Explain the following: i) Beta-1,3-diene gives 1,2- and 1,4-addition products. ii) Addition of HBr to `H_(2)C=CHCH_(2)C-=CH` gives `CH_(3)CHBrCH_(2)C-
Explain the following:
i) Beta-1,3-diene gives 1,2- and 1,4-addition products.
ii) Addition of HBr to `H_(2)C=CHCH_(2)C-=CH` gives `CH_(3)CHBrCH_(2)C-=CH`.
iii) Alkenes in decreasing order of reactivity towards electrophilic addition.
a) `CICH_(2)CH=CH_(2)`
b) `(CH_(3))_(2)C=CH_(2)`
c) `CH_(3)CH=CH_(2)`
d) `H_(2)C=CHCl` ltbgt iv) Stereochemical structure of the reaction product of `Br_(2)` with a) cis-2-butene and b) trans-2-butene.
v) Why alkynes are generally less reactive than alkenes shorter than that of n-butane.
vi) The central carbon-carbon bond in buta-1, 3-diene is shorter than that of n-butane.
vii) Arrange following alkenes in decreasing order of stability towards acid-catalysed hydrocarbons.
a) 1-phyenyl-1-butene, 1-phenyl-2-phenyl-2-butene
b) 2-methylpropene, cis-2-butene, trans-2-butene
c) 1-hexene, 2-methyl-1-pentene, 2-hexene.
1 Answers
ii) The addition occurs at double bond because isolated double bond is more reactive than an isolated triple bond in electrophilic additions.
iii) Electron releasing alkyl groups make pi-bond more electron rich and more reactive. While electrron with-drawing halogen atoms make the pi-bond more electron poor and less reactive. The order is:
b) Two R gt (C ) one R gt(a) one R and a Cl gt (d) Cl on a double bonded carbon.
iv) 2,3-Dibromobutane a) racemic b) meso.
v) The reaction of alkyne or alkene with elecrophilic reagent proceeds through the formation of carbocation. Since the alkyl carbocation from the alkene group is more stable than the vinyl carbonation from alkyne group, the `DeltaH` for its formation is less in case of alkene and hence alkene reacts faster than alkyne.
vi) Buta-1.3-diene `(H_(2)C=CH-CH=CH_(2))` has `sp^(2)-sp^(2) C-C` bond length, while n-butane has `sp^(3)-sp^(3) C-C` bond length. More `s` character in hybridisation, lesser is bond length.
vii) a) 2-phenyl-2-butene gt 1-phenyl-2-butene gt 1-phenyl-1-butene.
c) 2-methyl-1-pentene gt 2-hexanegt1-hexene.