Conformational, Steric and Stereoelectronic Effects

10/06/2006

(C&S A, Chapter 3, p. 117-178); Handouts 18 and 19

Conformation of acyclic molecules

Total energy related to geometry- can be divided up into specific structural features

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.

There are also stabilizing interactions that result from geometrical constraints.

• Most important are stereoelectronic effects i.e., a particular geometrical relationship needed for most stabilizing interaction - geometry dependent orbital interactions.

• Consider also H-bonding and dipolar interactions

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.

- Descriptions can be approached by classical-mechanical or from an MO view point

-Strain - caused by non-ideal geometry

Contribution to steric energy and molecular mechanics (read C&S A, section 3.1, p.

Non-bonded interactions difficult to estimate- Could be attractive or repulsive.

• Energy as a function of internuclear distance Morse potential (see CSA fig. 3. 2, p.

126). 10/06/2006

• London dispersion forces

• 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.

H 1.2, CH3 2.0; N 1.55; P 1.80; O 1.52; S. 1.80;

F 1.47; Cl 1.75; Br 1.85; I 1.98.

Barrier to rotation in ethane (CSA p. 127, Fig. 3.1) -overhead

3 maxima and 3 minima in energy vs dihedral angle.

Staggered-minimum Eclipsed -maximum at 2.9 kcal/mol or 12.1 kjoules/mol

(1cal = 4.184 Joules)

Origin of barrier - unknown - sigma bonds involved are cylindrical- Is it steric?- H’s passing one over the other? Probably not. H too small

MO calculations predict correct value

Known as Pitzer or tortional strain (Pitzer, R. M. Acc. Chem. Res. 1983, 16, 207)

-Barrier due to overlap (exchange) repulsions between the electron pairs in the coplanar C–H bonds.

(Other explanation; Coulombic repulsion)

Pitzer Strain or Torsional Strain: each C-H eclipsing interaction is worth ~ 1.0 kcal/mol

Propane 10/06/2006

2-methylpropane

Barrier to rotation in butane (C&S A, p. 121, Fig 3.3) - overhead

Resembles ethane - 3 minima, one deeper than the other two

Minima are staggered conformations; Maxima - eclipsed conformations

Staggered anti - lowest energy - other energies measured from here.

Gauche 0. 8 kcal/mol above anti staggered 10/06/2006

Me-Me eclipsed 6.1 Kcal/mol above staggered anti

Me-H eclipsed 3.4 kcal/mol above anti-staggered

• These are superimposed van der Waals and tortional (Pitzer) strains.

Calculate van der Waals contribution in the eclipsed conformations- For Me-Me eclipsed: 6.1(total) - 2.9

(due to Pitzer or tortional strain alone) = 3.2 kcal/mol

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.

Population of conformations are related to their energy

G0 = -RTlnK = -3.9 kcal/mol (work through the math)

Calculate Kequilibrium = (anti/gauche ) =1.9 i.e, 66 % anti and 34 5 gauche

Study Table 3.2 calculate % more stable isomer as a functional of G0

Free energy(G0 ) K % more stable isomer

-0.000 1 50

-0.502 2.33 70

- 1.302 9.00 90

-2.722 99.00 99

(practice calculations)

In general Staggered minima Eclipsed maxima

(ethane and butane) 10/06/2006

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

• Total strain can be calculated (molecular mechanics) and there is good correlation between intramolecular strain and bond dissociation energies (See table 3.3)

Rotational barriers of groups (Table 3.4)

Me - Me 2.9 kcal/mol Me - Et 3.4 (increase of 0.5 kcal/mol per methyl group eclipsing)

Me - CH (Me)2 3.9; calculated: 2.9 + 0.5 + 0.5 = 3.9

Me - C( Me)3 4.7; calculated. 2.9 + 0.5 + 0.5 + 0.5 = 4.4

Me - SiH3 1.7 (longer C-Si bond, decreased electron-electron repulsion in eclipsed.)

Si - C = 1.87 Å ; C - C = 1.54 Å ethane --> methylamine ----> methanol

2.9 1.98 1.07 (decrease with decreasing number of C–H/X–H eclipsing interactions)

CH3–CH2–CH3 (3.4) vs CH3–CH3 (2.8) difference ~ 0.6 Å

CH3–NH–CH3 (3.62) vs CH3–NH2 (1.98) difference ~ 1.6 Å

CH3–O–CH3 (2.70) vs CH3–OH (1.07) difference ~ 1.6 Å

(greater difference in the amine and ether because of shorter C–O and C–N bond distances)

Terminal Alkenes 10/06/2006

Carbonyl compounds

1,3-Dienes 10/06/2006

Unsaturated carbonyl compounds

Stereoelectronic factors favor coplanar arrangement

Aldehydes

Ketones

Esters