JEE Challenger
More from Aldehydes, Ketones and Carboxylic Acids
ChemMedAdv 2011 • Paper 2

Match Organic Reactions with Reaction Types and Reactive Intermediates

Match the reactions in column I with appropriate types of steps/reactive intermediate involved in these reactions as given in column II.

Column I

(A)

Diagram 0

(B)

Diagram 1

(C)

Diagram 2

(D)

Diagram 3

Column II

(p) Nucleophilic substitution

(q) Electrophilic substitution

(r) Dehydration

(s) Nucleophilic addition

(t) Carbanion

Options

A

(A)→(R,S),  (B)→(P,S),  (C)→(R,S),  (D)→(Q,R)(\text{A}) \rightarrow (\text{R}, \text{S}),\; (\text{B}) \rightarrow (\text{P}, \text{S}),\; (\text{C}) \rightarrow (\text{R}, \text{S}),\; (\text{D}) \rightarrow (\text{Q}, \text{R})

B

(A)→(R,S,T),  (B)→(P),  (C)→(S),  (D)→(Q,R)(\text{A}) \rightarrow (\text{R}, \text{S}, \text{T}),\; (\text{B}) \rightarrow (\text{P}),\; (\text{C}) \rightarrow (\text{S}),\; (\text{D}) \rightarrow (\text{Q}, \text{R})

C

(A)→(S,T),  (B)→(P,S),  (C)→(R),  (D)→(Q)(\text{A}) \rightarrow (\text{S}, \text{T}),\; (\text{B}) \rightarrow (\text{P}, \text{S}),\; (\text{C}) \rightarrow (\text{R}),\; (\text{D}) \rightarrow (\text{Q})

D

(A)→(R,S,T),  (B)→(P,S),  (C)→(R,S),  (D)→(Q,R)(\text{A}) \rightarrow (\text{R}, \text{S}, \text{T}),\; (\text{B}) \rightarrow (\text{P}, \text{S}),\; (\text{C}) \rightarrow (\text{R}, \text{S}),\; (\text{D}) \rightarrow (\text{Q}, \text{R})

Correct

Topics & Concepts

Aldehydes, Ketones and Carboxylic AcidsAldehydes and KetonesElectrophilic Aromatic SubstitutionElimination Reactions

Step-by-Step Solution

To match the reactions in Column I with the appropriate steps and intermediates in Column II, let us analyze the mechanism for each reaction:

  1. Reaction (A):
    Ph−C(=O)−CH2−CH2−C(=O)−CH3→aq. NaOH3-phenylcyclopent-2-en-1-one\text{Ph}-\text{C}(=\text{O})-\text{CH}_2-\text{CH}_2-\text{C}(=\text{O})-\text{CH}_3 \xrightarrow{\text{aq. NaOH}} \text{3-phenylcyclopent-2-en-1-one}
    • Mechanism: This is an intramolecular aldol condensation.
      • The base OH−\text{OH}^- abstracts an α\alpha-proton from the terminal methyl ketone group to generate a resonance-stabilized carbanion (t)(\textbf{t}).
      • The carbanion undergoes an intramolecular nucleophilic addition (s)(\textbf{s}) onto the benzoyl carbonyl group to form a 5-membered cyclic alkoxide/aldol intermediate.
      • Subsequent loss of a water molecule via base-catalyzed elimination constitutes a dehydration (r)(\textbf{r}) step yielding the conjugated cyclic α,β\alpha,\beta-unsaturated ketone.
    • Matching: A→r,s,t\text{A} \rightarrow \mathbf{r, s, t}
  1. Reaction (B):
    Ph−C(=O)−CH2CH2CH2Cl→CH3MgI2-methyl-2-phenyltetrahydrofuran\text{Ph}-\text{C}(=\text{O})-\text{CH}_2\text{CH}_2\text{CH}_2\text{Cl} \xrightarrow{\text{CH}_3\text{MgI}} \text{2-methyl-2-phenyltetrahydrofuran}
    • Mechanism:
      • The methyl group from the Grignard reagent (CH3MgI\text{CH}_3\text{MgI}) attacks the carbonyl carbon via nucleophilic addition (s)(\textbf{s}), forming a haloalkoxide intermediate Ph−C(O−)(CH3)−CH2CH2CH2Cl\text{Ph}-\text{C}(\text{O}^-)(\text{CH}_3)-\text{CH}_2\text{CH}_2\text{CH}_2\text{Cl}.
      • The alkoxide oxygen atom acts as an internal nucleophile and displaces the chloride ion in an intramolecular SN2\text{S}_\text{N}2 step (nucleophilic substitution, p\textbf{p}) to close the tetrahydrofuran ring.
    • Matching: B→p,s\text{B} \rightarrow \mathbf{p, s}
  1. Reaction (C):
    Ph−C(=O)−CH2CH2CH218OH→H2SO44,5-dihydro-2-phenylfuran\text{Ph}-\text{C}(=\text{O})-\text{CH}_2\text{CH}_2\text{CH}_2{^{18}\text{OH}} \xrightarrow{\text{H}_2\text{SO}_4} \text{4,5-dihydro-2-phenylfuran}
    • Mechanism:
      • The carbonyl oxygen is protonated in the acidic medium, and the labeled 18OH^{18}\text{OH} group attacks the carbonyl carbon via intramolecular nucleophilic addition (s)(\textbf{s}), forming a cyclic hemiacetal containing the 18O^{18}\text{O} atom in the ring.
      • Protonation of the remaining −OH-\text{OH} group followed by elimination of a water molecule (dehydration, r\textbf{r}) gives the enol ether.
    • Matching: C→r,s\text{C} \rightarrow \mathbf{r, s}
  1. Reaction (D):
    Ph−CH2CH2CH2−C(CH3)2OH→H2SO41,1-dimethyl-1,2,3,4-tetrahydronaphthalene (1,1-dimethyltetralin)\text{Ph}-\text{CH}_2\text{CH}_2\text{CH}_2-\text{C}(\text{CH}_3)_2\text{OH} \xrightarrow{\text{H}_2\text{SO}_4} \text{1,1-dimethyl-1,2,3,4-tetrahydronaphthalene (1,1-dimethyltetralin)}
    • Mechanism:
      • The tertiary alcohol group is protonated and loses a molecule of H2O\text{H}_2\text{O} (dehydration, r\textbf{r}) to generate a stable tertiary carbocation Ph−CH2CH2CH2−C+(CH3)2\text{Ph}-\text{CH}_2\text{CH}_2\text{CH}_2-\overset{+}{\text{C}}(\text{CH}_3)_2.
      • The carbocation acts as an electrophile and attacks the aromatic ring via an intramolecular electrophilic aromatic substitution (q)(\textbf{q}) to form the fused tetralin ring system.
    • Matching: D→q,r\text{D} \rightarrow \mathbf{q, r}

Final Match:

  • A→r,s,t\text{A} \rightarrow \mathbf{r, s, t}
  • B→p,s\text{B} \rightarrow \mathbf{p, s}
  • C→r,s\text{C} \rightarrow \mathbf{r, s}
  • D→q,r\text{D} \rightarrow \mathbf{q, r}
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