Question

Most stable carbonium ions ?
A) \[{{C}_{2}}^{+}{{H}_{5}}\]
B) \[{{(C{{H}_{3}})}_{3}}{{C}^{+}}\]
C) \[{{({{C}_{6}}{{H}_{5}})}_{3}}{{C}^{+}}\]
D) \[{{C}_{6}}{{H}_{5}}{{C}^{+}}{{H}_{2}}\]

Answer 1

Hint: The carbocation or a carbonium ion has a carbon atom which contains a positive charge and three bonds attached to it. Carbocation is a very reactive molecule which is unstable dur to its incomplete octet. The hybridisation of the carbon in the carbocation is \[s{{p}^{2}}\]and the geometry is trigonal planar.

Complete step-by-step answer:
There are different types of carbocation. The first is the methyl carbocation in which we have no carbon atom attached to the carbon with a positive charge. If we have one carbon attached to the carbon with positive charge then it is primary carbocation and if we have two carbons attached to the carbon with positive charge it is called secondary carbocation and if three carbons are attached to the carbon with positive charge then it is a tertiary carbocation.

In option A \[{{C}_{2}}^{+}{{H}_{5}}\] it can be written as \[C{{H}_{3}}-C{{H}_{2}}^{+}\]. So in this the methyl group will tend to donate its electron to the carbon with positive charge because it is a methyl group and shows +I effect. So it will help in stabilising the compound to some extent.

In option B we have three methyl groups attached to the positive carbon atom so all the three methyl groups will tend to give their electrons to the positive carbon atom being an alkyl group and showing +I effect. So it will be more stable than option A as it has a more alkyl group.

In option C we have a positively charged carbon atom attached to three benzene rings so it will show resonance effect. So it will be stabilised by three resonance benzene rings. As we know that the compound which is stabilized by +R effect is more stable than the compound which is stabilized by +I effect. So this compound is stabilized by +R effect so it will be more stable than option a and option b.

In option D we have a benzene ring attached to the \[{{C}^{+}}{{H}_{2}}\]so it will be stabilised by one benzene ring only while the compound of option c is stabilised by 3 benzene rings, so option c will be more stable than option d due to more resonance effect.

So the correct option is option ‘C’.

Note:Resonance effect is the releasing of electrons or the withdrawal effect of the electrons which is attributed to the particular substituent by the delocalisation of the pi electrons. There are two types of resonance effect, the one is positive in which the groups release their electrons to other molecules. There is a negative resonance effect in which the groups withdraw the electrons from other molecules by the process of delocalisation.