Which one is more reactive between vinyl bromide and allyl bromide in SN2 reaction ?

What is the function of Fe in chlorination or bromination of aromatic compounds?

Chlorination or bromination of aromatic compounds in the presence of iron( Fe ) is an example of electrophilic substitution reaction.

In these halogenation reaction iron dust is used as halogen carrier indirectly. The iron metal first reacts with halogen to form expected ferric halide which used as Lewis acid or halogen carrier.

For example, chlorination or bromination of benzene produced chlorobenzene or bromo benzene .The reaction are as follows,

Which one is more reactive between vinyl bromide and allyl bromide in SN2 reaction?

Primary alkyl halides are more effective towards SN2 reaction .Hence, allyl bromide under goes SN2 reaction easily. 

But, in case of vinyl bromide, the lone pair of bromine atom participate in resonance with the π electron of vinyl bromide molecule. 

Thus C – Br bond gets partially double bond character. So it is not easy to break the C –Br bond.

Consequently, allyl bromide is more reactive than vinyl bromide in SN2 reaction.

Which iodide of trans-1,5 diiodopent-2-ene is more effective in SN1 reaction?

Generally iodine is a good leaving group. But between this two iodine atom of trans-1,5diiodopent-2-ene,  iodine one acts as good leaving group than iodine two in SN1 reaction . 

Because ,on ionization of iodine one the carbo cation produced is more stable due to resonance . But ,on ionization of second iodine ,the carbo cation produced have no stability.

Hence, iodine one is more effective than iodine two in SN1 reaction .

Compare the ability of ethyl bromide, propyl bromide and neopentyl bromide in SN2 reaction?

Primary alkyl halide is more effective in SN2 reaction. Now all of the above three compounds are primary alkyl bromide. Yet their ability towards SN2 reaction are different . 

Among the three alkyl bromide, ethyl bromide is most effective due to less steric effect of methyl group . 

Now, the size of ethyl group is greater than methyl group, so the steric effect of ethyl group is slightly high . 

Hence propyl bromide is slightly less reactive than ethyl bromide . But in case of neopentyl bromide , the rate of SN2 reaction is very much less due to steric crowding of three methyl groups.  

Therefore, the ability order of ethyl bromide, propyl bromide and neopentyl bromide in SN2 reaction should be in the following order,

                          Ethyl bromide >  propyl bromide > neopentyl bromide.

Why iodo arene does not prepared by direct iodination of aromatic compounds?

Iodo arene can not prepared by direct iodination of aromatic ring . Because, through iodination reaction , the products are iodo arene and HI . 

Now, HI is a good reducing agent ,which reduced iodo arene to convert previous aromatic hydrocarbon and iodine . 

That is the reaction becomes reversible. Consequently, the expected iodo arene is not available.

But if the reaction is carried out in presence of oxidizing agents like nitric acid, iodic acid or mercuric oxide, the backward reaction does not occur . 

Because the used oxidizing agent oxidized HI to form I₂. In this way HI is eliminated from the reaction mixture.Thus, we can get the expected product iodo arene.

Which is the best method for preparation of iodo arene or aryl iodide?

If the mixture of benzene diazonium chloride and saturated solution of KI are heated together , the iodo arene or aryl iodide is obtained. This is the best method for the preparation of iodo arene.

For example, benzene diazonium chloride with KI solution gives iodo benzene .

Summary

Which one is more reactive, vinyl bromide or allyl bromide in SN2 reaction ?

Which iodide of trans-1,5 diiodopent-2-ene is more effective in SN1 reaction?

Ability of ethyl bromide, propyl bromide and neopentyl bromide in SN2 reaction ?

What is the function of iron in chlorination or bromination of aromatic compounds?

Why iodo arene does not prepared by direct iodination of aromatic compounds?

Which is the best method for preparation of iodo arene or aryl iodide?

What is Raschig process ? Click here .