Question

Which of the compound gives the most stable carbonium ion on dehydration?

• A. ${\mathrm{CH}}_3\mathrm{OH}$
• B. ${\mathrm{CH}}_3\mathrm{CHO}$
• C. ${\left({\mathrm{CH}}_3\right)}_3\mathrm{COH}$
• D. ${\mathrm{CH}}_2=\mathrm{CH}-{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{OH}$

Answer 1

The correct option is C

${\left({\mathrm{CH}}_3\right)}_3\mathrm{COH}$

Explanation of correct option

(C) ${\left({\mathrm{CH}}_3\right)}_{\mathbf{3}}\mathbf{COH}$

Carbocation

• Any even-electron cation with a significant positive charge on the carbon atom is referred to as a carbocation.
• The stability order of carbocation is as follows:

$3°>2°>1°>\stackrel{+}{\mathrm{C}}{\mathrm{H}}_3$

${\left({\mathrm{CH}}_3\right)}_{\mathbf{3}}\mathbf{COH}$

• The reaction is as follows:

${\left({\mathrm{CH}}_3\right)}_3\mathrm{COH}\stackrel{{\mathrm{H}}^{+}}{\to }{\left({\mathrm{CH}}_3\right)}_3{\mathrm{C}}^{+}+{\mathrm{H}}_2\mathrm{O}$

• ${\left({\mathrm{CH}}_3\right)}_3\mathrm{COH}$ form tertiary carbocation.

Explanation of incorrect options

(A) ${\mathbf{CH}}_{\mathbf{3}}\mathbf{OH}$

• The reaction is as follows:

${\mathrm{CH}}_3\mathrm{OH}\stackrel{{\mathrm{H}}^{+}}{\to }{\mathrm{H}}_3{\mathrm{C}}^{+}+{\mathrm{H}}_2\mathrm{O}$

• ${\mathrm{CH}}_3\mathrm{OH}$ form methyl carbocation.

(B) ${\mathbf{CH}}_{\mathbf{3}}\mathbf{CHO}$

• The reaction is as follows:

${\mathrm{CH}}_3\mathrm{CHO}\stackrel{{\mathrm{H}}^{+}}{\to }{\mathrm{CH}}_3\left(\mathrm{HO}\right)\stackrel{+}{\mathrm{C}}\mathrm{H}$

• ${\mathrm{CH}}_3\mathrm{CHO}$ form secondary carbocation.

(D) ${\mathbf{CH}}_{\mathbf{2}}\mathbf{=}\mathbf{CH}\mathbf{-}{\mathbf{CH}}_{\mathbf{2}}{\mathbf{CH}}_{\mathbf{2}}\mathbf{OH}$

• The reaction is as follows:

${\mathrm{CH}}_2=\mathrm{CH}-{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{OH}\stackrel{{\mathrm{H}}^{+}}{\to }{\mathrm{CH}}_2=\mathrm{CH}-{\mathrm{CH}}_2\stackrel{+}{\mathrm{C}}{\mathrm{H}}_2+{\mathrm{H}}_2\mathrm{O}$

• ${\mathrm{CH}}_2=\mathrm{CH}-{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{OH}$ form primary carbocation.

Therefore, the correct option is (C) ${\left({\mathrm{CH}}_3\right)}_{\mathbf{3}}\mathbf{COH}$.