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Preparation of Aryl Halides: Classification, Preparations, Uses, Practice problems and Frequently Asked Questions

Preparation of Aryl Halides: Classification, Preparations, Uses, Practice problems and Frequently Asked Questions

There are two friends, one with a bad body odor and the other with a very good odor.

Who do you want to spend time with?

Obviously, you'll go with the one who smells good.

Because he is using an artificially created product known as perfume.

Do you know how perfumes are formed?

Aromatic compounds are frequently used in the production of perfumes.

Many delectable scents, such as vanilla, cinnamon, and floral, are created by combining aromatic compounds. A fragrance compound is any chemical substance with a scent or odor that is volatile enough to be transported to the olfactory system in the upper part of the nose. Particles meeting this specification, also known as odorant and aromatic compounds, have molecular weights of less than 300.

Many aromatic compounds are benzene derivatives, which are widely used in perfumery due to their superior blending qualities. This element, known as phenylethanol, is a major component of rose oils, and its production necessitates the substitution of other groups for the hydrogen atoms.

Food, wine, spices, perfumes, fragrance oils, and floral scents all contain these aromatic compounds. Many form while fruits and other crops ripen. Most are byproducts of the fermentation process in wines. Women's perfumes contain more aromatic compounds than men's perfumes.

Table of content:

  • Classification of aryl halides
  • Preparations of aryl halides
  • Uses of aryl halides
  • Practice problems
  • Frequently asked questions

Classification of aryl halides:

Haloarenes have the general structural formula Ar-X, where Ar represents the aryl group and X denotes the halogen atom, which can be fluorine (F), chlorine (Cl), bromine (Br), or iodine (I). When one hydrogen atom of benzene is replaced by a halogen atom, we get monosubstituted haloarene. Similarly, the hydrogen atoms in benzene can be replaced by halogen atoms to form poly substituted haloarenes.

Based on the number of halogen groups attached to the benzene ring, haloarenes are classified as mono haloarene, di haloarene, or tri haloarene.

Mono haloarenes:

Mono Haloarenes are haloarenes with only one halogen group in their molecules.

Aryl Halides are mono haloarenes with a halogen group attached to an aromatic ring sp2 hybridized carbon atom.

1

Di haloarenes:

Di haloarenes are haloarenes with two halogen groups that are the same or different in their molecules. As an example –

The prefixes ortho (o-), meta (m-), and para (p-) indicate the relative positions of the halogens 1,2, 1,3, and 1,4 in di haloarenes, respectively.

Tri haloarenes:

Tri Haloarenes are haloarenes with three halogen groups that are either the same or different. As an example,

Preparations of haloarenes

1. Electrophilic Substitution of Hydrocarbons:

The halogenation process is the electrophilic substitution of arenes with halogen atoms in the presence of certain reagents.

Chlorobenzene can be made by reaction of chlorine and benzene at 25o in the presence of FeCl3.

Mechanism:

Step 1: Generation of an electrophile: To create an electrophile, chlorine dissociates and forms a bond with FeCl3.

Step 2: The benzene attacks the electrophile, causing an intermediate cation to form.

Step 3: The carbocation further deprotonates to produce the substituted product chlorobenzene.

2. From amines by Sandmayer’s reaction:

When primary aromatic amines are dissolved or suspended in cold aqueous mineral acid and treated with sodium nitrite at 0-5oC, a diazonium salt is formed, mixing the freshly prepared diazonium salt solution with cuprous chloride or cuprous bromide results in the diazonium group being replaced by -Cl or -Br with the elimination of nitrogen gas.


The presence of cuprous halide is not required for the formation of iodobenzene. The reaction takes place simply by shaking the diazonium salt with potassium iodide but this reaction is not considered as sandmeyer's reaction.


3. Bal-schiemann reaction:

Fluoro benzene is formed by the reaction of benzene diazonium chloride and fluoro boric acid. Diazonium fluoroborate is formed, and when heated, fluorobenzene is produced. This is known as the Bal–Schiemann reaction.

4. Gattermann's reaction:

Gattermann's reaction can also be used to prepare haloarenes (chloro and bromoarenes). Gattermann's reaction is the modification of Sandmayer’s reaction. Instead of cuprous halide(CuCl or CuBr) copper powder is used in Gattermann's reaction.

5. Raschig process:

Chlorobenzene is produced commercially by passing a mixture of benzene, hydrogen chloride, and oxygen over CuCl2 as a catalyst at 525 K.

Uses of haloarenes:

  • Haloarenes are used in the production of a variety of compounds. Chlorobenzene, for example, is used in the production of phenol, picric acid, DDT, and other chemicals.
  • Picric acid is an antiseptic as well as a dye.
  • DDT is an insecticide that is used to control mosquitoes, flies, moths, and agricultural pests.

Practice problems:

Q 1. What is the electrophile used in the halogenation process for the preparation of haloarenes?

(A) Nitrosonium ion
(B) Chloronium ion
(C) Carbonium ion
(D) none of the above

Answer: (B) 
To create an electrophile, chlorine dissociates and forms a bond with FeCl3.

Q 2. What is the most suitable temperature for the formation of a diazonium salt from aniline?

(A) 0oC
(B) 15oC
(C) 35oC
(D) 60oC

Answer: (A)
Diazonium salts are formed by treating ice-cold solution of aromatic amines in aqueous mineral acid with sodium nitrite at low temperatures of 273 K-278 K(0-5oC).

Q 3. Which of the following dichloro arenes is formed when excess chlorine is used in the electrophilic substitution of toluene?

(A) ortho and meta
(B) ortho and para
(C) meta and para
(D) ortho, meta and para

Answer: (B) 
When excess halogen is present, the second halogen is also incorporated into the aromatic ring at a para position relative to the first halogen. This is due to the ortho and para directing nature of the -CH3 group in toluene.

Q 4. Which compound causes the reduction of iodobenzene back to benzene during the iodination of benzene?

(A) Iodic acid
(B) Hydrogen iodide
(C) Nitric oxide
(D) Mercuric oxide

Answer: (B)
The hydrogen iodide formed during benzene iodination is a powerful reducing agent that reverses the reaction. To avoid this, oxidizing agents such as HIO4, HNO3, and HgO are used in the reaction to convert the HI to iodine.

Frequently asked questions:

Q 1. How come haloarenes are less reactive?
Answer: Because sp2 hybridized carbon is more electronegative, it has a lower tendency to release electrons to the halogen atom. As a result, the haloarenes C-X bond is less polar. The lower polarity of the C-X bond in haloarenes makes halide ion release more difficult. The lower reactivity is due to the low polarity of the C-X bond in haloarenes.

Q 2. Why aren't haloarenes prepared from phenol?
Answer: Because the C-O bond in phenols has a partial double bond character due to resonance, it is stronger and more difficult to cleave. As a result, haloarenes cannot be produced from phenols.

Q 3. Why is the presence of cuprous halide not required when the diazonium group is replaced by iodine?
Answer: Because iodide is quickly and reversibly oxidizable, cuprous halide catalysis is not required in this reaction. So in this reaction cuprous halide is not required as in the cases of sandmeyer reaction which uses cuprous halide as a catalyst to transform aryl diazonium salt to aryl halide.

Q 4. Why isn't fluorobenzene made directly from fluorination of benzene?
Answer: Fluorine is highly reactive than its halogen members. Fluorine reactions are extremely exothermic. Fluorine interacts with benzene (or other hydrocarbons) very quickly, making it difficult to manage the number of fluorine atoms that are introduced on the benzene ring. This reaction produces a lot of fluorinated compounds, thus it's not a suitable synthesis.

Related Topics:

Alkyl Halides

Polyhalogen compounds

Benzene

Tollens’ test

Aldehyde

Ketone

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