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1.

Ethane is subjected to combustion process. During the combustion the hybrid state of carbon changes from:A. `sp^(2)` to `sp^(3)`B. `sp^(3)` to `sp`C. `sp` to `sp^(3)`D. `sp^(2)` to `sp^(2)`

Answer» Correct Answer - b
`CH_(3)-underset(sp^(3))underset(uarr)(CH_(3))+(7)/(2)O_(2)overset(Delta)(to)underset(sp)underset(uarr)(2CO_(2))+3H_(2)O`
Discussion
2.

Arrange the following alkanes in decreasing order of their heats of combustion. (i) `CH_(3)-overset(CH_(3))overset(|)underset(("Neo-pentane")(i))underset(CH_(3))underset(|)(C)-CH_(3)` (ii) `CH_(3)-underset((Iso-pentane)(ii))underset(CH_(3))underset(|)(CH)-CH_(2)-CH_(3)` (iii) `CH_(3)-CH_(2)-underset(("n-pentane"))(CH_(2)-CH_(2)-CH_(3)`A. `(i) gt (ii) gt (iii)`B. `(iii) gt (i) gt (ii)`C. `(iii) gt (ii) gt (i)`D. `(i) gt (iii) gt (ii)`

Answer» Correct Answer - c
(iii) is most stable because of less steric repulsion `:.` least of combustion.
Discussion
3.

Arrange the compounds I,II and III decreasing order of their heats of combustion:A. `II gt I gt III`B. `I gt II gt III`C. `III gt II gt I`D. `IIIgt I gt II`

Answer» Correct Answer - c
More the carbon in alkane. More will heat of combustion.
Discussion
4.

`CH_(2)-CH_(2)-CH_(2)-CH_(3)overset(Br_(2)//hv)(to)` Major product in the above reaction is:A. Racemic mixtureB. MesoC. DiastereomersD. Constitutinal isomers

Answer» Correct Answer - a
`2^(@)` halide is major, due to formation of `sp^(2)`-hybridized free radical racemization take place.
Discussion
5.

A hydrocarbon A (V.D. = 36) forms only one monochloro substitution product. A will be:A. Iso-pantaneB. neo-butaneC. cyclopentaneD. methyl-cyclohexane

Answer» Correct Answer - b
Mol. Wt. `=2xxV.d = 72`
`:.` Mol. Formula `=C_(5)H_(12)`, structure `CH_(3)-underset(CH_(3))underset(|)overset(CH_(3))overset(|)(C)-CH_(3)` neo-pentane
Discussion
6.

Halogenation is a substitution reaction, where halogen replaces one or more hydrogens of hydrocarbon Chlorine free radical make `1^(@), 2^(@), 3^(@)` radicals with almost equal ease, whereas bromine free radicals have a clear preference for the formation of tertiary free radicals. So, bromine is less reactive, and more selectrive whereas chlorine is less selective and more reactive. The relative rate of abstraction of hydrogen by `Br^(**)` is `underset(1600)(3^(@)) gt underset(82)(2^(@)) gt underset(1)(1^(@))` `underset(5)(3^(@)) gt underset(3.8)(2^(@)) gt underset(1)(1^(@))` `CH_(3)-overset(CH_(3))overset(|)(CH)-CH_(3)overset(Cl_(2)//hv)(to)` product Major product in the above reactio is:A. `CH_(3)-overset(CH_(3))overset(|)(CH)-CH_(2)-Cl`B. `CH_(3)-overset(CH_(3))overset(|)underset(Cl)underset(|)(CH)-CH_(3)`C. `CH_(3)-CH_(2)-CH_(2)-Cl`D. `CH_(3)-underset(Cl)underset(|)(CH)-CH_(2)-CH_(3)`

Answer» Correct Answer - a
`o x-C-overset(C)overset(|)(C)-overset(C)overset(|)(C)-C`
`CH_(3)-overset(CH_(3))overset(|)(CH)-CH_(2)-Cl underset(1^(@))("Relative formation is") = 9xx1`, relative formation of `3^(@) = 5xx1`
`:.` %yield `= 9//14xx100`
Discussion
7.

Among the following free radical bromination reactions, select those in which `2^(@)` halide is the major product- A. P, Q, R, SB. P, R, UC. P, R, S, TD. P, Q, R, S, T

Answer» Correct Answer - b
Bromine is more selective `:.` it will form `3^(@)` halide if there is presence of `3^(@)` hydrogen. `:.` Q, S, T form `3^(@)` halide as major and P, R, U form `2^(@)` halide as major.
Discussion
8.

Which of the followiing compounds can be best prepared by Wurtz-reaction?A. Iso-butaneB. n-butaneC. n-pentaneD. Iso-pentane

Answer» Correct Answer - b
`2CH_(3)-CH_(2)-Cloverset(Na)underset("Dry ether")(to)CH_(3)-CH_(2)-CH_(2)-CH_(3)`
Open chain alkane product having even number of carbons and symmetrical can be achieved by Wurtz reaction
Discussion
9.

The correct order of reactivity of I, II & III towards addition reactions is:A. I gt III gt IIB. I gt II gt IIIC. III gt II gt ID. III gt I gt II

Answer» Correct Answer - b
Cyclopropane is most unstable therefore more reactive toward addition reaction. Cyclopentene is most stable `:.` less reactive.
Discussion
10.

Among the structures given, select the enantiomers:A. I and IIB. I and IIIC. II and IIID. I, II and III

Answer» Correct Answer - c
I is meso, II and III one mirror image of each other.
Discussion
11.

Draw an energy profile diagram for a three step reaction in which first step is slowest and last step is fastest. (Assume that reaction is exothermic)A. B. C. D. None of these

Answer» Correct Answer - c
Acc. To Arrehenious equation `K = Ae^(-E_(a)//Rt)`
`:.` greater is `E_(a)`, the smaller is K i.e., rate of reaction `prop (1)/("height of energy barrier")`
It should be noted that step leading to the height peak will not be rate determining step id the concentration used in some other step is sufficiency high. Since first step is rate determining step.
`:.` First peak should be high as is (c).
Discussion
12.

Order of the bond strength of C-H bonds involving `sp, sp^(2)` and `sp^(3)` hybridized carbon atom is:A. `sp gt sp^(2) gt sp^(3)`B. `sp^(3) gt sp^(2) gt sp`C. `sp^(2) gt sp^(3) gt sp`D. `sp^(2) gt sp gt sp^(3)`

Answer» Correct Answer - a
EN value `sp gt sp^(2) gt sp^(3)` (more the electronegativity more will be the bond strength)
Discussion
13.

Double bond equivalent of cubane is : A. `4`B. `5`C. `6`D. `7`

Answer» Correct Answer - b
Cubane has molecular formula `(C_(8)H_(8))` from the given structure.
Discussion
14.

Ethyle iodide and n-propyl iodide are allowed to undergo Wurtz reaction. The alkane which will not be obtained in this reaction is:A. butaneB. propaneC. pentaneD. hexane

Answer» Correct Answer - b
`Et-I+ CH_(3)-CH_(2)-CH_(2)-Ioverset(Na)underset(DE)(to)Et-Et+CH_(3)-CH_(2)-CH_(2)-Et+"hexane"`
Discussion