Question

There are some compounds which are as follows:
$1 - phenyl - 1 - butene$, $1 - phenyl - 2 - butene$, $2 - phenyl - 1 - butene$
Which of the following is the correct order for increasing order of reactivity for acid catalysed hydration?
(A) $1 - phenyl - 1 - butene < 1 - phenyl - 2 - buten < 2 - phenyl - 1 - butene$
(B) $1 - phenyl - 2 - butene < 1 - phenyl - 1 - buten < 2 - phenyl - 1 - butene$
(C) $2 - phenyl - 1 - butene < 1 - phenyl - 2 - buten < 1 - phenyl - 1 - butene$

Answer 1

The greater the number of alkyl groups attached to the carbon atoms of the whole chain increases so increases the degree of induction of effect. As the number of alkyl groups increases so increases the electron density on the alkene and makes it more ready for hydration.

Complete answer:
Acid catalysed hydration is defined as the process of hydration or the adding of water ions to the chain of unsaturated hydrocarbon so as to convert it into a saturated hydrocarbon and the whole process of addition takes place in an acidic medium so as to making it easier for the hydrogen ion and hydroxide ion to attack at the positive and negatively charged sites.
Now the compounds given to us are $1 - phenyl - 1 - butene$, $1 - phenyl - 2 - butene$, $2 - phenyl - 1 - butene$
Now the first thing we would be observing is which compound has the greatest number of alkyl groups attached to its carbon which due to the induction effect creates a partial positive charge on the carbon. Alkyl groups posse’s electron gaining effect rather than electron donating effect and also the phenyl group has more electron gaining effect rather than donating, therefore, to whichever carbon the alkyl group or phenyl group is attached that carbon will have partial positive charge.
Therefore, the answer should be: $1 - phenyl - 2 - butene < 1 - phenyl - 1 - buten < 2 - phenyl - 1 - butene$
The answer is such because as the number of alkyl groups increases attached to the carbon from left to right above the positive charge on the carbon atom increases and so increases its eagerness to react and form a compound. In the first compound there are less alkyl groups and the phenyl group are also far from the double bond or possesses a negative induction effect on the carbon. Therefore, the reactivity increases as the pi-electrons of the foible bond are donated to the proton or hydrogen ion and the positive charged intermediate is formed, the more the state intermediate the less reactivity it will have. therefore the $2 - phenyl - 1 - butene$ is the most reactive.
Hence the required answer is $1 - phenyl - 2 - butene < 1 - phenyl - 1 - buten < 2 - phenyl - 1 - butene$.

Note:
For the above given process in which you have to see the reactivity order of the alkenes, if the alkyl groups are the same then what we need to see is how stable the intermediate is. If the intermediate is stable, it will not undergo further reaction and the reactivity of the alkene is less compared to the other chain or compound.