Fri 6 Nov 09. More IR Mass spectroscopy. Hour exam 3 Fri Covers Chaps 9-12 Wednesday: Review

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1 Fri 6 Nov 09 our exam 3 Fri Covers Chaps 9-12 Wednesday: Review More IR Mass spectroscopy Good web site for IR, Mass, NMR spectra: 11/6/2009 1

2 IR is good for identifying functional groups, but not good for Mol Wt. 3 C C 3 3 C C 3 3 C C 3 11/6/ John W. Keller

3 IR is good for distinguishing some isomeric structures e.g. C 4 8 (NIST webbook) C- C- 3 C C 3 C= - C=C C=C- 2 C 11/6/ John W. Keller C-

4 IR frequencies Table Do not memorize handed out in exams John W. Keller 11/6/2009 4

5 IR frequencies Table Do not memorize handed out in exams. 11/6/ John W. Keller

6 The compound from the following list that exhibits the infrared spectrum below is a 2. b 3. c 4. d 5. e a b c d e 11/6/ John W. Keller

7 no no no no no no 11/6/ John W. Keller

8 1-hexanol 3 C 5-hexen-1-ol C-- C-- C-C C- C=C =C-- 11/6/2009 8

9 The compound from the following list that best exhibits the infrared spectrum below is a 2. b 3. c 4. d 5. e a 0 0 b c 0 d e /6/2009 9

10 The compound from the following list that best exhibits the infrared spectrum below is... The answer is... -C C- bend (alkyne bending: not in table) /6/

11 This compound must be a terminal alkyne, in spite of the fact that the C C stretch falls slightly outside the nominal range given in the WL table. 1) No other bond types absorb in the entire cm -1 region except C N, which is not among the choices. 2) The terminal C C stretch at 2110 cm -1 does show medium intensity, meaning roughly 50% transmission. 3) Can t be an alcohol, there is no strong C- stretch at 1100 cm -1. 4) Can t be an amine or amide, since none of compounds contain N. 5) Can t be a carboxylic acid, since there is no C=. 6) Can t be aromatic: no medium peaks at either 3100 or 1600 cm /6/

12 Mass spectrometry C Path of the heavier ion is bent less by mag field. C Vac pump removes neutrals Butane C 4 10 C e - C C C 3 11/6/

13 C 3 C 2 C 2 C 2 C 4 10 mw=74 Fragment ions Molecular ion M +. "Base peak" = highest x-axis in "m/z" = "mass-to-charge ratio" (usually z = +1, so effectively its m). 11/6/

14 3 C C C 3 Molecular ion M +. M+1 ion Natural carbon contains 1% 13 C isotope, so if there are 5 C s in a molecule, then 5% of the molecules will have one 13 C. If there are 20 C s, the M+1 peak will be ~20% and the M+2 peak (two 13 Cs) will be about 2%. (The exact % M+1 and M+2 ions are also influenced by other isotopes such as 2, 18, etc) 11/6/

15 Cl and Br are common elements that occur as mixed isotopes with high percentages. 35 Cl 75.5% 37 Cl 24.5% 79 Br 50.5% 81 Br 49.5% C C Br M 12 C Br M+2 12 C Br M+3 13 C 12 C Br M+1 13 C 12 C Br 11/6/

16 See isotopic abundance web applet at 11/6/

17 Routine mass spectra are resolved to about ±0.1 amu. igh resolution mass spectra may be resolved to ± amu N 2 N 2 C C 6 10 N C C Molecular weights using most common isotopes ( 1 / 12 C/ 14 N/ 16 ) are significantly different at high resolution. 11/6/

18 Which electron is lost? Non-bonding electron if (or other heteroatom) Sigma bond for alkanes The electron hole is actually delocalized by orbital overlap. Therefore different bonds are susceptible to breakage. These electron losses cause fragmentation. We will discuss fragmentation patterns of three types of molecules (and not amines): Alcohols....alpha cleavage and dehydration Alkanes C-C bond breakage Ketones (& aldehydes)..alpha cleavage and McLafferty rearrangment 11/6/

19 1. Alcohols A. alpha cleavage 3 C 31 1-butanol C 3 C 2 C 2 C 2 MW M - 43 M - 31 M+. 11/6/

20 ow does the fragmentation occur? Recall that only cations M +. or others are accelerated by the ms source. So each fragment peak must be due to a cation (+). 3 C 31 3 C 43 This is a homolysis that forms a neutral propyl radical fragment (it is lost), and a protonated formaldehyde. 3 C 3 C 43 This is a heterolysis that forms a neutral hydroxymethyl radical fragment (it is lost), and a propyl carbocation. 11/6/

21 3 C An alternative, and equivalent, way of looking at alpha cleavage process. This considers the "electron hole" to be in the alpha C-C bond. The fragmentation occurs when that single electron moves to one carbon or the other. 3 C 3 C 3 C 3 C 11/6/

22 3 C 3 C The initial alcohol cation-radical 1. Alcohols B. dehydration 3 C 3 C 3 C The atom removes a atom from the β-carbon, then splits off as water molecule... 3 C 3 C...leaving behind the dehydrated cation, which can be thought of as an alkene cation-radical butanol C 3 C 2 C 2 C 2 MW 74 M - 18 M+. 11/6/