ASR Interpretation of Spectra Tutorial

1 Anderson Serangoon Junior College 2025 H3 Chemistry Interpretation of Spectra 1 The spectra shown below were obtained from compound Q, which contains the element carbon, hydrogen, nitrogen and oxygen. Consider the NMR, mass and IR spectra in turn. Explain what information each spectrum gives about Q. Hence give a possible structure for the compound.

2 NMR  /ppm Multiplicity No of H Deduction 7-8 Multiplet 5 HH H H H Monosubstituted phenyl ring 2.1 Singlet 3 -CH3 with no H on adjacent C 3.1 Singlet 1 Since the peak disappear on adding D2O, it is a labile proton, attached to an electronegative atom. MS M : M+1 16.5 : 1.47 100 : 1.1 n No. of carbon atoms = (100/1.1)(1.47/16.5) = 8 Given Q contains C, H, O and N, Peak at m/e 135 could be molecular ion C8H9NO+. Structure of Q could be A or B. C O N CH3 H NH C CH3 O A B m/e Fragment 93 135 – 42 NH2 43 - C O CH3 Absence of m/e at 105 suggest absence of C6H5CO+. From the fragment ions, structure of Q is B. IR Presence of N-H at 3300 cm-1 Prsence of amide functional group.

3 2 Interpret the following spectra and use them to suggest a structure for the unknown compound.

4 MS peak at m/e 91 is [C6H5CH2]+ (benzyl or tropilium cation) peak at m/e 77 is [C6H5]+ peak at m/e 43 is [CH3CO] + UV/VIS Compound shows several absorptions within UV/VIS region, suggesting presence of a conjugated π-system IR C-H stretch at 3100cm-1 Strong absorption at 1750cm-1 due to C=O stretch. NMR δ / ppm No. of 1H Multiplicity Deductions 2.1 3 s -CH3 with no H on adj C Deshielded by electron -withdrawing ketone group 3.7 2 s -CH2- with no H on adj C Deshielded by ring current and electron- withdrawing ketone group 7.1 – 7.4 5 m Aromatic H on mono-substituted phenyl ring Structure:

5 3 Deduce the structure of compound A, giving your reasoning. [8]

6 IR absorption bond 1740 C=O 1200 C–O  Absence of peak at 3230 to 3500 cm–1 suggests absence of O–H stretch  No alcohol  The presence of a C–O (strong and broad) at 1200 cm–1 and C=O at 1740 cm–1 confirm the identity of an ester functional group. NMR MS m/e species 102 (C5H10O2)+ 57 (CH3CH2CO)+ 29 (CH3CH2)+ chemical shift /ppm splitting pattern No. of adjacent protons relative peak area group responsible 1.15 triplet 2 3 –CH3 (a) adj to a -CH2- 1.25 triplet 2 3 –CH3 (b) adj to a -CH2- 2.30 quartet 3 2 –CH2CO– (adjacent to a –CO– and –CH3) 4.15 quartet 3 2 –OCH2– (adjacent to a –O– and –CH3)

7 4 The 1H NMR signals of protons on carbon atoms adjacent to C=C bonds ca n be split by protons at the far end of the C=C bond. HC R R R HA R R HB For example, protons HA and HC with split each other with a small coupling constant JAC. Protons HB and HC will also split each other with coupling constant JBC. JA