Question

Four students, W, X , Y and Z made the following statements about alkanes and alkenes.
W Bromine is made to react with the alkanes by electrophilic substitution.
X Bromine is made to react with the alkenes by free-radical addition mechanism.
Y Alkenes can be oxidized by acidified manganate(VII) ions.
Z Alkenes are formed from alkanes by cracking.
Which two students are correct?
(a) W and X
(b) W and Z
(c) X and Y
(d) Yand Z

Answer 1

Hint: To find out which statements from the above given statements are incorrect, we should know about the reactions of the alkanes and the alkenes i.e. their chemical reactions and method of preparation and then, we can easily answer this statement.

Complete answer:
To know which of the following statements made by the students are correct, we will discuss the statements one by one as; According to the student W, bromine is made to react with the alkanes by electrophilic substitution. Now first of all let’s discuss what is electrophilic substitution. By the electrophilic substitution we mean replacement or the substitution of the functional group with the electrophile. But if it is not so, bromine reacts with the alkanes by a free radical mechanism in the presence of sunlight. So, this statement made by student W is incorrect.
According to the student X, bromine is made to react with the alkenes by free-radical addition mechanism. Now first of all let’s discuss what a free-radical addition mechanism is. By the free-radical addition mechanism, we mean that reaction which involves the free radicals and their addition. But if it is not so, bromine reacts with the alkenes by electrophilic substitution. So, this statement made by student W is incorrect.
According to the Y, alkenes on reaction when the acidified manganate(II) ions, it results in the formation of the alcohols. So, this statement made by student Y is correct. According to the student Z, when alkanes undergoes thermal decomposition, it results in the formation of the smaller alkanes along with the formation of the alkenes and this process is known as the cracking. So, this statement made by student Z is also correct. So, the correct statements are Y and Z.

Hence, option (d) is correct.

Note: Alkanes are the saturated hydrocarbons which don't consist of the double or triple bond whereas on the other hand, alkenes are the unsaturated hydrocarbons which consist of carbon- carbon double in its compounds.