• Many fuels like CNG (compress natural gas), LPG (liquified petroleum gas) are obtained form LNG (liquified natural gas). Many other fuels like petrol, diesel, kerosene, coal etc. have mixture of hydrocarbons.
  • Hydrocarbon also used in manufacture of polymers like polythene, also used as solvent for paints.

CLASSIFICATION-

  • The hydrocarbons are classify into 3-
    • Saturated Hydrocarbon- It is an organic compound which made form only carbon and hydrogen atoms where all are single bonded. These compounds also called alkanes, have general formula is CnH2n+2.
    • Unsaturated Hydrocarbons- It is an organic compound which made form only carbon and hydrogen atoms where they are at least one double or triple bonded. These compounds are classify into alkenes (with double bonds) or alkynes (with triple bonds).
    • Aromatic Hydrocarbons- It is an organic compound which contain a ring of carbon atoms with alternating single and double bonds.
      • These compounds follow a rule called Huckel ‘s rule which state that ring must have specific number of π-electrons(4n+2).
      • Aromatic Hydrocarbons are stable and have distinct smell.
      • ex.- Benzene ring(C6H6).

ALKANES-

  • It is saturated open chain hydrocarbons which contain single C-C bond.
  • The first member of its family is Methane(CH4).
  • It general formula is CnH2n+2.
  • It is mainly found in natural gas and petroleum.

NOMENCLATURE-

Nomenclature of Alkanes:

  • Alkanes are hydrocarbons that contain only single bonds. The first three alkanes – methane (CH₄), ethane (C₂H₆), and propane (C₃H₈) – have only one structural form.
  • As the number of carbon atoms increases, alkanes can have multiple structures (isomers). For example, butane (C₄H₁₀) has two structural isomers: n-butane (straight chain) and isobutane (branched chain).

Structural Isomers:

  • Alkanes with the same molecular formula but different structural arrangements are called structural isomers.
  • Isomers that differ only in the arrangement of carbon atoms in a chain are called chain isomers. For example:
    • C₄H₁₀ has 2 chain isomers (n-butane and isobutane).
    • C₅H₁₂ has 3 chain isomers.

Primary, Secondary, Tertiary, and Quaternary Carbon Atoms:

  • A carbon atom attached to no other carbon atom (as in methane, CH₄) or to only one other carbon atom (as in ethane, C₂H₆) is called a primary (1°) carbon.
  • A carbon atom attached to two other carbon atoms is a secondary (2°) carbon.
  • A carbon attached to three other carbon atoms is a tertiary (3°) carbon.
  • A carbon attached to four other carbon atoms is a quaternary (4°) carbon.

Example Problem 9.1:

  • Write structures for different chain isomers of alkanes with molecular formula C₆H₁₄ and their IUPAC names:
    1. n-Hexane: CH₃–CH₂–CH₂–CH₂–CH₂–CH₃
    2. 2-Methylpentane: CH₃–CH₂–CH–CH₂–CH₃
    3. 3-Methylpentane: CH₃–CH₂–CH₂–CH–CH₃
    4. 2,3-Dimethylbutane: CH₃–CH–CH₃–CH₂–CH₃
    5. 2,2-Dimethylbutane: CH₃–C(CH₃)₂–CH₂–CH₃

Alkyl Groups:

  • Alkyl groups are derived from alkanes by removing one hydrogen atom. General formula: CₙH₂ₙ₊₁. For example, the methyl group (CH₃) is derived from methane (CH₄), and the ethyl group (C₂H₅) is derived from ethane (C₂H₆).

Example Problem 9.2:

  • Write structures and IUPAC names of alcohols derived from C₅H₁₁ alkyl groups:
    1. Pentan-1-ol: CH₃–CH₂–CH₂–CH₂–CH₂–OH
    2. Pentan-2-ol: CH₃–CH–CH₂–CH₂–CH₃ (OH at C2)
    3. Pentan-3-ol: CH₃–CH₂–CH–CH₂–CH₃ (OH at C3)
    4. 3-Methylbutan-1-ol: CH₃–CH–CH₂–CH₂–OH (methyl at C3)
    5. 2-Methylbutan-1-ol: CH₃–CH–CH₂–CH₂–OH (methyl at C2)
    6. 2,2-Dimethylpropan-1-ol: CH₃–C(CH₃)₂–CH₂–OH (two methyl groups at C2)

General Nomenclature Rules:

  1. Identify the longest continuous carbon chain.
  2. Number the chain starting from the end nearest to the substituent.
  3. Name substituents (such as alkyl groups) and list them alphabetically.
  4. For compounds with multiple substituents, use prefixes like di-, tri-, etc., to indicate the number of identical substituents.

Example Problem 9.3:

  • Write IUPAC names of the following compounds:
    1. (CH₃)₃C CH₂C(CH₃)₃: 2,2,4,4-Tetramethylpentane
    2. (CH₃)₂C(C₂H₅)₂: 3,3-Dimethylpentane
    3. Tetra-tert-butylmethane: 3,3-Di-tert-butyl-2,2,4,4-tetramethylpentane

Key Takeaways:

  • Isomerism in alkanes increases with the number of carbon atoms, and this includes chain isomers and positional isomers.
  • Alkyl groups are formed by removing one hydrogen atom from an alkane.
  • Correct nomenclature follows the steps of identifying the longest chain, numbering it, and placing the substituents appropriately.

Preparation of Alkanes

Alkanes, which are primarily found in petroleum and natural gas, can be synthesized through several methods:

  1. From Unsaturated Hydrocarbons (Hydrogenation)
    • Dihydrogen (H₂) is added to alkenes or alkynes in the presence of finely divided catalysts such as platinum (Pt), palladium (Pd), or nickel (Ni).
    • This process is called hydrogenation and involves the activation of the hydrogen-hydrogen bond through the catalyst surface.
      • Example: Ethene (C₂H₄) + H₂ → Ethane (C₂H₆)
  2. From Alkyl Halides
    • Reduction with Zinc and Dilute Hydrochloric Acid: Alkyl halides (except fluorides) are reduced with zinc and dilute hydrochloric acid to yield alkanes.
      • Example: CH₃Cl (chloromethane) + H₂ → CH₄ (methane)
    • Wurtz Reaction: Alkyl halides react with sodium metal in dry ether to form higher alkanes with even numbers of carbon atoms.
      • Example: 2CH₃Br + 2Na → C₄H₁₀ (butane)
  3. From Carboxylic Acids
    • Decarboxylation: Sodium salts of carboxylic acids, when heated with soda lime (NaOH + CaO), lose a CO₂ molecule and form alkanes.
      • Example: CH₃COONa + NaOH → CH₄ (methane)
    • Kolbe’s Electrolytic Method: Electrolysis of sodium or potassium salts of carboxylic acids leads to the formation of alkanes with an even number of carbon atoms.

Properties of Alkanes

Physical Properties

  • Non-polarity: Alkanes are non-polar molecules due to the small difference in electronegativity between carbon and hydrogen.
  • Boiling Points: Boiling points of alkanes increase with molecular mass, as the van der Waals forces become stronger with increasing molecular size.
    • For example, pentane (C₅H₁₂) has a higher boiling point than its branched isomer, 2,2-dimethylpropane.
  • Solubility: Alkanes are generally insoluble in water because they are non-polar, but they are soluble in non-polar solvents like petrol.

Chemical Properties

  • Substitution Reactions: Alkanes undergo substitution reactions, replacing hydrogen atoms with halogens or other groups under heat or UV light.
    • Example: Chlorination of methane (CH₄ + Cl₂ → CH₃Cl + HCl)
    • The reaction proceeds via a free-radical chain mechanism involving initiation, propagation, and termination steps.
  • Combustion: Alkanes burn in the presence of oxygen to produce carbon dioxide and water, releasing a large amount of energy.
    • Example: CH₄ + 2O₂ → CO₂ + 2H₂O
    • Incomplete combustion results in the formation of carbon (soot) and water.
  • Controlled Oxidation: Under controlled conditions, alkanes can be oxidized to produce alcohols, aldehydes, or acids.
    • Example: CH₄ + O₂ → CH₃OH (methanol) with copper at high pressure.
  • Isomerisation: n-Alkanes can be converted into branched-chain alkanes when heated with anhydrous aluminum chloride (AlCl₃) and hydrogen chloride (HCl).
    • Example: n-Hexane → 2-Methylpentane
  • Aromatization: Long-chain alkanes (containing 6 or more carbon atoms) can undergo dehydrogenation and cyclization to form aromatic compounds like benzene when heated at high temperatures with catalysts like vanadium oxide (V₂O₅).
    • Example: Decane → Toluene (C₆H₆ with a methyl group)
  • Reaction with Steam: Methane reacts with steam at high temperatures to form carbon monoxide and hydrogen.
    • Example: CH₄ + H₂O → CO + 3H₂
  • Pyrolysis (Cracking): Higher alkanes decompose into smaller alkanes and alkenes when heated at high temperatures, typically in the presence of catalysts.
    • Example: C₁₂H₂₆ → C₇H₁₆ + C₅H₁₀ (from dodecane to heptane and pentene)

Alkanes are primarily used as fuels due to their high energy content upon combustion. They are also significant in industrial processes like cracking and hydrogenation to produce valuable chemical compounds.

Conformations of Alkanes and Ethane

  1. Conformations in Alkanes:
    • Alkanes contain carbon-carbon (C–C) sigma (σ) bonds. The electron distribution around these sigma bonds is symmetrical, meaning the electron density remains evenly spread along the internuclear axis of the C–C bond.
    • This symmetry allows for free rotation about the C–C single bond, which results in different spatial arrangements of atoms, known as conformations or conformers.
    • These conformations can convert into one another through rotation around the C–C single bond, allowing for an infinite number of spatial arrangements.
    • Rotation is not completely free due to an energy barrier (1-20 kJ/mol) caused by torsional strain, which arises from weak repulsive interactions between adjacent bonds.
  2. Conformations of Ethane (C₂H₆):
    • In ethane, a C–C single bond links two carbon atoms, each bonded to three hydrogen atoms.
    • By keeping one carbon stationary and rotating the other carbon around the C–C axis, various spatial arrangements of the hydrogen atoms occur, leading to different conformational isomers (conformers).
    • The two extreme conformations are:
      • Eclipsed Conformation: The hydrogen atoms attached to the two carbons are as close together as possible.
      • Staggered Conformation: The hydrogen atoms are as far apart as possible.
    • Any intermediate conformation between these extremes is called a skew conformation.
    • Bond Angles and Bond Lengths: In all conformations, the bond angles and bond lengths remain the same.
  3. Representation of Conformations:
    • Sawhorse Projections: In this projection, the molecule is viewed along the C–C bond axis. The C–C bond is drawn as a straight line, with the front carbon at the lower end and the rear carbon at the upper end. Each carbon has three bonds representing hydrogen atoms at 120° angles.
    • Newman Projections: The molecule is viewed directly along the C–C bond axis. The front carbon is shown as a point, and its three hydrogen atoms are arranged at 120° angles. The rear carbon is represented by a circle, with the hydrogen atoms also arranged at 120° angles.
  4. Relative Stability of Conformations:
    • The staggered conformation of ethane is more stable than the eclipsed conformation because the hydrogen atoms are as far apart as possible in the staggered form, minimizing electron cloud repulsions.
    • The eclipsed conformation results in higher repulsion between the electron clouds of the hydrogen atoms, leading to increased energy and reduced stability.
    • This repulsive interaction is called torsional strain, and the difference in energy between the two conformations is about 12.5 kJ/mol.
    • At room temperature, ethane molecules can overcome this energy barrier, making the rotation around the C–C bond almost free, and different conformations interconvert rapidly.

Alkenes: Structure and Properties

  1. Structure of Alkenes:
    • Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond.
    • The general formula for alkenes is CnH₂n, indicating that they have two fewer hydrogen atoms than alkanes.
    • The C=C double bond consists of one sigma (σ) bond formed by the head-on overlap of sp² hybridized orbitals and one pi (π) bond formed by the sideways overlap of two 2p orbitals.
    • The C=C double bond is shorter (134 pm) than a C–C single bond (154 pm).
    • The pi (π) bond is weaker than the sigma (σ) bond and is more reactive, making alkenes prone to attacks by electrophilic reagents.
  2. Stability of Alkenes:
    • Due to the presence of the pi (π) bond, alkenes are more reactive than alkanes.
    • The pi (π) bond is weaker than the sigma (σ) bond, which makes alkenes susceptible to electrophilic attack.
    • Alkenes are often used in reactions where the pi (π) bond is broken, and a new bond is formed with the attacking reagent.
  3. Bond Strengths:
    • The bond enthalpy of the C=C double bond is about 681 kJ/mol, which is higher than that of the C–C single bond in ethane (348 kJ/mol), but the presence of the pi bond makes alkenes more reactive.
  4. Nomenclature of Alkenes:
    • For IUPAC naming, the longest chain containing the double bond is selected, and numbering is done from the end nearest the double bond.
    • The suffix ‘ene’ replaces ‘ane’ from the alkane name.
    • Some examples of IUPAC names for alkenes:
      • Propene (C₃H₆)
      • But-1-ene (C₄H₈)
      • But-2-ene (C₄H₈)
      • Buta-1,3-diene (C₄H₆)
      • 2-Methylprop-1-ene (C₄H₈)
      • 3-Methylbut-1-ene (C₅H₁₀)

This explanation covers the key points regarding the conformations of alkanes, specifically ethane, and the structure, reactivity, and nomenclature of alkenes.

Isomerism in Alkenes

1. Structural Isomerism
Alkenes exhibit structural isomerism, which is also seen in alkanes. Alkenes such as ethene (C2H4) and propene (C3H6) have only one structure, while alkenes with more than three carbon atoms (such as C4H8) can exist as different isomers. For example, C4H8 can be represented as:

  • But-1-ene (C4H8): CH₂=CH-CH₂-CH₃
  • But-2-ene (C4H8): CH₃-CH=CH-CH₃
  • 2-Methylprop-1-ene (C4H8): CH₂=CH-C(CH₃)₂

The isomers above show two types of structural isomerism:

  • Chain isomerism (e.g., But-1-ene and 2-Methylprop-1-ene)
  • Position isomerism (e.g., But-1-ene and But-2-ene)

2. Geometrical Isomerism
Geometrical isomerism occurs due to the restricted rotation around the C=C double bond, resulting in distinct spatial arrangements of substituent groups around the double bond. This type of isomerism occurs when each carbon of the double bond is bonded to two different groups.

  • Cis isomer: Groups are on the same side of the double bond.
  • Trans isomer: Groups are on opposite sides of the double bond.

Geometrical isomerism leads to different physical properties such as melting point, boiling point, dipole moment, and solubility. For example, cis-but-2-ene has a dipole moment of 0.33 Debye, whereas trans-but-2-ene is almost non-polar due to the cancellation of dipoles.

3. Preparation of Alkenes

  • From Alkynes (Partial Hydrogenation):
    Alkyne molecules can undergo partial hydrogenation in the presence of a catalyst (such as Lindlar’s catalyst) to form cis-alkenes. For example, ethyne (C₂H₂) when hydrogenated in the presence of Lindlar’s catalyst forms ethene (C₂H₄).
  • From Alkyl Halides (Dehydrohalogenation):
    Alkyl halides undergo dehydrohalogenation (removal of halogen and hydrogen) when heated with alcoholic potassium hydroxide (KOH), leading to the formation of alkenes.
  • From Vicinal Dihalides (Dehalogenation):
    Vicinal dihalides (where two halogens are attached to adjacent carbons) can be converted to alkenes by treatment with zinc metal, resulting in the elimination of the halogen atoms and the formation of a double bond.
  • From Alcohols (Acidic Dehydration):
    Alcohols can undergo dehydration in the presence of concentrated sulfuric acid to form alkenes, with the elimination of a water molecule.

4. Physical Properties of Alkenes
Alkenes generally resemble alkanes in terms of physical properties such as being colorless and odorless. They are insoluble in water but soluble in non-polar solvents. The boiling point increases with the number of carbon atoms in the molecule. Straight-chain alkenes have higher boiling points than their branched isomers.

5. Chemical Properties of Alkenes

  • Addition Reactions:
    Alkenes are highly reactive due to the presence of the π bond, making them prone to addition reactions. In these reactions, electrophiles add to the carbon-carbon double bond, breaking the π bond and forming new single bonds.
  • Addition of Hydrogen (Hydrogenation):
    Alkenes can add hydrogen (H₂) in the presence of catalysts (nickel, palladium, platinum) to form alkanes.
  • Addition of Halogens:
    Halogens such as bromine (Br₂) or chlorine (Cl₂) add to the carbon-carbon double bond to form vicinal dihalides. Iodine does not typically add to alkenes under normal conditions.
  • Addition of Hydrogen Halides (HCl, HBr, HI):
    Alkenes react with hydrogen halides to form alkyl halides. The addition follows the Markovnikov rule, which states that the halide (negative part) adds to the carbon atom with fewer hydrogen atoms.
  • Anti-Markovnikov Addition (Peroxide Effect):
    In the presence of peroxides, the addition of HBr to unsymmetrical alkenes follows the opposite of Markovnikov’s rule. This reaction occurs via a free radical mechanism and results in the formation of the anti-Markovnikov product.
  • Addition of Sulfuric Acid:
    Alkenes add sulfuric acid (H₂SO₄) to form alkyl hydrogen sulfates, following the Markovnikov rule.
  • Addition of Water (Hydration):
    Alkenes undergo hydration in the presence of sulfuric acid, following the Markovnikov rule, to form alcohols.
  • Oxidation:
    Alkenes can be oxidized with potassium permanganate (KMnO₄) or potassium dichromate (K₂Cr₂O₇), leading to the formation of glycols or ketones, depending on the conditions. This is used to test for unsaturation.
  • Ozonolysis:
    Alkenes react with ozone (O₃) to form ozonides, which can be cleaved to yield smaller molecules. This is a useful reaction for determining the position of the double bond in alkenes.
  • Polymerization:
    Alkenes like ethene can undergo polymerization under high pressure and temperature with a catalyst to form long-chain polymers like polythene. Other alkenes, such as propene, can also undergo polymerization to form materials like polypropene.

This summary covers the key concepts of isomerism, preparation methods, and chemical properties of alkenes, with a focus on reactions and physical properties.

Alkynes

Alkynes are unsaturated hydrocarbons containing at least one triple bond between two carbon atoms. Their general formula is CnH2n–2, where n is the number of carbon atoms. Alkynes have fewer hydrogen atoms compared to alkenes or alkanes. The simplest and first stable member of the alkyne series is ethyne (acetylene), commonly used in oxyacetylene welding.

Nomenclature and Isomerism

  1. Common Nomenclature: Alkynes are named as derivatives of acetylene.
  2. IUPAC Nomenclature: Alkynes are named as derivatives of corresponding alkanes by replacing the suffix “-ane” with “-yne”. The position of the triple bond is indicated by the first carbon involved in the triple bond.Examples:
    • Ethyne (C2H2): Acetylene
    • Propyne (C3H4): Methylacetylene
    • Butyne (C4H6): But-1-yne and But-2-yne (Position isomers)
    For C5H8 (next alkyne homolog):
    • Pent-1-yne: HC≡C–CH2–CH2–CH3
    • Pent-2-yne: CH3–C≡C–CH2–CH3
    • 3-Methylbut-1-yne: CH3–C≡C–CH2–CH3 (Chain isomerism)
    For C6H10:
    • Hex-1-yne: HC≡C–CH2–CH2–CH2–CH3
    • Hex-2-yne: CH3–C≡C–CH2–CH2–CH3
    • Hex-3-yne: CH3–CH2–C≡C–CH2–CH3
    • Isomerism: Position and chain isomerism.

Structure of Triple Bond

  • Ethyne (C2H2) is the simplest alkyne. In ethyne, each carbon atom undergoes sp hybridization. The sigma (σ) bond between the two carbons is formed by the overlap of two sp hybridized orbitals, and the remaining sp orbitals of each carbon form sigma bonds with hydrogen atoms. The two unhybridized p orbitals of each carbon overlap laterally to form two pi (π) bonds between the carbons.
  • The C≡C bond is stronger than the C=C bond, with a bond enthalpy of 823 kJ/mol (shorter bond length of 120 pm). Ethyne is a linear molecule with an angle of 180° between the C-H bonds.

Preparation of Alkynes

  1. From Calcium Carbide:
    • Calcium carbide reacts with water to produce ethyne: CaC2+2H2O→Ca(OH)2+C2H2CaC_2 + 2H_2O \rightarrow Ca(OH)_2 + C_2H_2CaC2​+2H2​O→Ca(OH)2​+C2​H2​
    • Calcium carbide is made by heating quick lime with coke: CaO+3C→CaC2+COCaO + 3C \rightarrow CaC_2 + COCaO+3C→CaC2​+CO
  2. From Vicinal Dihalides:
    • Vicinal dihalides undergo dehydrohalogenation with alcoholic potassium hydroxide to form alkenyl halides, which, upon treatment with sodamide, give alkynes.

Properties of Alkynes

  1. Physical Properties:
    • Alkynes are colorless, with the first three members being gases, the next eight being liquids, and higher members being solids. They are immiscible in water but soluble in organic solvents like ethers and benzene. Their physical properties trend similarly to alkenes and alkanes.
  2. Chemical Properties:
    • Acidity: Ethyne (acetylene) is more acidic than ethene and ethane. The hydrogen atoms attached to the sp hybridized carbon in ethyne are more acidic due to the higher electronegativity of the sp hybridized carbon atoms. HC≡CH+Na→HC≡C–Na++12H2HC≡CH + Na \rightarrow HC≡C–Na^+ + \frac{1}{2} H_2HC≡CH+Na→HC≡C–Na++21​H2​
    • Addition Reactions: Alkynes readily undergo addition reactions:
      • Addition of Dihydrogen: Using a catalyst like Pt/Pd/Ni, ethyne adds hydrogen to form ethene, and propyne adds hydrogen to form propene.
      • Addition of Halogens: Alkynes react with halogens to form dihalides.
      • Addition of Hydrogen Halides: Alkynes react with hydrogen halides (HCl, HBr, HI) to form gem dihalides.
      • Addition of Water: Ethyne reacts with water in the presence of HgSO₄ and dilute H₂SO₄ to form carbonyl compounds.
  3. Polymerisation:
    • Linear Polymerisation: Under suitable conditions, ethyne polymerizes to form polyacetylene, a polymer with alternating single and double bonds.
    • Cyclic Polymerisation: Ethyne undergoes cyclic polymerisation at high temperatures to form benzene.

Aromatic Hydrocarbons (Arenes)

Aromatic hydrocarbons (arenes) are compounds that typically contain a benzene ring. Benzene is highly unsaturated, but in most reactions, its unsaturation is retained. These compounds are divided into two groups:

  1. Benzenoids: Compounds that contain a benzene ring.
  2. Non-benzenoids: Compounds that do not contain a benzene ring but still have aromatic character.

Examples of Aromatic Hydrocarbons:

  • Benzene (C₆H₆)
  • Toluene (C₆H₅CH₃)
  • Naphthalene (C₁₀H₈)
  • Biphenyl (C₆H₅-C₆H₅)

Nomenclature and Isomerism in Aromatic Hydrocarbons

  1. Monosubstituted: The hydrogen atoms in benzene are equivalent, leading to only one type of monosubstituted product.
  2. Disubstituted: When two hydrogen atoms are replaced by similar or different monovalent atoms or groups, three positional isomers are possible:
    • Ortho (o-): 1,2-positions
    • Meta (m-): 1,3-positions
    • Para (p-): 1,4-positions

Examples of Disubstituted Benzene:

  • Toluene: Methylbenzene
  • o-Xylene: 1,2-Dimethylbenzene
  • m-Xylene: 1,3-Dimethylbenzene
  • p-Xylene: 1,4-Dimethylbenzene

Structure of Benzene

Discovery
Benzene was first isolated by Michael Faraday in 1825. Its molecular formula, C₆H₆, indicates high unsaturation, but it doesn’t fit the pattern of alkanes, alkenes, or alkynes. The structure of benzene was unclear for many years due to its unique properties and unusual stability.

Kekulé’s Structure (1865)
Benzene was found to be unusually stable and formed a triozonide, indicating the presence of three double bonds. It was also found to produce only one monosubstituted derivative, suggesting that all six carbon and hydrogen atoms in benzene are equivalent. Based on this, August Kekulé proposed the cyclic structure with six carbon atoms, alternating single and double bonds, and a hydrogen atom attached to each carbon.

Oscillation of Bonds
Kekulé’s structure suggested two isomeric forms for 1,2-dibromobenzene, with the bromine atoms either on doubly bonded or singly bonded carbon atoms. However, this led to issues regarding the stability and behavior of benzene. Kekulé later modified his proposal by suggesting the oscillating nature of the double bonds.

Resonance and Stability
Valence Bond Theory later explained benzene’s structure through resonance, where benzene exists as a hybrid of various resonating structures. The two Kekulé structures, A and B, contribute equally to the hybrid, which is depicted by a circle inside the hexagon, representing delocalized electrons.

The carbon atoms in benzene are sp² hybridized, with each carbon forming sigma bonds with adjacent carbon atoms and a hydrogen atom. The remaining unhybridized p orbitals form π bonds by lateral overlap. The delocalized π electrons are shared across all six carbon atoms, enhancing stability and making benzene resistant to addition reactions.

Bond Lengths and Planarity
X-ray diffraction data confirms that all carbon-carbon bond lengths in benzene are identical and intermediate between single and double bonds (139 pm). This uniformity in bond length, along with the delocalized electron cloud, explains benzene’s reluctance to undergo addition reactions.

Aromaticity
Benzene is considered the prototype aromatic compound, which is characterized by:

  1. Planarity: The molecule must be planar.
  2. Delocalized π electrons: The π electrons in the ring must be completely delocalized.
  3. Hückel’s Rule: The molecule must contain 4n + 2 π electrons, where n is an integer (0, 1, 2,…).

This rule applies not only to benzene but also to other aromatic compounds.

Preparation of Benzene

Benzene can be prepared by:

  1. Cyclic polymerization of ethyne.
  2. Decarboxylation of aromatic acids: Heating sodium salt of benzoic acid with soda lime produces benzene.
  3. Reduction of phenol: Passing phenol vapors over heated zinc dust reduces it to benzene.

Properties of Benzene

Physical Properties
Aromatic hydrocarbons like benzene are non-polar, usually colorless liquids or solids with a characteristic aroma. They are immiscible with water but soluble in organic solvents. Benzene burns with a sooty flame.

Chemical Properties
Benzene undergoes electrophilic substitution reactions, such as:

  • Nitration: Benzene reacts with a mixture of nitric acid and sulfuric acid to form nitrobenzene.
  • Halogenation: Benzene reacts with halogens in the presence of a Lewis acid (FeCl₃, AlCl₃) to form haloarenes.
  • Sulphonation: Benzene reacts with fuming sulfuric acid (oleum) to form benzenesulfonic acid.
  • Friedel-Crafts Alkylation: Benzene reacts with alkyl halides in the presence of AlCl₃ to form alkylbenzenes.
  • Friedel-Crafts Acylation: Benzene reacts with acyl halides or acid anhydrides in the presence of AlCl₃ to form acylbenzenes.

Mechanism of Electrophilic Substitution
Electrophilic substitution reactions occur in three steps:

  1. Generation of the Electrophile: The electrophile (e.g., nitronium ion in nitration) is generated by combining the reagent with a Lewis acid.
  2. Formation of Arenium Ion: The electrophile attacks the benzene ring, forming a sigma complex (arenium ion), where one carbon atom temporarily becomes sp³ hybridized.
  3. Loss of Proton: The arenium ion loses a proton, restoring the aromatic character of the ring.

Addition Reactions
Under high temperature and pressure or in the presence of catalysts like nickel, benzene can undergo hydrogenation to form cyclohexane. In the presence of UV light, benzene can react with chlorine to form benzene hexachloride (BHC).

Directive Influence of Substituents in Benzene

When benzene is monosubstituted, further substitution occurs predominantly at either the ortho and para positions, or the meta position. The directing effect depends on the nature of the existing substituent.

  • Ortho/Para-directing Groups: These groups, such as hydroxyl (–OH), amine (–NH₂), and methyl (–CH₃), increase electron density at the ortho and para positions through resonance, making the ring more reactive at these positions.
  • Meta-directing Groups: These groups, such as nitro (–NO₂), carbonyl (–COOH), and cyano (–CN), decrease electron density at the ortho and para positions and direct substitution to the meta position.

Carcinogenicity and Toxicity

Benzene and polycyclic aromatic hydrocarbons (PAHs) are toxic and carcinogenic. PAHs, formed during the incomplete combustion of organic materials (like tobacco, coal, and petroleum), can enter the body, damage DNA, and lead to cancer.

This summarizes the key concepts related to the structure, properties, and reactions of benzene, providing a solid foundation for understanding its chemistry.

These are all the notes of this chapter. And after some time you get important questions, NCERT solutions HERE. *#THANKS FOR VISITING, VISIT AGAIN#*