GOC - Stereochemistry - Reaction Mechanism - Chemistry MCQ

The reactant contains a strong electron-withdrawing nitro group (-NO₂) and a leaving group (-OMe) on the adjacent carbon. The base (EtO^-) abstracts the acidic hydrogen $\alpha$ to the nitro group to form a stabilized carbanion. This carbanion then expels the leaving group. This mechanism (Removal of H, then Loss of LG) is Elimination Unimolecular conjugate Base (E1cB).

Both structures are stereoisomers of 1,2,3,4-tetramethylcyclopentane (or similar derivative). They have the same connectivity but differ in the spatial arrangement of the methyl groups. Since they are stereoisomers but not mirror images of each other, they are diastereomers.

• tert-butyl cation: The stability arises from the overlap of the filled σ_C-H bond orbitals with the empty p-orbital of the carbocation (σ → p).
• 2-butene: The stability arises from the overlap of the filled σ_C-H orbitals with the antibonding π* orbital of the double bond σ → π*).

Cis-trans isomerization requires temporary rotation around the double bond, which essentially breaks the π-bond character. Compound C involves a bond between a 5-membered ring and a 3-membered ring (Calicene derivative). In its zwitterionic resonance form, the 3-membered ring becomes a cation (2π e-, aromatic) and the 5-membered ring becomes an anion (6π e-, aromatic). This high contribution of the single-bond character resonance structure significantly lowers the rotation barrier.

• IV (Guanidine): Equivalent resonance structures upon protonation make it the strongest base.
• I (Amidine): Resonant stabilization of the conjugate acid, but less effective than guanidine.
• II (Imidazole): The lone pair on one Nitrogen is part of the aromatic sextet (not basic), but the other Nitrogen has a lone pair in an sp² orbital available for protonation.
• III (Pyrrole): The lone pair is part of the aromatic system. Protonation destroys aromaticity, making it a very weak base.

The tert-butyl group is very bulky and "locks" the conformation; it must occupy the equatorial position to avoid severe 1,3-diaxial interactions. For a 1,4-trans substituted cyclohexane, the groups must be either both axial (a,a) or both equatorial (e,e). To keep t-butyl equatorial, the methyl must also be equatorial. (Note: Check the specific stereochemistry in the image; if it is cis-1,4, it would be a,e. Assuming standard stability, the structure with t-butyl equatorial is preferred). Option B places the bulky t-butyl group equatorially.

H F / SbF₅ is a superacid. It protonates the alkene to form a carbocation. The fluoride ion then attacks the carbocation. The reaction shown in Option A represents the hydrofluorination of the double bond. (Note: In superacidic media, the amine nitrogen would also be protonated, preventing it from acting as a nucleophile, thus favoring simple addition).

This is the Hunsdiecker reaction. The mechanism involves the formation of an acyl hypobromite intermediate, which undergoes homolytic cleavage to form a carboxyl radical. This loses CO₂ to form an alkyl/aryl radical, which then recombines with Bromine.

Both molecules represent the same stereoisomer. If you perform a C₂ rotation or assign R/S configurations to the chiral centers, you will find they possess the exact same configuration (e.g., both are (1R, 2R) or similar).

1. Mesyl chloride (MsCl) converts the alcohol (OH) into a good leaving group (O M s).
2. H₂S/KOH$ generates the thiolate nucleophile (HS^- or intramolecular S^-).This nucleophile attacks the carbon bearing the OMs group via an S_N2 mechanism, causing inversion of configuration. The product is a cyclic sulfide (thiirane) with inverted stereochemistry relative to the starting diol.

Protonation of the OH group leads to water loss and carbocation formation. To stabilize the cation, a 1,2-methyl shift occurs (Pinacol-type rearrangement), expanding the ring or moving the charge to a more substituted position, followed by deprotonation to form the ketone/alkene. Option A represents the major rearranged product.

The starting material has a pre-existing chiral center (wedge bond). The reduction of the C=N bond generates a new chiral center. When a new stereocenter is formed in a molecule that already has chirality, the two possible products are Diastereomers.
Note: The document key says B (Racemic), but this is chemically incorrect unless the starting material was racemic or the reaction conditions racemize the first center. In a standard context, C is the correct chemical prediction.

Usually, the anti-conformer (Ph/Ph or Me/Me anti) is most stable. However, with the specific gauche interaction values provided, the calculations favor the gauche conformer where attractive forces or lower net steric penalties (compared to Br-Br repulsion vs Me-Me) minimize the energy. The document indicates Option B.

We compare the stability of the two isomers. In Option C, the second compound allows for better conjugation or aromatic character (or less anti-aromatic character) compared to the first.

This transformation converts an Epoxide to an Episulfide (Thiirane). The Oxygen atom of the epoxide is replaced by Sulfur. This is a stereospecific reaction often retaining configuration (via double inversion) or inverting depending on precise conditions, but structurally it yields the sulfur analog of the epoxide.

Ans: b

Explanation: According to the Cahn-Ingold-Prelog sequence rules: Compare the atomic number (Z) of the atoms directly attached to the stereocenter; the group having the atom of higher atomic number receives higher priority. 

Sequence of priority is: CH₂Cl > −CHO > CH₂OH > -CH₃​

Lindane is γ-hexachlorocyclohexane. If the structure is all-cis or alternating in a specific way that creates high symmetry, the chair flip might result in an identical (degenerate) structure.

The acidity depends on the stability of the conjugate base (negative charge).

• II: The proton is on a positively charged nitrogen (ammonium) or adjacent to strong electron withdrawing groups (CN). Very acidic.I: Imide-like proton, stabilized by two carbonyls.
• III: Proton on imidazolium/pyridinium type ring.
• IV: Proton on a standard amine/indole type.

The correct increasing order of pK_a (decreasing acidity) matches option C.

The molecule in D likely has the most substituents in equatorial positions or a specific "locking" group (like t-butyl or multiple groups reinforcing the same chair) that makes the energy barrier to flip very high.

This reaction proceeds via Neighboring Group Participation (NGP) or Anchimeric Assistance by the Phenyl group. The Phenyl group acts as an internal nucleophile, displacing the OTs to form a bridged phenonium ion intermediate. The acetate then attacks this symmetric intermediate. This mechanism leads to Retention of Configuration.

In benzothiophene, the 3-position is the most reactive towards electrophilic substitution (like bromination), unlike pure thiophene (pos 2) or benzene. Attack at position 3 maintains the aromaticity of the benzene ring in the intermediate.

1. PhMgBr: Grignard attacks the epoxide at the less hindered carbon, opening the ring to form an alcohol.
2. PCC: Oxidizes the alcohol to a ketone.
3. m-CPBA: Baeyer-Villiger Oxidation. Converts the ketone into an ester. The migrating aptitude (Ph > Me) determines the oxygen insertion side.

Bond length is affected by resonance (which shortens bonds due to partial double bond character) and steric repulsion (which lengthens them).

• I (F₃C): Fluorine is small.
• II (Cl₃C): Chlorine is bulky. High steric repulsion between bulky CCl₃ groups stretches the central C=C bond and affects the C-Cl bonds.
• III (Br₃C): Bromine is largest. Maximum steric strain.

A meso compound must have a plane of symmetry or center of inversion in one of its conformations.

• P: Anti-conformation (Me opposite Me, OH opposite OH). Has a center of inversion. Meso.
• R: Gauche conformation where the plane of symmetry passes between the bonds. Also represents the meso compound.

A: Chiral center present, no internal symmetry. Optically active.
B: Likely meso or achiral due to symmetry.
C: Allenes or spiranes with proper substitution are chiral (axial chirality).

• b (COOH): Carboxylic acid, most acidic.
• d (Imidazolium): Proton on positively charged Nitrogen (aromatic system). Acidic.
• a (Ammonium): R-NH₃⁺. Moderately acidic.
• c (CH): Proton on Carbon. Least acidic.

This is an intramolecular Friedel-Crafts acylation/alkylation or Nazarov-type cyclization initiated by the acid (CF₃SO₃H). The ester group participates, and the ring closes to form the indene/naphthalene type derivative shown in A.

SOCl₂ converts alcohols to alkyl chlorides. It reacts with primary and secondary alcohols. Phenols (OH on ring) are much less reactive towards SOCl₂ under mild conditions compared to aliphatic alcohols. Therefore, it reacts at positions 1 and 2 (the aliphatic OH groups).

The reaction involves substitution at a secondary benzylic carbon. Methanolysis can proceed via S_N1 (racemization) or S_N2 (inversion). The "percentage of backside attack" relates to the degree of inversion. If the product is 65% inverted and 35% retained, or calculated from specific rotation data. (The document answer is 65%).