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Q8.20:
What is the difference between distillation, distillation under reduced pressure and steam distillation ?
Solution :
1. Definitions and Operational Parameters
The selection of a separation technique depends on the thermal stability of the substances and the difference in their boiling points.
| Technique | Mechanism | Primary Application |
|---|---|---|
| Simple Distillation | Vaporization followed by condensation. | Liquids with large differences in boiling points ($> 20\,K$) and stable at their boiling points. |
| Reduced Pressure Distillation | Reduction of external pressure to lower the boiling point of the liquid. | Liquids that decompose at or below their normal boiling points. |
| Steam Distillation | Distillation of a mixture of water and an organic substance (immiscible). | Substances volatile in steam and immiscible with water. |
2. Comparative Analysis of Mechanisms
Step 1: Simple Distillation (Atmospheric Pressure)
This process relies on the principle that if two liquids have widely different boiling points, the more volatile component will vaporize first upon heating [Per Raoult's Law]. The vapor is then cooled to liquid form.
Step 2: Distillation Under Reduced Pressure (Vacuum Distillation)
Liquids boil when their vapor pressure equals the external atmospheric pressure. By employing a vacuum pump to decrease the pressure above the liquid, the vapor pressure required for boiling is reached at a significantly lower temperature [Clausius-Clapeyron Relation].
Equation: $ln\left(\frac{P_2}{P_1}\right) = \frac{\Delta H_{vap}}{R} \left(\frac{1}{T_1} - \frac{1}{T_2}\right)$
Step 3: Steam Distillation
This technique is applied when the compound is steam-volatile and insoluble in water. The total vapor pressure ($P_{total}$) of the system is the sum of the vapor pressure of water ($P_{H_2O}$) and the organic compound ($P_{org}$):
$P_{total} = P_{H_2O} + P_{org}$
Since $P_{total} = \text{Atmospheric Pressure}$, the mixture boils at a temperature lower than the boiling point of either pure water or the pure organic liquid [Dalton's Law of Partial Pressures].
3. Visual Representation of Boiling Point Relationships
Summary of Differentiators
- Simple Distillation: Used for purification of liquids boiling at high temperatures without decomposition.
- Reduced Pressure: Vital for thermally sensitive substances (e.g., Glycerol boiling at $563\,K$ at $760\,mm$ vs. $453\,K$ at $12\,mm$).
- Steam Distillation: Essential for separating organic compounds that decompose before reaching their normal boiling point but are stable in the presence of steam (e.g., Aniline, Essential oils).
Final Solution: The core difference lies in the thermodynamic state of the boiling process: Simple distillation uses atmospheric pressure, reduced pressure distillation lowers the boiling point by decreasing ambient pressure to prevent thermal decomposition, and steam distillation uses the partial pressure of water vapor to distill substances at temperatures below $373\,K$.
More Questions from Class 11 Chemistry Organic Chemistry – Some Basic Principles and Techniques EXERCISES
- Q8.1: What are hybridisation states of each carbon atom in the following compounds ? $CH_2=C=O$, $CH_3CH=CH_2$, $(CH_3)_2CO$, $CH_2=CHCN$, $C_6H_6$
- Q8.10: Explain why alkyl groups act as electron donors when attached to a $\pi$ system.
- Q8.11 (a): Draw the resonance structures for the following compounds. Show the electron shift using curved-arrow notation. (a) $C_6H_5OH$
- Q8.11 (b): Draw the resonance structures for the following compounds. Show the electron shift using curved-arrow notation. (b) $C_6H_5NO_2$
- Q8.11 (c): Draw the resonance structures for the following compounds. Show the electron shift using curved-arrow notation. (c) $CH_3CH=CHCHO$
- Q8.11 (d): Draw the resonance structures for the following compounds. Show the electron shift using curved-arrow notation. (d) $C_6H_5–CHO$
- Q8.11 (e): Draw the resonance structures for the following compounds. Show the electron shift using curved-arrow notation. (e) $C_6H_5–C^+H_2$
- Q8.11 (f): Draw the resonance structures for the following compounds. Show the electron shift using curved-arrow notation. (f) $CH_3CH=CHC^+H_2$
- Q8.12: What are electrophiles and nucleophiles ? Explain with examples.
- Q8.13 (a): Identify the reagents shown in bold in the following equations as nucleophiles or electrophiles: (a) $CH_3COOH + HO^– \rightarrow CH_3COO^–+H_2O$
- Q8.13 (b): Identify the reagents shown in bold in the following equations as nucleophiles or electrophiles: (b) $CH_3COCH_3+ C^–N \rightarrow (CH_3)_2C(CN)(OH)$
- Q8.13 (c): Identify the reagents shown in bold in the following equations as nucleophiles or electrophiles: (c) $C_6H_6 + CH_3C^+O \rightarrow C_6H_5COCH_3$
- Q8.14 (a): Classify the following reactions in one of the reaction type studied in this unit. (a) $CH_3CH_2Br + HS^– \rightarrow CH_3CH_2SH + Br^–$
- Q8.14 (b): Classify the following reactions in one of the reaction type studied in this unit. (b) $(CH_3)_2C = CH_2 + HCl \rightarrow (CH_3)_2ClC – CH_3$
- Q8.14 (c): Classify the following reactions in one of the reaction type studied in this unit. (c) $CH_3CH_2Br + HO^– \rightarrow CH_2 = CH_2 + H_2O + Br^–$
- Q8.14 (d): Classify the following reactions in one of the reaction type studied in this unit. (d) $(CH_3)_3C– CH_2OH + HBr \rightarrow (CH_3)_2CBrCH_2CH_2CH_3 + H_2O$
- Q8.15: What is the relationship between the members of following pairs of structures ? Are they structural or geometrical isomers or resonance contributors ? (c)
- Q8.15: What is the relationship between the members of following pairs of structures ? Are they structural or geometrical isomers or resonance contributors ? (b)
- Q8.15: What is the relationship between the members of following pairs of structures ? Are they structural or geometrical isomers or resonance contributors ? (a)
- Q8.16: For the following bond cleavages, use curved-arrows to show the electron flow and classify each as homolysis or heterolysis. Identify reactive intermediate produced as free radical, carbocation and carbanion. (c)
- Q8.16: For the following bond cleavages, use curved-arrows to show the electron flow and classify each as homolysis or heterolysis. Identify reactive intermediate produced as free radical, carbocation and carbanion. (d)
- Q8.16: For the following bond cleavages, use curved-arrows to show the electron flow and classify each as homolysis or heterolysis. Identify reactive intermediate produced as free radical, carbocation and carbanion. (b)
- Q8.16: For the following bond cleavages, use curved-arrows to show the electron flow and classify each as homolysis or heterolysis. Identify reactive intermediate produced as free radical, carbocation and carbanion. (a)
- Q8.17 (a): Explain the terms Inductive and Electromeric effects. Which electron displacement effect explains the following correct orders of acidity of the carboxylic acids? (a) $Cl_3CCOOH > Cl_2CHCOOH > ClCH_2COOH$
- Q8.17 (b): Explain the terms Inductive and Electromeric effects. Which electron displacement effect explains the following correct orders of acidity of the carboxylic acids? (b) $CH_3CH_2COOH > (CH_3)_2CHCOOH > (CH_3)_3C.COOH$
- Q8.18 (a): Give a brief description of the principles of the following techniques taking an example in each case. (a) Crystallisation
- Q8.18 (b): Give a brief description of the principles of the following techniques taking an example in each case. (b) Distillation
- Q8.18 (c): Give a brief description of the principles of the following techniques taking an example in each case. (c) Chromatography
- Q8.19: Describe the method, which can be used to separate two compounds with different solubilities in a solvent S.
- Q8.2: Indicate the $\sigma$ and $\pi$ bonds in the following molecules : $C_6H_6$, $C_6H_{12}$, $CH_2Cl_2$, $CH_2=C=CH_2$, $CH_3NO_2$, $HCONHCH_3$
- Q8.21: Discuss the chemistry of Lassaigne’s test.
- Q8.22: Differentiate between the principle of estimation of nitrogen in an organic compound by (i) Dumas method and (ii) Kjeldahl’s method.
- Q8.23: Discuss the principle of estimation of halogens, sulphur and phosphorus present in an organic compound.
- Q8.24: Explain the principle of paper chromatography.
- Q8.25: Why is nitric acid added to sodium extract before adding silver nitrate for testing halogens?
- Q8.26: Explain the reason for the fusion of an organic compound with metallic sodium for testing nitrogen, sulphur and halogens.
- Q8.27: Name a suitable technique of separation of the components from a mixture of calcium sulphate and camphor.
- Q8.28: Explain, why an organic liquid vaporises at a temperature below its boiling point in its steam distillation ?
- Q8.29: Will $CCl_4$ give white precipitate of AgCl on heating it with silver nitrate? Give reason for your answer.
- Q8.3: Write bond line formulas for : Isopropyl alcohol, 2,3-Dimethylbutanal, Heptan-4-one.
- Q8.30: Why is a solution of potassium hydroxide used to absorb carbon dioxide evolved during the estimation of carbon present in an organic compound?
- Q8.31: Why is it necessary to use acetic acid and not sulphuric acid for acidification of sodium extract for testing sulphur by lead acetate test?
- Q8.32: An organic compound contains 69% carbon and 4.8% hydrogen, the remainder being oxygen. Calculate the masses of carbon dioxide and water produced when 0.20 g of this substance is subjected to complete combustion.
- Q8.33: A sample of 0.50 g of an organic compound was treated according to Kjeldahl’s method. The ammonia evolved was absorbed in 50 ml of 0.5 M $H_2SO_4$. The residual acid required 60 mL of 0.5 M solution of NaOH for neutralisation. Find the percentage composition of nitrogen in the compound.
- Q8.34: 0.3780 g of an organic chloro compound gave 0.5740 g of silver chloride in Carius estimation. Calculate the percentage of chlorine present in the compound.
- Q8.35: In the estimation of sulphur by Carius method, 0.468 g of an organic sulphur compound afforded 0.668 g of barium sulphate. Find out the percentage of sulphur in the given compound.
- Q8.36: In the organic compound $CH_2 = CH – CH_2 – CH_2 – C \equiv CH$, the pair of hydridised orbitals involved in the formation of: $C_2 – C_3$ bond is:
- Q8.37: In the Lassaigne’s test for nitrogen in an organic compound, the Prussian blue colour is obtained due to the formation of:
- Q8.38: Which of the following carbocation is most stable ?
- Q8.39: The best and latest technique for isolation, purification and separation of organic compounds is:
- Q8.4: Give the IUPAC names of the following compounds : (a)
- Q8.4: Give the IUPAC names of the following compounds : (b)
- Q8.4: Give the IUPAC names of the following compounds : (c)
- Q8.4: Give the IUPAC names of the following compounds : (d)
- Q8.4: Give the IUPAC names of the following compounds : (e)
- Q8.4 (f): Give the IUPAC names of the following compounds : (f) $Cl_2CHCH_2OH$
- Q8.40: The reaction: $CH_3CH_2I + KOH(aq) \rightarrow CH_3CH_2OH + KI$ is classified as :
- Q8.5 (a): Which of the following represents the correct IUPAC name for the compounds concerned ? (a) 2,2-Dimethylpentane or 2-Dimethylpentane
- Q8.5 (b): Which of the following represents the correct IUPAC name for the compounds concerned ? (b) 2,4,7-Trimethyloctane or 2,5,7-Trimethyloctane
- Q8.5 (c): Which of the following represents the correct IUPAC name for the compounds concerned ? (c) 2-Chloro-4-methylpentane or 4-Chloro-2-methylpentane
- Q8.5 (d): Which of the following represents the correct IUPAC name for the compounds concerned ? (d) But-3-yn-1-ol or But-4-ol-1-yne.
- Q8.6 (a): Draw formulas for the first five members of each homologous series beginning with the following compounds. (a) H–COOH
- Q8.6 (b): Draw formulas for the first five members of each homologous series beginning with the following compounds. (b) $CH_3COCH_3$
- Q8.6 (c): Draw formulas for the first five members of each homologous series beginning with the following compounds. (c) H–CH=CH2
- Q8.7 (a): Give condensed and bond line structural formulas and identify the functional group(s) present, if any, for : (a) 2,2,4-Trimethylpentane
- Q8.7 (b): Give condensed and bond line structural formulas and identify the functional group(s) present, if any, for : (b) 2-Hydroxy-1,2,3-propanetricarboxylic acid
- Q8.7 (c): Give condensed and bond line structural formulas and identify the functional group(s) present, if any, for : (c) Hexanedial
- Q8.8: Identify the functional groups in the following compounds (c)
- Q8.8: Identify the functional groups in the following compounds (b)
- Q8.8: Identify the functional groups in the following compounds (a)
- Q8.9: Which of the two: $O_2NCH_2CH_2O^–$ or $CH_3CH_2O^–$ is expected to be more stable and why?
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