2 no writing Google G Owww.lib.nthu.edu.tw.library/department 2304.per 2604.pdf

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2 no writing

Google G Owww.lib.nthu.edu.tw.library/department 2304.per 2604.pdf C 100/2604.pdf ') Google 1221 ') wikip dia D LinkedIn 122e D iCloud e 28 D due digence!! D1 L ng-9: For le. Hit C 02 Phase 2 PUSH » 2604pdf I. KOH, CH,OH. heat CH3 2. Hy S03H Benzene, heat CH3 CH3 3. Explain the differences in appearance of two 'H NMR spectra of 1-chloropentane shown at 60 MHz and 500 MHz. Please assign the signals to specific hydrogens in the molecules. (5%) 4. SN2 reactions of halocyclopropane and halocyclobutane substrates are very much slower than those of analogous acyclic secondary haloalkanes. Suggest an explanation for this phenomenon. (5%)

Explanation / Answer

3. 1H NMR spectrum of a compound at higher frequency instrument increases resolution of peaks.

So If we were to see a singlet for CH3 protons in CH3CH2- at 60 MHz instrument, we would get a triplet at 500 MHz instrument

Therefore the peaks for 1-chloropropane at 60 MHz would appear at,

0.9 ppm (singlet, 3H) for CH3 next to CH2

1.1 ppm (singlet, 2H) for CH2 between CH3 and CH2

1.2 ppm (singlet, 4H) for CH2's between two CH2

3.4 ppm (singlet, 2H) for CH2 between CH2 and Cl

and,

the peaks for 1-chloropropane at 500 MHz would appear at,

0.9 ppm (triplet, 3H) for CH3 next to CH2

1.1 ppm (sextet, 2H) for CH2 between CH3 and CH2

1.2 ppm (multiplet, 4H) for CH2's between two CH2

3.4 ppm (triplet, 2H) for CH2 between CH2 and Cl

4. Sn2 reaction involves the concerted mechanism wherein, the nucleophile attacks the carbon having the leaving group from backside and the leaving group departs from the front, all in a single step. The reaction proceeds through a high energy transition state where carbon is bonded to five groups. In case of halocycloalkanes (cyclopropane and cyclobutane) the resulting transition state has very less freedom (lower entropy) of groups, lower entropy results into higher transition state energy and thus reaction is not preferred or is very slow. Whereas, in case of acyclic systems, the groups can readjust so as a to give a transition state with minimal possible energy and therefore reaction works faster compared to the cyclic systems.

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