Haloalkane & Haloarenes Class 12/CHEMICAL REACTION HALOALKANE & haloarenes/ SN1 & SN2/ elimination

SN1 & SN2 reaction
Chemical reactions of Haloalkanes and Haloarenes
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–
Formation of Alcohols
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.

CH3​−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.
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.

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

Saytzef’s rule
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)

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.
SN1 and SN2 reaction
Nucleophilic substitution reaction

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