Hydrocarbons Test

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Hydrocarbons Test - 14

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Weekly Quiz Competition

Question 1
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Direction (Q. Nos. 1 - 11) This section contains 11 multiple choice questions. Each question has four choices (a), (b), (c) and (d), out of which ONLY ONE option is correct.
Q. When light is shined on a mixture of chlorine and ethane, chloroethane is formed besides dichloroethane, trichloroethane and several other products. What reaction condition can optimise the yield of chloroethane?

A
Higher reaction temperature

B
High concentration of chlorine gas

C
Excess of ethane reactant

D
Low reaction temperature

Solution

When chlorine and Ethane are taken with chlorine in excess only then we have more than one product like chloroethane, dichloroethane, trichloroethane etc. To avoid this we should take Ethane in excess because when we will take it then in excess then we will have only single time chlorination and we will get monochloroethane.
Question 2
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Which of the following is not a possible termination step in the free radical chlorination of methane?

A

B

C

D

Solution

Termination is the last step. So there shouldn't be any free radical atom remaining. In first option there is Cl• remaining it can't be termination step.The steps in free radical halogenation are as
Question 3
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The major monobromination product which results when ethyl cyclohexane is subjected to free radical bromination, is

A

B

C

D

Solution

It's a tertiary bromide . Br₂ converts into Br+ Be because of free radical abstracts Br form new Br₂ to continue the chain
Question 4
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What is relative reactivity of secondary versus primary hydrogens in free radical bromination of n-butane if the ratio of 1-bromo to 2-bromobutane formed is 7 : 39?

A
200 :1

B
20 :1

C
5 :1

D
1 : 20

Solution

Correct Answer :- b
Explanation : The 2o hydrogens are 20 times more reactive than the 1o ones.
Question 5
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What is the major monobromination product in the following reaction?

A

B

C

D

Solution

Bromination in the presence of light is done by free radical mechanism. The radical on the carbon chain is formed where the radical is most stable. We can’t put radical on the ring as the aromaticity of the ring will be lost. So, it should be put either on the CH₃. However, the CH₃ which is attached to NO₂ ring will not bear stable radical as NO will decrease the stability of radical.(NO₂ is an electron withdrawing group). So the only position is the other CH₃ in option d. There also we have stability due to resonance.
Question 6
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Which is the best description of Hammond postulate?

A
In an exothermic reaction, the transition state is closer in energy of the product

B
The structure of transition state in a reaction is always modeled on the structure of reactants leading to that transition state

C
Transition states are molecular species of finite life time whose properties can be probed through free radical reactions

D
Related species that are similar in energy are also similar in structure

Solution

Hammond's postulate states that the transition state of a reaction resembles either the reactants or the products, to whichever it is closer in energy. In an exothermic reaction, the transition state is closer to the reactants than to the products in energy
Question 7
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What is the major bromination product in the following reaction?

A

B

C

D

Solution

The correct answer is option c

Thus, a racemic mixture is obtained. A racemic mixture is one that has an equal amount of left and right handed enantiomers of a chiral molecule.
Question 8
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During free radical bromination of isobutane, an alkene is produced as by product via disproportionation of the intermediate alkyl free radical. What is this alkene?

A
1-butene

B
2-butene

C
2-methyl propene

D
Both ‘b’ and ‘c’

Solution

As shown in the above mechanism 1-butene and 2-butene cannot be formed by this free radical. release of 1 hydrogen radical gives 2-methyl propene as disproportionated product.
Hence, the correct option is C.
Question 9
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How many dichlorocyclohexane would be produced upon free radical chlorination of chlorocyclohexane?

A
4

B
6

C
8

D
9

Solution

Product 1-Does not have any chiral carbon so only one product
Product 2-Two chiral centres so RR, RS and SR possible
Product 3-Two chiral centres so RR, RS and SR possible
Product 4-Two chiral centres but plane of symmetry exists, so only two isomers.
Question 10
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Which of the following statem ents regarding free radical halogenation of alkane is not true?

A
Hydrogen abstraction by halogen radical in the propagation step is exothermic in both chlorination and bromination

B
Hydrogen abstraction by halogen radical in propagation step is exothermic in chlorination but endothermic in bromination

C
A single halogen radical may bring about halogenation of thousands of alkane molecules

D
Continuous source of energy is required to continue the free radical halogenation reaction

Solution

Hydrogen abstraction by halogen radical in the propagation step is exothermic in chlorination but endothermic in bromination. Hence, the option (A) is an incorrect statement.
Question 11
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Arrange the following in increasing order of boiling points.
I. 3 -methyl pentane
II. 3-chloropentane
III. 3-bromopentane
IV. 3,3-dichloropentane

A
IV < III < II < I

B
I < II < IV < III

C
I < II < III < IV

D
II < I < IV < III

Solution

For boiling point, we have to consider both branching and Molecular mass. In 4 bromopentane molecular mass is nearly the same as compared to 3 chloro pentane but we have 3,3-dichloropentane extended into 2 directions so the boiling point of 3,3-dichloropentane will be more and the other order will be followed by option C.