Lab 8. Questions to Answer in Your Report.

Please do not write your report as a set of answers to the following questions. Instead, use these questions for guidance of what important points should not be missed in presenting your results and discussion

Section A. Identification of compounds.

• Present all your experimental NMR spectra and briefly describe how you identified the compounds A, B, C, D, E, F, G, and G. Use your knowledge of characteristic chemical shifts and J-couplings. Fill in the table below (check with the instructor if the list has not been changed):

  

  Name of compound	Letter Label
  methanol (CDCl3)
  ethanol (CDCl3)
  ethylbenzene (CDCl3)
  1,4-dimethylbenzene(CDCl3)
  1,2-dimethylbenzene (CDCl3)
  benzhydrol , (C6H5)2CHOH, (CDCl3)
  acetone (CDCl3)
  anthracene (CDCl3)
  2-propanol (CDCl3)
  phenanthrene (CDCl3)
  diethylamine (CDCl3)

Explain your assignment based on: relative intensities, chemical shifts and J-coupling patterns. Identify all these values.

Section B. Slow kinetics.

• Present the full NMR spectrum of thiamine hydrochloride at room temperature (you may limit it only to the range of 10 ppm to 8 ppm) and identify the spectral lines
• Describe which line intesities you monitored and why
• Explain why it was necessary to use a buffer and why it was chosen to be the acetate/acetic acid buffer
• Present the semilogarithmic plot of the normalised intesity for C-(2)-H proton (I₂/I_6') as a functon of time; determine the pseudo first order rate of exchange, k_obs, from the observed linear slope
• Calculate k_OD from the k_obs and the concetration of deuteroxide ion, [OD^-]:

k_OD = k_obs / [OD^-]

the latter can be calculated using :

[OD^-] = 10 ^{(pD -pK}w⁾

Value for pK_w can be found in the literature¹ :

t/°C	pKw
	H2O	D2O
10	14.53	15.53
15	14.34	15.32
20	14.16	15.13
25	14.00	14.95
30	13.84	14.78
35	13.69	14.62

and pD can be calculated² from the measured pH,

pD = pH + 0.4

• Extra (+ 10 points): Since a base, [B], can catalyse the proton exchange reaction^3,4, the rate law more accurately is explained by:

k_obs = k_OD [OD^-] + k_B[B]

Catalysis is weak and substantial change in the buffer concentration is required. If you did a series of measurements at the buffer concentration varying from ~0.05 to 1.0 M, you may view the effect by plotting k_obs - k_OD [OD^-] as a function of [B].

• Extra (+ 3points): How can you explain/calculate the difference between pK_w for H₂O and D₂O? Would you expect the rates of H/D (hydrogen by deuterium) exchange to be the same as D/H (deuterium by hydrogen)? Why?

Section C. Fast kinetics.

• Present your NMR spectra at different temperatures with identification (at lowest temperature) of the lines: (axial and equatorial protons for cyclohexane, different methyl protons for aminoacrolein). Which one is which?
• Are their intensities different at low temperature? What does it mean?
• Explain the mechanism of the observed lineshape variation stating at low T.
• Using either Eqs.(11-15) or Eq.(16) with the fitting program in Excel, calculate the rates of spin exchange at different temperatures. If using Eqs. (11-15), first decide which of the equations are relevant to your data.
• Plot the logarithms of rates as a function of the inverse temperature. Assuming that the rate of conformation change follows Arrhenius law:

n = n_o e ^-Ea^/RT

determine the activation barrier, E_a (from the slope), and the preexponential factor n_o (from the intercept).

• What would you expect the E_a, and the n_o to be? Can you relate them to parameters that you can obtain from measurements using other techniques? Describe your speculations.
• If you did the measurements in more than one solvent, what parameters (E_a, n_o, or both) are changing? What solvent properties can the changes be correlated with? What molecular properties in the ground and the transition state are responsible for such changes?
• You can go a step further and apply Eyring's formula from transition state theory: n = k_BT/h e ^-DSo#/R e ^-DHo#/RT, according to which one should plot ln(n/T) vs 1/T. Then the slope will be DH^o#/RT and the intercept 23.76 - DS^o#/R if is n in s^-1.

References.

1. CRC Handbook of Chemistry and Physics, D.R. Lide Ed., 74^th Edition, p.8-48
2. Glasoe, P.K.; Long, F.A. J. Phys. Chem. 1960, 64 , 188
3. Washabaugh, M.W.; Jencks, W.P. Biochemistry 1988, 27 , 5044-5053
4. Washabaugh, M.W.; Jencks, W.P. JACS 1989, 111 , 674-683
5. Wiberg, K.B., Rablen, P.R., Rush D.J., Keith T.A., JACS 1995, 117 , 4261-4270

Last updated 05/08/09.