$.
• For cycloalkanes with a single ring, the general formula is $C_nH_{2n}$ where $n\ge 3$.
• Hydrogen atoms in alkanes are classified as primary, secondary, tertiary, or quaternary based on whether they are attached to a primary, secondary, tertiary, or quaternary carbon atom.

💡 Memory Hook

Paraffins = “little affinity”: they resist most reactions, but combustion with oxygen readily produces CO2 and H2O.

📖 5. Physical Properties of Alkanes

🔑 Key Concepts & Definitions

• Constitutional isomers : Constitutional isomers are compounds with the same molecular formula but different atomic connectivity, so they differ in physical properties.
• Structural isomerism : Structural isomerism is the situation where compounds share a molecular formula but differ in how atoms are connected, giving different properties.

📝 Essential Points

• Constitutional isomers always show different physical properties such as melting point, boiling point, density, and refractive index.
• For the C4H10 pair, boiling points of -1.0 °C and -11.7 °C indicate the two compounds are related by structural (constitutional) differences.
• Isomers can be identified when their melting/boiling points or other measured properties differ even though they share the same molecular formula.

📖 6. Alkane Nomenclature

📖 7. Benzene and Aromatic Compounds

🔑 Key Concepts & Definitions

• Benzene : Benzene is the simplest aromatic hydrocarbon with molecular formula C6H6.
• Resonance hybrid : A resonance hybrid is the modern representation of benzene as an average of two equivalent Kekulé structures.

📝 Essential Points

• Aromatic hydrocarbons are generally obtained from petroleum and coal tar.
• Benzene’s early structure used a six-membered ring with alternating single and double bonds proposed by Kekulé in 1872.
• Benzene is represented today as a resonance hybrid of two equivalent contributing Kekulé structures.
• In benzene, all C–C bonds are equivalent with intermediate character between single and double bonds and equal length.

💡 Memory Hook

Aromatic = aroma-ring from tar/petroleum; Benzene is C6H6 and its bonds are an “average” (resonance hybrid).

📖 8. Natural Sources of Hydrocarbons

🔑 Key Concepts & Definitions

• Natural gas : Natural gas is a mixture of gases whose main component is methane, with other hydrocarbons and small impurities often present.
• Petroleum crude oil : Crude oil is a dark, viscous mixture of fossil hydrocarbons that must be separated and refined into useful fractions.
• Fractional distillation : Fractional distillation is the process that separates crude oil into fractions using differences in boiling points.
• Petroleum fractions : Petroleum fractions are the separated crude-oil products, each with a typical carbon-number range, boiling range, and use.

📝 Essential Points

• Natural gas consists chiefly of methane (>90%) and may also contain ethane, propane, butane, plus gases like CO2, N2, O2, and H2S.
• Crude oil mainly contains alkanes, cycloalkanes, and aromatic hydrocarbons, and it also includes nitrogen-, sulphur-, and oxygen-containing compounds in small amounts.
• Fractional distillation first gives three principal cuts by boiling point: straight-run gasoline (30–200 °C), kerosene (175–300 °C), and heating oil/diesel (275–400 °C).
• Typical fraction ranges include natural gases (C1→C4, below 20 °C), gasoline (C5→C10, 40–200 °C), and lubrication oils plus wax/paraffin (higher carbon numbers).
• Straight-run gasoline can cause engine knock, an uncontrolled combustion in a hot engine, so it is a poor automobile fuel.

💡 Memory Hook

Natural gas = methane-rich; crude oil = mix of hydrocarbon types; distillation = split by boiling point into gasoline/kerosene/diesel.

📖 9. Petroleum Fractions and Octane Rating

🔑 Key Concepts & Definitions

• Octane number : The octane number is a fuel rating that measures its antiknock performance in internal combustion engines.
• 2,2,4-trimethylpentane : 2,2,4-trimethylpentane is the highly branched hydrocarbon used as the octane rating standard of 100.
• Heptane : Heptane is the straight-chain hydrocarbon used as the octane rating standard of 0.

📝 Essential Points

• Petroleum refining starts with fractional distillation and produces straight-run gasoline (bp 30–200 °C), kerosene (bp 175–300 °C), and heating oil/diesel (bp 275–400 °C).
• Typical petroleum fractions include natural gases (C1→C4, below 20 °C), petroleum ether (C5→C7, 20→60 °C), and lubricating oils and waxes from higher-carbon cuts.
• Straight-run gasoline causes engine knock because its combustion is uncontrolled in a hot engine.
• Octane rating is based on mixtures of 2,2,4-trimethylpentane (100) and heptane (0) that match the same knocking behavior in an engine.

💡 Memory Hook

Branched = 100 (isooctane), straight = 0 (heptane): octane tracks how well the fuel resists knocking.

📖 10. Cracking and Coal Distillation

🔑 Key Concepts & Definitions

• Cracking : Cracking is the breakdown of large hydrocarbon molecules into smaller ones using heat (thermal cracking) or catalysts (catalytic cracking).
• Catalytic cracking : Catalytic cracking is cracking carried out in the presence of catalysts to produce smaller branched molecules for higher-quality fuels.
• Destructive distillation of coal : Destructive distillation of coal is heating coal in the absence of air (no oxygen) to produce volatile products and coke.
• Coal tar : Coal tar is the condensed liquid product formed when coal’s volatile gases are cooled during destructive distillation.

📝 Essential Points

• Cracking splits a large hydrocarbon into smaller products, e.g., hexadecane forms octane and octene on heating.
• During cracking, hydrogen can be added under suitable conditions to saturate the alkenes formed.
• Catalytic cracking is used to convert the high-boiling kerosene cut (C11–C14) into smaller branched molecules suitable for gasoline.
• Reforming converts C6–C8 alkanes into aromatic compounds like benzene and toluene, which have much higher octane numbers.
• Heating coal without air produces volatile products that can be separated into coal gas and a liquid that is called coal tar.
• Coal tar can be fractionally distilled to isolate aromatic hydrocarbons, while coke is used as fuel in blast furnaces and to make water gas (H2+CO) and producer gas (N2+CO).

💡 Memory Hook

Cracking = break big molecules (often into gasoline), Destructive distillation = cook coal without air (gas + tar + coke).

📅 Key Dates

📊 Synthesis Tables

Saturated vs unsaturated hydrocarbons (functional group focus)

⚠️ Common Pitfalls & Confusions

1. Confusing vitalism’s claim (“only living things”) with the 1828 urea synthesis that disproved it.
2. Mixing up structural representations: in bond-line structures, carbons/hydrogens are omitted and only heteroatoms are written explicitly.
3. Using “organic” to mean “natural only”: the definition includes both synthetic and natural compounds.
4. Treating aromatic compounds like alkenes: benzene does not decolorize bromine water/KMnO4 and typically does not undergo alkene addition.
5. Applying the wrong homologous-series increment: alkanes differ by a constant -CH2- unit, but alkenes differ differently and follow CnH2n.
6. Forgetting which bonds imply saturation: only single bonds mean saturated; any double/triple bond makes it unsaturated.
7. Applying Markovnikov’s rule incorrectly: hydrogen goes to the double-bond carbon with more hydrogens, not the more substituted carbon.

✅ Exam Checklist

1. State the vitalism idea and explain how Wöhler’s 1828 urea synthesis disproved its main claim.
2. Define catenation and use it to explain carbon’s ability to form diverse organic compounds.
3. Distinguish Lewis structures, structural formulas, condensed structural formulas, bond-line representations, and polygon formulas (including what polygon corners/sides represent).
4. Classify hydrocarbons as aliphatic vs aromatic, and as saturated vs unsaturated, using the functional group/bond rules.
5. Write the alkane general formula CnH2n+2, describe the homologous-series -CH2- pattern, and recall that alkanes have no functional group.
6. Use n-, iso-, and neo- naming prefixes to identify how branched alkanes are arranged (continuous chain vs near-end branching vs four-carbon central branching).
7. Apply IUPAC principles for naming straight-chain vs branched-chain alkanes, including locants, longest chain selection, and substituent alphabetical order.
8. For alkanes, define isomerism/structural isomerism and interpret the “2n-4+1” isomer count for alkanes with 4–7 carbon atoms.
9. For alkenes and alkynes, write the general formulas (CnH2n and CnH2n−2), determine key physical behavior trends, and name using -ene/-yne and double/triple-bond locants.
10. Predict major products of alkene addition reactions using the correct reaction type (addition vs substitution), and apply Markovnikov’s rule for H-X and H-OH cases.
11. Describe major natural sources (natural gas, crude oil, coal), fractional distillation main cuts and uses, and cracking/reforming/destructive distillation outcomes (including octane standards).