Question

(a) Account for the following :
(i) Electrophilic substitution reactions in haloarenes occur slowly.
(ii) Haloalkanes, through polar, are insoluble in water.
(b) Arrange the following compounds in increasing order of reactivity towards `S_(N)2` displacement: 2-Bromo-2-Methylbutane, 1-Bromopentane, 2-Bromopentane

Answer 1

(a) (i) Halogen atoms of haloarenes has -I effect which has some tendency to withdraw electrons from benzene ring. As a result, the ring gets somewhat deactivated as compared to benzene and hence electrophilic substituation reactions in haloarenes occur slowly.
(ii) Haloalkanes, though polar, are insoluble oon water because less energy is released when new attractins are set up between the haloalkanes and water molecules because these are not as strong as the hydrogen bonds already existing between `H_(2)O` molecules. As a result, holoalkanes are insoluble in water.
(b) `{:(" "CH_(3)),(" "|),(H_(3)C-HC-C-CH_(3)H_(3)CH_(2)CH_(2)C-Br","H_(3)CH_(2)CH_(2)CH_(2)C-CHCH_(3)),(""" |" " "(B)(1^(@))" "|" "(C)(2^(@))),(" "Br " "Br),(" "(A)(3^(@))):}`
For `SN_(2)` mechanism, order of reactivity is
`1^(@)gt2^(@)gt3^(@)`
i.e ` BgtCgtA`
i.e 1-Bromopentane `gt2-` Bromopentane `gt2-`Bromo-2-Methylbutane.