Question

The decreasing order of boiling points of the following alcohols

• A. $3-\mathrm{methylbutan}-2-\mathrm{ol}>2-\mathrm{methylbutan}-2-\mathrm{ol}>\mathrm{pentan}-1-\mathrm{ol}$
• B. $\mathrm{pentan}-1-\mathrm{ol}>3-\mathrm{methylbutan}-2-\mathrm{ol}>2-\mathrm{methylbutan}-2-\mathrm{ol}$
• C. $2-\mathrm{methylbutan}-2-\mathrm{ol}>3-\mathrm{methylbutan}-2-\mathrm{ol}>\mathrm{pentan}-1-\mathrm{ol}$
• D. $2-\mathrm{methylbutan}-2-\mathrm{ol}>\mathrm{pentan}-1-\mathrm{ol}>3-\mathrm{methylbutan}-2-\mathrm{ol}$

Answer 1

The correct option is B

$\mathrm{pentan}-1-\mathrm{ol}>3-\mathrm{methylbutan}-2-\mathrm{ol}>2-\mathrm{methylbutan}-2-\mathrm{ol}$

The explanation for the correct option :

Option (b) $\mathrm{pentan}-1-\mathrm{ol}>3-\mathrm{methylbutan}-2-\mathrm{ol}>2-\mathrm{methylbutan}-2-\mathrm{ol}$

• 3-methylbutan-2-ol is a secondary alcohol

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

• ​2-methylbutan-2-ol is a tertiary alcohol

${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{C}\left({\mathrm{CH}}_3\right)\left(\mathrm{OH}\right){\mathrm{CH}}_3$

• pentan-1-ol is a primary alcohol

${\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{OH}$

• The ease and quantity of Hydrogen-bonding among alcohol molecules determine the boiling point of alcohols.
• Tertiary alcohols have less Hydrogen bonding and consequently a lower boiling point than secondary alcohols that are less inhibited.
• As a result, the boiling point sequence is

Primary alcohol $>$ Secondary alcohol $>$ Tertiary alcohol

• As follows: $\mathrm{pentan}-1-\mathrm{ol}>3-\mathrm{methylbutan}-2-\mathrm{ol}>2-\mathrm{methylbutan}-2-\mathrm{ol}$

So, the correct option is (b).