Organic Chemistry - Some Basic Principles and Techniques Previous Year Questions — JEE Main Chemistry
101 past-year Organic Chemistry - Some Basic Principles and Techniques questions from JEE Main (Chemistry) — page 18.
Q74 (2026)
Sodium fusion extract of an organic compound (Y) with $CHCl_{3}$ and chlorine water gives violet color to the $CHCl_{3}$ layer. 0.15g of (Y) gave 0.12 g of the silver halide precipitate in Carius method. Percentage of halogen in the compound (Y) is ____. (Nearest integer).
(Given: molar mass $g\text{ mol}^{-1}$ C: 12, H: 1, Cl: 35.5, Br: 80, I: 127)
### Related Formula
$$\% \text{ of Halogen (I)} = \frac{\text{Atomic weight of I}}{\text{Molecular weight of AgI}} \times \frac{\text{Mass of AgI}}{\text{Mass of organic compound}} \times 100$$
### Core Logic
1. Identify the halogen: The violet color in the chloroform layer upon addition of chlorine water is the classic test for Iodine ($I_2$). Chlorine oxidizes $I^-$ to $I_2$, which dissolves in $CHCl_3$ with a violet/purple color.
2. Molar mass calculations:
Atomic weight of I = 127
Molar mass of AgI = $108 (Ag) + 127 (I) = 235\text{ g mol}^{-1}$. (Since Ag is not provided in data, assume standard 108. The PDF explicitly uses 235).
### Step 1: Apply Carius Method Formula
Given:
Mass of organic compound ($w$) = 0.15 g
Mass of AgI precipitate ($m$) = 0.12 g
$$\% \text{ of I} = \frac{127}{235} \times \frac{0.12}{0.15} \times 100$$
$$\% \text{ of I} = \frac{127}{235} \times 0.8 \times 100$$
$$\% \text{ of I} = 0.5404 \times 80 = 43.234\%$$
### Step 2: Rounding
The nearest integer to 43.23 is 43.
### Pattern Recognition
Always use the qualitative test (chloroform layer color) to definitively identify the halogen before executing the quantitative Carius formula.
### Evaluation Rubric / Model Answer
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### Chapter Mix
Class 11 Chemistry: Organic Chemistry Some Basic Principles and Techniques
Q56 (2026)
When $1\text{ g}$ of compound (X) is subjected to Kjeldahl's method for estimation of nitrogen, $15\text{ mL}$, $1\text{M } \text{H}_2\text{SO}_4$ was neutralized by ammonia evolved. The percentage of nitrogen in compound (X) is:
- 21
- 0.42
- 42
- 0.21
### Related Formula
$$\%\text{ N} = \frac{1.4 \times M \times 2 \times V}{\text{Mass of compound (g)}}$$
Alternatively:
$$\text{Meq of } \text{H}_2\text{SO}_4 = V \times M \times n_{\text{factor}}$$
### Core Logic
Step 1: Calculate equivalents of acid neutralized:
$$\text{Equivalents of } \text{H}_2\text{SO}_4 = \frac{15 \times 1 \times 2}{1000} = 0.03$$
Step 2: Calculate moles of nitrogen atoms:
$$\text{Moles of } \text{NH}_3 = 0.03 \implies \text{Moles of N} = 0.03$$
$$\text{Mass of N} = 0.03 \times 14 = 0.42\text{ g}$$
Step 3: Calculate percentage of nitrogen:
$$\%\text{ N} = \frac{0.42}{1} \times 100 = 42\%$$
### Pattern Recognition
Sees: Kjeldahl's method with $\text{H}_2\text{SO}_4$ volume and molarity.
Shortcut: Use standard formula $\%\text{ N} = \frac{1.4 \times M \times 2 \times V}{W} = \frac{1.4 \times 1 \times 2 \times 15}{1} = 42\%$.
### Evaluation Rubric / Model Answer
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### Chapter Mix
Class 11 Chemistry: Organic Chemistry - Some Basic Principles & Techniques
Q58 (2026)
The IUPAC name of the following compound is:
{{IMG1}}
- n-propyl-2-bromo-5-methylheptanoate
- 2-bromo-5-methylhexylpropanoate
- 2-bromo-5-methylpropanoate
- n-propyl-1-bromo-4-methylhexanoate
### Related Formula
$$\text{Ester IUPAC Nomenclature: Alkyl + [Substituents] + alkanoate}$$
### Core Logic
Step 1: Identify the alkyl group attached to oxygen: $n\text{-propyl}$.
Step 2: Number the longest carbon chain containing the ester carbonyl carbon from C1 to C7.
Step 3: Substituents present: Br at C2 and Methyl at C5.
Step 4: Combine into IUPAC name: n-propyl-2-bromo-5-methylheptanoate.
{{SOL_IMG1}}
### Pattern Recognition
Sees: Ester esterified with propyl alcohol and heptanoate chain.
Shortcut: Always start naming ester with alkyl group attached to oxygen (n-propyl), followed by numbered parent carboxylate chain.
### Evaluation Rubric / Model Answer
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### Chapter Mix
Class 11 Chemistry: Organic Chemistry - Some Basic Principles & Techniques
Q66 (2026)
Match List-I with List-II.
<div style="overflow-x: auto; margin: 1rem 0;"><table style="width: 100%; border-collapse: collapse; text-align: left; font-size: 14px;"><thead><tr><th style="border: 1px solid #888; padding: 8px;">List-I (Functional group detection)</th><th style="border: 1px solid #888; padding: 8px;">List-II (Change observed during detection)</th></tr></thead><tbody><tr><td style="border: 1px solid #888; padding: 8px;">(A) Unsaturation (Baeyer's test)</td><td style="border: 1px solid #888; padding: 8px;">(I) Red colour Appears</td></tr><tr><td style="border: 1px solid #888; padding: 8px;">(B) Alcoholic group (Ceric ammonium nitrate test)</td><td style="border: 1px solid #888; padding: 8px;">(II) Silver mirror appears</td></tr><tr><td style="border: 1px solid #888; padding: 8px;">(C) Aldehyde group (Tollen's reagent)</td><td style="border: 1px solid #888; padding: 8px;">(III) Violet colour appears</td></tr><tr><td style="border: 1px solid #888; padding: 8px;">(D) Phenolic group ($FeCl_3$ test)</td><td style="border: 1px solid #888; padding: 8px;">(IV) Discharge of pink colour</td></tr></tbody></table></div>
Choose the correct answer from the options given below:
- $\text{A-III, B-IV, C-II, D-I}$
- $\text{A-III, B-IV, C-I, D-II}$
- $\text{A-IV, B-I, C-II, D-III}$
- $\text{A-IV, B-III, C-II, D-I}$
### Core Logic
Recall standard qualitative laboratory tests for functional groups:
(A) Unsaturation (Baeyer's Test): Dilute alkaline $KMnO_4$ is pink. When it reacts with an alkene/alkyne, it gets reduced to $MnO_2$ (brown ppt), discharging the pink colour. Matches with (IV).
(B) Alcoholic group: Primary and secondary alcohols react with Ceric ammonium nitrate to form a red-coloured coordination complex. Matches with (I).
(C) Aldehyde group (Tollens' Test): Aldehydes reduce Tollens' reagent ($[Ag(NH_3)_2]^+$) to metallic silver, forming a silver mirror. Matches with (II).
(D) Phenolic group: Phenols form strongly colored (often violet) complexes with neutral $FeCl_3$. Matches with (III).
### Step 1: Final Mapping
(A) - (IV)
(B) - (I)
(C) - (II)
(D) - (III)
### Pattern Recognition
Baeyer's = Pink to colorless. Tollens' = Silver mirror. $FeCl_3$ = Violet/Purple. Ceric Ammonium Nitrate = Red.
### Evaluation Rubric / Model Answer
null
### Chapter Mix
Class 11 Chemistry: Organic Chemistry Some Basic Principles and Techniques
Class 12 Chemistry: Aldehydes Ketones and Carboxylic Acids
Q67 (2026)
Given below are two statements:
Statement-I : Sublimation is used for the separation and purification of compounds with low melting point.
Statement-II : The boiling point of a liquid increases as the external pressure is reduced.
In the light of the above statements, choose the correct answer from the options given below :
- $\text{Statement-I is false but Statement-II is true.}$
- $\text{Statement-I is true but Statement-II is false.}$
- $\text{Both Statement-I and Statement-II are true.}$
- $\text{Both Statement-I and Statement-II are false.}$
### Core Logic
Assess theoretical principles of purification processes.
Statement-I: Sublimation is a process used for separating sublimable compounds from non-sublimable impurities. It does not strictly depend on a 'low melting point'. Sublimable solids bypass the liquid phase altogether when heated. (False)
Statement-II: The boiling point of a liquid is the temperature at which its vapor pressure equals the external atmospheric pressure. If external pressure is reduced, the liquid needs less vapor pressure (and thus lower temperature) to boil. Hence, boiling point decreases with reduced external pressure. (False)
### Step 1: Final Conclusion
Both statements are fundamentally false based on basic thermodynamics and purification principles.
### Pattern Recognition
Lower pressure = Lower boiling point (used in vacuum distillation). Sublimation relies on vapor pressure of solid overcoming external pressure without melting, independent of specifically 'low' melting points.
### Evaluation Rubric / Model Answer
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### Chapter Mix
Class 11 Chemistry: Organic Chemistry Some Basic Principles and Techniques
