Chapter Notes

HYDROCARBONS

Hydrocarbons are organic compounds made up of only carbon and hydrogen atoms. They are essential to our daily lives, primarily as sources of energy. Fuels like LPG (liquified petroleum gas), CNG (compressed natural gas), petrol, diesel, and kerosene are all mixtures of hydrocarbons. Beyond fuel, they are the starting materials for manufacturing polymers (like polythene), paints, dyes, and drugs.

CLASSIFICATION

Hydrocarbons are broadly classified based on the types of carbon-carbon bonds they contain.

• Saturated Hydrocarbons: These compounds contain only carbon-carbon single bonds.
  • Alkanes: Saturated hydrocarbons with an open chain of carbon atoms.
  • Cycloalkanes: Saturated hydrocarbons where carbon atoms form a closed ring.
• Unsaturated Hydrocarbons: These compounds contain at least one carbon-carbon double bond or triple bond.
  • Alkenes: Contain at least one carbon-carbon double bond.
  • Alkynes: Contain at least one carbon-carbon triple bond.
• Aromatic Hydrocarbons: A special class of cyclic compounds, with benzene being the most common example. They have unique properties due to their electronic structure.

ALKANES

Alkanes are saturated, open-chain hydrocarbons with the general formula $C_nH_{2n+2}$. They contain only carbon-carbon single bonds. The simplest alkane is methane ($CH_4$).

Because alkanes are relatively unreactive towards acids, bases, and other common reagents, they were historically called paraffins (from Latin parum affinis, meaning "little affinity").

Structure of Alkanes

In alkanes, each carbon atom is $sp^3$ hybridized, resulting in a tetrahedral geometry with bond angles of approximately $109.5^\circ$.

• The C-C bond length is about $154$ pm.
• The C-H bond length is about $112$ pm.
• All bonds are sigma ($\sigma$) bonds, formed by the head-on overlap of hybrid orbitals.

Nomenclature and Isomerism

The first three alkanes—methane ($CH_4$), ethane ($C_2H_6$), and propane ($C_3H_8$)—have only one possible structure. However, higher alkanes can exist as isomers, which are compounds with the same molecular formula but different structural arrangements.

1. n-butane: A straight, continuous chain. $CH_3-CH_2-CH_2-CH_3$
2. 2-Methylpropane (isobutane): A branched chain. $CH_3-CH(CH_3)-CH_3$

These are called chain isomers because they differ in the arrangement of their carbon chains. As the number of carbon atoms increases, the number of possible isomers grows rapidly. For example, $C_6H_{14}$ has five isomers, and $C_{10}H_{22}$ has 75 possible isomers.

Classification of Carbon Atoms Carbon atoms in an alkane are classified based on how many other carbon atoms they are attached to:

• Primary ($1^\circ$): Attached to one other carbon atom (or none, in methane). Terminal carbons are always primary.
• Secondary ($2^\circ$): Attached to two other carbon atoms.
• Tertiary ($3^\circ$): Attached to three other carbon atoms.
• Quaternary ($4^\circ$): Attached to four other carbon atoms.

Preparation of Alkanes

1. From Unsaturated Hydrocarbons (Hydrogenation) Alkenes and alkynes react with dihydrogen gas ($H_2$) in the presence of a catalyst (like platinum, palladium, or nickel) to form alkanes. This process is called hydrogenation. $CH_2=CH_2 + H_2 \xrightarrow{Pt/Pd/Ni} CH_3-CH_3 \quad (\text{Ethene} \rightarrow \text{Ethane})$ $CH_3-C \equiv CH + 2H_2 \xrightarrow{Pt/Pd/Ni} CH_3-CH_2-CH_3 \quad (\text{Propyne} \rightarrow \text{Propane})$

2. From Alkyl Halides

   • Reduction: Alkyl halides (except fluorides) are reduced to alkanes using zinc and dilute hydrochloric acid. $CH_3-Cl + H_2 \xrightarrow{Zn, H^+} CH_4 + HCl \quad (\text{Chloromethane} \rightarrow \text{Methane})$
   • Wurtz Reaction: Alkyl halides react with sodium metal in dry ether to form higher alkanes with an even number of carbon atoms. This reaction is useful for creating larger molecules by joining two alkyl groups. $CH_3Br + 2Na + BrCH_3 \xrightarrow{\text{dry ether}} CH_3-CH_3 + 2NaBr \quad (\text{Bromomethane} \rightarrow \text{Ethane})$ [!note] The Wurtz reaction is not suitable for preparing alkanes with an odd number of carbon atoms because using two different alkyl halides results in a mixture of products that is difficult to separate.

3. From Carboxylic Acids

   • Decarboxylation: Sodium salts of carboxylic acids, when heated with soda lime (a mixture of NaOH and CaO), lose a molecule of carbon dioxide to form an alkane with one less carbon atom. $CH_3COO^-Na^+ + NaOH \xrightarrow{CaO, \Delta} CH_4 + Na_2CO_3 \quad (\text{Sodium ethanoate} \rightarrow \text{Methane})$
   • Kolbe's Electrolytic Method: The electrolysis of an aqueous solution of a sodium or potassium salt of a carboxylic acid produces an alkane with an even number of carbon atoms at the anode. $2CH_3COO^-Na^+ + 2H_2O \xrightarrow{\text{Electrolysis}} CH_3-CH_3 + 2CO_2 + H_2 + 2NaOH$ [!note] Methane cannot be prepared by this method because the reaction involves the coupling of two alkyl groups. A single methyl radical cannot form methane this way.

Properties of Alkanes

Physical Properties

• Polarity: Alkanes are non-polar molecules due to the small electronegativity difference between carbon and hydrogen.
• Forces: They are held together by weak van der Waals forces.
• State: At room temperature (298 K), $C_1$ to $C_4$ are gases, $C_5$ to $C_{17}$ are liquids, and those with 18 or more carbons are solids.
• Solubility: They are insoluble in water but soluble in non-polar solvents like benzene ("like dissolves like").
• Boiling Point: The boiling point increases with molecular mass because van der Waals forces become stronger with a larger surface area. Branched-chain isomers have lower boiling points than their straight-chain counterparts because branching makes the molecule more spherical, reducing the surface area of contact.

Chemical Properties

Alkanes are generally inert but undergo a few specific reactions under certain conditions.

1. Substitution Reactions One or more hydrogen atoms in an alkane can be replaced by other atoms or groups, such as halogens.

   • Halogenation: This reaction occurs at high temperatures ($573-773$ K) or in the presence of UV light. For example, the chlorination of methane produces a mixture of products. $CH_4 + Cl_2 \xrightarrow{h\nu} CH_3Cl + HCl$ $CH_3Cl + Cl_2 \xrightarrow{h\nu} CH_2Cl_2 + HCl$ The reaction proceeds via a free radical chain mechanism involving three steps:
     1. Initiation: The Cl-Cl bond breaks homolytically to form chlorine free radicals ($Cl^\cdot$).
     2. Propagation: A chlorine radical attacks a methane molecule, creating a methyl radical ($CH_3^\cdot$), which then attacks another chlorine molecule. This cycle continues.
     3. Termination: The reaction stops when free radicals combine with each other (e.g., $Cl^\cdot + Cl^\cdot \rightarrow Cl_2$ or $CH_3^\cdot + CH_3^\cdot \rightarrow C_2H_6$). The formation of ethane is a result of this termination step.

2. Combustion Alkanes burn in the presence of sufficient oxygen to produce carbon dioxide, water, and a large amount of heat, making them excellent fuels. $CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(l) \quad \Delta_cH^\ominus = -890 \text{ kJ mol}^{-1}$ Incomplete combustion (with insufficient oxygen) produces carbon black (soot) and water. $CH_4(g) + O_2(g) \xrightarrow{\text{incomplete combustion}} C(s) + 2H_2O(l)$

3. Controlled Oxidation With a regulated supply of oxygen and specific catalysts, alkanes can be oxidized to form useful products like alcohols, aldehydes, or carboxylic acids.

   • $2CH_4 + O_2 \xrightarrow{Cu/523K/100atm} 2CH_3OH$ (Methanol)
   • $CH_4 + O_2 \xrightarrow{Mo_2O_3/\Delta} HCHO + H_2O$ (Methanal)

4. Isomerisation n-Alkanes, when heated with anhydrous aluminum chloride and HCl gas, rearrange to form their branched-chain isomers. $CH_3CH_2CH_2CH_3 \xrightarrow{Anhyd. AlCl_3/HCl} CH_3CH(CH_3)CH_3$

5. Aromatization n-Alkanes with six or more carbon atoms, when heated to high temperatures (773 K) and pressures over a catalyst (e.g., oxides of Cr, Mo), cyclize and dehydrogenate to form aromatic compounds like benzene. $n\text{-Hexane} \xrightarrow{Cr_2O_3/\Delta} \text{Benzene} + 4H_2$

6. Reaction with Steam Methane reacts with steam at 1273 K with a nickel catalyst to produce carbon monoxide and dihydrogen gas. This is an industrial method for preparing $H_2$. $CH_4 + H_2O \xrightarrow{Ni/\Delta} CO + 3H_2$

7. Pyrolysis (Cracking) At high temperatures, higher alkanes break down into smaller, more volatile alkanes and alkenes. $C_6H_{14} \xrightarrow{773K} C_6H_{12} + H_2 \quad \text{or} \quad C_4H_8 + C_2H_6 \quad \text{etc.}$

Conformations

Conformations are the different spatial arrangements of atoms in a molecule that can be converted into one another by rotation around a C-C single bond. These different arrangements are also called conformers or rotamers.

Rotation around a C-C single bond is almost free but is slightly hindered by a small energy barrier due to torsional strain, a weak repulsive interaction between adjacent bonds.

Conformations of Ethane ($C_2H_6$)

• Eclipsed Conformation: The hydrogen atoms on the two carbons are as close as possible. This conformation has maximum repulsive forces and is the least stable (highest energy).
• Staggered Conformation: The hydrogen atoms are as far apart as possible. This conformation has minimum repulsive forces and is the most stable (lowest energy).
• Skew Conformation: Any intermediate conformation between eclipsed and staggered.

Representing Conformations

1. Sawhorse Projections: The C-C bond is drawn as a diagonal line, showing the spatial relationship of all atoms.
2. Newman Projections: The molecule is viewed along the C-C axis. The front carbon is a dot, and the back carbon is a circle.

The energy difference between the staggered and eclipsed forms of ethane is very small (about $12.5 \text{ kJ mol}^{-1}$), so at room temperature, the molecules can easily rotate between conformations. For this reason, conformers cannot be isolated.

ALKENES

Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond. They are also known as olefins. Their general formula is $C_nH_{2n}$. The simplest stable alkene is ethene ($C_2H_4$).

Structure of the Double Bond

A carbon-carbon double bond consists of:

• One strong sigma ($\sigma$) bond formed by the head-on overlap of $sp^2$ hybrid orbitals.
• One weak pi ($\pi$) bond formed by the sideways overlap of unhybridized $2p$ orbitals.

The $\pi$ bond is weaker and its electrons are more exposed, making alkenes more reactive than alkanes. They readily undergo addition reactions, where reagents add across the double bond. The C=C bond length (134 pm) is shorter than the C-C single bond (154 pm).

Nomenclature

In the IUPAC system, the suffix -ene replaces the -ane of the corresponding alkane. The carbon chain is numbered to give the double bond the lowest possible number.

Isomerism in Alkenes

Alkenes exhibit both structural and geometrical isomerism.

1. Structural Isomerism:

   • Chain Isomerism: Different carbon skeletons (e.g., 2-Methylprop-1-ene and But-1-ene).
   • Position Isomerism: The double bond is in a different position (e.g., But-1-ene and But-2-ene).

2. Geometrical Isomerism (cis-trans isomerism) This occurs because there is restricted rotation around the C=C double bond. For this isomerism to exist, each carbon atom of the double bond must be attached to two different atoms or groups.

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

   Geometrical Isomers of But-2-ene
   • cis-But-2-ene: The two methyl groups are on the same side. This molecule is polar ($\mu = 0.33$ D).
   • trans-But-2-ene: The two methyl groups are on opposite sides. This molecule is non-polar ($\mu = 0$) because the bond dipoles cancel each other out.

   Generally, trans isomers are more stable and have higher melting points than cis isomers due to better packing in the crystal lattice. Cis isomers often have slightly higher boiling points due to being more polar.

Preparation of Alkenes

1. From Alkynes (Partial Reduction)

   • To get a cis-alkene, an alkyne is hydrogenated using Lindlar's catalyst (palladised charcoal partially deactivated with sulfur or quinoline). $CH_3-C \equiv C-CH_3 + H_2 \xrightarrow{\text{Lindlar's catalyst}} \text{cis-But-2-ene}$
   • To get a trans-alkene, an alkyne is reduced using sodium in liquid ammonia. $CH_3-C \equiv C-CH_3 + 2Na + 2NH_3 \rightarrow \text{trans-But-2-ene}$

2. From Alkyl Halides (Dehydrohalogenation) Alkyl halides, when heated with alcoholic potassium hydroxide (KOH), eliminate a molecule of hydrogen halide (HX) to form an alkene. This is a $\beta$-elimination reaction. $CH_3-CH_2-Br + \text{alc. KOH} \xrightarrow{\Delta} CH_2=CH_2 + KBr + H_2O$

3. From Vicinal Dihalides (Dehalogenation) Vicinal dihalides (where halogens are on adjacent carbons) react with zinc metal to form an alkene. $CH_2Br-CH_2Br + Zn \xrightarrow{\Delta} CH_2=CH_2 + ZnBr_2$

4. From Alcohols (Acidic Dehydration) Alcohols, when heated with a strong acid like concentrated sulfuric acid, eliminate a water molecule to form an alkene. $CH_3-CH_2-OH \xrightarrow{\text{conc. } H_2SO_4, 443K} CH_2=CH_2 + H_2O$

Properties of Alkenes

Physical Properties

• The first three members are gases, the next fourteen are liquids, and higher ones are solids.
• They are nearly non-polar, insoluble in water but soluble in non-polar solvents.
• Boiling points increase with molecular mass. Straight-chain alkenes have higher boiling points than their branched isomers.

Chemical Properties

Alkenes are chemically reactive due to the presence of the weak $\pi$ bond. They primarily undergo electrophilic addition reactions.

1. Addition of Dihydrogen: Forms alkanes (hydrogenation).

2. Addition of Halogens: Forms vicinal dihalides. The reaction with bromine water (reddish-orange) is used as a test for unsaturation, as the color is discharged. $CH_2=CH_2 + Br_2 \xrightarrow{CCl_4} CH_2Br-CH_2Br$

3. Addition of Hydrogen Halides (HX)

   • Markovnikov's Rule: When adding an unsymmetrical reagent (like HBr) to an unsymmetrical alkene, the negative part of the addendum (Br⁻) attaches to the carbon atom with the fewer number of hydrogen atoms. This rule is based on the formation of the more stable carbocation intermediate (tertiary > secondary > primary). $CH_3-CH=CH_2 + HBr \rightarrow CH_3-CHBr-CH_3 \quad (\text{2-Bromopropane, major product})$
   • Anti-Markovnikov Addition (Peroxide Effect): In the presence of a peroxide (e.g., benzoyl peroxide), the addition of HBr occurs contrary to Markovnikov's rule. The Br atom attaches to the carbon with more hydrogen atoms. This reaction proceeds via a free-radical mechanism and works only for HBr. $CH_3-CH=CH_2 + HBr \xrightarrow{\text{Peroxide}} CH_3-CH_2-CH_2Br \quad (\text{1-Bromopropane, major product})$

4. Addition of Water (Hydration) In the presence of a few drops of concentrated sulfuric acid, water adds across the double bond according to Markovnikov's rule to form an alcohol. $CH_3-CH=CH_2 + H_2O \xrightarrow{H^+} CH_3-CH(OH)-CH_3$

5. Oxidation

   • With Baeyer's Reagent: Cold, dilute, aqueous KMnO₄ oxidizes alkenes to form vicinal glycols (diols). The purple color of KMnO₄ disappears, serving as another test for unsaturation. $CH_2=CH_2 + H_2O + [O] \xrightarrow{\text{dil. }KMnO_4} HO-CH_2-CH_2-OH \quad (\text{Ethane-1,2-diol})$
   • With Acidic KMnO₄: Strong oxidation with acidic KMnO₄ cleaves the double bond, forming ketones and/or carboxylic acids depending on the alkene's structure.

6. Ozonolysis Alkenes react with ozone ($O_3$) to form an ozonide, which is then cleaved by zinc and water ($Zn/H_2O$) to produce smaller carbonyl compounds (aldehydes or ketones). This reaction is very useful for determining the location of a double bond in an unknown alkene. $CH_3-CH=CH-CH_3 \xrightarrow{1. O_3} \text{Ozonide} \xrightarrow{2. Zn/H_2O} 2CH_3-CHO \quad (\text{Ethanal})$

7. Polymerisation Under high temperature and pressure with a catalyst, many small alkene molecules (monomers) link together to form a large molecule called a polymer. $n(CH_2=CH_2) \xrightarrow{\text{High T, P, Catalyst}} -(CH_2-CH_2)_n- \quad (\text{Polythene})$

ALKYNES

Alkynes are unsaturated hydrocarbons containing at least one carbon-carbon triple bond. Their general formula is $C_nH_{2n-2}$. The simplest alkyne is ethyne ($C_2H_2$), commonly known as acetylene.

Structure of the Triple Bond

A carbon-carbon triple bond consists of:

• One strong sigma ($\sigma$) bond from the overlap of $sp$ hybrid orbitals.
• Two weak pi ($\pi$) bonds from the sideways overlap of two pairs of unhybridized $p$ orbitals.

The carbon atoms in a triple bond are sp-hybridized, which gives alkynes a linear geometry with a bond angle of $180^\circ$. The C≡C bond length (120 pm) is the shortest of all carbon-carbon bonds.

Nomenclature and Isomerism

The IUPAC suffix for alkynes is -yne. The chain is numbered to give the triple bond the lowest number. Alkynes show position and chain isomerism. For example, But-1-yne and But-2-yne are position isomers.

Preparation of Alkynes

1. From Calcium Carbide On an industrial scale, ethyne is produced by reacting calcium carbide ($CaC_2$) with water. $CaC_2 + 2H_2O \rightarrow C_2H_2 + Ca(OH)_2$

2. From Vicinal Dihalides Vicinal dihalides undergo dehydrohalogenation with alcoholic KOH to form an alkenyl halide, which is then treated with a stronger base, sodamide ($NaNH_2$), to form an alkyne. $CH_2Br-CH_2Br \xrightarrow{\text{alc. KOH}} CH_2=CHBr \xrightarrow{NaNH_2} CH \equiv CH$

Properties of Alkynes

Physical Properties

Similar to alkanes and alkenes. They are weakly polar, insoluble in water, and their boiling points increase with molar mass. Ethyne has a characteristic odor.

Chemical Properties

1. Acidic Character of Alkynes The hydrogen atom attached to a triply bonded carbon is acidic. This is because the sp-hybridized carbon is highly electronegative (50% s-character) and pulls the C-H bond electrons closer, making it easier for the hydrogen to leave as a proton ($H^+$).

   • Terminal alkynes (like ethyne and propyne) react with strong bases like sodium metal or sodamide ($NaNH_2$) to form sodium acetylides and liberate hydrogen gas. $HC \equiv CH + Na \rightarrow HC \equiv C^-Na^+ + \frac{1}{2}H_2$
   • This reaction is not shown by alkanes, alkenes, or non-terminal alkynes (like But-2-yne) and can be used to distinguish terminal alkynes from other hydrocarbons.
   • Acidic strength order: $HC \equiv CH > H_2C=CH_2 > CH_3-CH_3$

2. Addition Reactions Alkynes add two molecules of reagents like $H_2$, halogens, or HX across the triple bond. The addition follows Markovnikov's rule.

   • Addition of Dihydrogen: Complete hydrogenation gives an alkane. $CH \equiv CH + 2H_2 \xrightarrow{Pt/Pd/Ni} CH_3-CH_3$
   • Addition of Halogens: $CH_3-C \equiv CH + 2Br_2 \rightarrow CH_3-CBr_2-CHBr_2$
   • Addition of Hydrogen Halides: Forms gem-dihalides (halogens on the same carbon). $HC \equiv CH + 2HBr \rightarrow CH_3-CHBr_2$
   • Addition of Water: In the presence of mercuric sulphate ($HgSO_4$) and dilute $H_2SO_4$, water adds to form a carbonyl compound (an aldehyde or a ketone). $CH_3-C \equiv CH + H_2O \xrightarrow{HgSO_4, H_2SO_4} [CH_3-C(OH)=CH_2] \rightarrow CH_3-CO-CH_3 \quad (\text{Propanone})$

3. Polymerisation

   • Linear Polymerisation: Ethyne polymerizes to form polyacetylene, a polymer that can conduct electricity.
   • Cyclic Polymerisation: When ethyne is passed through a red-hot iron tube at 873 K, three molecules polymerize to form benzene. $3C_2H_2 \xrightarrow{\text{Red hot Fe tube, 873K}} C_6H_6$

AROMATIC HYDROCARBONS

Aromatic hydrocarbons, or arenes, are cyclic compounds that possess a special stability associated with a delocalized pi electron system. Most contain a benzene ring.

• Benzenoids: Aromatic compounds containing a benzene ring (e.g., benzene, toluene, naphthalene).
• Non-benzenoids: Aromatic compounds that do not contain a benzene ring but still satisfy the conditions for aromaticity.

Nomenclature and Isomerism

For disubstituted benzene, the positions of the substituents are indicated by:

• ortho (o-) for 1,2 positions.
• meta (m-) for 1,3 positions.
• para (p-) for 1,4 positions.

Structure of Benzene ($C_6H_6$)

Benzene has a unique structure that accounts for its unusual stability.

• It is a planar, hexagonal ring of six carbon atoms, with one hydrogen atom attached to each carbon.
• Each carbon atom is $sp^2$ hybridized.
• The C-C bonds form a sigma framework. Each carbon has one unhybridized p-orbital perpendicular to the plane of the ring.
• These six p-orbitals overlap sideways to form a continuous, delocalized $\pi$ electron cloud containing six electrons, with one ring of electron density above the plane and one below.
• This delocalization of electrons is represented by a circle inside the hexagon.
• Due to resonance, all six C-C bonds in benzene are identical, with a bond length of 139 pm, which is intermediate between a single bond (154 pm) and a double bond (133 pm).
• This delocalized structure makes benzene extraordinarily stable and causes it to undergo substitution reactions rather than the addition reactions typical of alkenes.

Aromaticity

For a compound to be aromatic, it must meet the following criteria:

1. It must be planar.
2. It must be cyclic with complete delocalization of $\pi$ electrons in the ring.
3. It must obey Hückel's Rule, meaning it must have $(4n + 2) \pi$ electrons, where n is an integer (0, 1, 2, ...).
   • Benzene has $6 \pi$ electrons, which fits the rule for $n=1$.

Preparation of Benzene

1. Cyclic Polymerisation of Ethyne: (As described in the alkynes section).
2. Decarboxylation of Aromatic Acids: Heating sodium benzoate with sodalime produces benzene. $C_6H_5COO^-Na^+ + NaOH \xrightarrow{CaO, \Delta} C_6H_6 + Na_2CO_3$
3. Reduction of Phenol: Passing phenol vapor over heated zinc dust reduces it to benzene. $C_6H_5OH + Zn \xrightarrow{\Delta} C_6H_6 + ZnO$

Properties of Benzene

Physical Properties

• Aromatic hydrocarbons are non-polar, colorless liquids or solids with a characteristic smell.
• They are immiscible with water but soluble in organic solvents.
• They burn with a sooty flame due to their high carbon content.

Chemical Properties

The main characteristic reaction of arenes is electrophilic substitution.

Electrophilic Substitution Reactions An electrophile ($E^+$) attacks the electron-rich benzene ring, replacing one of the hydrogen atoms.

1. Nitration: Benzene reacts with a mixture of concentrated nitric acid and concentrated sulfuric acid (nitrating mixture) to form nitrobenzene. $C_6H_6 + \text{conc. } HNO_3 + \text{conc. } H_2SO_4 \xrightarrow{323-333K} C_6H_5NO_2 + H_2O$
2. Halogenation: Benzene reacts with halogens (Cl₂, Br₂) in the presence of a Lewis acid catalyst (like $FeCl_3$ or $AlCl_3$) to form halobenzenes. $C_6H_6 + Cl_2 \xrightarrow{\text{Anhyd. } AlCl_3} C_6H_5Cl + HCl$
3. Sulphonation: Reaction with fuming sulfuric acid (oleum) produces benzenesulphonic acid. $C_6H_6 + H_2SO_4 (\text{oleum}) \xrightarrow{\Delta} C_6H_5SO_3H + H_2O$
4. Friedel-Crafts Alkylation: Reaction with an alkyl halide in the presence of a Lewis acid catalyst forms an alkylbenzene. $C_6H_6 + CH_3Cl \xrightarrow{\text{Anhyd. } AlCl_3} C_6H_5CH_3 + HCl$
5. Friedel-Crafts Acylation: Reaction with an acyl halide or acid anhydride in the presence of a Lewis acid catalyst forms an acyl benzene (a ketone). $C_6H_6 + CH_3COCl \xrightarrow{\text{Anhyd. } AlCl_3} C_6H_5COCH_3 + HCl$

Mechanism of Electrophilic Substitution The reaction occurs in three steps:

1. Generation of the Electrophile: The catalyst helps generate a strong electrophile (e.g., $Cl^+$, $NO_2^+$, $R^+$).
2. Formation of a Carbocation Intermediate: The electrophile attacks the $\pi$ electron cloud of the benzene ring, forming a resonance-stabilized carbocation called an arenium ion or sigma complex. In this intermediate, the aromaticity is temporarily lost.
3. Removal of a Proton: A base removes a proton ($H^+$) from the carbon atom that the electrophile attached to, restoring the stable aromatic ring.

Addition Reactions Under vigorous conditions, benzene can be forced to undergo addition reactions.

• Hydrogenation: With high pressure/temperature and a Ni catalyst, benzene adds three molecules of $H_2$ to form cyclohexane. $C_6H_6 + 3H_2 \xrightarrow{Ni, \text{High T, P}} C_6H_{12}$
• Chlorination: In the presence of UV light, benzene adds three molecules of $Cl_2$ to form benzene hexachloride (BHC), an insecticide.

Directive Influence of a Functional Group

When a monosubstituted benzene undergoes further substitution, the existing group directs the incoming electrophile to a specific position.

1. Ortho and Para Directing Groups: These groups direct the incoming electrophile to the ortho and para positions. They are typically activating groups because they donate electron density to the ring, making it more reactive towards electrophiles.

   • Examples: $-OH$, $-NH_2$, $-OCH_3$, $-CH_3$, $-C_2H_5$.
   • These groups increase electron density at the ortho and para positions through resonance or inductive effects.
   • Halogens ($-F, -Cl, -Br, -I$) are an exception: they are deactivating (due to their strong electron-withdrawing inductive effect) but are still ortho, para-directing (due to their electron-donating resonance effect).

2. Meta Directing Groups: These groups direct the incoming electrophile to the meta position. They are deactivating groups because they withdraw electron density from the ring, making it less reactive.

   • Examples: $-NO_2$, $-CN$, $-CHO$, $-COR$, $-COOH$, $-SO_3H$.
   • These groups withdraw electron density, especially from the ortho and para positions, making the meta position the most electron-rich (or least electron-poor) site for electrophilic attack.

CARCINOGENICITY AND TOXICITY

Benzene and polynuclear hydrocarbons (compounds with multiple fused benzene rings, like benzpyrene) are toxic and can be carcinogenic (cancer-causing). These compounds are often formed from the incomplete combustion of organic materials like tobacco, coal, and petroleum. When they enter the body, they can be metabolically activated and damage DNA, leading to cancer.