Friedel–Crafts Reactions and Directive Influence of Substituents

Hello students! Today, we are diving into a crucial topic in Organic Chemistry: Friedel–Crafts reactions and understanding how substituents on an aromatic ring influence where new groups attach. These reactions are fundamental for building more complex organic molecules from simple aromatic compounds like benzene.

Friedel–Crafts Alkylation and Acylation

Friedel–Crafts reactions are a set of reactions that involve the alkylation or acylation of aromatic compounds. They were first described by Charles Friedel and James Crafts in 1877. These reactions are examples of electrophilic aromatic substitution.

Friedel–Crafts Alkylation

In Friedel–Crafts alkylation, an alkyl group is introduced onto an aromatic ring. This is typically achieved by reacting the aromatic compound with an alkyl halide in the presence of a Lewis acid catalyst, such as aluminum chloride (AlCl₃) or ferric chloride (FeCl₃).

The general reaction is: Ar-H + R-X $\xrightarrow{\text{Lewis Acid}}$ Ar-R + H-X Where: Ar-H represents the aromatic compound (e.g., benzene) R-X represents an alkyl halide Ar-R represents the alkylated aromatic compound Lewis Acid is the catalyst (e.g., AlCl₃, FeCl₃)

Let's consider the mechanism using benzene, methyl chloride, and AlCl₃ as an example:

1. Formation of the electrophile: The Lewis acid catalyst (AlCl₃) reacts with the alkyl halide (CH₃Cl) to generate a carbocation, which is a strong electrophile. CH₃Cl + AlCl₃ $\rightleftharpoons$ [CH₃⁺] [AlCl₄^-]
2. Electrophilic attack: The carbocation attacks the electron-rich aromatic ring (benzene), forming a resonance-stabilized carbocation intermediate called a sigma complex (or arenium ion). C₆H₆ + CH₃⁺ $\rightarrow$ [C₆H₆CH₃]⁺
3. Proton removal and rearomatization: A base (often the [AlCl₄^-] anion) removes a proton from the carbon atom bearing the alkyl group. This restores the aromaticity of the ring, forming the alkylated product (toluene) and regenerating the catalyst. [C₆H₆CH₃]⁺ + [AlCl₄^-] $\rightarrow$ C₆H₅CH₃ + HCl + AlCl₃

Limitations of Friedel–Crafts Alkylation:

Friedel–Crafts alkylation has several limitations:

• Carbocation rearrangements: Primary alkyl halides can rearrange to form more stable secondary or tertiary carbocations, leading to a mixture of products. For example, using 1-chloropropane can lead to isopropylbenzene as the major product due to the rearrangement of the propyl carbocation to the isopropyl carbocation. CH₃CH₂CH₂Cl $\xrightarrow{\text{AlCl}_3}$ CH₃⁺CHCH₃ (isopropyl carbocation)
• Multiple alkylations: The alkylated product is often more reactive towards further alkylation than the starting aromatic compound. This is because alkyl groups are activating substituents. As a result, polysubstitution can occur, leading to a mixture of mono-, di-, and tri-alkylated products.
• Reaction with deactivated rings: Friedel–Crafts alkylation does not work with aromatic compounds that have strongly electron-withdrawing groups (deactivating groups) attached to the ring, such as nitro (-NO₂), cyano (-CN), or carbonyl (-C=O) groups. This is because these groups deactivate the ring towards electrophilic attack.
• Rearrangement of alkyl halides: As mentioned, carbocation rearrangements can occur, leading to undesired products.

Friedel–Crafts Acylation

Friedel–Crafts acylation introduces an acyl group (-COR) onto an aromatic ring. This is accomplished by reacting the aromatic compound with an acyl halide or acid anhydride in the presence of a Lewis acid catalyst, usually AlCl₃.

The general reaction is: Ar-H + R-COCl $\xrightarrow{\text{Lewis Acid}}$ Ar-COR + HCl Where: R-COCl represents an acyl halide Ar-COR represents the acylated aromatic compound

The mechanism is similar to alkylation, but with a key difference in the electrophile formation:

1. Formation of the electrophile: The Lewis acid catalyst (AlCl₃) reacts with the acyl halide to form an acylium ion, which is a resonance-stabilized cation. This ion is a potent electrophile. R-COCl + AlCl₃ $\rightarrow$ [R-C≡O⁺ ↔ R-C=O⁺] + [AlCl₄^-]
2. Electrophilic attack: The acylium ion attacks the aromatic ring, forming a sigma complex. C₆H₆ + R-CO⁺ $\rightarrow$ [C₆H₆COR]⁺
3. Proton removal and rearomatization: A base removes a proton to restore aromaticity, yielding the acylated product. [C₆H₆COR]⁺ + [AlCl₄^-] $\rightarrow$ C₆H₅COR + HCl + AlCl₃

Important Note: In Friedel–Crafts acylation, the Lewis acid catalyst (AlCl₃) is required in stoichiometric amounts (or even slightly more) because it forms a complex with the product ketone. This complexation deactivates the AlCl₃, preventing it from catalyzing further reactions. To work up the reaction, water is added to hydrolyze the complex and liberate the ketone product.

Advantages of Friedel–Crafts Acylation over Alkylation:

Friedel–Crafts acylation is generally preferred over alkylation due to its advantages:

• No rearrangements: The acylium ion is resonance-stabilized and does not undergo rearrangement.
• No polysubstitution: The acyl group (-COR) is an electron-withdrawing group, which deactivates the aromatic ring towards further electrophilic substitution. This ensures that monosubstitution is the predominant outcome.

After acylation, the ketone product can be reduced to an alkane using methods like the Clemmensen reduction (using zinc amalgam and HCl) or the Wolff-Kishner reduction (using hydrazine and a strong base like KOH at high temperatures). This allows for the introduction of alkyl groups without the problems associated with direct Friedel–Crafts alkylation.

Directive Influence of Substituents on Electrophilic Aromatic Substitution

When an aromatic ring already has one or more substituents, these substituents influence the rate of further electrophilic substitution and also direct the incoming electrophile to specific positions on the ring. This is known as the directive influence of substituents.

Substituents are broadly classified into two categories based on their effect on electrophilic aromatic substitution:

1. Activating groups: These groups increase the electron density of the aromatic ring, making it more reactive towards electrophilic attack. They also direct the incoming electrophile to the ortho (o-) and para (p-) positions. Examples include -OH, -NH₂, -OR, -NHCOR, -R, -Ar.
2. Deactivating groups: These groups decrease the electron density of the aromatic ring, making it less reactive towards electrophilic attack. They direct the incoming electrophile to the meta (m-) position. Examples include -NO₂, -CN, -SO₃H, -COR, -COOR, -NR₃⁺.

Understanding the Mechanism of Directing Effects:

The directive influence is explained by considering the stability of the resonance-stabilized carbocation intermediate (sigma complex) formed during the electrophilic attack.

Ortho, Para Directors (Activating Groups):

Activating groups are typically electron-donating groups (EDGs). They can donate electron density to the ring through resonance or inductive effects. When an electrophile attacks the ortho or para positions, the positive charge in the sigma complex can be delocalized onto the atom directly attached to the ring, if that atom has a lone pair of electrons (e.g., oxygen in -OH, nitrogen in -NH₂). This delocalization provides extra resonance structures that stabilize the intermediate.

Consider phenol (C₆H₅OH) reacting with an electrophile (E⁺):

• Ortho attack: The positive charge can be placed on the oxygen atom.
• Para attack: The positive charge can also be placed on the oxygen atom.
• Meta attack: The positive charge cannot be placed on the oxygen atom.

Since the intermediates formed from ortho and para attack are more stable due to the resonance contribution involving the lone pair on oxygen, these positions are favored.

Electron-donating groups (-R, -OR, -NH₂, -OH) are ortho, para directors. They activate the ring because they increase electron density.

Meta Directors (Deactivating Groups):

Deactivating groups are typically electron-withdrawing groups (EWGs). They withdraw electron density from the ring through resonance or inductive effects. When an electrophile attacks the ortho or para positions of a ring substituted with a deactivating group, the positive charge in the sigma complex gets placed on the atom directly attached to the ring. If this atom is part of an electron-withdrawing group (e.g., the carbon of a carbonyl group in -COR, or the nitrogen in -NO₂), it further destabilizes the intermediate because it is already electron-deficient.

However, when the electrophile attacks the meta position, the positive charge in the sigma complex is delocalized to positions that do not involve the electron-withdrawing atom directly attached to the ring. This results in a more stable intermediate compared to ortho/para attack.

Consider nitrobenzene (C₆H₅NO₂) reacting with an electrophile (E⁺):

• Ortho attack: The positive charge is placed on the carbon adjacent to the nitro group. This structure has a resonance form where the positive charge is on the nitrogen atom of the nitro group, which is highly unfavorable.
• Para attack: Similar to ortho attack, a resonance form places the positive charge on the nitrogen atom, which is unfavorable.
• Meta attack: The positive charge is delocalized to positions that do not place it on the nitrogen atom. These intermediates are more stable than those formed from ortho/para attack.

Therefore, deactivating groups like -NO₂, -CN, -SO₃H, -COR, -COOR, -X (halogens) direct the incoming electrophile to the meta position.

Halogens (X): A Special Case

Halogens (-F, -Cl, -Br, -I) are an interesting case. They are deactivating groups because of their strong inductive electron-withdrawing effect. However, they are ortho, para directors due to their lone pairs of electrons, which can donate electron density to the ring through resonance. The resonance effect, though weaker than the inductive effect, is sufficient to stabilize the ortho and para sigma complexes more than the meta complexes. Because they are deactivating, they slow down the reaction rate but still direct to ortho and para positions.

Summary Table of Directing Effects:

Let's summarize the directive influence and activating/deactivating nature of common substituents.

Combined Directive Effects

When a benzene ring has more than one substituent, the directive influence of all substituents is considered. The incoming electrophile will preferentially attack the position that is activated by the most activating group and is ortho or para to it.

If the directing effects of the substituents are in conflict (e.g., one directs ortho/para and the other meta), the position directed by the stronger activator is usually favored. If both are deactivating, the position directed by the less deactivating group is favored. If one is activating and one is deactivating, the activating group's influence dominates.

Consider o-nitrotoluene. The methyl group (-CH₃) is an activating o,p-director, and the nitro group (-NO₂) is a deactivating m-director.

• The -CH₃ group directs to positions 2 (already substituted), 4, and 6.
• The -NO₂ group directs to positions 4 and 6 (relative to itself, which are positions 3 and 5 on the ring).

The positions 4 and 6 are ortho/para to the methyl group and meta to the nitro group. Position 5 is meta to the methyl group and ortho to the nitro group. Position 3 is meta to the methyl group and para to the nitro group.

The methyl group is activating, and the nitro group is deactivating. Therefore, the directing influence of the methyl group will dominate. The incoming electrophile will attack at positions 4 or 6 (which are ortho/para to -CH₃ and meta to -NO₂). Steric hindrance from the methyl group might favor position 4 over position 6.

Example: Nitration of Toluene

Toluene (C₆H₅CH₃) has a methyl group, which is an activating o,p-director. When toluene is nitrated using a mixture of concentrated nitric acid and concentrated sulfuric acid (HNO₃/H₂SO₄), the nitro group (-NO₂) is directed to the ortho and para positions.

The reaction yields a mixture of o-nitrotoluene and p-nitrotoluene. The para product is usually obtained in a higher yield due to less steric hindrance compared to the ortho product. C₆H₅CH₃ + HNO₃ $\xrightarrow{\text{H}_2\text{SO}_4}$ o-C₆H₄(CH₃)(NO₂) + p-C₆H₄(CH₃)(NO₂) + H₂O

Example: Nitration of Chlorobenzene

Chlorobenzene (C₆H₅Cl) has a chlorine atom, which is a deactivating o,p-director. Nitration of chlorobenzene with HNO₃/H₂SO₄ yields a mixture of o-chloronitrobenzene and p-chloronitrobenzene. The para isomer is favored due to steric reasons. The reaction is slower than the nitration of benzene itself because chlorine is deactivating. C₆H₅Cl + HNO₃ $\xrightarrow{\text{H}_2\text{SO}_4}$ o-C₆H₄(Cl)(NO₂) + p-C₆H₄(Cl)(NO₂) + H₂O

Example: Nitration of Benzoic Acid

Benzoic acid (C₆H₅COOH) has a carboxyl group (-COOH), which is a deactivating m-director. Nitration of benzoic acid with HNO₃/H₂SO₄ will direct the incoming nitro group to the meta position. The reaction is also slower than the nitration of benzene because the -COOH group is deactivating. C₆H₅COOH + HNO₃ $\xrightarrow{\text{H}_2\text{SO}_4}$ m-C₆H₄(COOH)(NO₂) + H₂O