Alkynes and Alkanes

Hydrocarbons

• Composed of carbon and hydrogen
• Common hydrocarbon fuels include petrol, kerosene, coal gas, CNG, and LPG

Sources of Hydrocarbons

• Petroleum and natural gas: major sources of aliphatic hydrocarbons
• Coal: important source of aromatic hydrocarbons
• Petroleum is oil trapped inside rocks
  • Rocks are referred to as "Petra"
  • Oil referred to as "Oleum"
• Natural gas
  • Gaseous mixture covering the oil in petroleum fields
  • Main constituents include methane, ethane, propane, and butane

Classification of Hydrocarbons

• Hydrocarbons are divided into two main categories:
  • Acyclic or Aliphatic (Open chain)
  • Carbocyclic or Cyclic

Acyclic or Aliphatic Hydrocarbons

• Further divided into:
  • Alkanes
  • Alkenes
  • Alkynes

Carbocyclic or Cyclic Hydrocarbons are divided into

• Alicyclic:
  • Cycloalkanes
  • Cycloalkenes
  • Cycloalkynes
• Aromatic

Alkanes

• Also known as paraffins
• General formula: CnH2n+2
• Hybridization: sp³
• Carbon-carbon bond length: 1.154 Å
• Chemically unreactive
• Show chain, position, and optical isomerism
• Isomers
  • Heptane: 9 isomers
  • Octane: 18 isomers
  • Decane: 75 isomers

Preparation of Alkanes

Wurtz Reaction

• 2R-X + 2Na → R-R + 2NaX
• Involves free radical mechanism
• Useful for preparing alkanes with an even number of carbon atoms
• Serves as a chain-stepping reaction

Frankland Reaction

• RX + Zn + RX → R-R + ZnX2

Grignard Reagent

• RMgX + HOH → RH + Mg(OH)X
• RMgX + R'OH → RH + Mg(OR')X
• RMgX + R'NH2 → RH + Mg(NHR')X

Unsaturated Hydrocarbons: Sabatier-Senderens Reduction

• R-CH=CH₂ + H₂ → R-CH₂-CH₃
• R-C≡CH + H₂ → R-CH₂-CH₃

Carboxylic Acids: Decarboxylation

• CH₃COO⁻Na⁺ + NaOH → CH₄ + Na₂CO₃

Kolbe's Electrolytic Method

• 2CH₃COO⁻Na⁺ + 2H₂O → CH₃-CH₃ + 2CO₂ + H₂ + 2NaOH

Physical Properties of Alkanes

Nature

• Non-polar due to the covalent nature of C-C and C-H bonds
• C-C bond energy 83 kJ/mole
• C-H bond energy 99 kJ/mole

State

• C₁-C₄: gases
• C₅-C₁₇: colorless, odorless liquids
• ​C₁₇+: solids

Solubility

• Like dissolves like
• Polar compounds dissolve in polar solvents, and non-polar compounds dissolve in non-polar solvents

Boiling Point

• Low boiling point due to non-polar nature

Intermolecular Forces

• Molecules held together by weak Van der Waals forces
• Magnitude of Van der Waals forces is directly proportional to molecular size
  • Boiling point increases with the number of carbon atoms.
• Branched-chain alkanes have lower boiling points than straight-chain isomers
  • Branching reduces molecular compactness, decreasing the surface area and Van der Waals forces.

Chemical Properties of Alkanes

Combustion

• CH₄ + 2O₂ → CO₂ + 2H₂O
• ΔH = -217.0 K cal/mole

Oxidation

• CH₄ + O₂ → 2CH₃OH
• ​CH₄ + O₂ → HCHO + H₂O

Substitution: Halogenation

• CH₄ + Cl₂ → CH₃Cl + HCl
  • proceeds through multiple steps forming CH₂Cl₂, CHCl₃, and CCl₄

Iodination Considerations

• It is a reversible reaction
• Conducted with an oxidizing agent to oxidize HI formed, such as iodic acid (HIO₃), nitric acid (HNO₃), or mercuric oxide (HgO)

Fluorination

• It takes place explosively, potentially rupturing C-C bonds in higher alkanes

Features of Halogenations

• Reactivity of halogens: F₂ > Cl₂ > Br₂ > I₂
• Rate of hydrogen replacement: 3° > 2° > 1°

Halogenation Mechanism

• Proceeds through a free radical mechanism
• Steps: - Initiation: Cl-Cl → 2Cl• - Propagation: CH₄ + Cl• → CH3• + HCl, CH3• + Cl₂ → CH₃Cl + Cl• - Termination: Combination of radicals

Nitration

• It is a free radical mechanism at high temperatures (450°C)
• C-C bonds can break, resulting in a mixture of nitroalkanes

Sulphonation

• Replacement of hydrogen atom of alkane by -SO₃H group

Aromatization

• Converts alkanes to aromatic compounds using catalysts like Cr₂O₃ at high temperatures
• Also called dehydrogenation or hydroforming

Thermal Decomposition (Pyrolysis/Cracking/Fragmentation)

- Higher alkanes heated to high temperatures (700-800K) with alumina or silica catalysts break down into smaller alkanes and alkenes

Action of Steam

• Methane reacts with steam at 1000°C in the presence of nickel and alumina to produce carbon monoxide and hydrogen

Conformational Isomerism

• Different molecular arrangements arising from rotation around carbon-carbon single bonds
• Two Extreme Conformations:
  • Eclipsed Conformation
  • Staggered Conformation

Alkenes

• Unsaturated hydrocarbons containing a double bond
• General formula: CnH2n
• Carbon-carbon bond length: 1.34 Å
• sp² hybridization
• Also known as olefins (Greek for "oil-forming")
• Exhibit chain, positional, and geometrical isomerism

Preparation of Alkenes

From Alkynes

• Partial reduction of alkynes using a partially deactivated palladised charcoal catalyst (Lindlar's catalyst)

From Haloalkanes: Dehydrohalogenation

• Involves E2 or 1,2-elimination using alcoholic KOH
• Saytzeff's rule guides the formation of substituted alkenes

From Dihaloalkanes: Dehalogenation

• Uses Zn/HOAc or NaI in acetone

From Alcohols: Dehydration

Requires concentrated H₂SO₄ at 160°C or Al₂O₃

Chemical Properties of Alkenes

Addition Reactions

• Show electrophilic addition reactions

Addition of Hydrogen

• RCH=CH₂ + H₂ → RCH₂CH₃

Addition of Halogens

• CH₂=CH₂ + Br₂ → CH₂Br-CH₂Br (using CCl₄ solvent)
• CH₂=CH₂ + Br₂ → Br-CH₂-CH₂-OH + HBr (using H₂O solvent)

Addition of Hydrogen Halides

• Follows Markovnikov's rule for unsymmetrical alkenes

Markovnikov's Rule

• The negative part of the addendum attaches to the carbon with fewer hydrogen atoms

Peroxide Effect (Kharasch Addition)

• In the presence of organic peroxides, HBr adds opposite to Markovnikov's rule

Addition of Water (Hydration)

• Acid-catalyzed reaction

Oxidation

• Combustion: Produces CO₂ + H₂O
• Hydroboration-Oxidation: Alkenes react with diborane to form trialkyl boranes, which oxidize to alcohols

Alkynes

• Unsaturated hydrocarbons with a triple bond
• General formula: CnH2n-2
• sp hybridization
• Show chain, positional, and functional isomerism

Preparation of Alkynes

From Vicinal Dihalides

• Involves dehalogenation using alcoholic KOH
• React water with calcium carbide

Chemical Properties

• Alkynes show electrophilic addition reactions

Addition of Hydrogen

• Requires Nickel catalyst

Addition of Halogens

Addition of Hydrogen Halides

Addition of Water (Hydration)

• Acid catalyzed addition

Aromatic Hydrocarbons

• Compounds containing a benzene ring (benzenoids) or those without a benzene ring (non-benzenoids)
• Structure of benzene: Described by Kekulé structure
• Benzene: hybrid of various resonating structures
• All six carbon atoms in benzene are sp² hybridized, forming sigma bonds

Characteristic of Benzene Rings

• Six π electrons are delocalized and move freely about the six carbon nuclei
• Delocalized π electron cloud is attracted more strongly by the carbon nuclei
• Delocalized π electrons in benzene make it more stable

Aromaticity Criteria:

• Planarity
• Complete delocalization of π electrons in the ring
• Presence of (4n + 2) π electrons (Hückel Rule)

Preparation of Benzene:

Cyclic polymerization of ethyne Decarboxylation of aromatic acids Reduction of phenol using heated zinc dust

Physical Properties:

• Aromatic hydrocarbons are non-polar
• Usually colorless liquids or solids with a characteristic aroma
• Immiscible with water but miscible with organic solvents
• Burn with a sooty flame

Chemical Properties:

• Undergo electrophilic substitution reactions

Nitration

Introduction of a nitro group (NO₂) using concentrated nitric acid and sulfuric acid

Halogenation

Introduction of a halogen atom using chlorine or bromine and a Lewis acid catalyst

Sulfonation

• Introduction of a sulfonic acid group (SO₃H) using fuming sulfuric acid

Friedel-Crafts Alkylation

• Introduction of an alkyl group using an alkyl halide and a Lewis acid catalyst

Friedel-Crafts Acylation

• Introduction of an acyl group using an acyl halide and a Lewis acid catalyst

Directive Influence in Monosubstituted Benzene

• Functional groups influence the position of incoming substituents

Directing Groups and Activating

• -Ortho and para: OH, -NH₂, -NHR, -NHCOCH₃, -OCH₃, -CH₃, -C₂H₅

Meta Directing and Deactivating

• -NO₂, -CN, -CHO, -COR, -COOH, -COOR, -SO₃H

Halogens

• Halogens (Ortho and para directing and deactivating)
• They decrease the overall electron density on the benzene ring due to their strong -I effect
• However, resonance increases electron density at the ortho and para positions

Carcinogenicity and Toxicity

• Benzene and polynuclear hydrocarbons are toxic and carcinogenic when containing more than two fused benzene rings

Questions cover reaction types, catalysts, acidity, and product formation in alkynes and alkanes. Topics include double bond detection, hybridization, and polymerization.