question

What happens in a electrophilic substitution Reaction

answer

a hydrogen atom is replaced by an electrophile.

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Characteristic Reactions of Benzene

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electrophilic aromatic substitution Benzene has six ? electrons delocalized in six p orbitals that overlap above and below the plane of the ring. These loosely held ? electrons make the benzene ring electron rich, and so it reacts with electrophiles.

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Why doesn't Benzene undergo Addition reactions

answer

Because addition would yield a product that is not aromatic. It likes to keep its aromatic ring intact.

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Electrophilic aromatic substitution: basic steps

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two-step mechanism: 1. addition of the electrophile E? to form a resonance-stabilized carbocation 2. deprotonation with a base

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electrophilic aromatic substitution forms what intermediary?

answer

A carbocation, for which three resonance structures can be drawn: • Always draw in the H atom on the carbon bonded to E. This serves as a reminder that it is the only sp³ hybridized carbon in the carbocation intermediate. • Notice that the positive charge in a given resonance structure is always located ortho or para to the new C-E bond. In the hybrid, therefore, the charge is delocalized over three atoms of the ring.

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Energies of electrophilic aromatic substitution RXNs

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• The mechanism has two steps, so there are two energy barriers. • Step [1] is rate-determining; its transition state is at higher energy.

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How to the different electrophilic aromatic substitution reactions differ?

answer

how the electrophile is generated. Then the electrophile reacts with the aromatic compound with the same two step process

question

Halogenation of an aromatic compound

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Benzene reacts with Cl? or Br? in the presence of a Lewis acid catalyst, such as FeCl? or FeBr?, to give the aryl halides chlorobenzene or bromobenzene, respectively. Reactions with I? and F? aren't possible because I? is too unreactive and F? reacts too violently.

question

Generation of the electrophile in both nitration and sulfonation requires What?

answer

A strong acid

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Formation of the Nitronium Ion (+NO?) for Nitration

answer

The electrophile is NO?? (the nitronium ion), formed by protonation of HNO? followed by loss of water

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Formation of the Electrophile +SO?H for Sulfonation

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In sulfonation, protonation of sulfur trioxide, SO?, forms a positively charged sulfur species (?SO?H) that acts as an electrophile

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alkylation

answer

transfer of an alkyl group from one atom to another (from Cl to benzene ).

question

acylation

answer

The transfer of an acyl group from one atom to another

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Friedel-Crafts acylation,

answer

A benzene ring is treated with an acid chloride (RCOCl) and AlCl? to form a ketone.

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Unique feature of each Electrophilic Aromatic Substitution Reactions

answer

How the electrophile is generated. The rest is the same

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Formation of the Electrophile in Friedel-Crafts Alkylation—Two Possibilities (Mechanism)

answer

• For CH?Cl and 1° RCl, the Lewis acid-base complex itself serves as the electrophile for electrophilic aromatic substitution. • With 2° and 3° RCl, the Lewis acid-base complex reacts further to give a 2° or 3° carbocation, which serves as the electrophile. Carbocation formation occurs only with 2° and 3° alkyl chlorides, because they afford more stable carbocations.

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Friedel-Crafts Alkylation Using a 3° Carbocation (Mechanism)

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• Addition of the electrophile (a 3° carbocation) forms a new carbon-carbon bond in Step [1]. • AlCl?- removes a proton on the carbon bearing the new substituent, thus re-forming the aromatic ring in Step [2].

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Other Facts About Friedel-Crafts Alkylation (3)

answer

[1] Vinyl halides and aryl halides do not react in Friedel-Crafts alkylation [2] Rearrangements can occur. [3] Other functional groups that form carbocations can also be used as starting materials.

question

Do Vinyl halides and aryl halides react in Friedel-Crafts alkylation?

answer

Most Friedel-Crafts reactions involve carbocation electrophiles. Because the carbocations derived from vinyl halides and aryl halides are highly unstable and do not readily form, these organic halides do not undergo Friedel-Crafts alkylation.

question

A Rearrangement Reaction Beginning with a 1° Alkyl Chloride (Mechanism)

answer

Rearrangements can occur even when no free carbocation is formed initially. a 1,2-hydride shift forms a 2° carbocation, which then serves as the electrophile in the two-step mechanism for electrophilic aromatic substitution

question

What other functional groups that form carbocations can also be used as starting materials in Friedel-Crafts Alkylation/Acylation?

answer

any compound that readily forms a carbocation can be used instead. The two most common alternatives are alkenes and alcohols, both of which afford carbocations in the presence of strong acid.

question

How do you use an alkene for Alkylation?

answer

Protonation of an alkene forms a carbocation, which can then serve as an electrophile in a Friedel-Crafts alkylation.

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How do you use an alcohol for Alkylation?

answer

Protonation of an alcohol (From an acid), followed by loss of water, likewise forms a carbocation

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Intramolecular Friedel-Crafts Reactions

answer

Starting materials that contain both a benzene ring and an electrophile are capable of intramolecular reaction, and this forms a new ring

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Inductive Effects

answer

• Atoms more electronegative than carbon—including N, O, and X—pull electron density away from carbon and thus exhibit an electron-withdrawing inductive effect. • Polarizable alkyl groups donate electron density, and thus exhibit an electron-donating inductive effect.

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Resonance Effects

answer

• A resonance effect is electron donating when resonance structures place a negative charge on carbons of the benzene ring. • A resonance effect is electron withdrawing when resonance structures place a positive charge on carbons of the benzene ring.

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When is electron-donating resonance effect seen?

answer

whenever an atom Z having a lone pair of electrons is directly bonded to a benzene ring (general structure—(C?H? - Z:) atoms: N, O, and halogen.

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When is electron-withdrawing resonance effect seen?

answer

in substituted benzenes having the general structure C?H? -Y=Z, where Z is more electronegative than Y

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Is an alkyl group electron-donating group or withdrawing?

answer

An alkyl group is an electron-donating group and an alkyl benzene is more electron rich than benzene.

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Common Electron Donating groups

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Alkyl groups or groups with an N or O atom (with a lone pair) bonded to the benzene ring.

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Common Electron withdrawing groups

answer

Common electron-withdrawing groups are halogens or groups with an atom Y bearing a full or partial positive charge (+ or ??) bonded to the benzene ring. Also, C6H5 -Y=Z (with Z more electronegative than Y) are electron withdrawing

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Considering Both Inductive and Resonance Effects

answer

• When a neutral O or N atom is bonded directly to a benzene ring, the resonance effect dominates and the net effect is electron donation. • When a halogen X is bonded to a benzene ring, the inductive effect dominates and the net effect is electron withdrawal.

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All ortho, para directors are what?

answer

R groups or have a nonbonded electron pair on the atom bonded to the benzene ring.

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All meta directors are what?

answer

All meta directors have a full or partial positive charge on the atom bonded to the benzene ring.

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Activators

answer

make a benzene ring react faster than benzene itself All activators are either R groups or they have an N or O atom with a lone pair bondeddirectly to the benzene ring. These are the electron-donor groups

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Deactivators

answer

Make a benzene ring react slower than by itself. All deactivators are either halogens or they have an atom with a partial or full positive charge bonded directly to the benzene ring. These are the electron-withdrawing groups

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Why is the CH? Group—An ortho, para Director?

answer

attack ortho or para to CH? generates a resonance structure that places a positive charge on a carbon atom with the CH? group. The electron-donating CH? group stabilizes the adjacent positive charge.

question

Why is The NH? Group—An ortho, para Director?

answer

Because of the lone pair on the N atom, attack at the ortho and para positions generates a fourth resonance structure, which is stabilized because every atom has an octet of electrons. This additional resonance structure can be drawn for all substituents that have an N, O, or halogen atom bonded directly to the benzene ring.

question

Halogenation of Activated Benzenes

answer

benzene rings activated by strong electron-donating groups—OH, NH?, and their alkyl derivatives (OR, NHR, and NR?)—undergo polyhalogenation when treated with X? and FeX?.

question

Monosubstitution of H by Br in Benzene

answer

occurs with Br? alone without added catalyst to form a mixture of ortho and para products.

question

Friedel-Crafts reactions with NH? groups

answer

Doesn't occur. NH? groups are strong Lewis bases (due to the nonbonded electron pair on N), so they react with AlCl?, the Lewis acid needed for alkylation or acylation. The resulting product contains a positive charge adjacent to the benzene ring, so the ring is now strongly deactivated and therefore unreactive in Friedel-Crafts reactions.

question

Friedel-Crafts reactions with NO? groups

answer

No Reaction. A benzene ring deactivated by a strong electron-withdrawing group—that is, any of the meta directors—is not electron rich enough to undergo Friedel-Crafts reactions.

question

Treatment of benzene with an alkyl halide and AlCl?

answer

places an electron-donor R group on the ring. Because R groups activate a ring, the alkylated product (C?H?R) is now more reactive than benzene itself towards further substitution, and it reacts again with RCl to give products of polyalkylation.

question

Polysubstitution with Friedel-Crafts acylation

answer

does not occur because the product now has an electron-withdrawing group that deactivates the ring towards another electrophilic substitution.

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What happens when the directing effects of two groups REINFORCE each other on Disubstituted Benzenes

answer

the new substituent is located on the position directed by both groups.

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What happens when the directing effects of two groups OPPOSE each other on Disubstituted Benzenes?

answer

the more powerful activator "wins out."

question

What happens with the directing effects of two meta substituents on Disubstituted Benzenes?

answer

No substitution occurs BETWEEN two meta substituents because of crowding

question

How would you synthesize p-bromonitrobenzene?

answer

the Br group is an ortho, para director and the NO? group is a meta director. Because the two substituents are para to each other, the ortho, para director must be introduced first

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Oxidation of Alkyl Benzenes

answer

Arenes containing at least one benzylic C- H bond are oxidized with KMnO? to benzoic acid,

question

Reduction of Aryl Ketones

answer

Ketones formed as products in Friedel-Crafts acylation can be reduced to alkyl benzenes by two different methods

question

Clemmensen reduction

answer

uses zinc and mercury in the presence of strong acid to reduce an aryl ketone to a alkyl benzene

question

Wolff-Kishner reduction

answer

uses hydrazine (NH?NH?) and strong base (KOH) to reduce an aryl ketone to a alkyl benzene

question

two different ways to introduce an alkyl group on a benzene ring

answer

• A one-step method using Friedel-Crafts alkylation • A two-step method using Friedel-Crafts acylation to form a ketone, followed by reduction

question

How do you prepare propylbenzene?

answer

propylbenzene cannot be prepared by a Friedel-Crafts alkylation. Instead, when benzene is treated with 1-chloropropane and AlCl?, isopropylbenzene is formed by a rearrangement reaction. Propylbenzene can be made, however, by a two-step procedure using Friedel-Crafts acylation followed by reduction.

question

Reduction of Nitro Groups

answer

the nitro group is readily reduced to an amino group (NH?) under a variety of conditions. The most common methods use H? and a catalyst, or a metal (such as Fe or Sn) and a strong acid like HCl

question

To be reactive in Friedel Crafts alkylation the halogen must be bonded to what?

answer

An Sp³ hybridized carbon. If its connected to a double bond its unreactive. Vinyl halides (CH?=CHCl) and aryl halides (Ph-Cl)

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