solve part a 20 14. The NMR spectrum of propanol is shown. Chem. Rel. expanded v
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solve part a
20 14. The NMR spectrum of propanol is shown. Chem. Rel. expanded views shift area 0.93 3.00 1.56 2.00 3.17 1.00 3.58 2.00 (b) TMS CH3 CH2CH2OH (d) (c) (a) 0 ppm Chemical shift (6) a) Match the protons in the molecule to the corresponding peaks in the spectrum. Explain how you come to your conclusions. (base your explanation on the spin-spin splitting and your understanding of chemical shifts). (6 pts) b) Explain why the peak at 3.58 6 is shifted farthest downfield? (4 pts) c) What is the purpose of the TMS peak? (4 pts) d) In NMR why, in terms of magnet strength, is bigger better? (4 pts)Explanation / Answer
(a) We have to solve this question based on the splitting pattern.
(i) Consider the signal at 1.0 ppm. The peak is split into a triplet. We know that the splitting of a peak in 1H NMR is governed by the (n+1) rule where n is the number of adjacent protons. Since the peak has a multiplicity (i.e, splitting of 3), there must be 2 neighboring protons. This peak also has the lowest downfield shift which means that it should be the farthest from the most electronegative atom which here is O. Thus, this peak corresponds to proton a in the figure.
(ii) The peak near 1.5 ppm is split into a sextet (6 sub-peaks). As per the (n+1) rule, there must be 5 adjacent hydrogens. This peak therefore corresponds to proton b in the figure. Proton b has 5 adjacent hydrogens (3 from the terminal methyl group and two from the –CH2 group attached to the oxygen atom); hence split into sextet.
(iii) The peak near 3.0 ppm is due to –OH proton. This peak is not split since there are no adjacent hydrogen atoms. The nearest hydrogen atom(s) to this proton are separated by 2 atoms, i.e, oxygen and carbon (carob atom bears the protons). Such coupling doesn’t occur in simple compounds and hence this peak is a singlet.
(iv) The peak at 3.5 ppm is due to c protons. c protons are on the carbon attached to the oxygen atom. Due to the electronegativity of the oxygen atom, these protons will appear downfield (i.e, high value). Moreover, the peak is split into a triplet. This aligns well with the (n+1) rule since c protons have 2 adjacent protons to couple to.
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