Low reactivity of haloarene as compound to haloalkane in term of nucleophilic substitution reaction

Nucleophilic substitution reactions

In the C-X bond there is a partial positive charge on the carbon atom and negative on the halogen atom. Thus nucleophilies attack the electron deficient carbon resulting in the displacement of the weaker nucleophile, the halide ion. Reactions of alkyl halides are generally nucleophilic substitution reactions.

The halide ions are substituted only if the attacking nucleophile is stronger. As the halide ion itself is a very weak nucleophile, the attacking nucleophile should be stronger than it. The order of reactivity of various alkyl halides towards nucleophilic substitution is:

RI > RBr > RCl > RF

This order of reactivity can be explained on the basis of strength of C-X bond. The C-X bond is the weakest in R-I and strongest in R-F.

These reactions take place either by the S_N¹ and S_N² mechanisms.

Types of nucleophilic substitution reactions

The nucleophilic substitution reactions can be classified into two types:

First is the S_N¹ reaction (substitution nucleophilic, first order). This type of reaction proceeds in two steps as:

The first step is slow and is the rate-determining step. As the nucleophile (Z^-) is not involved in the rate-determining step, the reaction depends only upon the concentration of alkyl halide (RX) and is, therefore, a first order reaction.

Rate = k [RX]

The order of reactivity depends upon the stability of carbonium ion formed in the first step. Since the 3° carbonium ion is most stable, the ionization of tertiary alkyl halide is favored. The order of reactivity for S_N¹ reaction is, tertiary > secondary > primary.

The second type is the S_N² reaction (substitution nucleophilic, second order). This type of reaction occurs in one step through the formation of transition state as:

Here, the rate of reaction depends upon the concentration of both the alkyl halide and the nucleophile.

Rate = k[RX] [Z^-]

The transition state from tertiary alkyl halide is less stable due to steric hindrance i.e., crowding of bulky groups. The order of reactivity is: primary > secondary > tertiary.

Thus the reactivity by S_N¹ versus S_N² increases as follows:

Factors favouring S

_N¹ and S_N² mechanisms

The reaction mechanism of S_N¹ or S_N² substitutions depends upon a number of factors as follows:

Nature of alkyl halide

If the alkyl halide is primary, it reacts through S_N² and if it is tertiary, it reacts through S_N¹ mechanism. 2° Alkyl halide react through both mechanisms.

Nature of nucleophile

Strong nucleophiles favor S_N² mechanism whereas weak nuclephiles favor S_N¹mechanism.

Concentration of nucleophile

High concentration of nucleophile favors S_N², while low concentration favors SN¹ mechanism.

Nature of solvent

Polar solvents favor S_N¹ mechanism as they can cause dissociation of alkyl halide and thus facilitate the formation of carbocation. Solvents of low polarity favor S_N² mechanism.

Nucleophilic reactions of haloalkanes are:

Replacement by hydroxyl group (Formation of alcohols)

On treatment with aqueous solution of KOH or moist silver oxide (Ag₂O/H₂O) haloalkanes give alcohols.

Replacement by alkoxy group

(Formation of ethers) - Williamson Synthesis

This reaction is called 'Williamson's synthesis' and is quite useful for preparing ethers. Haloalkanes can also be converted into ethers by heating with dry silver oxide.

Substitution by cyano group

Haloalkanes react with alcoholic solution of potassium cyanide (KCN) to give alkane nitriles or alkyl cyanides as the major products along with a small amount of alkyl isocyanides.

Alkyl cyanides can further be converted into acid amides, carboxylic acids and primary amines under different conditions. Therefore, they are useful starting materials for these compounds.

The reaction of haloalkanes with alcoholic KCN is important because the product formed has one more carbon atom than the alkyl halide. Therefore, the reaction is a good method for ascending the homologous series (increasing the length of the carbon chain by one carbon atom).

Substitution by isocyanide group

Isocyanides are obtained when haloalkane is treated with alcoholic silver cyanide (AgCN). These are also called carbylamines and have extremely unpleasant smell.

(Alkane isonitrile)

On reduction with sodium or alcohol, alkyl isocyanides give secondary amines.

Substitution by amino group (formation of amines)

A primary amine is formed when haloalkane is heated with alcoholic ammonia solution in a sealed tube at 383 K; the halogen is substituted by -NH₂ group.

However, when haloalkane is in excess, a mixture of primary, secondary and tertiary amines is formed, as one or both the hydrogen atoms of the amino alkane (primary amine) are replaced by alkyl groups.

The tertiary amines which result, can also form quaternary ammonium salt by combining with another molecule of alkyl halide.

This reaction is called Hoffmann ammonolysis reaction.

Substitution by nitrite group

The halogen atom of haloalkane gets substituted by nitrite group (-O-N=O) when it is treated with sodium or potassium nitrite and forms alkyl nitrites.

The bond between K-O in KNO₂ (alkyl nitrites) is ionic and therefore the negative charge on oxygen is the attacking site: it therefore forms nitrites (R-O-N = O).

Substitution by nitro group

The halogen atom in haloalkane is replaced by nitro group (-NO₂) when it is treated with silver nitrite (AgNO₂) and give nitroalkanes.

The bond between Ag-O is covalent and therefore, the nucleophilic attack here occurs through the lone pair on nitrogen: nitroalkanes (R-NO₂) are therefore, formed.

Substitution by carboxyl group (formation of esters)

When Haloalkanes are heated with an ethanolic solution of silver salt of a fatty acid these form esters.

Substitution by hydrosulphide group

The formation of thioalcohols or thiols takes place when haloalkane is treated with-sodium or potassium hydrogen sulphide: the halogen atom gets replaced by hydrosulphide (-SH) group.

Substitution by mercaptide group

On treatment with mercaptide ion (RS^-) haloalkanes give thioethers.

Thioethers are also formed by heating the haloalkanes with sodium or potassium sulphide.

Substitution by alkynyl group (formation of alkynes)

The formation of higher alkynes occurs when haloalkane is treated with sodium salt of alkynes (sodium alkynides): the halogen atom is replaced by alkyl group (-CC-).

The sodium alkynides needed for the above reaction are formed by the reaction of sodamide (or sodium in liquid NH₃) with alkynes containing terminal triple bond.