Conformation and Reactivity in Acyclic compounds
The term conformation is used to denote any one of the infinite number of momentary arrangements of the atoms in space that results from rotation about single bonds.
It is now recognized that the differences in energies betn staggered and eclipsed are is some way associated with interaction of the electron clouds in the C –H and C-C bond.
Conformation & physical Properties
Thermodynamic properties e.g. enthalpy , entropy and related experimental properties e.g. specific heat, heat of formation & several other physically measurable properties are dependent on conformation and in turn , their measurement may be used to determine conformation.
The physical methods are; IR, Raman, microwave, NMR spectra, dipole moment measurements, & data derived from electron diffraction & X-ray diffraction.
- Dipole moments: 1,2-dibromoethane
In the anti C-Br bond is anti parallel & μ =0 , whereas in the gauche form it is finite (µ>0 ) . The actual µ is approx. 1 Debye (D) indicating that the molecule can not exist entirely in the anti form. The fact that µ varies with temperature indicates that the molecule can not exist entirely in gauche form . Two explanations; 1) Either anti & gauche forms are in equilibrium ( fig.6.5) & the equilibrium consts varies with temperature 2) or the molecule exists in either form but librates ( torsional oscillation ie. twisting to and fro about the bond in question, giving dipole moment ) around the C –C bond.The µ of the diastereoisomeric stilbene dichloride are incompatible with free rotation ( ie. there is barrier).
- Once the principle here is recognized, the argument may be reversed and used to decide on which μ value belongs to which isomer ( diastereomer ).
- μ measurement rules out free rotation ( rotation is not really free) but can not give unequivocal proof of the existence of several conformers in equilibrium.
Definite evidence for the existence of several conformations comes from IR & Raman spectra ( at least for liquids & gas). Molecules e.g.. 1,2-dibromoethane shows more than the allowed no of lines in gas , or liquid phase in its IR & Raman spectra. All the lines found in the solid state spectra are also found in the gas or liquid state but additional lines appear in the latter state.
Strength from Weakness:
Conformational Divergence between Solid and Solution States of Substituted Cyclitols Facilitated by CH···O Hydrogen Bonding
IR spectrum of ethylene glycol soln( dil) in CCl4 shows two bands.
- 3644 cm-1 ( due to unbonded OH)
- 3612cm-1 ( intra molecularly H-bonded OH)
Now H- bonded OH band is very strong means considerable fraction of the molecules are in the gauche form despite the steric & dipolar repulsion of the OH group.