Q.NO. 19
Given below are Newman projections of ethylene glycol and 1,2-difluoroethane
about their respective C-C bonds. The most stable conformations (lowest energy)
of ethylene glycol and 1,2-difluoroethane are
(A) I and III respectively.
(B)I and IV respectively.
(C)II and III respectively.
(D)II and IV respectively.
The most stable conformations of ethylene glycol and 1,2-difluoroethane are identified from their Newman projections along the C-C bond, where staggered forms minimize torsional strain. For ethylene glycol (HOCH₂CH₂OH), the gauche arrangement of OH groups prevails due to intramolecular hydrogen bonding. In 1,2-difluoroethane (CH₂FCH₂F), the gauche form is favored by the gauche effect despite fluorine electronegativity.
Newman Projections Analysis
Newman projections I-IV depict staggered conformations viewed along the central C-C bond. For ethylene glycol (left column), I shows gauche OH (60° dihedral), II anti (180°), III gauche (opposite rotation), and IV another anti or variant. For 1,2-difluoroethane (right), I anti F-C-C-F (180°), II gauche (60°), III gauche, IV anti.
Ethylene Glycol Stability
The gauche conformation (I or III) is most stable for ethylene glycol as the OH groups form an intramolecular H-bond (O-H···O), stabilizing by ~2-4 kcal/mol over anti (II or IV). Anti lacks this interaction and faces higher steric repulsion between CH₂OH groups. Experimental and computational studies confirm gauche dominance in gas and solution phases.
1,2-Difluoroethane Stability
1,2-Difluoroethane prefers the gauche conformation (II or III) over anti (I or IV) due to the gauche effect, where hyperconjugative σ_C-H → σ*_C-F interactions (antiperiplanar in gauche) provide ~0.6-3.4 kJ/mol stabilization. Electrostatics and orbital overlap outweigh F-F repulsion. Gas-phase data shows gauche as lowest energy.
Option Evaluation
- (a) I and III: Correct for ethylene glycol (gauche I/III) and 1,2-difluoroethane (gauche III), matching lowest energy forms.
- (b) I and IV: Incorrect; IV likely anti for difluoroethane, higher energy.
- (c) II and III: Incorrect; II (anti) unstable for ethylene glycol.
- (d) II and IV: Incorrect; both anti, unstable for both molecules.
Correct Answer: (a) I and III respectively. This aligns with CSIR NET-level conformational analysis emphasizing gauche preferences.
