5 1 electronegative atoms or electron withdrawing

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Unformatted text preview: (5) 1. Electronegative atoms or electron-withdrawing groups cause the observed nucleus to be deshielded. Example: F C H δ (ppm): > RO C H > N > O CCH O O 2N C H δ (ppm): > COCH C H 13 What factors affect the chemical shift? 2. Electron-donating groups (either by resonance or by being electron-rich) cause the observed nucleus to be shielded. 3. Unsaturated groups (alkenes, phenyl rings, etc) act as electron-withdrawing groups and deshield the observed nucleus. Example: H > C C > C C H δ (ppm): 4. Acidic protons (OH, NH, SH, CO2H) have highly variable chemical shifts, but often are fairly wide and rounded peaks. 14 H Chemical equivalence/nonequivalence (part A) • Nuclei in the same chemical environment will have the same chemical shift. H H H C H H O C C C H O H H Number of signals expected: _____________ 15 Chemical equivalence, eg’s eg 1) H3C H2 C O CH3 O 4 3 2 PPM 1 0 eg 2) How many signals are expected for propanal? __________ 16 Typical proton chemical shifts R RC H n CH 0.7 - 1.7 1.6 - 2.6 R R O S R I CH R N 2.1-2.5 R Br N C C H R 2.1 - 3.0 Cl C R H 2.3 - 2.7 F R R C C H 1.7 - 2.7 R O R O2 N R C C C C C C C C H R 2.2 - 2.9 H 4.5 - 7.0 R H R H R 2.0 - 3.0 H 6.5 - 8.0 2.0 - 4.0 O R H R 9.0 - 10.0 2. 7 - 4. 1 R H H O 3.1 - 4.1 R OH 11.0 - 12.0 R H 4.2 - 4.8 R H 3.0 - 5.0 R H R 4.1 - 4.3 17 Integration of signal area • The area under each signal is proportional to the number of hydrogen atoms producing that signal. • The area is measured by the computer and is represented by the integral line over each signal H3C A H2 C C O O CH3 C B 4 3 2 PPM 1 18 0 Steps to determining #H’s under a peak • Measure the height of the INTEGRAL / SIGNAL with a ruler (in mm) • Add the integral heights together • Mm per H = Total height of integrals (mm) / # H’s in formula • # H’s per signal = integral height ÷ mm/H 19 Example 1: unknown C2H6O C B A 5 4 3 PPM 2 1 0 Step 1: Determine the degrees of unsaturation from the formula Step 2: Determine the number of protons under each peak using the integrals Step 3: Analyze the multiplicity to determine the neighboring groups of each signal – not required for this question Step 4: Use chemical shifts as clues to put the pieces together 20 Step 5: Time to put the molecule together—this usually requires trial and error! Example 1: unknown C2H6O Peak δ Integration Multiplicity Comments/Ideas A B C 21 Coupling/Multiplicity/Splitting • The small magnetic fields generated by neighboring atoms affect the nucleus and cause the signal to “split” into multiple peaks. • Number of peaks = multiplicity = n + 1 • Where “n” = number of non-equivalent neighbors 22...
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