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 13.
Q1577 (2024)
IUPAC name of following compound (P) is:
- 1-Ethyl-5, 5-dimethylcyclohexane
- 3-Ethyl-1,1-dimethylcyclohexane
- 1-Ethyl-3, 3-dimethylcyclohexane
- 1,1-Dimethyl-3-ethylcyclohexane
### Core Logic
Number the ring to give substituents the lowest possible locants. Setting locant 1 at the carbon carrying the two methyl groups provides a locant list of $(1,1,3)$, whereas setting it at the ethyl-bearing carbon gives $(1,3,3)$. The set $(1,1,3)$ wins by the lowest locant rule. Then arrange alphabetically: 3-ethyl precedes 1,1-dimethyl. Hence, the correct systematic tag is 3-Ethyl-1,1-dimethylcyclohexane. {{SOL_IMG1}}
### Pattern Recognition
Lowest locant grouping set takes ultimate priority before checking alphabetical organization rules.
### Chapter Mix
Class 11 Chemistry: Organic Chemistry - Some Basic Principles and Techniques
Q1585 (2024)
3-Methylhex-2-ene on reaction with $\text{HBr}$ in presence of peroxide forms an addition product (A). The number of possible stereoisomers for 'A' is $\text{\quad\quad}$.
### Core Logic
In the presence of peroxide, $\text{HBr}$ undergoes an anti-Markovnikov free-radical addition mechanism across the alkene bond of 3-methylhex-2-ene. Bromine appends to carbon-2 while hydrogen attaches at carbon-3.
The generated dynamic addition product (A) corresponds structurally to 2-bromo-3-methylhexane. {{SOL_IMG1}}
### Step 1: Check chiral centers count
Inspecting the structure of 2-bromo-3-methylhexane reveals two asymmetric chiral centers:
- Carbon-2 carrying $\text{-H, -CH}_3\text{, -Br, and -CH(CH}_3\text{)CH}_2\text{CH}_2\text{CH}_3$
- Carbon-3 carrying $\text{-H, -CH}_3\text{, -CH(Br)CH}_3\text{, and -CH}_2\text{CH}_2\text{CH}_3$
Since both stereocenters are unsymmetrical ($n=2$):
$$\text{Total Stereoisomers} = 2^n = 2^2 = 4$$
### Pattern Recognition
Anti-Markovnikov hydrobromination often generates multiple stereocenters. Count unsymmetrical $C^*$ elements systematically.
### Chapter Mix
Class 11 Chemistry: Organic Chemistry - Some Basic Principles and Techniques
Class 11 Chemistry: Hydrocarbons
Q1586 (2024)
Among the given organic compounds, the total number of aromatic compounds is $\text{\quad\quad}$.
### Core Logic
To satisfy Huckel's rules for aromaticity, a system must be planar, cyclic, completely conjugated, and possess $(4n+2)\pi$ delocalized electrons. Evaluating structures B, C, and D against these criteria shows they follow the $(4n+2)\pi$ electron counting rules successfully. Thus, three structures are fully aromatic.
### Pattern Recognition
Verify ring conjugation continuity and apply Huckel's electron count constraints ($2, 6, 10, 14\dots$).
### Chapter Mix
Class 11 Chemistry: Organic Chemistry - Some Basic Principles and Techniques
Class 11 Chemistry: Hydrocarbons
Q87 (2024)
Among the following, total number of meta directing functional groups is (Integer based):
$-\text{OCH}_3$, $-\text{NO}_2$, $-\text{CN}$, $-\text{CH}_3$, $-\text{NHCOCH}_3$, $-\text{COR}$, $-\text{OH}$, $-\text{COOH}$, $-\text{Cl}$
### Core Logic
Meta-directing functional groups are electron-withdrawing groups via resonance or induction effects ($-M$ or $-I$). Let's audit the list:
- $-\text{OCH}_3 \rightarrow \text{ortho/para}$ ($+M$)
- $-\text{NO}_2 \rightarrow \text{meta}$ ($-M, -I$)
- $-\text{CN} \rightarrow \text{meta}$ ($-M, -I$)
- $-\text{CH}_3 \rightarrow \text{ortho/para}$ ($+I$, hyperconjugation)
- $-\text{NHCOCH}_3 \rightarrow \text{ortho/para}$ ($+M$)
- $-\text{COR} \rightarrow \text{meta}$ ($-M, -I$)
- $-\text{OH} \rightarrow \text{ortho/para}$ ($+M$)
- $-\text{COOH} \rightarrow \text{meta}$ ($-M, -I$)
- $-\text{Cl} \rightarrow \text{ortho/para}$ ($+M$ counteracted by strong $-I$)
The meta-directing groups are $-\text{NO}_2$, $-\text{CN}$, $-\text{COR}$, and $-\text{COOH}$.
### Pattern Recognition
Groups whose attachment atom carries a double or triple bond to an electronegative element typically function as meta-directors.
### Chapter Mix
Class 11 Chemistry: Organic Chemistry - Some Basic Principles and Techniques
Class 12 Chemistry: Haloalkanes and Haloarenes
Q64 (2024)
The difference in energy between the actual structure and the lowest energy resonance structure for the given compound is
- $\text{electromagnetic energy}$
- $\text{resonance energy}$
- $\text{ionization energy}$
- $\text{hyperconjugation energy}$
### Core Logic
By definition, a resonance hybrid (the actual structure) is always more stable than any of its contributing canonical structures (resonance structures). The difference in potential energy between the most stable contributing resonance structure and the actual resonance hybrid is called the resonance energy.
### Evaluation Rubric / Model Answer
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### Chapter Mix
Class 11 Chemistry: Organic Chemistry Some Basic Principles and Techniques
