Experiment 14: IR AND NMR IDENTIFICATION OF AN UNKNOWN

Objectives:  To learn how to interpret IR and NMR spectra.  To use IR and NMR spectra to propose a structure for an unknown, given the molecular formula.

Before coming to lab…  Go to the website:  From the left menu, select TUTORIALS.  From the right column, PRELECTURES, scroll ¾ of the way down the page.  Watch the following: SPECTROSCOPY (Part 3 of 4) SPECTROSCOPY (Part 4 of 4) (YOU’LL BE GLAD YOU DID! )

IR SPECTROSCOPY THINGS TO CONSIDER… What kinds of bonds do I have? If they appeared in the IR spectrum, where would they be? Now, look at the spectrum. Are they there?

IR SPECTROSCOPY

Base values for Absorptions of Bonds (cm -1 ) O-H C-O (Esters have two!) C-H (sp 2 ) C-H (sp 3 ) Aldehyde C-H2700 & 2800 (there are two!) Amide N-H C=O C-X Full IR Absorption Correlation Table in Appendix J

CALCULATING DEGREE OF UNSATURATION CcHhNnOoXxCcHhNnOoXx DU = (2c + 2) – (h – n + x) 2 1 o unsaturation = 1 C=C or 1 ring 2 o unsaturation = 2 C=C, 2 rings, or CΞC, or combination of C=C & rings 3 o unsaturation = combination of double bonds, triple bonds, rings 4 o unsaturation = typically indicates an aromatic ring

13 C-NMR SPECTROSCOPY Information provided: A.Functionality (Chemical Shift) tells the type of carbon via position of signal on x-axis B.Presence of symmetry via # of signals C.Presence of non-protonated carbons via small signals sometimes useful, not always!

TYPICAL CHEMICAL SHIFTS   aldehydes, ketones   esters, amides, carboxylic acids, acyl halides   arenes, alkenes   alkynes, sp 3 C attached to functional groups  0-50  sp 3 C-Csp 3, where 4 o >3 o >2 o >1 o

13 C NMR CHEMICAL SHIFT CORRELATION CHART p. 118 in lab manual

1 H-NMR SPECTROSCOPY Information provided: A. Functionality chemical shift tells the type of hydrogen Via position on x-axis B. Presence of symmetry via the # signals C. Number of protons of each type per signal Integration D. Number of neighboring protons per signal via the splitting patterns n+1 rule, where n=# of protons on neighboring carbons

TYPICAL CHEMICAL SHIFTS   carboxylic acid  9-10  aldehyde   aromatic   alkene   alkyne, hydrogens on carbons attached to functional groups   sp 3 C-H, typically 3 o >2 o >1 o

1 H NMR CHEMICAL SHIFT CORRELATION CHART

SYMMETRY & EQUIVALENCE  Notice that there are 8H according to the MF, but we only see 3 signals.  This is because some of the hydrogens are equivalent.  Number of signals = number of different types of hydrogens present  There are 3 different signals because the hydrogens are in 3 different environments. C4H8O2C4H8O2

INTEGRATION  The integration is proportional to the # hydrogens causing that signal. There are 3 H a protons There are 2 H b protons There are 3 H c protons C4H8O2C4H8O2 3H 2H

SPLITTING  Splitting of a signal occurs b/c the chemical shift of a signal can be affected by neighboring protons.  Splitting = n + 1, where n = # neighboring protons. Must be within 3 bonds or less.  Equivalent hydrogens DO NOT split each other, so H a would not split each other.  Protons cannot “see” through atoms such as oxygens or nitrogens. n=0, so n+1 = 1 (singlet) n=3, so n+1 = 4 (quartet) n=2, so n+1 = 3 (triplet) q s t

TABLE 14.1 NameMolecular Formula StructureDegree of Unsaturation 4- acetamidophenol Look up using Look up using Must be calculated using MF propyl acetate benzaldehyde

COMBINED SPECTRAL PROBLEMS  Once you have completed Table 14.1, you will notice 3 questions, followed by 3 sets of combined spectral problems.  Each page contains a 1 H NMR spectrum, a 13 C NMR spectrum, and an IR spectrum pertaining to one of the compounds in Table  You must identify which set of spectra belong to which compound, and complete the tables by recording actual chemical shift values and IR absorptions.

EXAMPLE TABLE C 3 C 2 C1C1 C 5 H 4,5 H 3 H6H6 C 4 C 6 IR Data*Structure Type of Absorption Frequency (cm -1 ) sp 3 CH stretch sp 2 CH stretch C-O stretch C=O stretch OH stretch Enter ACTUAL IR frequencies from spectra…NOT base values! Enter ACTUAL NMR chemical shifts from spectra…NOT base values! Do not include integration or multiplicity!

PROPOSING A STRUCTURE  The final part of the experiment is to practice the ability to propose a structure, given only the molecular formula (MF) and a set of IR and NMR spectra.  The best place to start is to calculate the degree of unsaturation using the provided MF, and use this information as a starting point in the proposal of possible structures.

PROPOSING A STRUCTURE  Using the MF, calculate the degrees of unsaturation.  Propose possible structures based on the MF and information gathered from the degrees of unsaturation.  Identify how many signals would appear in the spectra of each of the possible proposed structures based on symmetry and equivalent protons.  Identify approximate chemical shifts where these types of carbons/protons would appear based on correlation tables.  Identify splitting patterns of each type of proton present in each of your proposed structures.  Observe the spectra. Using the information you have, eliminate possible structures until you have identified the actual structure.

For next lab…  The FINAL LAB REPORT for Experiment 14 will be due at the beginning of class!  The PRE-LAB notebook entry for Experiment 15 will be due at the beginning of class!  Safety goggles and closed toe shoes are MANDATORY!