Q20 of 37 Page 1

Assertion: Aryl halides undergo nucleophilic substitution reactions with ease.

Reason: The carbon halogen bond in aryl halides has partial double bonds character.


The electron pairs on halogen atom of aryl halides are in conjugation with the benzene ring. This causes delocalisation of electrons on C-X bond (where X is a halogen) due to which a partial double bond character arises from the resonance structures given below. As a result bond cleavage is difficult, making it hard for the substituent to react. Thus these aryl halides very less reactive towards nucleophilic substitution reactions.


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18

Assertion: The acidic strength of halogen acids varies in the order HF>HCl>HBr>HI

Reason: The bond dissociation enthalpy of halogen acids decreases in the order HF>HCl>HBr>HI


 19

Assertion: C2H5OH is a weaker base than phenol but is a stronger nucleophile than phenol. (1)

Reason: In phenol the lone pair of electrons on oxygen is withdrawn towards the ring due to resonance.


 21

Calculate the number of lone pairs on central atom in the following molecule and predict the geometry.

XeF4


 22

The rate of a reaction depends upon the temperature and is quantitatively expressed as

k = A e


i) If a graph is plotted between log k and 1/T, write the expression for the slope of the reaction?


ii) If at under different conditions Ea1 and Ea2 are the activation energy of two reactions. If Ea1 = 40 J / mol and Ea2 = 80 J / mol. Which of the two has a larger value of the rate constant?