Chapter 15: Reactions of Aromatic Compounds

Question 1

What is An Activating Group?

Answer 1

An Activating Group is an electron donating group that will make the aromatic ring more reactive, and render the electrophile aromatic substitutions to occur more quickly

Question 2

What are Arenes?

Answer 2

Arenes are hydrocarbons that contain alternating double and single bonds between carbons
ie) A benzene ring is an Arene

Question 3

What is an Arenium Ion?

Answer 3

An Arenium ion is a cyclohexadienyl cation that acts as a reactive intermediate in an electrophilic aromatic substitution reaction

Question 4

What is a Deactivating Group?

Answer 4

A Deactivating Group is an electron withdrawing group which will make the aromatic ring less reactive and render electrophilic aromatic substitutions to occur more slowly

Question 5

What is an Electrophile?

Answer 5

An electrophile is a chemical specie that is electron-deficient or lacks electrons

Question 6

What is Electrophilic Aromatic Substitution (EArS)?

Answer 6

One of the hydrogen atoms on an aromatic ring can be replaced by an electrophile

Ph-H + E-A —> Ph-E + HA

1. The benzene pi electrons will react with a strong electrophile that will lead to a carbocation intermediate
2. The Carbocation will react with the nucleophile to form a product
3. Benzene undertakes a substitution reaction rather than an addition reaction like alkenes
4. A nonaromatic cyclohexadienyl carbocation is formed when the electrophile attacks the benzene double bond. This cyclohexadienyl is also known as an arenium ion
5. The electrophile grabs two electrons from a double bond in the benzene ring, so that it can form a single bond (sigma) with one of the carbon atoms on the benzene ring
6. The Proton that is attached to the same position as the electrophile will be removed by a nucleophile, so that aromaticity can be re-established once again
7. The first step of the electrophile aromatic substitution step is endothermic, thus, it is the rate determining step. The second step is fast and exothermic (More favorable) because aromaticity is restored

Question 7

What is Halogenation of a Benzene?

Answer 7

Br2 or Cl2 with FeCl3 or AlCl3 (Lewis Acid)

• One of the hydrogen atoms of the ring is replaced by a Br or Cl atom
  1. One can use either aluminum chloride or Iron (|||) chloride as the catalyst in this reaction. If bromination is desired, use iron (|||) bromide instead
• Aromatic Fluroination occur extremely rapidly and needs to be carrie out under careful conditions
• Aromatic Iodination is very unreactive, can occur under special conditions

Question 8

What is Nitration of Benzene?

Answer 8

Nitration os Benzene occurs when one of the Hydrogen atoms on the benzene ring is replaced by a Nitro Group (NO2)

1. Benzene is treated with a mixture of concentrated with Nitric Acid (HNO3) and Sulfuric Acid (H2SO4) at a temperature between 50-55C for this substitution reaction to occur
2. Sulfuric Acid reacts as a catalyst to increase the concentration of the electrophile (Nitronium Ion; NO2), thus it does not appear in the mechanism

Question 9

What is Sulfonation of Benzene?

Answer 9

Benzene will react with fuming sulfuric acid (H2SO4) at room temperature to give benzenesulfonic acid
1. The fuming sulfuric acid (Oleum) that is used in this reaction contains dissolved sulfur trioxide
2. Instead of fumin sulfuring acid, once can use concentrated sulfuric acid, but the reaction occurs extremely slowly
3. Sulfuric Acid operates as both an acid and a base since its in equilibrium with the sulfur trioxide
4. If the final desired product is a sulfonate on the ring, then we can use either fuming sulfuric acid or concentrated sulfuric acid as the reagent
5. If one wants to remove, the sulfonic acid group (Desulfonation) from the ring, one can use dilute sulfuric acid followed by running this mixture in steam
THE SULFONATE CAN ACT AS A DIRECTING GROUP OR A PROJECTING GROUP

Question 10

What is Friedel-Crafts Alkylation?

Answer 10

Friedel-Crafts (1877)

1. A primary alkyl halide is not used because it will generate another complex. The complex will have a partial positive charge on the alkyl group, and a partial negative charge on the Al (Alkyl halide + AlCl3)
2. Rearrangements will mostly likely occur if primary halides were used
3. Hydroxy and Amine are very strong activators. In Bromination of phenol and aniline, reaction can be accomplished with Br2, H2O in the absence of a catalysts, and it does not stop at the level of monosubstitution. A Trisubstituted product is obtained instead. Polyhalogenation occurs very often with strong activating groups
4. Strong withdrawing groups such as aniline does not undergo Friedel Craft Alkylation reactions

Question 11

What is Friedel-Crafts Acylation?

Answer 11

An Acyl group has the general form of: R-C=O-CH3
1. Both Benzoyl and Acetyl groups are common acyl groups as shown:
Ph-C=O-CH3 or R-C=O-CH3
2. This reaction allows the synthesis of monoacylated products from the reaction between benzene and acyl chlorides (Acid chlorides)
3. The acylation reaction requires a Lewis acid (AlCl3) and will generate aryl ketones as one of the products
4. Acid chlorides can be produced from many different reactions
5. In step 1 of Friedel-Crafts Acylation reaction, an acylium ion (RCO+) is produced from the removal of the halide by the Lewis acid catalyst

Question 12

What are some restrictions of Friedel-Crafts Reactions?

Answer 12

1. Rearrangement can occur if the carbocation that was formed from an alcohol, alkene, or alkyl halide is able to generate a more stable carbocation
2. If there is the presence of an electron withdrawing group on the benzene ring, it will give low yield for Friedel-Crafts acylation and alkylation reactions; ie) NR3+, PR3+, SR2 and SO2R
3. The cause of this low yield is due to the lack of electrons. The electron withdrawing group (Deactivate) removes electrons from the ring and renders it less aromatic
4. Vinylic halides and aryl compounds will not be used as an alkyl-halide group in Friedel-Crafts reactions because they are unable to generate good carbocations, thus it will yield no reaction
5. Alkyl groups such as CH(CH3)2 among many other groups can donate electrons to the benzene ring, and cause polyalkylation and additional substitution effects THIS ONLY OCCURS IN ALKYLATION, NOT ACYLATION
6. Acyl Groups are strong withdrawing groups and do not allow additional substitution reactions to occur

Question 13

What is a Vinyl Halide?

Answer 13

a vinyl halide is an any alkene with atleast one halide substituent (Benzene) bonded directly on one of the alkene carbons

Question 14

What is Aryl Compound?

Answer 14

An aryl compound is phenyl C6H5; it is derived from benzene

Question 15

What is Another exception to Friedel-Crafts Reactions?

Answer 15

When there is a strong deactivator on the ring, Friedel-Crafts reaction do not occur, because meta directors are too unreactive

Question 16

What are Clemmensen Reduction Reactions (Aromatic)?

Answer 16

1. Rearrangements/shifting does not occur in Friedel-Crafts Acylation because resonance stabilization of the acylium ion is stronger than that for a carbocation
2. One can first go through an acylation reaction followed by a reduction reaction to obtain unbranched alkyl benzenes, instead of using the direct route of alkylation (Eliminate Rearrangement Issues)
3. The Clemmensen Reduction allows the deoxygeneration of ketones or aldehydes, to produce hydrocarbons
   - The is Reduction is especially useful if a primary alkyl substitution is desired

Ph-C=O-R + Zn(Hg); HCl reflux –> Ph-R

Question 17

What are Ortho/Para Directors?

Answer 17

They will direct incoming groups to the ortho and para positions with respect to itself

Question 18

What are Meta directors?

Answer 18

They will direct incoming groups to the meta position with respect to itself

Question 19

What are Halogens in terms of directors?

Answer 19

Halogens are extremely weak deactivating groups that they act as ortho/para directors instead of meta director

Question 20

Substitution of Toluene will always lead an attack at what direction?

Answer 20

Substitution of Toluene will always lea an attack at the ortho/para positions using nitration, halogenation and sulfonation reactions

Question 21

Determining priority with Activating Directors?

Answer 21

Activating Groups that have an unshared pair of electron is stronger than one without unshared electrons

Question 22

What are some Ortho/Para Directors?

Answer 22

O-, NR2, NHR, NH2, OH = Strong Directors
OR, R-C=O-O, R-C=O-NH = Moderate Directors
R, C6H5, H = Weak Directors

Question 23

What are some Meta Directors?

Answer 23

NO2, NR3+, CF3, CCl3 = Strong Deactivating

C-C=O-OH, C-C=O-OR, C-C=O-H, C-C=O-R = Moderate Directors

Question 24

What is Inductive Effect?

Answer 24

Are effects that occur through the sigma system due to the differences in electronegativities

• If the substituent is an electron withdrawing group, then the ring will be more positive
• Electrophile addition is slower because electrophiles are also positively charged

Question 25

What is the Resonance Effect?

Answer 25

Is the ability to form resonance structures to stabilize the overall structure
- The order of ability to donate electrons are as follows
X < OR < OH < NR2 < NH2 (X = Halogens)

Question 26

Meta Directing Groups?

Answer 26

1. Since Meta directors are electron withdrawing groups, they will generate an unstable arenium ion
2. The arenium ion of the ortho and para positions have a positive charge on the carbon atom attached to the electron withdrawing group, which is very unstable
3. Meta position is not particularly stable, it is simply the most stable amongst the 3 positions

Question 27

Ortho/Para Directing Groups?

Answer 27

1. 3 resonance structures for arenium ions are generated with the meta position, but 4 resonance structures were generated at the ortho and para positions
2. An added covalent bond is formed with the nitrogen atom, which then increases stability of the ortho and para arenium ion

Question 28

Oxidation of an Alkylbenzene will generate Carboxylic Acids (1) KMnO4; OH- with heat 2) H3O+) Solvent?

Answer 28

1. Ph-CH3 —–> Ph-COOH
2. Ph-CH2R —–> Ph-COOH
3. Ph-C=O-CH3 —–> Ph-COOH
4. Ph-CH=CHCH3 —–> Ph-COOH
5. Ph-CCCH3 —–> Ph-COOH
6. Ph-R —-> Ph-COOH

Question 29

If there are 2 substituents on the benzene ring, then the direction of the addition relies on what?

Answer 29

1. The stronger activating group
2. Ortho-para directors predominate over meta directors
3. If the disubstituted benzene is on position 1 and 3, there will be steric effects if one tries to add another group on position 2. Thus, the position that yields less strain will predominate. This other positions will likely dominate

Question 30

Allylic and Benzylic halide groups undergo what?

Answer 30

They undergo nucleophilic substitution reactions as well

• 3° halides will undergo SN1
• 1° and 2° halides undergo both SN1 and SN2. If stable allylic and benzylic carbocations can be formed, then SN1 is favoured.

Question 31

What are Birch Reduction Reactions?

Answer 31

Reduction of aromatic substrates with alkali metal (Li, or Na), alcohol in liquid ammonia is called Birch Reduction reaction
1. The Protons are abstracted from the ethanol and added to the position of the carbanion

Ph + Na; NH3, EtOH —> Ph(2 double bonds)