Nitro Compounds: Preparation of Nitro Compounds: From Alkyl Halides, Alkyl Amines and Hydrocarbons, Practice Problems and FAQs

Can you tell which chemical compounds are periodically and naturally formed? They are oxides of nitrogen from the atmosphere that contains nitrogen and oxygen gasses in plenty. These oxides on reaching earth is converted into nitro compound of both inorganic and organic substances. Organic compounds containing the Nitro group are the starting material for many useful nitrogen compounds in fertilizer, pharmaceuticals dyes, etc

Every year community look for the award ‘NOBLE PRIZE’. Do you the the story behind Noble prize. Alfred Nobel a scientist discovered the powerful explosive nitroglycerin and unimaginable money in this destruction oriented world. He was so being called as a merchant of of death. Feeling bad of such an ignominious nickname, he wanted to make use of the money of nitroglycerine for good use and formed Noble prize to promote human benefitting work.

Let us know about the nitro organic compounds here.

Table of Content

• Nitro Group
• Preparation of Nitro Compounds
• Practice Problems
• Frequently Asked Questions (FAQs)

Nitro Group

Organic substances that include one or more nitro functional groups (NO₂) are known as nitro compounds. One of the most often used explosophores (functional groups that make a compound explosive) on a global scale is the nitro group. Additionally, the nitro group exhibits significant electron withdrawal nature.. This characteristic allows C-H bonds that are alpha to the nitro group and next to it to be acidic. The inclusion of nitro groups in aromatic compounds slows down electrophilic aromatic substitution but speeds up nucleophilic aromatic substitution for related reasons.

Preparation of Nitro Compounds

From Alkyl Halides

Primary alkyl halides especially bromides and iodides react on heating with an aqueous alcoholic solution of silver nitrite to give excellent yields (70-80%) of nitroalkanes along with small amounts (10-15%) of alkyl nitrites.

From tertiary alkyl amines

Primary amines containing a tertiary alkyl group on oxidation with KMnO₄, give excellent yields of the corresponding nitroalkanes.

From hydrocarbons :

Aromatic nitro compounds:

Aromatic nitro compounds can be easily prepared by the action of a mixture of conc. HNO₃and conc. H₂SO₄on an arene or any other aromatic compound. For example,

This reaction which involves the replacement of one or more hydrogen atoms of a compound by an equal number of nitro groups is called nitration. A mixture of HNO₃ and H₂SO₄ acids used for nitration is called the nitrating mixture. Besides nitrating mixture (conc. HNO₃+ conc. H₂SO₄), other reagents which have been used for nitration of aromatic compounds are: nitric acid - acetic anhydride mixture [HNO₃-(CH3CO)₂0], acetyl nitrate (CH_₂COONO_₂), nitronium perchlorate (NOClO₄), nitronium tetrafluoroborate (NOBF), N₂O₅etc.

Mechanism:

This electrophilic substitution reaction occurs by the following three-step mechanism.

Step 1: Formation of nitronium electrophile

Nitration is a typical example of an aromatic electrophilic substitution reaction in which nitronium ion (NO^₂+) acts as the electrophile. The function of H₂SO₄is that of a dehydrating agent and

acts on HNO₃to form the reactive nitronium ion as shown below :

Step 2: Electrophilic attack:

Benzene and other arenes are electron rich molecules. In the first step, the electron rich benzene molecule (nucleophile) attacks nitronium ion (electrophile) leading to the formation of a carbocation (1) which is stabilized by resonance. During this step, the aromatic character (resonance energy of 150 kJ mol^-1) is sacrificed. As a result, this step is slow and hence is the rate-determining step of the reaction.

Step 3. In the third step, the carbocation (1) formed in step 2 loses a proton to yield nitrobenzene

Since during this step, the aromatic character of the benzene ring is restored. Therefore, this step is fast and hence does not affect the rate of the reaction.

Evidence in support of the mechanism. This mechanism is supported by isotope effect. It has been

found that when hexadeuterobenzene is nitrated, the rate of nitration is exactly the same as that of benzene. Since a C-D bond breaks more slowly than a C-H bond, therefore, the loss of a proton is not the rate-determining step. Had it been so, the rates of nitration of benzene and hexadeuterobenzene would have been different?

Choice of the Nitrating Agents

The choice of nitrating mixture depends upon the reactivity of the aromatic compound. Electron donating substituents like -CH₃, -OCH₃, -OH, -NH₂, etc. activate the ring and stabilize the intermediate carbocation. In contrast, electron withdrawing groups such as -NO₂, -CN, -SO,H, X (halogens), etc., deactivate the ring by destabilizing the carbocation intermediate. Therefore, nitration of these deactivated compounds is carried out with a mixture of conc. HNO₃ + conc. H₂SO₄ at higher temperatures.

(b) Aliphatic Nitro Compounds.

In contrast to arenes, alkanes do not undergo nitration easily. However, mixture of nitroalkanes resulting through cleavage of carbon-carbon bonds along with the oxidation products with fuming HNO3in the vapour phase at 673-793 K under pressure, alkanes do undergo nitration to give a like CO₂, NO₂, H₂O, etc. Thus,This reaction occurs by a free radical mechanism and the ease of substitution of hydrogens follows the sequence : tertiary > secondary> primary.

Though this method is not suitable for the laboratory preparation of nitroalkanes, it is widely used in industry for the manufacture of nitroalkanes (since they are used as solvents only).

Practice Problems

Q1. Which of the following is used as a reactant for the nitration of benzene to form nitrobenzene?

1. HNO₂
2. HNO₃
3. Mixture of HNO₃ and H₂SO₄
4. None of the above.

Answer: (C)
Nitration is achieved using a mixture of nitric acid and sulfuric acid, which produce the nitronium ion (NO²⁺), with the electrophile.

Q2. Which of the following reagent is used for the preparation of nitroalkane from alkyl halides?

1. CuCl₂/HBr
2. HNO₃
3. AgNO₂
4. None of the above

Answer: (C)
Primary alkyl halides especially bromides and iodides react on heating with an aqueous alcoholic solution of silver nitrite to give excellent yields (70-80%) of nitroalkanes along with small amounts (10-15%) of alkyl nitrites.

Q3. Which of the following stabilise the intermediate carbocation?

1. -NO₂
2. -CN
3. -SO
4. -CH₃

Answer: (D)
Electron donating substituents like -CH₂, -OCH₂, -OH, -NH₂, etc. activate the ring and stabilize the intermediate carbocation. In contrast, electron withdrawing groups such as -NO₂, -CN, -SO,H, X (halogens), etc., deactivate the ring by destabilizing the carbocation intermediate.

Q4. In the nitration of benzene using a mixture of concentrated nitric acid and sulphuric acid, the species which initiates the reaction is

1. NO₂
2. NO⁺
3. NO²⁺
4. NO^2-

Answer: (C)
The species that starts the reaction in the nitration of benzene using a mixture of strong nitric acid and sulfuric acid is NO²⁺.

Frequently Asked Questions (FAQs)

Q1. Why Toluene is easily nitrated than benzene?
Answer: Nitration is an electrophilic substitution reaction and thus occurs more readily at a site where the electron density is more. Now in toluene, there is electron-donating -CH₃ group. In other words, the electron density in the toluene ring is much more than in the benzene ring. As a result, toluene is nitrated more easily than benzene.

Further since hyperconjugative effect increases the electron-density at o- and p-positions, relative to m-positions, therefore, nitration of toluene gives a mixture of O- and p-nitrotoluenes.

Q2. Why Nitrobenzene doesn’t dissolve in water?
Answer: In spite of the presence of an “electronegative bomb” as a substituent group (-NO_₂), nitrobenzene displays a fairly big benzene ring which is basically C and H tightly bonded together.

That is what gives such a molecule a marked hydrophobic character and makes it soluble in nonpolar solvents such as (let me make an educated guess) hexane, cyclohexane, toluene, benzene, and so forth.

Q3. Nitrobenzene is acidic or basic in nature?
Answer: nitro group is a strong withdrawing group and deactivating also so the positive charge on the benzene increases. if the positive charge increases the acidic nature increases.. therefore nitrobenzene is more acidic than benzene

Q4. During nitration of benzene with a mixture of concentrated nitric acid and concentrated sulphuric acid, nitric acid acts as a base. Explai
Answer: Both HNO₃ and H₂SO₄ are strong acids but H₂SO₄ is a stronger acid than HNO₃. Therefore, during nitration. H₂SO₄ acts as an acid releasing a proton. HNO₃, on the other hand, accepts this proton and us acts as a base. The protonated nitric acid then loses a proton to form nitronium ion which then brings about the nitration.

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