<p> 10/06/2006</p><p>Conformational, Steric and Stereoelectronic Effects</p><p>(C&S A, Chapter 3, p. 117-178); Handouts 18 and 19 </p><p>Conformation of acyclic molecules</p><p>Total energy related to geometry- can be divided up into specific structural features</p><p>Recognizable connections between energy and geometry: non-bonded repulsions, ring strain in cyclic systems, tortional strain from non-optimum rotational alignment, destabilization from distortions of bond lengths and bond angles.</p><p>There are also stabilizing interactions that result from geometrical constraints. </p><p>• Most important are stereoelectronic effects i.e., a particular geometrical relationship needed for most stabilizing interaction - geometry dependent orbital interactions.</p><p>• Consider also H-bonding and dipolar interactions </p><p>When a bond can be rotated, it happens and the molecule exists in the lowest energy form as determined by the rotation of various bonds- The various shapes the molecule takes with certain energy minima are called conformations.</p><p>- Descriptions can be approached by classical-mechanical or from an MO view point</p><p>-Strain - caused by non-ideal geometry</p><p>Contribution to steric energy and molecular mechanics (read C&S A, section 3.1, p. </p><p>124) </p><p>Non-bonded interactions difficult to estimate- Could be attractive or repulsive.</p><p>• Energy as a function of internuclear distance Morse potential (see CSA fig. 3. 2, p. </p><p>126). 10/06/2006</p><p>• London dispersion forces</p><p>• van der Waals radii -van de Waals repulsion- When two atoms (groups) are closer in distance than the sum of their van der Waals radii.</p><p>H 1.2, CH3 2.0; N 1.55; P 1.80; O 1.52; S. 1.80;</p><p>F 1.47; Cl 1.75; Br 1.85; I 1.98.</p><p>Barrier to rotation in ethane (CSA p. 127, Fig. 3.1) -overhead</p><p>3 maxima and 3 minima in energy vs dihedral angle.</p><p>Staggered-minimum Eclipsed -maximum at 2.9 kcal/mol or 12.1 kjoules/mol</p><p>(1cal = 4.184 Joules)</p><p>Origin of barrier - unknown - sigma bonds involved are cylindrical- Is it steric?- H’s passing one over the other? Probably not. H too small</p><p>MO calculations predict correct value </p><p>Known as Pitzer or tortional strain (Pitzer, R. M. Acc. Chem. Res. 1983, 16, 207)</p><p>-Barrier due to overlap (exchange) repulsions between the electron pairs in the coplanar C–H bonds. </p><p>(Other explanation; Coulombic repulsion)</p><p>Pitzer Strain or Torsional Strain: each C-H eclipsing interaction is worth ~ 1.0 kcal/mol</p><p>Propane 10/06/2006</p><p>2-methylpropane</p><p>Barrier to rotation in butane (C&S A, p. 121, Fig 3.3) - overhead</p><p>Resembles ethane - 3 minima, one deeper than the other two</p><p>Minima are staggered conformations; Maxima - eclipsed conformations</p><p>Staggered anti - lowest energy - other energies measured from here.</p><p>Gauche 0. 8 kcal/mol above anti staggered 10/06/2006</p><p>Me-Me eclipsed 6.1 Kcal/mol above staggered anti</p><p>Me-H eclipsed 3.4 kcal/mol above anti-staggered</p><p>• These are superimposed van der Waals and tortional (Pitzer) strains.</p><p>Calculate van der Waals contribution in the eclipsed conformations- For Me-Me eclipsed: 6.1(total) - 2.9 </p><p>(due to Pitzer or tortional strain alone) = 3.2 kcal/mol</p><p>For Me-H eclipsed: 3.4 - 2.9 = 0.5 kcal/mol; but now there are two such interactions . Therefore the van der Waals contribution per Me-H interaction is 0.5/2 = 0.25 kcal/mol.</p><p>Population of conformations are related to their energy</p><p>G0 = -RTlnK = -3.9 kcal/mol (work through the math)</p><p>Calculate Kequilibrium = (anti/gauche ) =1.9 i.e, 66 % anti and 34 5 gauche </p><p>Study Table 3.2 calculate % more stable isomer as a functional of G0 </p><p>Free energy(G0 ) K % more stable isomer</p><p>-0.000 1 50</p><p>-0.502 2.33 70</p><p>- 1.302 9.00 90</p><p>-2.722 99.00 99</p><p>(practice calculations)</p><p>In general Staggered minima Eclipsed maxima</p><p>(ethane and butane) 10/06/2006</p><p>Staggered anti need not be the minimum. e.g. sometimes attractive forces can prevail - 1-chloropropane gauche more stable Me-Cl attraction! - by H 0.3 ± 0.3 kcal/mol</p><p>• Total strain can be calculated (molecular mechanics) and there is good correlation between intramolecular strain and bond dissociation energies (See table 3.3)</p><p>Rotational barriers of groups (Table 3.4)</p><p>Me - Me 2.9 kcal/mol Me - Et 3.4 (increase of 0.5 kcal/mol per methyl group eclipsing)</p><p>Me - CH (Me)2 3.9; calculated: 2.9 + 0.5 + 0.5 = 3.9</p><p>Me - C( Me)3 4.7; calculated. 2.9 + 0.5 + 0.5 + 0.5 = 4.4</p><p>Me - SiH3 1.7 (longer C-Si bond, decreased electron-electron repulsion in eclipsed.)</p><p>Si - C = 1.87 Å ; C - C = 1.54 Å ethane --> methylamine ----> methanol</p><p>2.9 1.98 1.07 (decrease with decreasing number of C–H/X–H eclipsing interactions)</p><p>CH3–CH2–CH3 (3.4) vs CH3–CH3 (2.8) difference ~ 0.6 Å</p><p>CH3–NH–CH3 (3.62) vs CH3–NH2 (1.98) difference ~ 1.6 Å </p><p>CH3–O–CH3 (2.70) vs CH3–OH (1.07) difference ~ 1.6 Å</p><p>(greater difference in the amine and ether because of shorter C–O and C–N bond distances) </p><p>Terminal Alkenes 10/06/2006</p><p>Carbonyl compounds</p><p>1,3-Dienes 10/06/2006</p><p>Unsaturated carbonyl compounds</p><p>Stereoelectronic factors favor coplanar arrangement</p><p>Aldehydes</p><p>Ketones</p><p>Esters</p>