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UNIT I

Introduction to Arenes

• Alkyl substituted benzene compounds are known as arenes.

• Characteristic reactions of benzene include electrophilic substitution reactions such as nitration, halogenation, and sulphonation.

• Benzene does not undergo addition reactions.

Benzene Electrophilic Substitution Reaction

• Benzene's formula is $C_6H_6$; all six hydrogens are equivalent.

• When benzene undergoes an electrophilic substitution reaction, it yields only mono-substituted products.

  • Example: The reaction with electrophile (E) is as follows:

  egin{align*}
  C_6H_6 &+ E \

ightarrow & ext{Mono-substituted benzene}

  ext{(All hydrogens are replaced by E)}

\ ext{Example: if E is NO}_2 ext{:}
C_6H_6 + NO_2
ightarrow C_6H_5NO_2
\ ext{(Nitrobenzene)}
\ ext{Only one hydrogen is replaced, yielding a mono-substituted product.}

\end{align*}

• If mono-substituted benzene undergoes further electrophilic substitution, it can generate different isomers such as:

  • Ortho isomer (1,2-disubstituted)

  • Meta isomer (1,3-disubstituted)

  • Para isomer (1,4-disubstituted)

Directive or Orientation Effect

• A pre-existing group on the benzene ring (the 'key atom') influences where the incoming electrophile will attach.

  • It directs to the ortho, para, or meta positions based on the nature of the substituent.

• Activating groups increase reactivity and are electron-donating.

  • Examples include $–OH$, $–OR$, $–NH_2$, $–NHR$, $–NHCOR$.

• Deactivating groups decrease reactivity and are electron-withdrawing.

  • Examples include $–NO_2$, $–CN$, $–COOH$, $–COCl$.

Orientation of Electrophilic Substitution

Ortho-Para Directing Groups

• Groups that direct incoming electrophiles to the ortho (1,2) or para (1,4) positions are termed ortho-para directing.

• Examples include:

  • $–OH$ (hydroxyl), $–O^R$ (alkoxy), $–NH_2$ (amino), etc.

  • Notably, all activating groups except halogens are ortho-para directing.

Meta Directing Groups

• Groups that direct incoming electrophiles to the meta (1,3) position are termed meta directing.

• Examples:

  • $–NO_2$, $–CN$, $–COOH$, $–COCl$, etc.

• All deactivating groups (except halogens) are meta directing.

Directive Influence of Functional Groups

Activating Groups Influence

• The directive influence of electron-donating groups such as $–OH$ is illustrated:

  • The $–OH$ group donates electrons via a resonance (+R) effect, leading to increased electron density at ortho and para positions.

Deactivating Groups Influence

• The $–NO_2$ group, for instance, withdraws electrons through a resonance (−R) effect. This results in decreased electron density at ortho and para positions, favoring substitution at the meta position.

Halogen Anomalies

• Halogens display unique properties as they are ortho-para directing but weakly deactivating.

• Example: Chlorobenzene has a lone pair on Cl, which donates electrons (+R effect), yet it shows a strong inductive effect (−I effect) that decreases benzene ring reactivity.

Ortho-Para Ratio

• When benzene rings with ortho-para groups undergo electrophilic aromatic substitution, mainly ortho and para substrates form, typically in the ratio of 2:1.

• However, real ratios vary due to effects like steric hindrance.

• The steric hindrance of larger alkyl groups may lower ortho products due to steric strain.

Preparation and Reactions of Toluene

Preparations of Toluene

1. From Benzene (Friedel-Crafts Reaction):

   • $C_6H_6 + CH_3Cl <br>ightarrow ext{Toluene} + HCl$

   • Catalyzed by anhydrous $AlCl_3$.

2. From Bromobenzene (Wurtz-Fittig Reaction):

   • $Br-C_6H_5 + 2 ext{Na} + Br-CH_3 <br>ightarrow ext{Toluene}$

3. From Grignard Reaction:

   • $C_6H_5-MgBr + CH_3Br <br>ightarrow ext{Toluene} + Mg(Br)2$

4. From n-Heptane (Aromatization):

   • $C_7H_{16} <br>ightarrow ext{Toluene at } Al_2O_3/C_2O_3.$

   • High temp/process.

Chemical Reactions of Toluene

1. Electrophilic Substitution:

   • Halogenation:
     $C_7H_8 + Cl_2 o o-, p- ext{chlorotoluene}$
     Catalyzed by $FeCl_3$.

   • Nitration:
     $C_7H_8 + HNO_3 + H_2SO_4 o o-, p- ext{nitrotoluene}$.

   • Sulphonation:
     $C_7H_8 + H_2SO_4 <br>ightarrow o-, p- ext{toluene sulphonic acid}$.

2. Reactions on Side Chain:

   • Halogenation:
     Toluene reacts under UV light.

   • Nitration and Sulphonation: Similar to electrophilic substitutions.

   • Friedel-Crafts Alkylation and Acylation: Similar pathways.

Reactions and Mechanism of Nucle

UNIT I

Introduction to Arenes

• Alkyl substituted benzene compounds are known as arenes.
• Characteristic reactions of benzene include electrophilic substitution reactions such as nitration, halogenation, and sulphonation.
• Benzene does not undergo addition reactions.

Benzene Electrophilic Substitution Reaction

• Benzene's formula is $C<em>6H</em>6$; all six hydrogens are equivalent.

• When benzene undergoes an electrophilic substitution reaction, it yields only mono-substituted products.

• Example: The reaction with electrophile (E) is as follows:
  egin{align*}
  C6H6 + E
  ightarrow ext{Mono-substituted benzene} ext{(All hydrogens are replaced by E)}
  ext{Example: if E is NO}2 ext{:} C6H6 + NO2
  ightarrow C6H5NO2 ext{(Nitrobenzene)} ext{(Only one hydrogen is replaced, yielding a mono-substituted product.)} \ ext{Example: if E is Cl} ext{:} \ C6H6 + Cl2
  ightarrow C6H5Cl \
  ext{(Chlorobenzene)}
  \
  ext{Only one hydrogen is replaced, yielding a mono-substituted product.}
  ext{When further electrophilic substitution on mono-substituted benzene, different isomers are generated:}

  • Ortho isomer (1,2-disubstituted)
  • Meta isomer (1,3-disubstituted)
  • Para isomer (1,4-disubstituted)

Directive or Orientation Effect

• A pre-existing group on the benzene ring (the 'key atom') influences where the incoming electrophile will attach.
• It directs to the ortho, para, or meta positions based on the nature of the substituent.
• Activating groups increase reactivity and are electron-donating.
  • Examples include –OH$,$–OR$,$–NH_2$,$–NHR$,$–NHCOR.
• Deactivating groups decrease reactivity and are electron-withdrawing.
  • Examples include –NO_2$,$–CN$,$–COOH$,$–COCl.

Orientation of Electrophilic Substitution

Ortho-Para Directing Groups

• Groups that direct incoming electrophiles to the ortho (1,2) or para (1,4) positions are termed ortho-para directing.
• Examples include:
  • –OH$(hydroxyl),$–O^R$(alkoxy),$–NH_2 (amino), etc.
• Notably, all activating groups except halogens are ortho-para directing.

Meta Directing Groups

• Groups that direct incoming electrophiles to the meta (1,3) position are termed meta directing.
• Examples:
  • –NO_2$,$–CN$,$–COOH$,$–COCl, etc.
• All deactivating groups (except halogens) are meta directing.

Directive Influence of Functional Groups

Activating Groups Influence

• The directive influence of electron-donating groups such as –OH is illustrated:
• The –OH group donates electrons via a resonance (+R) effect, leading to increased electron density at ortho and para positions.

Deactivating Groups Influence

• The –NO_2 group, for instance, withdraws electrons through a resonance (−R) effect. This results in decreased electron density at ortho and para positions, favoring substitution at the meta position.

Halogen Anomalies

• Halogens display unique properties as they are ortho-para directing but weakly deactivating.
• Example: Chlorobenzene has a lone pair on Cl, which donates electrons (+R effect), yet it shows a strong inductive effect (−I effect) that decreases benzene ring reactivity.

Ortho-Para Ratio

• When benzene rings with ortho-para groups undergo electrophilic aromatic substitution, mainly ortho and para substrates form, typically in the ratio of 2:1.
• However, real ratios vary due to effects like steric hindrance. The steric hindrance of larger alkyl groups may lower ortho products due to steric strain.

Preparation and Reactions of Toluene

Preparations of Toluene

1. From Benzene (Friedel-Crafts Reaction):
   • C6H6 + CH_3Cl
     ightarrow ext{Toluene} + HCl
   • Catalyzed by anhydrous AlCl_3.
2. From Bromobenzene (Wurtz-Fittig Reaction):
   • Br-C6H5 + 2 ext{Na} + Br-CH_3
     ightarrow ext{Toluene}
3. From Grignard Reaction:
   • C6H5-MgBr + CH_3Br
     ightarrow ext{Toluene} + Mg(Br)2
4. From n-Heptane (Aromatization):
   • C7H{16}
     ightarrow ext{Toluene at } Al2O3/C2O3.$$ -