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13.25  Why is Wurtz reaction not preferred for the preparation of alkanes containing an odd number of carbon atoms? Illustrate your answer by taking one example.

Wurtz reaction- Wurtz reaction not preferred for the preparation of alkanes containing an odd number of carbon atoms because if we take two dissimilar alkyl halide as a reactant, the product will be a mixture of alkane but the reaction is by a free radical mechanism it will produce an alkene also. example- bromomethane and iodoethane. all the products in the mixture have nearly close...

13.24  Suggest the name of a Lewis acid other than anhydrous aluminium chloride which can be used during ethylation of benzene.

Lewis acid like anhydrous ferric chloride, stannic chloride,  etc. can be used instead of aluminium chloride.

13.23     Out of benzene, m–dinitrobenzene and toluene which will undergo nitration most easily and why?

Nitration is occurred by an electrophilic substitution reaction, in which an electron rich species is attacked by an electron deficient molecule known as an electrophile. In nitration  is used as an electrophile. Here methyl group is electron donating group and the nitro group is electron withdrawing group. So, benzene ring attached with the  group has high electron density and the ring which...

13.22     Arrange the following set of compounds in order of their decreasing relative reactivity with an electrophile, E+

          (b)         Toluene, p-H_{3}C-C_{6}H_{4}-NO_{2},p-O_{2}N-C_{6}H_{4}-NO_{2.} 

Electrophiles are electron deficient species, so they want a nucleophile which donates electrons to them. The higher the electron density on a benzene ring, the higher is the reactivity towards electrophile. since the methyl group is electron donating group it increases the electron density on the benzene ring. And more the number of EWG lesser reactive towards electrophile. therefore, the...

13.22     Arrange the following set of compounds in order of their decreasing relative reactivity with an electrophile, E+

         (a)      Chlorobenzene, 2,4-dinitrochlorobenzene, p-nitrochlorobenzene

Electrophiles are electron deficient species, so they want a nucleophile which donates electrons to them. The higher the electron density on a benzene ring, the higher is the reactivity towards electrophile.  is an electron withdrawing group, it deactivates the benzene ring towards electrophile by decreasing the electron density from the ring. Decreasing order of their reactivity with an...

13.21     Write structures of all the alkenes which on hydrogenation give 2-methylbutane.

Structures of all the alkenes which on hydrogenation give 2-methylbutane. the general structure of the 2-methylbutane is shown here; As per the above structure, the following alkene compounds produces  2-methylbutane by hydrogenation.

13.20     How would you convert the following compounds into benzene?

         (iii)      Hexane

The cyclisation of hexane in the presence of  produce cyclohexane, which on aromatisation gives benzene.

13.20     How would you convert the following compounds into benzene?

         (ii)      Ethene 

By converting ethene to ethyene by reacting with bromine in the presence of carbon tetrachloride. And then heating in presence of alc. KOH followed by . Cyclic polymerisation of ethyne gives us benzene.

13.20     How would you convert the following compounds into benzene?

      (i)          Ethyne

By cyclic polymerisation of ethyne, Ethyne on passing by red hot tube(made of iron) at 873K. Three molecules of ethyne polymerises to form benzene.

13.19     Why does benzene undergo electrophilic substitution reactions easily and nucleophilic substitutions with difficulty?

In benzene, the -electrons are delocalised above and below of the ring. Thus it is an electron rich species. In nucleophilic substitution, the attacking species is an electron-rich in nature, so it becomes challenging to attack benzene because they are repelled by benzene. On the other hand, when the attacking agent is electrophile, which is electron deficient, are easily attracted by the benzene.

13.18     Arrange benzene, n-hexane and ethyne in decreasing order of acidic behaviour. Also give reason for this behaviour.

As the  character increases, the electronegativity of the carbon atom also increases. As a result, there is a development of partial charges in the C-H bond. increasing order of  character- More is the  character, more is the acidic nature of the compound. So, the decreasing order of acidic behaviour is- ethyne(50% ) >benzene(33% ) > hexane(25% )

13.17     Write down the products of ozonolysis of 1,2-dimethylbenzene (o-xylene). How does the result support Kekulé structure for benzene?

Ortho-xylene has two resonant structure so, Since all three products, methylglyoxal, 1, 2-methylglyoxal and glyoxal are cannot be obtained from any one of the two structure (i and ii). Hence we can say that o-xylene is a resonant hybrid of two Kekule structure (I and II)  

13.16     Addition of HBr to propene yields 2-bromopropane, while in the presence of benzoyl peroxide, the same reaction yields 1-bromopropane. Explain and give mechanism.

Addition of  to propene- In this addition, an electrophile  attacks the double bond of the alkene to form  and  carbocations- a secondary carbocation is more stable than the primary carbocation. Thus bromide ion attacks the carbocation to form 2-bromopropane as a major product. (This mechanism is followed by Markovnikov's rule)  

13.15     What effect does branching of an alkane chain has on its boiling point?

On an increase in the branching of the alkane, the boiling point of the alkane is decreased. Alkane experience inter-molecular van der Waals forces. The strong is the force, strong will be the boiling point. When we increase the branching, the surface area of the molecule decreases, as a result, of the van der Waals force also decreases.

13.14     In the alkane H3C- CH2-C(CH3)2-CH2-CH(CH3), identify 1°,2°,3° carbon atoms and give the number of H atoms bonded to each one of these.

 Carbon are those which are directly bonded with only one carbon atoms, the given structure has five  carbon atom and 15 hydrogens attached to it.  carbon are those which are connected with only two carbon atoms. In the above structure, there is two carbon present, and four hydrogens attached to it.  are those which connect with three carbon atom. In the above structure, 1 carbon atom...

13.13         How will you convert benzene into

          (iv)       acetophenone

Benzene on reacting with an acyl chloride in the presence of anhydrous aluminium chloride gives acetophenone and hydrochloric acid as a by-product.

13.13     How will you convert benzene into

       (iii)       p - nitrotoluene

Alkylation of benzene gives in the presence of anhydrous aluminium chloride give methylbenzene and . When methylbenzene reacts with conc. nitric acid and sulphuric acid gives a mixture of para and ortho product of nitrotoluene, which on distillation gives -nitrotoluene.

13.14     How will you convert benzene into

         (ii)      m- nitrochlorobenzene 

Benzene on treatment with conc. nitric acid and sulphuric acid gives nitrobenzene which on further treatment with chlorine in the presence of anhydrous aluminium chloride () gives m-nitrochlorobenzene.

13.13     How will you convert benzene into

       (i)        p-nitrobromobenzene 

Bromination of a benzene ring in the presence of anhydrous  and  gives bromobenzene and after that treating bromobenzene with conc. nitric acid in the presence of sulphuric acid followed by fractional distillation gives p-nitrobenzene.

13.12     Explain why the following systems are not aromatic?

         (iii)     Explain why the following systems are not aromatic (iii)

It disobeys the Huckle rule of (4n+2) electron. According to this rule, it has 2, 6, 10 .... number of  electron but it has 8  electrons.
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