Infrared Spectroscopy: Functional Group Determination

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1 nfrred pectroscopy: Functionl Group Determintion tch the infrred spectr below with the following compounds. ote tht one of the compounds does not mtch ny of the spectr. dentify ny spectrl fetures tht ssisted you in your determintion. B o mtch! (Would need both = nd =) o - sp 2 - o = = pectrum WVUB o - o sp 2 - = pectrum B WVUB o sp 2 - o = or = - pectrum WVUB 1

2 nfrred pectroscopy: tructure Determintion he infrred spectr for the following six compounds pper on the next two pges. Fill in ech box with the correct structure tht corresponds to ech spectrum. t () (b) t (c) 2

3 nfrred pectroscopy: tructure Determintion (continued) t (d) (e) (f) 3

4 nfrred pectroscopy: ore tructure Determintion Determine the structure of ech of the compounds on this nd the next pge using only the given moleculr formul nd infrred spectrum. Plce your nswer in the box below the corresponding spectrum. () oleculr formul: WVUB (b) oleculr formul: WVUB 4

5 nfrred pectroscopy: ore tructure Determintion (continued) (c) oleculr formul: WVUB (b) oleculr formul: B B WVUB 2. 5

6 nfrred pectroscopy: heory 1. nk the following three compounds in order of decresing crbonyl stretching frequency (1 = highest stretching frequency, 3 = lowest stretching frequency). xplin your nswer. e We consider the bility of ech heterotom to weken the neighboring crbonyl bond vi resonnce: e his resonnce structure cn never chieve the required overlp of p orbitls (they re effectively orthogonl), nd plys no role in reducing the crbonyl's double-bond chrcter. trongest crbonyl bond, so highest stretching frequency Between these two resonnce structures, the positive chrgeismorestbleonthenitrogenthnonthe oxygen. (lterntely, the nitrogen lone pir is higher in energy thn the oxygen lone pir, nd is thus more effective t donting electron density into the crbonyl.) hus, the mide crbonyl hs more single bond chrcter thn the ester crbonyl. ence, the mide crbonyl is weker bond nd hs lower stretching frequency thn the ester crbonyl. 2. Ketene, shown below, hs much higher crbonyl stretching frequency thn typicl ketone such s cetone. xplin why ketene's crbonyl stretching frequency is so high. ketene 18 cm -1 he crbonyl crbon of ketene is sp-hybridized, so the crbonyl bond should be shorter nd stronger. his strength of the crbonyl is lso reflected by the triple-bond resonnce structure shown below. hese fctors result in much higher crbonyl stretching frequency s compred to typicl ketone. 6

7 nfrred pectroscopy: ore heory 1. Phenylcetylene exhibits strong, shrp peks t 33 cm -1 nd cm -1 in the spectrum. dentify the bonds responsible for the presence of these peks. onversely, the compound diphenylcetylene, which you prepred in lb, shows neither of these peks in the infrred! xplin this observtion. Phenylcetylene shows shrp peks t 33 cm -1 nd cm -1 Diphenylcetylene shows neither pek in spectrum he f irst one is esy -- the 33 cm -1 pek results f rom terminl lkyne - stretch, nd diphenylcetylene is n internl lkyne nd lcks this type of - bond. For the second pek, spectroscopy relies on bond vibrtions tht chnge the dipole moment of those bonds. Bonds whose dipole moments do not chnge during vibrtion (nonpolr bonds) will not show pek in the spectrum. ince the triple bond in diphenylcetylene is completely symmetric, it should be no surprise tht this bond does not pper in the spectrum. 2. he infrred spectr shown below correspond to the compounds dicyclopropyl ketone nd dicyclohexyl ketone -- but which is which? xplin how cler understnding of bsic principles of spectroscopy cn mke this tsk less dunting. trin increses the s-chrcter of cyclopropne crbons f rom pure sp 3 lmost to sp 2. hus the - bonds re correspondingly stronger (s in lkenes) nd vibrte t higher Dicyclopropyl ketone f requency in the spectrum. Dicyclohexyl ketone Dicyclopropyl ketone WVUB (ote tht there is essentilly no dif f erence in their respective = peks!) Dicyclohexyl ketone WVUB 7

8 pectroscopy: Proton quivlence 1. Predict the number of 1 signls tht would be observed for ech of the following compounds. () (b) 3 6 signls 7signls (c) F (d) 2 3 signls 6signls (e) (f) 7 signls 3signls (g) (h) 5 signls 2signls (i) (j) 3 signls 5signls 8

9 pectroscopy: hemicl hift 1. By nlyzing the chemicl shifts nd integrtions, ssign the proton peks to the protons in ech molecule. b c c b 3 c 1 b f e d b 3 c 3 b 1 (exchnges with D 2 ) f e 1 2 c 3 d b d 3 c e b 2 e 3 c d

10 pectroscopy: pin/pin plitting nd the oupling onstnt 1. nlyze the expnded multiplets in the following proton spectrum, nd explin the observed splitting. hen write in the integrtion you expect for ech multiplet. b c 2 c b portion of the proton spectrum of methyl vinyl ketone is given below. ssign the three lkenyl proton resonnces in the spectrum by filling in ech box with, B,or s pproprite. methyl vinyl ketone B 3 B 3. n 2, Professor Brry rost nd coworkers t tnford University developed new method for prtil reduction of lkynes to lkenes. Given the coupling constnt between the two indicted hydrogens in the product, determine whether this rection produces the cis lkene or the trns lkene. n-bu e 1. [p*u(e) 3 ]PF 6,(t) 3 i 2. u, BF n-bu e J=15z he coupling constnt tells us tht the lkene product in this rection must end up trns.

11 opics in tructure Determintion: ymmetry dentify the unknown compound bsed on the spectrl dt given below. he correct structure will possess prticulr element of symmetry. From mss spectrometry, you deduce tht the moleculr formul is he infrred spectrum is: WVUB he 1 spectrum (with expnsions) is: Provide your best choice for the structure of the compound in the box below. emember tht the correct structure will possess n element of symmetry! ,2,3,3-tetrmethylbutne 11

12 opics in tructure Determintion: ymmetry dentify the unknown compound bsed on the spectrl dt given below. he correct structure will possess prticulr element of symmetry. From mss spectrometry, you deduce tht the moleculr formul is he infrred spectrum is: WVUB he 1 spectrum (with expnsions) is: Provide your best choice for the structure of the compound in the box below. emember tht the correct structure will possess n element of symmetry! dimethyl 2,2-diethylmlonte 12

13 opics in tructure Determintion: ings dentify the unknown compound bsed on the spectrl dt given below. he correct structure will possess non-romtic ring. From mss spectrometry, you deduce tht the moleculr formul is 4 9. he infrred spectrum is: WVUB he 1 spectrum (with expnsions) is: (exchnges with D 2 ) Provide your best choice for the structure of the compound in the box below. emember tht the correct structure will possess non-romtic ring! morpholine 13

14 opics in tructure Determintion: ings dentify the unknown compound bsed on the spectrl dt given below. he correct structure will possess non-romtic ring. From mss spectrometry, you deduce tht the moleculr formul is he infrred spectrum is: WVUB he 1 spectrum (with expnsions) is: 2 (ne dditionl pek t ; 1; exchnges with D 2 ) Provide your best choice for the structure of the compound in the box below. emember tht the correct structure will possess non-romtic ring! 1,1-cyclopropnedicrboxylic cid 14

15 opics in tructure Determintion: romtic ubstitution Ptterns dentify the unknown compound bsed on the spectrl dt given below. he correct structure will possess substituted romtic ring. From mss spectrometry, you deduce tht the moleculr formul is 8 9. he infrred spectrum is: WVUB he 1 spectrum (with expnsions) is: (exchnges with D 2 ) Provide your best choice for the structure of the compound in the box below. emember tht the correct structure will possess substituted romtic ring! 3 2 p-minocetophenone 15

16 opics in tructure Determintion: romtic ubstitution Ptterns dentify the unknown compound bsed on the spectrl dt given below. he correct structure will possess substituted romtic ring. From mss spectrometry, you deduce tht the moleculr formul is he infrred spectrum is: WVUB he 1 spectrum (with expnsions) is: Provide your best choice for the structure of the compound in the box below. emember tht the correct structure will possess substituted romtic ring! isophthlonitrile 16

17 opics in tructure Determintion: Geminl oupling dentify the unknown compound bsed on the spectrl dt given below. he correct structure will exhibit non-vinyl geminl coupling. From mss spectrometry, you deduce tht the moleculr formul is he infrred spectrum is: WVUB he 1 spectrum (with expnsions) is: (exchnges with D 2 ) Provide your best choice for the structure of the compound in the box below. emember tht the correct structure will exhibit non-vinyl geminl coupling! 3 1,2-propnediol 17

18 opics in tructure Determintion: Geminl oupling dentify the unknown compound bsed on the spectrl dt given below. he correct structure will exhibit non-vinyl geminl coupling. From mss spectrometry, you deduce tht the moleculr formul is he infrred spectrum is: WVUB he 1 spectrum (with expnsions) is: Provide your best choice for the structure of the compound in the box below. emember tht the correct structure will exhibit non-vinyl geminl coupling! 3 -butyrolctone 18

C Nuclear Magnetic Resonance

C Nuclear Magnetic Resonance Inherent Difficulties Low bundnce of 13 C (1.1% vs, 99.9% for 1 H) Lower gyromgnetic rtio (1/4 tht of 1 H) - 13 C: 67.28 vs. 1 H 267.53-1/2 1,000,000 nuclei t 60 MHz E ΔE +1/2 1,000,009 nuclei t 60 MHz