#3) For each of the following IR spectra (1-5) there is a choice of three possib
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#3) For each of the following IR spectra (1-5) there is a choice of three possible types of compounds. For each spectrum, choose the most appropriate class of com- pound. Explain your reasoning by noting the presence or absence of characteristic bands in the spectrum. 1 Alcohol Carboxylic acid Phenol Micrometers um) 2.2 2.4 2.6 2.8.3 3.5 4 45 5 SS 6 8 10 t, 1212141S%1 202225 0.06 0.1 0.2 0.3 04 0.5 08 2.0 -10 400 1000 2400 2aco 2200 2000 18001600 1400 1200, Wavenumber (cm-1) 2 Aldehyde Ester Ketone 22 24 26 28 3 35 4 45 5 55 67 8910 11 12 13 14 1516 18 20 22 25 Micrometers (um) 100 0.05 0.1 02 03 0.5 0.8 80 D 70 60 50 30 10 4000 400 3400 2000 220020001800 1000140013001 1 0 Wavenumber (cm-1)Explanation / Answer
Q 1
It is a carboxylic acid. The presence of carboxylic acid can be inferred by the presence of a broad peak between 2880 and 3500 and carbonyl stretching frequency at 1600 cm-1.
Q2 .It is an aldehyde. It can be inferred by the presence of a doublet around 2800 and carbonyl stretching at around 1700cm-1
Q3. It is a nitrile. It can be inferred by the presence of CN triple bond strech at around 2200 and absence of sp-C-H stretching frequency at around 3300 cm-1
Q4. It is a primary amide. It can be inferred by the presence of a doublet at 2800 and carbonyl stretching frequecy at aroun 1600 to1700 cm-1
Q5 It is an acid anhydride. It can be inferred by the presence of a doublet at around 1600 and absence of a broad peak at 3000 cm-1
Q87. The signal at 3.2ppm would be singlet (3H) bonded to the oxygen of the methoxy group. The signal at 6.4 ppm is the hydrogen attached to C-1 ( methoxy group attached carbon) and would be a doublet of doublet with two coupling constants because of its coupling to E and Z vicinal hydrogens. The signal at 4ppm is of the hydrogen bonded to C-2 and is E- ( trans) to methoxy group. The signal is a doublet of doublet with large coupling constant with vicinal hydrogen and very small couplinh to the geminal hydrogen. Similarily the signal at 3.9 ppm is of the hydrogen bonded to C-2 and Z- (cis) to methoxy group. This signal will also be a doublet of doublet with the same coupling constants as of signal at 4.0 ppm.
Q88. The compound shown by the spectrum labelled A is of 1-phenyl-1-ethanol. The doublet observed at 2.6 which shifts with D2O is of the OH and the splitting is due the couplinh of the hydrogen bonded to O with the single hydrogen at C-1.
The compound shown by spectrum B is of 2-phenylethanol. The signal is of the -OH hydrogen. The triplet pattern is due to the coupling with the two methylene hydrogens at C-1.
If water is added to the alcohols the splitting pattern will disappear and both will appear as singlets. This is due to hydrogen bonding of -OH with water.
Q84 The IR spectrum 1 is of 6-phenyl-2-hexyne. The characteristic terminal CC triple bond strech at 2100 and sp- C-H streth at 3300 cm-1 are absent in 1. Therefore it is IR spectrum of non terminal alkyne.
But spectrum 2 shows both signals characteristic of terminal CC triple bond strech at 2100 and sp- C-H streth at 3300 cm-1 . Therefore it is of terminal alkyne 5-phenyl-1-pentyne.
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