Q49E. - Organic Chemistry (Mcmurry)

Question

Choose the alcohol from each pair below that would react faster with HX to form the corresponding alkyl halide.

Step-by-Step Solution

Answer

(a) Propan-2-ol reacts faster than Propan-1-ol.

(b) 1-methylcyclopentan-1-ol reacts much faster than 2-methylcyclopentan-1-ol.

(c)2-methylbutan-1-ol reacts faster than 2, 2-dimethylpropan-1-ol.

Types of alcohol

If the –OH group is present on the carbon atom, which is bonded to one carbon atom—it is known as primary alcohol.                                                                                       

If the –OH group is present on the carbon atom, which is bonded to two carbon atoms—it is known as secondary alcohol.

If the –OH group is present on the carbon atom, which is bonded to three carbon atoms—it is known as tertiary alcohol.

Reactivity of Alcohols

Primary and secondary alcohol reacts much more slowly at high temperatures. The reaction of HX with tertiary alcohol is so rapid that it’s often carried out simply by bubbling pure HCl into a cold ether solution of the alcohol. The reactivity order of alcohol is given as:

              Methyl <Primary <Secondary<Tertiary

Identifying the alcohol from pair that would react faster with HX

(a)

                             Propan-1-olPropan-2-ol

(a) Propan-2-ol reacts faster than Propan-1-ol, because secondary alcohol reacts faster than primary alcohol with HX to form the corresponding alkyl halide.

                              1-methylcyclopentan-1-ol2-methylcyclopentan-1-ol

1-methylcyclopentan-1-ol reacts much faster than 2-methylcyclopentan-1-ol, because tertiary alcohol reacts faster than secondary alcohol with HX to form the corresponding alkyl halide.

                               2, 2-dimethylpropan-1-ol2-methylbutan-2-ol

2-methylbutan-2-ol reacts faster than 2, 2-dimethylpropan-1-ol, because tertiary alcohol reacts faster than primary alcohol with HX to form the corresponding alkyl halide.