University:
York University
Course:
CHEM 1000 | Chemical Structure
Academic year:

2021
Views:

440

Pages:

3
Author:

Niklas

Newman Projections: Ethane: H H H C H H C H H H H Staggered Lower Energy Eclipsed Higher Energy Configuration: denotes 3D arrangement of atoms in space. Conformer: infinite possible arrangements that result from rotation around a single bond. Conformation: conformational isomers cannot be separated! Free Rotation: spontaneous process, but there is an activation energy (0.96 kcal/mol) Q: Do Conformer Populations ∆ w/ Temp? A: YES! (Reactants ⇌ Products) Endothermic Rx: A+B+heat ⇌ C (K↑ [C]↑) Exothermic Rx: A+B ⇌ C+heat (K↓ [C]↓) Q: What is the Origin of Ethane's Conformation preference? A: TWO competing theories! Older View: Glass Half Empty: Eclipsed is DISFAVORED H H C C H H Eclipsed Q: Are these H atoms REALLY that close in space? A: NO. TOO FAR apart FOR VDR to interact. NEW VIEW: Stereoelectronic Event Staggered is stabilized → Favored! Hyperconjugation: Donation of C-H bond (filled) to σ*(C-H) orbitals Stabilization Key to this Hypothesis: (stabilizing effect) Overlap can only occur in staggered conformation Propane: E profile similar to Ethane (3.34 kcal/mol) Higher Barrier (Rotation) Add'l eclipsing interactions Steric Strain Eclipsed H/Me = 1.4 kcal/mol Eclipsed H/H = 2.94 / 3 = 1 kcal/mol n-Butane: Me H Me Me H 0° Syn Torsional/Steric Strain Me H Me H 60° Gauche Torsional/Steric Strain Me Me H H 120° Eclipsed Torsional Strain Me H H Me 180° Anti or Anti periplanar Torsional Strain Me H H Me 240° Eclipsed Torsional Strain Me H Me H 300° Gauche Torsional/Steric Strain Strain: Any Energy which raises the PE of a molecule over that of its idealized version. Torsional Strain: Ts & Stabiling + Ts & Stabiling Steric Strain: LOWEST → Highest Internal E: anti gauche eclipsed fully eclipsed Energy Breakdown: Me H Me Me H 4.5 kcal/mol (1 + 2.5) kcal/mol) Steric Strain B/W Eclipsed Methyl group Bigger the two groups, the greater the contribution is to the strain/PE Eclipsing "costs": H/Me = 1.4 kcal/mol H/H = 1 kcal/mol Me/Me (Anti) = 2.6 kcal/mol Me/Me (Gauche) = 0.9 kcal/mol Staggered-Gauche Conformation