Chemistry Class-11: Chapter –13. Hydrocarbons Part – 4

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III. Short Answer Type

Question: 20

Why do alkenes prefer to undergo electrophilic addition reaction while arenas prefer electrophilic substitution reactions? Explain

Answer:

Both alkenes and arenas are electron-rich. Therefore undergo electrophilic reactions. Olefins undergo addition reactions because addition of a reagent to an olefin gives a more stable product as sp2 hybridisation changes to sp3 hybridisation. Addition to the double bond of an arena would give a product with less or no resonance stability hence addition is difficult arenas. On the other hand in substitution reaction resonance stabilisation is retained therefore, arenas undergo substitution reaction.

Question: 21

Alkynes on reduction with sodium in liquid ammonia form trans alkenes. Will the butane thus formed on reduction of 2-butyne show the geometrical isomerism?

Answer:

2-Butene is capable of showing geometrical isomerism.

Question: 22

The rotation about bond is restricted because of repulsion between electron cloud of bonds on either carbon atoms.

Answer:

The rotation about bond is restricted because of repulsion between electron cloud of bonds on either carbon atoms.

Question: 23

Draw Newman and Sawhorse projections for the eclipsed and staggered conformations of ethane. Which of these conformations is more stable and why?

Answer:

Question: 24

The intermediate carbocation formed in the reactions of HI, and with propene is the same and the bond energy of , and is , and respectively. What will be the order of reactivity of these halogen acids?

Answer:

Bond dissociation energy is least for and maximum for therefore, order of reactivity will be .

Question: 25

What will be the product obtained as a result of the following reaction and why?

Product obtained

Product Obtained

Product obtained

Answer:

Propyl chloride forms less stable carbocation with anhydrous which rearranges to a more stable carbocation and gives isopropyl benzene as the product of the reaction.

Question: 26

How will you convert benzene into (i) (ii)

Answer:

Question: 27

Arrange the following set of compounds in the order of their decreasing relative reactivity with an electrophile. Give reason.

Set of compounds in the order

Decreasing Reactivity

Set of compounds in the order

Answer:

The +R effect of and has a – R effect. Relative reactivity of the substituted benzene rings is as follows:

Question: 28

Despite their - I effect, halogens are o- and p-directing in haloarenes. Explain

Answer:

Halogens attached to benzene rings exert – I and +R effect. +R effect dominates – I effect and increases the electron density at ortho and para positions of the benzene ring with respect to halogens.

Question: 29

Why does presence of a nitro group make the benzene ring less reactive in comparison to the unsubstituted benzene ring. Explain.

Answer:

Question: 30

Suggest a route for the preparation of nitrobenzene starting from acetylene?

Answer:

Question: 31

Predict the major product (s) of the following reactions and explain their formation

Answer:

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