Comprehensive Organic Chemistry Notes – Hydrocarbons: Alkanes, Alkenes & Alkynes

1. Organization & Classification of Organic Compounds

Organic compounds are millions in number, so chemists classify them to understand their properties and behaviors. They are primarily classified using two main schemes:

• By functional group: These are specific arrangements of atoms (like C bonded to H, N, O, S, or halogens) that determine chemical reactivity.

• By molecular framework: This classifies compounds based on the overall shape of their carbon skeleton (open chain or closed ring).

1.1 Functional-Group Classes & General Formulae

• Hydrocarbons: Made up only of carbon ($C$) and hydrogen ($H$) atoms.

  • Alkanes: "Saturated" hydrocarbons with only single (σ) carbon-carbon bonds. General formula: $C<em>{n}H</em>{2n+2}$. Examples: methane ($CH<em>4$), propane ($C</em>3H*8$).

  • Alkenes: Contain at least one carbon-carbon double bond ($C=C$). "Unsaturated." General formula: $C<em>{n}H</em>{2n}$. Example: ethene ($C<em>2H</em>4$).

  • Alkynes: Contain at least one carbon-carbon triple bond ($C≡C$). Even more "unsaturated." General formula: $C<em>{n}H</em>{2n-2}$. Example: ethyne ($C<em>2H</em>2$).

• Oxygenated Compounds: Contain oxygen, carbon, and hydrogen.

  • Alcohols: Contain a hydroxyl group (-OH). General formula $C<em>{n}H</em>{2n+2}O$.

  • Ethers: Contain an oxygen atom bonded to two alkyl/aryl groups (R-O-R).

  • Aldehydes: Carbonyl group ($C=O$) at chain end.

  • Ketones: Carbonyl group ($C=O$) within a chain.

  • Carboxylic acids: Carboxyl group (-COOH).

  • Esters: Derived from carboxylic acids.

• Nitrogen / other heteroatom Compounds: Contain nitrogen, sulfur, or halogens.

  • Amines: Nitrogen atom bonded to alkyl/aryl groups (e.g., $R-NH*2$).

  • Amides: Nitrogen atom attached to a carbonyl group.

1.2 Molecular-Framework Classes

• Acyclic: 'Open-chain' compounds; carbon atoms form a straight or branched chain.

• Cyclic: Carbon atoms form one or more closed rings.

  • Homocyclic: Ring entirely of carbon atoms (e.g., benzene).

  • Heterocyclic: Ring contains at least one atom other than carbon (O, N, S).

The smallest stable ring is a 3-membered ring (e.g., cyclopropane).

2. Hydrocarbons: General Overview

Hydrocarbons are compounds solely of carbon and hydrogen. They are divided into:

1. Aliphatic: Includes straight, branched, or non-aromatic rings (alkanes, alkenes, alkynes, cycloalkanes, cycloalkenes, cycloalkynes).

2. Aromatic (arenes): Special cyclic compounds with a highly stable, delocalized system of pi (π) electrons (like benzene).

There are useful general formula patterns:

• Chain alkanes: $C<em>{n}H</em>{2n+2}$

• Cycloalkanes: $C<em>{n}H</em>{2n}$

• Chain alkenes: $C<em>{n}H</em>{2n}$

• Cycloalkenes: $C<em>{n}H</em>{2n-2}$

• Cycloalkynes: $C<em>{n}H</em>{2n-4}$

3. Alkanes (Saturated Hydrocarbons)

3.1 Key Features

• Simplest hydrocarbons, 'saturated' with only C-C single (σ) bonds.

• Free rotation around single bonds.

• General formula: $C<em>{n}H</em>{2n+2}$.

• Naming: Based on carbon count (methane, ethane, propane, butane, then prefixes pent-, hex-, etc. + '-ane').

• Alkyl groups (substituents R): Alkane minus one $H$; named by changing '-ane' to '-yl' (e.g., methyl, ethyl).

3.2 IUPAC Nomenclature Rules (overview)

1. Locate the longest continuous carbon chain: This is the 'parent' name.

2. Number the carbon atoms: Start from the end that gives the lowest number to the first substituent.

3. Identify and name substituents.

4. Use prefixes for multiple identical substituents: di-, tri-, tetra-, etc.

5. Alphabetize different substituent names: Ignore di-, tri-, sec-, t-; consider iso-, neo-, cyclo-.

6. Combine with hyphens (numbers and letters) and commas (numbers): Final name is a single word (e.g., '2-methylpropane').

• Common hetero substituents: fluoro (-F), chloro (-Cl), bromo (-Br), iodo (-I), hydroxy (-OH), amino (-NH2), nitro (-NO2), methoxy (-OCH3), cyano (-CN), nitroso (-NO), phenyl.

• Conventions: Example: 4-ethyl-2,4-dimethylhexane. For a chain length tie, choose the chain with the most substituents.

3.3 Cycloalkanes

• Alkanes where carbon atoms form a ring, saturated.

• General formula: $C<em>{n}H</em>{2n}$.

• Named by adding 'cyclo-' prefix (e.g., cyclopropane, cyclohexane).

• Numbering for multiple substituents: Lowest possible set of locants; use alphabetical priority for carbon '1'.

3.4 Physical Properties Trend

• Low melting and boiling points, insoluble in water.

• Only weak London dispersion forces, requiring little energy to overcome.

• Insoluble in polar solvents; miscible with non-polar solvents.

• Boiling point increases with molecular weight (more carbons) due to stronger London forces.

• Small alkanes (C1–C4) are gases; larger ones are liquids or solids.

3.5 Natural Sources & Refining

• Major sources: petroleum crude oil and natural gas (primarily methane).

• Fractional distillation separates crude oil by boiling points:

  • Gases (<20 °C): heating, cooking.

  • Gasoline (C5–C12, 20–200 °C): car fuel.

  • Kerosene (C12–C16, 175–275 °C): jet fuel, heating oil.

  • Fuel oil (C15–C18, 250–400 °C): diesel, heating oil.

  • Lubricating oil (>C16, >350 °C): machinery.

  • Asphalt (>C20): roads, roofing.

3.6 Laboratory/Industrial Preparation

1. Catalytic hydrogenation of alkenes/alkynes: Adding $H<em>2$ across double/triple bonds with metal catalysts (Pt, Pd, Ni, $PtO</em>2$).

2. Reduction of alkyl halides: Using Zn in AcOH, Grignard reagents ($RMgX$), or Gilman reagents ($R*2CuLi$).

3. Carbonyl reductions: Aldehydes and ketones to alkanes.

   • Clemmensen reduction: Using Zn/Hg and HCl.

   • Wolff–Kishner reduction: Using $NH<em>2NH</em>2$ and KOH at high temperatures.

3.7 Reactivity Overview

• Generally unreactive ('inert') due to strong C-C and C-H single bonds.

• Key reactions:

  1. Combustion: Rapid reaction with $O<em>2$ forming $CO</em>2$ and $H<em>2O$, releasing heat (fuels). General equation: $C</em>{n}H<em>{2n+2} + O</em>2 → CO<em>2 + H</em>2O$ (unbalanced).

  2. Ring-opening hydrogenation of small rings: Cyclopropane and cyclobutane open with $H*2$ and Ni due to ring strain. Larger rings are unreactive.

  3. Radical halogenation: Substitution of $H$ by halogen (Cl2, Br2) with UV light (hv) or heat (Δ).

     • Mechanism: radical chain (Initiation, Propagation, Termination).

     • Selectivity: $Br·$ are less reactive and more selective, preferentially reacting with more substituted hydrogens (3° > 2° > 1°). Leads to racemic products if a chiral center is formed.

     • Excess alkane favors monohalide formation.

4. Alkenes (Olefins)

4.1 Fundamentals

• 'Unsaturated' hydrocarbons with at least one carbon-carbon double bond ($C=C$).

• The double bond has a strong sigma (σ) bond and a weaker pi (π) bond, which is electron-rich and attacked by electrophiles.

• General formula: $C<em>{n}H</em>{2n}$ (for acyclic with one double bond).

• Vinyl carbon: $sp^2$ carbon in C=C double bond.

• Allylic carbon: $sp^3$ carbon adjacent to C=C double bond.

4.2 Nomenclature Highlights

• Suffix '–ene'.

• Parent chain must include the double bond; carbons numbered to give lowest locant to double bond.

• Multiple double bonds: 'diene', 'triene'.

• Stereochemistry (E/Z): Used for isomers due to restricted rotation around C=C bond (E = opposite, Z = together).

• Rings: 'cyclo-' prefix; double bond carbons are 1 and 2 automatically.

4.3 Physical Property Patterns

• Similar to alkanes, boiling/melting points increase with molecular weight.

• Geometry (cis vs. trans, branching) affects values (e.g., cis-2-butene vs. trans-2-butene).

4.4 Industrial Significance

• Key monomers for producing polymers (plastics).

4.5 Laboratory Preparation (Eliminations)

Alkenes are typically prepared via elimination reactions (removal of atoms from adjacent carbons to form a pi bond).

• From alcohols (acid-catalysed dehydration): Alcohol ($R-OH$) loses $H<em>2O$ with acid catalyst ($H</em>2SO*4$).

• From alkyl halides (E2 dehydrohalogenation): Alkyl halide ($R-X$) loses $HX$ with strong base (KOH(alc)).

• From vicinal (vic-) or geminal (gem-) dihalides: With Zn in AcOH or NaI in acetone.

• Partial hydrogenation of alkynes: Adding one molecule of $H*2$ to alkynes.

  • Syn addition (Lindlar catalyst): Pd/BaSO4 with quinoline → cis-alkene.

  • Anti addition (dissolving metal reduction): Na or Li in liquid $NH*3$ at -78 °C → trans-alkene.

4.6 Stability Order

• More substituted alkenes are more stable: Tetrasubstituted > trisubstituted > disubstituted > monosubstituted (due to hyperconjugation).

• Trans isomers are generally more stable than cis isomers: Minimizes steric repulsion.

• Conjugated double bonds are more stable than isolated ones: Due to electron delocalization.

4.7 General Electrophilic Addition Mechanism

Characteristic reaction of alkenes: Electrophilic addition.

1. Step 1: Electrophilic attack: Pi (π) bond attacks an electrophile (e.g., $H^+$), forming a carbocation (reactive intermediate with positive charge).

2. Step 2: Nucleophilic capture: A nucleophile (e.g., $X^-$ or $H*2O$) attacks the carbocation.

• Markovnikov Rule: In $H-X$ addition to unsymmetrical alkenes, $H$ attaches to the carbon with more hydrogen atoms, forming the more stable carbocation (tertiary > secondary > primary).

4.8 Key Reactions & Products

• HX addition (Hydrohalogenation): Markovnikov addition of HCl, HBr, HI to yield alkyl halide.

• Hydration (Adding Water): Markovnikov addition of $H<em>2O$ (with $H</em>2SO*4$) to yield alcohol.

  • Alternatives (superior yields, no rearrangement):

    • Oxymercuration-reduction: Markovnikov addition using $Hg(OAc)<em>2$ then $NaBH</em>4$.

    • Hydroboration-oxidation: Anti-Markovnikov and syn addition using $BH<em>3·THF$ then $H</em>2O*2$/KOH.

• X2 addition (Halogenation): Adding $Cl<em>2$ or $Br</em>2$ to yield vicinal dihalide via halonium ion intermediate, resulting in anti addition. Decolorizes bromine water.

• X2/H2O (Halohydrin Formation): Adding $X<em>2$ in $H</em>2O$ forms halohydrin (halogen on less substituted, -OH on more substituted carbon).

• Hydroxylation (Diol Formation): Adds two -OH groups.

  • Syn hydroxylation: Cold, dilute, basic $KMnO<em>4$ or $OsO</em>4$ (with $H<em>2O</em>2$) yields cis-diol.

  • Anti hydroxylation: Peracid (e.g., mCPBA) to form epoxide, then acid hydrolysis yields trans-diol.

• Ozonolysis ($O*3$): Cleaves $C=C$ bond, forming two fragments.

  • Reductive work-up (Zn in $H*2O$): Aldehydes or ketones.

  • Oxidative work-up ($H<em>2O</em>2$ in NaOH): Carboxylic acids or ketones.

• Hydrogenation ($H<em>2$/Pt, Pd, Ni): Adds $H</em>2$ across double bond to yield alkane (syn addition).

5. Alkynes

5.1 Basics

• Contain at least one carbon-carbon triple bond ($C≡C$).

• Even more unsaturated; general formula: $C<em>{n}H</em>{2n-2}$.

• $sp$ hybridized carbons; linear geometry around triple bond.

• Terminal alkyne: Triple bond at chain end ($R-C≡C-H$); acidic hydrogen.

• Internal alkyne: Triple bond within chain ($R-C≡C-R$).

5.2 Nomenclature

• Suffix '–yne'. Numbering gives lowest locant to triple bond.

5.3 Acidity & Metal Acetylides

• Terminal alkynes are acidic ($pK_a$ ≈ 25) due to $sp$ hybridization stabilizing carbanion.

• Strong bases ($NaNH*2$, $RLi$, $RMgBr$) deprotonate terminal alkynes to form acetylide anions ($R-C≡C^-$), which are strong nucleophiles/bases.

• Form insoluble precipitates with heavy metal salts ($AgNO*3$, $CuCl$) to distinguish from internal alkynes.

5.4 Synthesis

• Double dehydrohalogenation: Removing two $HX$ from a dihalide, often with strong bases (KOH, $NaNH*2$) and heat. (Can stop at vinyl halide under milder conditions).

• Alkylation of acetylide anions: Acetylide anions attack primary alkyl halides ($R'CH*2X$) to form longer alkyne chains.

5.5 Electrophilic Addition Reactions (2 equivalents for saturation)

Alkynes undergo electrophilic addition, adding two equivalents of reagent to become saturated.

• Hydrogenation: Adding $H*2$ to a triple bond.

  • Partial hydrogenation to alkenes: stops at alkene stage.

    • Lindlar catalyst (Pd/BaSO4, quinoline): syn addition → cis-alkene.

    • Sodium (Na) or Lithium (Li) in liquid ammonia ($NH*3$) at −78 °C: *anti* addition → trans-alkene.

  • Complete hydrogenation to alkanes: Reactive catalysts (Pt) convert directly to alkane.

• Halogenation (X2):

  • 1 equivalent: Dihaloalkene, typically anti addition.

  • 2 equivalents (or excess X2): Tetrahalide.

• Hydrohalogenation (HX):

  • 1 equivalent: Markovnikov addition forms vinyl halide.

  • Excess HX: Geminal dihalide (both halogens on same carbon).

• Hydration: Adding water to alkynes initially forms an enol, which rearranges to a carbonyl compound.

  1. Acid/HgSO4 catalysis (Markovnikov): Strong acid ($H<em>2SO</em>4$) and $HgSO*4$ catalyst.

     • Internal alkynes: Ketones.

     • Terminal alkynes (except acetylene): Methyl ketones. Acetylene: Acetaldehyde.

     • Mechanism: Forms unstable enol, which undergoes keto–enol tautomerism to stable keto form.

  2. Hydroboration-oxidation (anti-Markovnikov): Uses borane ($R<em>2BH$) then $H</em>2O*2$/KOH.

     • Terminal alkynes: Aldehydes.

     • Internal alkynes: Ketones.

6. Radical & Mechanistic Notes

• Selectivity: Less reactive bromine radicals ($Br·$) are more selective in choosing reaction pathways than chlorine radicals ($Cl·$).

• Stereogenic carbon: Radical substitution at a chiral center typically produces racemic products (50:50 mixture of enantiomers).

• Stereospecificity: Halogenation of cis/trans alkenes is stereospecific (e.g., cis-2-butene + $Br<em>2$ yields racemic 2,3-dibromobutane; trans-2-butene + $Br</em>2$ yields meso-2,3-dibromobutane).

7. Key Numeric / Formula Reminders

• General Formulas:

  • Alkanes: $C<em>{n}H</em>{2n+2}$

  • Alkenes / Cycloalkanes: $C<em>{n}H</em>{2n}$

  • Alkynes / Cycloalkenes: $C<em>{n}H</em>{2n-2}$

  • Cycloalkynes: $C<em>{n}H</em>{2n-4}$

• Fractionation Boiling Point Ranges:

  • Gases: <20 °C

  • Gasoline: 20–200 °C

  • Kerosene: 175–275 °C

  • Fuel oil: 250–400 °C

  • Lubricants: >350 °C

• Small-ring strain: Cyclopropane (60° internal angles) and cyclobutane (90°) have significant strain, driving reactivity.

8. Ethical, Industrial & Practical Implications

• Combustion of alkanes: Crucial for energy, but contributes to $CO*2$ (climate change); raises concerns about fossil fuel dependence.

• Halogenation: Provides alkyl halides as synthetic intermediates, but many halogenated compounds are toxic or ozone-depleting (e.g., CFCs). Waste management is a concern.

• Ozonolysis: Industrially important, but ozone ($O*3$) is hazardous and can cause air pollution/ozone layer depletion.

• Polymer precursors: Alkenes are vital for plastics, leading to serious sustainability issues like waste accumulation and pollution.

9. Cross-Connections & Concept Integration

• Dehydration and hydration: Reversible reactions involving alcohols and alkenes under acid catalysis. Le Châtelier's principle explains equilibrium shifts.

• Hydrogenation: Alkyne can be selectively converted to alkene or fully to alkane by catalyst choice, demonstrating reaction control.

• Radical halogenation: Introduces a functional group (halogen) onto unreactive alkanes, forming versatile starting materials for further reactions, illustrating synthetic strategy.