Why CH3Cl is more active than C6H5Cl towards hydrolysis and What is Ulmann reaction ?

Why CH₃Cl is more active than C₆H₅Cl towards hydrolysis?

In case of chloro benzene, the lone pair of chlorine atom takes part in resonance with the π electrons of benzene ring . As a result, the C –Cl bond in chloro benzene gets partially double bond character . 

So, C –Cl bond becomes more strong than the normal C –Cl bond. Hence, to break the C –Cl bond by the nucleophile attack is not so easy.

Therefore, the hydrolysis C₆H₅Cl does not occurs easily . Although ,the hydrolysis of chloro benzene occurs under high temperature and high pressure.

On the other hand, in case of methyl chloride the C –Cl bond has no chance to gain double bond character due to absence of conjugation. Hence, the hydrolysis occurs easily.

Why alkyl halides give alkyl cyanide with KCN but alkyl isocyanides with AgCN?

Cyanide ion is an ambident nucleophile . Because , it can attack electrophilic centre either through the ‘C’- atom or through ‘N ‘atom. When alkyl halide react with AgCN , the reaction proceed through SN1 mechanism. In this method , Ag⁺ ion is co-ordinate with bromine atom first. 

Then, the carbo cation is formed through the breaking of C – Br bond. The AgBr is also obtained as precipitate.

 Now, more electronegative and more electron dense ‘N’-atom of cyanide nucleophile attack the carbocation, resulting in the formation  of alkyl isocynide ( R – NC ).

On the other hand, in the KCN, the reaction follow , SN2 mechanism . Because , Na⁺ ion is weak acid than Ag⁺ ion.

So it can not break the C – Br bond.  In this case more polarizable ( big size and less electronegative ) and strong nucleophile ‘C’-atom attack the alkyl halide. As a result , alkyl cyanide ( R– CN ) is the main product.

For the above reason, alkyl halides give alkyl cyanide with KCN but alkyl isocyanides with AgCN.

Compare the basic hydrolysis ability of m-chloro nitro benzene , p-chloro nitro benzene and chloro benzene.

The basic hydrolysis of chloro benzene and o-chloro nitro benzene, m-chloro nitro benzene , p-chloro nitro benzene are an example of nucleophilic substitution reaction.

Now , the ability of these compounds depends on the relative stability of the inter mediate carbo anions.

In case of m-chloro nitro benzene , the intermediate carbo-anion can not be stabilized by the –NO2 group due to absent of conjugation between negative charge and – NO2 group . 

The carbo-anion stabilized through the resonance of benzene ring only .Hence, the carbo-anion is produced from the m-chloro nitro benzene is less stable than the carbo-anion produced from o-chloro nitro benzene and p-chloro nitro benzene.

For the above reason , the nitro group present in the meta position can not increase the activity of benzene ring .

Although , the – NO2 group can not stabilize the carbo-anion produced from the m-chloro nitro benzene yet it more stable than carbo-anion produced from chloro benzene. Because –NO2 group has  strong  – I effect .

Consequently, m-chloro nitro benzene is more effective than chloro benzene but less effective than p-chloro nitro benzene.

What is Ulmann reaction?

When a mixture of copper powder and aryl iodide is heated in sealed tube , the desired product is diaryl. This reaction is known as Ulmann reaction.

For example,when the mixture of copper powder and iodo benzene is heated , bi phenyl is obtained. The reaction is as follows,

Summary

Why alkyl halides give alkyl cyanide with KCN but alkyl isocyanides with AgCN?

Compare the basic hydrolysis ability of m-chloro nitro benzene, p-chloro nitro benzene and chloro benzene.

What is Ulmann reaction? Give example

Why CH₃Cl is more active than C₆H₅Cl towards hydrolysis?

Why benzyl chloride is highly reactive in SN1 reaction ?