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

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

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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