Chemical Reactions of Haloalkanes and Haloarenes, FAQs

When a hydrogen atom in an aromatic or aliphatic hydrocarbon is replaced with a halogen atom, haloalkanes and haloarenes are produced. Haloalkanes are generated when an H-atom is replaced in an aliphatic hydrocarbon, whereas Haloarenes are formed when the same H-atom is replaced in an aromatic hydrocarbon. 

When one or more hydrogen aliphatic hydrocarbons are replaced by a corresponding number of halogen atoms like chlorine, bromine, or iodine, the resulting compounds are called halogen derivatives of alkanes (i.e., haloalkanes) example (CH₃Cl – Methyl-chloride).

Saturated aliphatic hydrocarbons are called alkanes, represented by the general formula C_nH_2n+2.  Example – CH₄ – Methane.

Chemical Reactions of Haloalkanes

Substitution reaction

Many additional functional groups can readily substitute the halide in alkyl halides. Substitution reactions occur when an atom or a group of atoms is substituted by another atom or group of atoms, respectively.

R – X  +  Y^–   ⇢    R – Y  + X^– 

When alkyl halides are heated with aqueous alkalis such as sodium hydroxide or potassium hydroxide, they hydrolyze and yield the corresponding alcohols. In this reaction, the halide (-X) group is replaced by the hydroxyl (-OH) group.

For Example: When heated with aqueous potassium hydroxide, methyl iodide hydrolyzes and generates methyl alcohol.

CH_3​−I  +  KOH    ⇢    ​CH₃​−OH  +  KI

This reaction is also known as hydrolysis reaction.

In general, we can also write,

R_​−X  +  KOH    ⇢    R​−OH  +  KI

• Reaction with Moist silver oxide

When alkyl halides are cooked with damp silver oxide (Ag₂O), they hydrolyze and yield the appropriate alcohols. Despite the fact that silver hydroxide does not exist, silver oxide floating in water behaves similarly to silver hydroxide. 

Example: When ethyl bromide is heated with damp silver oxide (Ag₂O), it undergoes hydrolysis and yields ethyl chloride.

In general, we can write,

• Formation of alkyl cyanides or alkane nitriles

Alkyl halides when boiled with an alcoholic solution of potassium cyanide from the corresponding alkyl cyanides or alkane nitriles. In this reaction halide (-X) of alkyl halide is substituted by a cyanide group (-C= N). 

For Example, Methyl iodide when boiled with an alcoholic solution of potassium cyanide forms methyl cyanide or ethane.

CH₃​−I  +  KCN    ⇢    ​CH₃​−C≡N  +  KI

In general, we can write,

R​−X  +  KCN    ⇢    ​R−N  +  KX

• Formation of alkyl isocyanides

Alkyl halides when heated with silver cyanide from alkyl isocyanides. In this reaction halide (-X) of alkyl halide is substituted by an isocyanide group ( – NC).

For Example, Methyl chloride when heated with silver cyanide forms methylisocyanide.

In general, we can write,

• Formation of primary amines: Ammonolysis

Alkyl halides on boiling with an excess alcoholic solution of ammonia, under pressure, form the corresponding primary amines. This reaction is also known as ammonolysis of alkyl halide. In this reaction halide (-X) of alkyl halides is substituted by an amino group (-NH₂). 

For Example, Methyl chloride on boiling with an excess amount of alcoholic solution of ammonia, under pressure, forms methylamine.

In general, we can write,

Alkyl halides when boiled with sodium alkoxide undergo a substitution reaction to form the corresponding ethers. In this reaction halide (-X) of alkyl halides is substituted by an alkoxy group (- O – R). This reaction is known as Williamson’s synthesis of ethers.  The RO- group is an alkyl group attached to oxygen hence called alkoxide e.g. CH, O- is methoxide, and C, H, O- is ethoxide.

Sodium alkoxide is prepared by the action of sodium on alcohol,

2R – OH + 2Na    ⇢    2RONa + H₂

For Example: Methyl bromide when boiled with sodium ethoxide undergoes substitution reaction and forms ethyl methyl ether

CH₃​−Br  +  Na−O−C₂​H₅    ⇢    CH₃​​−O−C₂​H₅​  +  NaBr

Alkyl halides on heating with silver salt of carboxylic acid form corresponding esters. In this reaction, halide (-X) of alkyl halides is substituted by a carboxylate (R-COO-) group.

For Example, methyl iodide on heating with silver acetate forms methyl acetate.

In general, we can write,

Elimination reaction

The reaction in which two atoms or groups are removed from an adjacent carbon atom in a molecule to form an unsaturated compound is called elimination reaction

• Dehydrohalogenation reaction

Alkyl halides when heated with an alcoholic solution of potassium or sodium hydroxide undergo dehydrohalogenation and alkenes are formed.  Dehydrohalogenation involves the removal of the halogen atom from the a-carbon and a hydrogen atom from the adjacent ß-carbon atom [refer to below image].

For Example, Ethyl bromide when heated with an alcoholic solution of potassium hydroxide forms ethene.

CH₃​−CH₂​−Br  +  KOH (alc.)    ⇢    ​H₂​C=CH₂​  +  KBr  +  H₂​O

In a dehydrohalogenation reaction, the preferred product is alkene, which has the greater number of alkyl groups attached to the doubly bonded carbon atoms. (more substituted double bond is formed)

For Example: 

• Mono-substituted – (CH₂ = CH – R)
• Di-substituted – (R – CH = CH – R)

In general,

Reaction with metals

• Reaction with sodium: (Formation of higher alkanes)

Alkyl halides when treated with sodium in presence of dry ether form higher alkanes. 

Example: Methyl bromide when reacted with sodium in presence of dry ether forms ethane.

If a mixture of two different alkyl halides is treated with sodium in presence of dry ether, a mixture of alkanes is obtained. Self-coupling products are formed in preference of cross-coupling products

• Reaction with magnesium: Formation of Grignard reagent

The Grignard reagent is an organometallic compound in which the divalent magnesium is directly linked to an alkyl group and a halogen atom. It is represented by the general formula R-Mg – X. The carbon magnesium bond is highly polar and the magnesium-halogen bond is ionic in nature. The Grignard reagent is highly reactive. It reacts with numerous organic as well as inorganic compounds. Thus, it is an important reagent having wide applications. It is used in the preparation of a large number of organic compounds.

Preparation of Grignard reagent

An alkyl halide when treated with pure and dry magnesium in the presence of pure and dry ether forms an alkyl magnesium halide known as a Grignard reagent.

For Example, methyl bromide when treated with magnesium in presence of dry ether forms methyl magnesiumArnab Mandal

Chemical reaction of Haloarenes

Introduction of halogen in the benzene ring. Chlorobenzene reacts with chlorine in presence of anhydrous ferric chloride to give a mixture of ortho and paradichlorobenzene.

Introduction of -NO, group in the benzene ring. Chlorobenzene when heated with the nitrating mixture (conc nitric acid + conc. sulphuric acid) yields 1- Chloro-4-nitro-benzene and 1-chloro-2-nitrobenzene. Para derivative is a major product.

Introduction of -SO, H group in the benzene ring. Chlorobenzene when heated with concentrated sulphuric acid yields 4-chlorobenzene sulphonic acid (major product) and 2-chlorobenzene sulphonic acid.

Introduction of the alkyl group or acyl group in the benzene ring. Chlorobenzene when treated with methyl chloride in presence of anhydrous aluminum chloride forms a mixture of 1-chloro-2-methyl benzene and I-chloro-4-methylbenzene.

Chlorobenzene also reacts with acetyl chloride in presence of anhydrous aluminum chloride to form o-and p-Chloro- acetophenones.

• Reaction with sodium metal

Aryl halides react with alkyl halides and undergo a coupling reaction when treated with sodium metal in presence of dry ether to form alkylbenzene. The reaction is known as the Wurtz Fittig reaction.

By coupling of two aryl groups, it gives diaryl and by coupling of two alkyls (methyl) groups, ethane is also obtained along with toluene.

Sample Questions(FAQs)

Question 1:  What is the action of Moist Ag₂O on ethyl bromide?

Answer:

On moist Ag₂O: When ethyl bromide is boiled with moist Ag₂O, ethyl alcohol is formed.

(Ag₂O + H₂O ⇢ 2AgOH)

Question 2: Write a note on Wurtz’s reaction with an example?

Answer: 

When alkyl halides re reacted with sodium in presence of dry ether, higher alkanes are formed. This reaction is called Wurtz reaction. Alkanes obtained by this method contain double the number of carbon atoms than that of the alkyl halides.

Example:

When ethyl bromide is reacted with sodium in the presence of dry ether, n -butane is formed.

C₂​H_5​−Br  +  2Na  +  Br−C₂​H₅    ⇢     ​C₂​H₅−C₂​H₅  ​+  2NaBr

Question 3: What is Grignard’s reagent? How is it prepared?

Answer:

The Grignard reagent is an organometallic compound in which the divalent magnesium is directly linked to an alkyl group and a halogen atom. It is represented by the general formula R-Mg – X. The carbon magnesium bond is highly polar and the magnesium-halogen bond is ionic in nature. The Grignard reagent is highly reactive. It reacts with numerous organic as well as inorganic compounds. Thus, it is an important reagent having wide applications. It is used in the preparation of a large number of organic compounds.

Preparation of Grignard reagent

An alkyl halide when treated with pure and dry magnesium in the presence of pure and dry ether forms an alkyl magnesium halide known as a Grignard reagent.

For Example: methyl bromide when treated with magnesium in presence of dry ether forms methyl magnesium

Question 4: How will you convert n-propyl bromide to iso-propyl bromide?

Answer:

n -propyl bromide when boiled with alcoholic KOH gives propene, which on treatment with HBr gives iso-propyl bromide.

Question 5: State and explain Saytzeff`s rule?

Answer: 

In ß-elimination reaction hydrogen atom from that adjacent carbon atom is preferentially eliminated having less number of hydrogen atoms. Or in other words, in B-elimination reaction the preferred product is that alkene which has greater number of alkyl groups attached to the double bonded carbon atom (more substituted alkene).