Problem 74 - A Complete Resource Book in Chemistry for JEE Main

Question

The correct order of boiling points for primary \(\left(1^{\circ}\right)\), secondary \(\left(2^{\circ}\right)\) and tertiary \(\left(3^{\circ}\right)\) alcohols is (a) \(1^{\circ}>2^{\circ}>3^{\circ}\) (b) \(3^{\circ}>2^{\circ}>1^{\circ}\) (c) \(2^{\circ}>1^{\circ}>3^{\circ}\) (d) \(2^{\circ}>3^{\circ}>1^{\circ}\)

Step-by-Step Solution

Answer

The correct order is (a) \(1^{\circ}>2^{\circ}>3^{\circ}\).

Step 1: Understanding Alcohol Structures

Primary alcohols (\(1^{\circ}\)) have one alkyl group attached to the carbon with the hydroxyl (-OH) group. Secondary alcohols (\(2^{\circ}\)) have two alkyl groups attached, and tertiary alcohols (\(3^{\circ}\)) have three. Greater substitution leads to a decrease in intermolecular hydrogen bonding.

Step 2: Analyzing Boiling Point Trends

Boiling points are largely determined by intermolecular forces. In alcohols, hydrogen bonding is the main intermolecular force. The less the steric hindrance (bulkiness around -OH), the stronger the hydrogen bonds formed. Thus, primary alcohols can form stronger hydrogen bonds than secondary, which are stronger than tertiary.

Step 3: Identifying Impact of Substitution on Hydrogen Bonding

With increased substitution, steric hindrance increases, weakening hydrogen bonding (primary > secondary > tertiary). Due to this, primary alcohols have the highest boiling points compared to secondary and tertiary alcohols.

Step 4: Determining the Correct Order of Boiling Points

Considering the impact of hydrogen bonding and steric hindrance, the order of boiling points from highest to lowest is primary > secondary > tertiary.

Key Concepts

Primary Alcohols

Primary alcohols are the simplest type of alcohol, with only one alkyl group attached to the carbon atom that is bonded to the hydroxyl group (-OH). This basic structure allows for less steric hindrance, meaning the -OH group can more easily form hydrogen bonds with neighboring molecules. These hydrogen bonds are particularly strong in primary alcohols, contributing significantly to higher boiling points compared to more substituted alcohols.
The ability to form these strong hydrogen bonds is a key reason why primary alcohols usually have higher boiling points than their secondary and tertiary counterparts. If you're comparing boiling points, always consider the simplicity and efficiency with which primary alcohols establish hydrogen bonding.

Secondary Alcohols

Secondary alcohols have a structure that includes two alkyl groups attached to the carbon that carries the hydroxyl group. This moderate level of substitution results in some steric hindrance, though not as much as in tertiary alcohols.
Due to these two alkyl groups, secondary alcohols show an intermediate strength in hydrogen bonding between molecules, generally resulting in moderate boiling points.
It's important to remember that while secondary alcohols can still form hydrogen bonds, their increased steric hindrance compared to primary alcohols somewhat decreases the effectiveness of these interactions, which in turn affects their boiling point that lies between primary and tertiary alcohols.

Tertiary Alcohols

Tertiary alcohols are characterized by having three alkyl groups attached to the carbon holding the -OH group, making them the most substituted among the alcohol types. This heavy substitution introduces significant steric hindrance.
One consequence of this increased bulk is the considerable weakening of hydrogen bonds between the alcohol molecules.
Due to the strength of these weakened interactions, tertiary alcohols tend to have the lowest boiling points among alcohols. The sheer size and complexity of the molecule prevent the -OH group from forming strong, consistent hydrogen bonds.

Hydrogen Bonding

Hydrogen bonding is a critical intermolecular force in alcohols, substantially influencing their boiling points. This type of bonding occurs when a hydrogen atom is attracted to an electronegative atom such as oxygen in nearby molecules.
In alcohols, the hydroxyl group (-OH) is the primary site for these interactions, allowing molecules to "stick" to one another more effectively. The more hydrogen bonding that occurs, the higher the boiling point of the alcohol.
Primary alcohols, with less steric hindrance, can form these bonds more efficiently, following secondary and then tertiary alcohols, where the bulkiness limits this bonding. Understanding the role of hydrogen bonding will help you predict boiling points and other physical properties.

Steric Hindrance

Steric hindrance refers to the spatial constraints that occur from the presence of bulky groups around a functional group such as the -OH in alcohols.
When discussing steric hindrance, think of it as the 'blockage' that prevents molecular interactions, particularly hydrogen bonding.
As more alkyl groups are attached to the carbon bearing the hydroxyl group, steric hindrance increases. This is why tertiary alcohols, with three bulky groups, show significant hindrance, leading to weaker hydrogen bonding and, consequently, lower boiling points compared to primary and secondary alcohols. Keeping this concept in mind helps clarify why primary alcohols boast stronger intermolecular forces and why boiling points decrease as alcohols become more substituted.