Hydrocarbons (Aromatic Hydrocarbon)

Aromatic Hydrocarbon

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Aromatic hydrocarbons are a class of hydrocarbons characterized by the presence of one or more benzene rings or similar ring structures. They are known for their unique stability and reactivity.

Nomenclature And Isomerism

Nomenclature of Aromatic Hydrocarbons:

• Benzene: The parent compound is benzene ($C_6H_6$).
• Monosubstituted Benzene: The substituent name is prefixed to 'benzene' (e.g., Chlorobenzene, Nitrobenzene, Benzaldehyde, Benzoic acid). Common names like Toluene ($C_6H_5CH_3$) and Phenol ($C_6H_5OH$) are also accepted IUPAC names.
• Disubstituted Benzene: Positions are indicated by numbers (1,2-; 1,3-; 1,4-) or prefixes:
• ortho (o-): 1,2- substitution
• meta (m-): 1,3- substitution
• para (p-): 1,4- substitution
• Polysubstituted Benzene: Numbering starts from the carbon attached to the principal functional group or substituent with the highest priority. Substituents are listed alphabetically.

Isomerism in Aromatic Compounds:

• Position Isomerism: Common in disubstituted and polysubstituted benzenes (e.g., o-, m-, p-xylenes).
• Chain Isomerism: Possible in alkyl-substituted benzenes (e.g., ethylbenzene vs. cumene (isopropylbenzene)).
• Functional Isomerism: Possible between aromatic hydrocarbons and other classes of compounds (e.g., phenol $C_6H_5OH$ and anisole $C_6H_5OCH_3$ are isomers).

Structure Of Benzene

Molecular Formula: $C_6H_6$.

Kekulé Structure: Proposed by August Kekulé, it consists of a six-membered ring of carbon atoms, each bonded to one hydrogen atom. The carbons are arranged hexagonally.

Bonding:

• Each carbon atom is $sp^2$ hybridized.
• Three $sp^2$ hybrid orbitals on each carbon atom form $\sigma$ bonds with adjacent carbon atoms and with hydrogen atoms.
• The remaining unhybridized $p$ orbital on each carbon atom is perpendicular to the plane of the ring.
• These six $p$ orbitals overlap laterally above and below the plane of the ring, forming a delocalized $\pi$ electron system.
• The $\pi$ electrons are spread evenly over the entire ring, contributing to its stability.

Bond Lengths: All carbon-carbon bonds in benzene are identical, with a length of 139 pm, intermediate between a typical $C-C$ single bond (154 pm) and a $C=C$ double bond (134 pm).

Resonance: Benzene is a resonance hybrid of two contributing Kekulé structures, showing delocalization of $\pi$ electrons.

Stability: Benzene is unusually stable due to this electron delocalization, a phenomenon known as resonance stabilization or aromaticity.

Aromaticity

Definition: Aromaticity is a property of cyclic, planar, conjugated systems of atoms that leads to unusual stability and characteristic reactivity (electrophilic substitution). It is due to the delocalization of $\pi$ electrons.

Hückel's Rule: A compound is considered aromatic if it meets the following criteria:

• Cyclic: The molecule must contain a ring structure.
• Planar: The ring must be planar so that the p-orbitals can overlap effectively.
• Fully Conjugated: Every atom in the ring must have an unhybridized p-orbital that can overlap with neighboring p-orbitals, forming a continuous $\pi$ system around the ring.
• $(4n+2) \pi$ Electrons: The cyclic $\pi$ system must contain $(4n+2)$ delocalized $\pi$ electrons, where $n$ is a non-negative integer (0, 1, 2, ...).
• $n=0 \Rightarrow 2 \pi$ electrons (e.g., $H_3O^+$, $NH_4^+$ if considered cyclic).
• $n=1 \Rightarrow 6 \pi$ electrons (Benzene).
• $n=2 \Rightarrow 10 \pi$ electrons (e.g., Naphthalene).
• $n=3 \Rightarrow 14 \pi$ electrons.

Consequences of Aromaticity:

• Unusual Stability: Aromatic compounds are more stable than expected based on their structure.
• Characteristic Reactions: They tend to undergo electrophilic substitution reactions rather than addition reactions, which would disrupt the stable aromatic $\pi$ system.

Preparation Of Benzene

1. From Alkenes/Alkynes:

• Cyclic Polymerization of Ethyne: Heating ethyne ($C_2H_2$) in a red-hot iron or nickel tube.

$3C_2H_2(g) \xrightarrow{Fe \ or \ Ni \ catalyst, \ heat} C_6H_6(g)$

• Catalytic Reforming of Alkanes: Heating alkanes (like hexane or heptane) in the presence of catalysts like platinum at high temperatures.

$C_6H_{14}(g) \xrightarrow{Pt, \ heat} C_6H_6(g) + 4H_2(g)$

2. From Phenol: Dry distillation of sodium benzoate ($C_6H_5COONa$) or heating phenol with zinc dust.

$C_6H_5COONa(s) \xrightarrow{NaOH, \ heat} C_6H_6(l) + Na_2CO_3(s)$

$C_6H_5OH(l) \xrightarrow{Zn \ dust, \ heat} C_6H_6(g) + ZnO(s)$

Properties

Physical Properties:

• Benzene is a colorless liquid with a characteristic sweet smell.
• It is flammable and burns with a sooty flame due to its high carbon content.
• It is a non-polar solvent, insoluble in water but soluble in many organic solvents.
• Boiling point: 353 K (80°C).
• Freezing point: 278.7 K (5.5°C).
• Toxic: Benzene is toxic and a known carcinogen.

Chemical Properties: Benzene is unusually stable for a hydrocarbon with a high degree of unsaturation, due to its aromaticity.

1. Electrophilic Substitution Reactions: Benzene preferentially undergoes substitution reactions where an electrophile ($E^+$) replaces a hydrogen atom on the ring, rather than addition reactions that would destroy the aromaticity.

• Halogenation: Reaction with halogens ($Cl_2$, $Br_2$) in the presence of a Lewis acid catalyst ($FeCl_3$, $AlCl_3$, $FeBr_3$).

$C_6H_6 + Cl_2 \xrightarrow{FeCl_3} C_6H_5Cl + HCl$

• Nitration: Reaction with a mixture of concentrated nitric acid and concentrated sulfuric acid ("nitrating mixture").

$C_6H_6 + HNO_3 \xrightarrow{conc. H_2SO_4} C_6H_5NO_2 + H_2O$

• Sulfonation: Reaction with fuming sulfuric acid ($H_2SO_4 + SO_3$).

$C_6H_6 + H_2SO_4(\text{fuming}) \rightarrow C_6H_5SO_3H + H_2O$

• Friedel-Crafts Alkylation: Reaction with alkyl halides in the presence of a Lewis acid catalyst ($AlCl_3$).

$C_6H_6 + CH_3Cl \xrightarrow{AlCl_3} C_6H_5CH_3 + HCl$

• Friedel-Crafts Acylation: Reaction with acyl halides or acid anhydrides in the presence of a Lewis acid catalyst ($AlCl_3$).

$C_6H_6 + CH_3COCl \xrightarrow{AlCl_3} C_6H_5COCH_3 + HCl$

2. Addition Reactions: Benzene can undergo addition reactions under drastic conditions (high temperature and pressure, or strong catalysts), which destroy its aromaticity.

• Hydrogenation: Addition of $H_2$ over Ni catalyst at 573 K forms cyclohexane.

$C_6H_6 + 3H_2 \xrightarrow{Ni, 573K} C_6H_{12}$

• Halogenation: Addition of $Cl_2$ or $Br_2$ in the presence of UV light forms benzene hexachloride ($C_6H_6Cl_6$).

$C_6H_6 + 3Cl_2 \xrightarrow{UV \ light} C_6H_6Cl_6$

3. Combustion: Burns with a sooty flame.

Directive Influence Of A Functional Group In Monosubstituted Benzene

Activating and Deactivating Groups: Substituents already present on the benzene ring influence the rate and position of further electrophilic substitution.

1. Activating Groups:

• These groups increase the electron density of the benzene ring, making it more reactive towards electrophilic substitution.
• They direct the incoming electrophile to the ortho and para positions.
• Examples: $-OH$, $-OR$, $-NH_2$, $-NHR$, $-NR_2$, $-R$ (alkyl), $-OR$. These groups donate electron density via resonance (+R effect) or inductive effect (+I effect).

2. Deactivating Groups:

• These groups decrease the electron density of the benzene ring, making it less reactive towards electrophilic substitution.
• They direct the incoming electrophile to the meta position.
• Examples: $-NO_2$, $-CN$, $-COOH$, $-CHO$, $-COR$, $-SO_3H$, halogens ($F, Cl, Br, I$). These groups withdraw electron density via resonance (-R effect) or inductive effect (-I effect).

Halogens: Halogens are an exception. They are deactivating groups (due to -I effect) but are ortho, para directors (due to resonance +R effect, though weaker than -I effect). Their deactivating influence is less than groups like $-NO_2$ or $-CN$.

Multiple Substituents: If multiple substituents are present, their directive influences might reinforce or oppose each other, directing the incoming electrophile to the most activated position.

Versatile Nature Of Carbon (Nomenclature from Carbon And Its Compounds)

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This section reiterates the connection between carbon's versatile bonding and the need for a systematic nomenclature system.

Nomenclature Of Carbon Compounds

Systematic Naming: The IUPAC system provides a universal language to identify and name the millions of carbon compounds.

Reflection of Versatility: The rules of IUPAC nomenclature are designed to accommodate carbon's unique ability to:

• Catenate: Form chains, branches, and rings. The parent name is determined by the longest carbon chain or ring.
• Form Multiple Bonds: Suffixes like '-ene' and '-yne' indicate the presence and position of double and triple bonds.
• Bond with Functional Groups: A vast array of suffixes and prefixes represent the diverse functional groups that carbon can form, each dictating specific reactivity and properties.
• Show Stereoisomerism: Prefixes like 'cis-', 'trans-', 'R-', 'S-' are used to describe the spatial arrangement of atoms, reflecting the three-dimensional nature of organic molecules.

Importance: A clear and systematic nomenclature is essential for organizing, communicating, and advancing the study of organic chemistry, given the sheer number and complexity of carbon compounds.