Q.Arrange the following compounds in the increasing order of their boiling points: $\mathrm{CH_3CH_2CH_2CHO}$, $\mathrm{CH_3CH_2CH_2CH_2OH}$, $\mathrm{H_5C_2}$-O-$\mathrm{C_2H_5}$, $\mathrm{CH_3CH_2CH_2CH_3}$

Concept: Boiling Point Trends — Boiling point depends on intermolecular forces: hydrogen bonding > dipole-dipole > London dispersion. For similar molar mass, alcohols have the highest boiling points due to strong H-bonding, followed by aldehydes (dipole-dipole), then ethers (weak dipole-dipole), and finally alkanes (only London forces).

Reasoning:

1. $CH_3CH_2CH_2CH_3$ (butane) — only weak London forces; lowest boiling point.
2. $H_5C_2\text{-}O\text{-}C_2H_5$ (diethyl ether) — weak dipole-dipole; slightly higher than alkane.
3. $CH_3CH_2CH_2CHO$ (butanal) — stronger dipole-dipole due to polar carbonyl group. …

Why boiling point trends matter

Boiling point is a direct measure of how strongly molecules stick to each other in the liquid phase. To break free into vapour, you must overcome these intermolecular attractions. The stronger the forces, the more energy (higher temperature) needed.

The key players, in order of strength:

• Hydrogen bonding — the strongest (requires H bonded to N, O, or F)
• Dipole-dipole interactions — moderate (polar molecules without O–H or N–H)
• London dispersion forces — weakest (present in all molecules, but increase with molecular size and surface area)

So the first question to ask: Which of these compounds can hydrogen bond? Then, among those that cannot, which has the largest dipole or the largest molecule?

Step-by-step reasoning

1. Identify the compounds and their functional groups

   Compound	Formula	Functional group	Type of molecule
   Butanal	$CH_3CH_2CH_2CHO$	Aldehyde ($-$CHO)	Polar, no O–H
   Butan-1-ol	$CH_3CH_2CH_2CH_2OH$	Alcohol ($-$OH)	Polar, has O–H
   Diethyl ether	$H_5C_2\text{-}O\text{-}C_2H_5$	Ether ($-$O$-$)	Polar, no O–H
   Butane	$CH_3CH_2CH_2CH_3$	Alkane	Non-polar

   All four have the same number of carbon atoms (4 carbons) and roughly similar molecular masses. That’s important — it means differences in boiling point come almost entirely from the type of intermolecular force, not from size.

2. Rank by the strongest intermolecular force present

   • Butan-1-ol has an O–H group. That means it can form hydrogen bonds — the strongest force here. Its boiling point will be the highest.
   • Butanal has a polar C=O bond, giving a permanent dipole. It experiences dipole-dipole interactions, but no hydrogen bonding (the H in $-$CHO is not attached to O, N, or F). So it boils lower than the alcohol.
   • Diethyl ether also has a polar C–O–C linkage, so it too has dipole-dipole interactions. But the dipole is weaker than that of an aldehyde (the C=O bond is more polar than C–O). Also, ether molecules are slightly more “bent” and less able to pack closely. So ether’s boiling point is lower than butanal’s.
   • Butane is non-polar. Only London dispersion forces hold its molecules together. That gives the lowest boiling point of the four.

   So the qualitative order is already clear:

   butane < diethyl ether < butanal < butan-1-ol.

3. Check actual boiling points (for confirmation)

   Compound	Boiling point (°C)
   Butane	−0.5
   Diethyl ether	34.6
   Butanal	74.8
   Butan-1-ol	117.7

Method: Intermolecular Force Analysis

This method ranks boiling points by identifying and comparing the strength of intermolecular forces present in each molecule.

Step 1: Identify the functional group and molecular forces for each compound

Step 2: Rank by force strength

General rule:

Hydrogen bonding > dipole-dipole > London forces only

• Lowest boiling point: Butane — only London forces
• Next: Diethyl ether — dipole-dipole, but no H-bonding …

Common Mistakes in Boiling Point Trends (Aldehyde, Alcohol, Ether, Alkane)

Here are the most frequent errors students make when arranging these compounds, and how to avoid each.

Mistake 1: Forgetting that Alcohols have the strongest intermolecular forces

The error: Students often rank the aldehyde ($CH_3CH_2CH_2CHO$) above the alcohol ($CH_3CH_2CH_2CH_2OH$) because they see the aldehyde has a carbonyl group and think "polar = high boiling point."

Why it's wrong: Alcohols form hydrogen bonds (O–H···O) — the strongest type of intermolecular force among neutral organic molecules. Aldehydes only have dipole-dipole interactions (C=O is polar, but no O–H bond). Hydrogen bonding requires a hydrogen attached to O, N, or F.

How to avoid: Always check for an –OH or –NH group first. If present, that compound will have the highest boiling point in a set of similar-sized molecules.

Correct order: Alcohol > Aldehyde > Ether > Alkane

Mistake 2: Confusing Ether with Aldehyde polarity

The error: Students think diethyl ether ($H_5C_2$–O–$C_2H_5$) has a higher boiling point than the aldehyde because "oxygen is electronegative."

Why it's wrong: Ethers have a bent structure around oxygen, and the two alkyl groups partially cancel the dipole. The net dipole is weak. Aldehydes have a strong, permanent dipole from the C=O bond, giving stronger dipole-dipole interactions.

How to avoid: Compare the dipole moment magnitude:

• Aldehyde: $\approx 2.5\text{ D}$
• Ether: $\approx 1.2\text{ D}$

Stronger dipole → stronger intermolecular forces → higher boiling point.

Mistake 3: Ignoring molecular mass / surface area

The error: Students rank butane ($CH_3CH_2CH_2CH_3$) above ether because "alkanes have London forces."

Why it's wrong: While both are nonpolar, molecular mass and surface area matter. Butane (MW = 58) and diethyl ether (MW = 74) — ether is heavier and has a longer chain, so its London forces are stronger. Butane has the lowest boiling point in this set.

How to avoid: When comparing nonpolar molecules, check:

• Molecular weight (higher = higher boiling point)
• Chain length / branching (more surface area = higher boiling point)

Mistake 4: Misreading the formulas (especially the ether)

The error: Students think $H_5C_2$–O–$C_2H_5$ is ethanol or some other alcohol.

Why it's wrong: The notation $H_5C_2$ means an ethyl group ($-CH_2CH_3$). So this is diethyl ether ($CH_3CH_2OCH_2CH_3$), not an alcohol. There is no O–H bond. …