Which of the following isomers is steam volatile and why? p-Nitrophenol and o-Nitrophenol

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Q. Which of the following isomers is steam volatile and why? p-Nitrophenol and o-Nitrophenol

Meghalaya MboseMBOSE Meghalaya Intermediate Board 2020Subjective· 1mImportance★★★★★

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Concept understanding — Steam Distillation Volatility

Steam Distillation Volatility – From Intuition to Precision

Imagine you have a pot of water boiling on the stove. Now imagine you drop a few drops of a fragrant oil — say, clove oil — into the water. The oil doesn't dissolve; it floats as a separate layer. Yet, as the water boils, you smell the clove oil strongly in the steam. How did that oil, which boils at a much higher temperature than water, get carried into the vapour?

That is the core puzzle that steam distillation volatility explains.

The Intuition: Two Liquids That Don't Mix

When two immiscible liquids (like water and oil) are heated together, they do not behave like a single liquid. Each liquid exerts its own vapour pressure independently, as if the other weren't there. The total vapour pressure above the mixture is simply the sum of the two individual vapour pressures.

Note

This is completely different from a solution of two miscible liquids (like ethanol and water), where the vapour pressure of each is lowered by the presence of the other (Raoult's law). For immiscible liquids, each acts alone.

Now, boiling occurs when the total vapour pressure equals the surrounding atmospheric pressure. Because the two vapour pressures add up, the mixture reaches atmospheric pressure at a temperature lower than the boiling point of either pure liquid.

That is the key: the mixture boils at a temperature below 100°C (if water is one component) — often well below the boiling point of the organic compound. The organic compound, which would normally require a much higher temperature to boil, now gets carried over in the steam at this lower temperature.

The Precise Statement

P_{\text{total}} = P_{\text{water}} + P_{\text{organic}} = P_{\text{atm}}

When $P_{\text{total}}$ equals atmospheric pressure, the mixture boils. The temperature at which this happens is always less than the boiling point of pure water (100°C at 1 atm) and far less than the boiling point of the pure organic compound.

The vapour that distills over contains both water and the organic compound. The ratio of the masses of the two components in the distillate is given by:

\frac{m_{\text{organic}}}{m_{\text{water}}} = \frac{P_{\text{organic}} \times M_{\text{organic}}}{P_{\text{water}} \times M_{\text{water}}}

where $P$ is the vapour pressure of each component at the distillation temperature, and $M$ is the molar mass.

Why This Matters for Exams

Steam distillation volatility is not a property of the compound alone — it is a property of the mixture with water. A compound is "steam volatile" if it is immiscible with water and has a measurable vapour pressure at 100°C (or below).
The compound does not need to have a low boiling point. Many high-boiling natural oils (like eugenol from clove, boiling point ~254°C) are steam volatile because they have enough vapour pressure at ~99°C to be carried over.
The key condition: the compound must be immiscible with water. If it dissolves even slightly, the simple additive vapour pressure model breaks down. …

Why this formula?

Steam Distillation Volatility: Why the Formula Holds

Steam distillation is a technique used to separate immiscible liquids — typically an organic compound (like an essential oil) and water. The key idea is that the mixture boils when the sum of the vapor pressures equals the external pressure, even though each component's individual boiling point is higher.

The Core Formula

For a mixture of two immiscible liquids (A and water), the total vapor pressure at a given temperature is:

P_{\text{total}} = P_A^\circ + P_{\text{water}}^\circ

where $P_A^\circ$ and $P_{\text{water}}^\circ$ are the vapor pressures of the pure components at that temperature.

The mixture boils when:

P_{\text{total}} = P_{\text{atm}}

Why This Works — The Reasoning

1. Immiscibility → No Mutual Solubility

Since the two liquids do not mix, each exists as a pure phase (not a solution). There is no Raoult's law deviation — each liquid exerts its own pure vapor pressure independently.

In a solution, the vapor pressure of a component is lowered by the presence of the other (Raoult's law).
In an immiscible mixture, each liquid behaves as if the other is not there — they are separate layers.

2. Vapor Pressure Adds Independently

Because the liquids are immiscible, the vapor above the mixture contains molecules from both pure phases. The total pressure is simply the sum:

P_{\text{total}} = P_A^\circ + P_{\text{water}}^\circ

This is Dalton's law of partial pressures applied to two independent pure vapors.

3. Boiling Occurs When Total Pressure Equals Atmospheric Pressure

Boiling happens when the vapor pressure of the liquid equals the external pressure. Here, the "liquid" is the two-phase system. So:

P_A^\circ + P_{\text{water}}^\circ = P_{\text{atm}}

This temperature is lower than the boiling point of either pure component — because each contributes only part of the required pressure.

The Composition of the Distillate

The mole fraction of each component in the vapor (and hence in the distillate) is given by:

y_A = \frac{P_A^\circ}{P_{\text{total}}}, \quad y_{\text{water}} = \frac{P_{\text{water}}^\circ}{P_{\text{total}}}

Since the vapor is in equilibrium with the pure liquids, the mass ratio in the distillate is:

\frac{m_A}{m_{\text{water}}} = \frac{P_A^\circ \cdot M_A}{P_{\text{water}}^\circ \cdot M_{\text{water}}}

where $M_A$ and $M_{\text{water}}$ are molar masses. …

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Whether a nitrophenol isomer forms intramolecular or intermolecular hydrogen bonds decides its volatility — chelated molecules stay separate and volatile, associated molecules do not.

In o-nitrophenol, the $-OH$ and $-NO_2$ groups are on adjacent (ortho) carbons, close enough for the $-OH$ hydrogen to hydrogen-bond intramolecularly with an oxygen of the nearby $-NO_2$ group, forming a stable six-membered ring (chelation). Because this hydrogen bond is used up within each molecule, o-nitrophenol molecules do not associate strongly with one another; they remain relatively small, discrete units with a comparatively low boiling point, and can therefore be carried over with steam — i.e., it is steam volatile.

…

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CBSE 2025Set ANNUAL1 mark
$Q. Assertion [A] : The ortho and para isomers of nitrophenol can be separated by steam distillation. Reason [R] : o-nitrophenol is steam volatile due to intra-molecular hydrogen bonding while p-nitrophenol is less volatile due to inter-molecular hydrogen bonding which causes the association of molecules.$
›Reveal solutionSolution

o- and p-nitrophenol genuinely are separable by steam distillation (Assertion true), and the reason given — intramolecular H-bonding making o-nitrophenol steam-volatile while intermolecular H-bonding (association) makes p-nitrophenol far less volatile — correctly explains why.

o-Nitrophenol: the -OH and -NO2 groups are close enough (ortho) to form an INTRAmolecular hydrogen bond (a 6-membered chelate ring), which prevents the molecule from associating with other molecules; this keeps it relatively volatile, so it steam-distils over.

…

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CBSE 2022Set ANNUAL1 mark
$Q. Fill in the blank: _______ is less volatile among o-nitrophenol and p-nitrophenol.$
›Reveal solutionSolution

o-Nitrophenol forms an intramolecular hydrogen bond (chelation), which keeps its molecules separate and makes it steam-volatile; p-nitrophenol can only form intermolecular hydrogen bonds, which associate its molecules and make it much less volatile.

In o-nitrophenol, the -OH group and the -NO2 group are adjacent (ortho) on the ring, so the -OH hydrogen forms an intramolecular hydrogen bond with the nearby -NO2 oxygen (forming a stable 6-membered ring, chelation). Because this hydrogen bond is 'used up' within the single molecule, o-nitrophenol molecules do not associate strongly with each other, so it has a lower boiling point and is steam volatile.

…

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CBSE 2020Set OC1 mark
$Q. Which of the following isomers is steam volatile and why? p-Nitrophenol and o-Nitrophenol$
›Reveal solutionSolution

Whether a nitrophenol isomer forms intramolecular or intermolecular hydrogen bonds decides its volatility — chelated molecules stay separate and volatile, associated molecules do not.

In o-nitrophenol, the $-OH$ and $-NO_2$ groups are on adjacent (ortho) carbons, close enough for the $-OH$ hydrogen to hydrogen-bond intramolecularly with an oxygen of the nearby $-NO_2$ group, forming a stable six-membered ring (chelation). Because this hydrogen bond is used up within each molecule, o-nitrophenol molecules do not associate strongly with one another; they remain relatively small, discrete units with a comparatively low boiling point, and can therefore be carried over with steam — i.e., it is steam volatile.

…

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Q.Which of the following isomers is steam volatile and why? p-Nitrophenol and o-Nitrophenol

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