The equilibrium for the keto-enol tautomerism of 2, 4-pentanedione is shown belo

Question

The equilibrium for the keto-enol tautomerism of 2, 4-pentanedione is shown below: Examine the structure of each tautomer and for each in turn record The number of signasl you expect in the ^1H NMR spectrum. Which groups of equivalent protons would give rise to each of the signals. The relative intensity (integral) you would expect for each of the signals. The approximate chemical shift you would expect for each of the signals. The multiplicity expected for each signal Record the dielectric constant and rank the solvents CHCI_3, toluene and MeOH i order of increasing polarity. With regard to the approximate bond strength ranges in (kJ/mol) place the following non-covalent interactions - hydrophobic, electrostatic, pi-pi aromatic stacking, van der Waals, Hydrogen bonds in order of strongest to weakest.

Explanation / Answer

A.

(a) Number of signals expected in 1H-NMr for,

keto form : two

enol form : four

(b) For keto form : 2-CH3 would give one signal and 1CH2 another signal

For enol form : signal would be for 1CH3, 1CH2, 1CH and 1OH

(c) The relative intensity expected for the signals would be thus,

For keto : CH3 (6H), CH2 (2H)

For enol : CH3 (3H), CH2 (2H), CH (1H), OH (1H)

(d) Approximate chemical shift

for keto : 2.1 ppm (CH3, 6H), 2.6 ppm (CH2, 2H)

for enol : 2.1 ppm (CH3, 3H), 4.2 ppm (CH2, 2H), 5.3 ppm (CH, 1H), 2.4 ppm (OH, 1H)

(e) Multiplicity expected

for keto : CH3 (singlet), CH2 (singlet)

for enol : CH3 (singlet), CH2 (doublet of doublet), CH (doublet), OH (singlet)

B. Dielectric constant values for CH2Cl2 (9.08), MeOH (32.70), Toluene (2.38)

So the polarity in increasing order would be : Toluene < CH2Cl2 < MeOH

C. In terms of bond strength ranges the following is the order : pi-pi aromatic stacking > hydrogen bonds > van der waals > electrostatic > hydrophobic > non-covalent interactions

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