ChemActivity L4: Proton ( 1 H) NMR

Transcription

1 hemactivity L: Proton ( ) NMR (What can a NMR spectrum tell you aout the structure of a molecule?) This activity is designed to e completed in a ½ hour laoratory session or two classroom sessions. Model : aron NMR versus ydrogen NMR ( Proton NMR ) In NMR the signals are generated y caron nuclei. NMR signals are generated y hydrogen nuclei. Most NMR peaks appear etween ppm. NMR peaks appear etween ppm. a peak cluster a a l peak cluster a reference peak (TMS) peak area = peak area = ppm The peak at ppm is a reference peak associated with the additive TMS, not the molecule in the ox. ritical Thinking Questions. (E) ow many different types of chemically distinct 's are there on the structure in Model?. (E) ow many peak clusters are there on the spectrum in Model?. (E) omplete the following sentence: In a proton NMR spectrum there is one peak cluster for each chemically distinct type of or [circle one].. The numer listed elow a peak cluster gives you the area of the peak cluster. Peak cluster area is a measure of the size of the peak cluster. a. (E) Which peak cluster a or [circle one] is igger (as measured y area)?. (E) What is the ratio of the size of peak cluster a : size of peak cluster? c. (E) ow many s are there of type a? type? Does this match the ratio aove? d. ypothesize: What structural information is conveyed y peak cluster areas in NMR? e. onstruct an explanation for why peak cluster area, and not peak height, is used to determine the numer of s associated with a peak cluster. (int: an you imagine a tall thin peak cluster with a smaller area than a shorter, ut roader peak cluster?) opyright oughton Mifflin arcourt ompany. All rights reserved.

2 98 hemactivity L: Proton ( ) NMR Memorization Task L.: Peak area tells you numer of s associated with peak cluster Peak area is calculated (y the instrument) using a mathematical technique called integration. For this reason peak cluster area is often referred to as a peak s integral, integration, or integrated area. The area of each peak cluster is often represented y a numer written aove or elow a peak cluster. This integration numer tells you the relative numer of s associated with each peak cluster. Because it is a relative numer, the ratio : would also apply to a molecule with [ & 6 s] or [6 & 9 s], etc. Be careful! Peak area (not peak height) tells you the numer of s associated with a peak cluster. That is, sometimes a tall, thin peak will have a smaller area than a short, road peak. 5. Explain why the integration ratio on a proton NMR spectrum of either of the following molecules is :. (The same as the ratio for the molecule in Model.) F F l 6. For now, don't worry aout the numer of peaks in a peak cluster. We will deal with that on the next page. Based only on the integration of each peak cluster (shown as a old numer elow the cluster) assign a letter to each peak cluster matching it to the correct type of on the structure. a l 6 7. a. onstruct an explanation for why three 's attached to the same caron (a methyl group) are equivalent (for example, see the structures aove and elow).. onstruct an explanation for why the two groups on the structure aove are equivalent (making all 6 s laeled equivalent), ut the two groups on the structure at right are NT equivalent. a c l d opyright oughton Mifflin arcourt ompany. All rights reserved.

3 hemactivity L: Proton ( ) NMR 99 Memorization Task L.: Finding NMR Equivalent s using Symmetry ur rules for finding equivalent carons also apply to finding equivalent hydrogens. These rules state that two hydrogens are equivalent if any of the following hold true. The hydrogens are directly across a mirror plane (symmetry plane) from one another. can e rotated into one another without changing the overall representation of the molecule. can e rotated into one another via a rotational symmetry axis. For those who have completed hemactivity Interestingly, it turns out that 's on different molecules are NMR equivalent if there is an external mirror plane that links them. This means that a molecule and its enantiomer will produce the exact same NMR spectrum. That is, enantiomers are indistinguishale from one another y NMR. S external mirror plane Br Br R 8. The spectrum from Model is reproduced elow. Recall that a cluster with one peak is called a singlet, two peaks = doulet, three peaks = triplet, and four peaks = quartet. Any cluster with five or more peaks will e called a multiplet. Lael peak clusters a and, elow, with singlet (s), doulet (d), triplet (t), quartet (q), or multiplet (m), as appropriate a a l a reference peak (TMS) ppm 9. Look ack at Memorization Task L. in the previous hemactivity. Recall that in NMR one attached hydrogen could split an NMR peak into a doulet, two attached hydrogens could split it into a triplet, etc.. By counting peaks in a cluster you could determine the numer of hydrogens attached to a given caron. It turns out that in NMR counting peaks in a cluster tells you the numer of nearest neighor hydrogens that are not equivalent. For the spectrum aove a. (E) ow many times was peak cluster a split/cut to product the four peaks shown?. Identify the three neighoring s on the structure which split cluster a into four peaks? c. (E) ow many times was peak cluster split/cut to product the three peaks shown? d. Identify the two neighoring s on the structure which split cluster into three peaks? opyright oughton Mifflin arcourt ompany. All rights reserved.

4 hemactivity L: Proton ( ) NMR Memorization Task L.: Multiplicity tells you the numer of foreign s within onds In NMR, 's split the signals of foreign (= non-equivalent) hydrogens within three onds. This means chemically equivalent s D NT split each other. (Review) In NMR, 's split the signal of carons one ond away.. onfirm that Memorization Task L. explains the splitting patterns of the spectra in TQ s 6 & 8.. For each individual drawing, indicate whether the circled will split the signal of the oxed. (When counting onds for splitting, ignore whether a ond is a single, doule, or triple ond. For example, the circled and oxed s in the first column are,, and onds apart, respectively.) l l F l l F Molecule I Molecule II Molecule III. n the first row of structures aove a. Lael each hydrogen on the structure with a numer, giving identical hydrogens the same numer. (The first one is done for you.). For molecules II and III, complete a tale, like the one for Molecule I, elow, y reporting the integration and multiplicity of the peak due to each type of hydrogen. Molecule I Integration Multiplicity 6* d * q *Ratio of to could also e reported as :. (heck your work) Explain why, for Molecule II, the s on carons and do not split one another even though they are only three onds apart. opyright oughton Mifflin arcourt ompany. All rights reserved.

5 hemactivity L: Proton ( ) NMR. (heck your work) There are no (methylene) groups on Molecule III, yet the signal due to the on caron is a triplet. Explain how a triplet can occur with no group next door. Model : Doulet of Doulets versus Triplet If and are similar, the peak due to will e a triplet. If not, a different cluster called a doulet of doulets can occur. The cartoon elow illustrates how, depending on spacing, either a doulet of doulets or a triplet will e found for. F Molecule III peak due to efore any splitting splits this peak into a doulet. splits each of these peaks, producing a doulet of doulets if middle two peaks are pushed together, it looks like a triplet ritical Thinking Questions 5. (E) onfirm that each in Molecule III is split y oth an and 6. (E) onfirm that in Molecule III is NT equivalent to 7. (E) Explain why in Molecule III could appear as a doulet of doulets. 8. Two s attached to the same caron (in rare cases) can e inequivalent. onstruct an explanation for why the circled on the structure at right is NT chemically equivalent to the oxed. (See the next question for a hint.) a. (int for aove) Is the circled cis or trans to the methyl group? What aout the oxed?. Lael each on the structure aove with a numer, giving identical s the same numer. Memorization Task L.: Doulet of Doulets When exactly two inequivalent s split a given, report the multiplicity as dd for doulet of doulets (dd). c. Fill in the tale reporting the multiplicity and integration of the peak due to each type of hydrogen. propene Integration Multiplicity opyright oughton Mifflin arcourt ompany. All rights reserved.

6 hemactivity L: Proton ( ) NMR 9. n a proton NMR spectrum, the chemical shift (ppm/placement along the x axis) conveys similar information as in NMR. Explain why peak cluster a is farther to the left (higher ppm) than peak cluster on the spectrum in Model. Memorization Task L.5: Memorize the following NMR hemical Shifts (ppm ranges) Type of aron Examples Marked s are within the given NMR ppm ranges ( ) ppm Range three or more onds from a functional group - two onds from π ond (= or =) allylic enzylic "alpha" to a caronyl (.8-.6) two onds from, N, or halogen N Br (.5-.5) attached to = (not enzene) 7 (.5-7) attached to enzene ring ( ) 7 9 attached to caron of = (caronyl) (9.7-) attached to or N Ar N water alcohol amine phenol - 8 caroxylic acid - opyright oughton Mifflin arcourt ompany. All rights reserved.

7 hemactivity L: Proton ( ) NMR ritical Thinking Questions. Alcohols do not always follow the rules for predicting multiplicities. Based on integration and chemical shift alone, assign each of the three (nonreference) peaks in the proton NMR of ethanol at right. a c c c TMS relative peak areas. Write the expected multiplicity aove each peak cluster associated with ethanol. Which clusters in the spectrum aove have a multiplicity that does not fit the rules we have learned so far? Memorization Task L.6: An attached to N or usually does not participate in splitting.. Illustrate how Memorization Task L.6 explains the inconsistencies you noted aove. Model : Proton NMR of s attached to N or An attached to N or often appears as a road singlet on a proton NMR spectrum. This is true even if the is within three onds of a foreign (as is the case with ethanol). Also watch out for the fact that the integration of an attached to or N can e inaccurate. These irregularities are attriutale to hydrogen onding etween sample molecules and the small amount of water present in all samples prepared in air. (There is a significant amount of water in air). Not only do s on or N hydrogen ond to water, they actually exchange with s on water. ritical Thinking Question. onstruct an explanation for why the peak at.8 ppm in the spectrum at the top of the page disappears completely if a second NMR is taken a few minutes after adding a drop of D to the NMR sample. (Note that deuterium does not show up on a proton NMR spectrum.) opyright oughton Mifflin arcourt ompany. All rights reserved.

8 hemactivity L: Proton ( ) NMR Memorization Task L.7: Deuterium Exchange D can e added to a NMR sample to determine which peaks are due to an attached to an or N. Model : NMR Solvents Typically, a solvent is present in much higher concentrations than the molecule eing analyzed. This means NMR solvents must not have any hydrogens. If an NMR solvent has even one hydrogen, the huge solvent peak will oscure all the peaks of the analyte molecule. Dl (deuterated chloroform, or chloroform-d) is the most common NMR solvent ecause it is cheap relative to other deuterated solvents, dissolves most organic molecules, and has no hydrogens. Most Dl is contaminated with a very tiny amount of l. This turns out to e a good thing since the tiny l peak (which appears at 7. ppm) can e used as a reference peak instead of adding TMS. ritical Thinking Questions. ross out the two molecules elow that cannot serve as a proton NMR solvent. D l D D l l D D S D D l D enzene-d 6 caron tetrachloride water DMS-d 6 acetone D D D D TF-d 8 5. Identify/assign each peak (including any solvent or reference peaks) on the following spectrum. 6. Aove each peak cluster due to -propanol, write the expected integration value. 7. (heck your work) Are your assignments aove consistent with the fact that only the peak at. ppm disappears when the sample is treated with D ( deuterium exchange)? Explain. opyright oughton Mifflin arcourt ompany. All rights reserved.

9 hemactivity L: Proton ( ) NMR 5 Exercises. Assign each peak on the following spectrum. 5. Predict the integration value associated with each peak cluster on the spectrum aove.. ere are proton NMR data for -romopropane: a : triplet ().ppm; : multiplet ().8ppm; : triplet ().9ppm. (Relative integrations shown in parentheses.) c a. Through how many onds can a hydrogen split another hydrogen?. According to this splitting rule, does a split c? c. Is your answer in part a) consistent with the multiplicity listed for peak clusters a and c? d. ow many hydrogens split? e. Upon very close inspection of the proton NMR spectrum of -romopropane, you would find that peak cluster has at least six peaks. Is this consistent with your answer in part d)? f. Speculate as to why any peak cluster with more than four peaks is listed simply as a "multiplet.". For each structure elow, use letters or numers to indicate chemically equivalent and distinct hydrogens, and make a tale showing the predicted integration and multiplicity of each peak cluster. l l l Br l Br opyright oughton Mifflin arcourt ompany. All rights reserved.

10 6 hemactivity L: Proton ( ) NMR 5. For each structure elow, use numers to indicate chemically equivalent and distinct hydrogens, and make a tale showing the predicted integration and multiplicity of each peak cluster. Br l l l l l l l l 6. (If you have completed hemactivity ) Imagine you have two ottles: one with (R) and the other with (S)--romoutane. Unfortunately, your la partner messed up and laeled oth ottles simply "-romoutane." an you use NMR to sort out this prolem? Explain why or why not. 7. A researcher wants to take a proton NMR of a molecule that dissolves only in water. What solvent should she use to make the NMR sample? opyright oughton Mifflin arcourt ompany. All rights reserved.

11 hemactivity L: Proton ( ) NMR 7 8. Based on the ppm range in which the peaks appear on the spectrum elow, what can you infer aout the molecule associated with the spectrum? 9. Propose a structure to go with the spectrum aove assuming the unknown has molecular formula 6 5 N, and the integrations of the peaks are ::, from left to right.. Propose a structure to go with the spectrum elow assuming the unknown has molecular formula 8, and the integrations are as shown. (int: First determine the degrees of unsaturation.) opyright oughton Mifflin arcourt ompany. All rights reserved.

12 8 hemactivity L: Proton ( ) NMR. (heck your work) The peaks at. and.5 ppm on the previous page represent the signature pattern of an ethyl group. This pattern (a quartet worth s and a triplet worth s) is so common it is worth memorizing. a. Find this pattern on the proton NMR spectrum of ethanol in TQ. ).. Sketch the proton NMR spectrum expected for diethyl ether (. Propose a structure to go with the proton NMR spectrum and the following mass spectral data: [M] + = 6 (), [M+] + = 7 (.), [M+] + = 8 (97.) 8 6. onstruct an explanation for why the peak in the spectrum aove found at.5 ppm is outside the typical range for a hydrogen alpha to a caronyl (-) and the typical range for a hydrogen ound to the same caron as a halogen (-).. Draw the structure of an unknown to go with this proton NMR and the following mass spectral data: [M] + = 66 (5), [M+] + = 67 (5.5), [M+] + = 68 (6.) opyright oughton Mifflin arcourt ompany. All rights reserved.

13 hemactivity L: Proton ( ) NMR 9 5. Peak clusters in the 7-9 ppm region are almost always indicative of s attached to a enzene ring. (As we will learn later, the region from 7-9 ppm is called the aromatic region, and molecules containing a enzene ring are y far the most common type of aromatic molecule). Peak clusters in the aromatic region often overlap one another. For example, the mass of peaks etween 7. and 7. ppm on the spectrum elow is actually two overlapping peak clusters: a doulet of doulets worth s overlapping with a triplet worth. Draw one structure that goes with oth spectra on this page (heck your work) ow can you tell the top spectrum on this page is a proton NMR spectrum and the ottom spectrum is a caron NMR spectrum? 7. (heck your work) According to the MR spectrum, how many unique s are there? 8. (heck your work) Explain why, on the proton NMR spectrum, the peak at 6. is expected to e a doulet of quartets. opyright oughton Mifflin arcourt ompany. All rights reserved.

14 hemactivity L: Proton ( ) NMR 9. The spectrum elow goes with one of the diasteriomers shown. The other diasteriomer is the answer to the previous question. Memorization Task L.8: Splitting etween trans s is larger than etween cis s It turns out that splitting etween two s that are trans to each other is larger than splitting etween two s that are cis to each other. This is often used to identify whether a molecule is E or Z. Splitting is usually reported as a J value in a unit called ertz (z). [peak difference in ppm] = [peak difference in z]/[frequency of the spectrometer in megahertz (Mz)] A typical NMR spectrometer operates at a set frequency somewhere etween -6 megahertz (Mz), though some are as large as Gz ( Mz). For example: If J = z on a -Mz instrument, the splitting will e.5 ppm. This same z splitting translates to a.ppm splitting on a 5-Mz instrument, and a.ppm splitting on a -Gz instrument. That is, the relative space etween peak clusters increases, while the space etween peaks within a given cluster, decreases. a. Splitting etween the circled and oxed s elow can e measured y looking at either the peak cluster of the circled or the oxed ; however, the cluster associated with the oxed is much harder to interpret since it is a doulet of quartets! Identify the peak cluster that goes with the circled, and confirm that NLY the oxed is near enough to split this peak.. n which proton NMR spectrum (the one elow or the one on the previous page) is the splitting etween the circled and oxed s largest? c. ross out the structure elow that does NT go with the spectrum, and explain your reasoning. d. Numer or letter the s on the remaining structure and use these to assign each unique type of to a cluster on the spectrum. (Note that there are some overlapping clusters near 7. ppm.) e. (heck your work) The structure that you crossed out elow goes with the NMR spectra on the previous page. Draw this structure on the previous page, and assign each to a peak cluster on the proton NMR spectrum and each to a peak on the caron NMR spectrum Read the assigned pages in the text and do the assigned prolems. opyright oughton Mifflin arcourt ompany. All rights reserved.

15 hemactivity L: Proton ( ) NMR The Big Picture Proton NMR works in much the same way as caron NMR, ut again interpretation of the spectra is a much more important skill for an organic chemist than understanding the complex physics ehind the instrument. The key elements of a proton NMR spectrum are reviewed elow. ommon Points of onfusion The following is a summary of the key elements of proton NMR: ppm or chemical shift (given y location along the x axis) tells you the amount of electron density around an. The closer the is to an electronegative element, the more deshielded it is and therefore the higher the ppm numer of its peak cluster (farther left on the spectrum). Multiple onds also cause the signal of neary 's to e shifted to the left. Memorization Task L.5 gives "chemical shifts" for common functional groups. Each chemically distinct should have a unique chemical shift, though in practice different peak clusters sometimes overlap just y coincidence. This is a igger prolem in proton NMR than in caron NMR (especially in the socalled aromatic region from 7-9 ppm), since most proton NMR peaks are squeezed into just an 8 ppm range (-9 ppm). Peak area, not peak height, tells you the relative numer of s associated with a peak cluster. Integration or peak cluster area (given y a numer aove or elow a peak cluster, or y a line stepping up from left to right called the integration line) tells you the relative area of each peak and therefore the relative numer of equivalent s represented y each peak. Note that integration gives you only a ratio of peak areas, making it impossile to tell the difference etween a to ratio and a to 6 ratio. Splitting in caron NMR tells you the numer of s attached to a given caron. For this reason, students incorrectly assume that splitting in proton NMR tells you the numer of s associated with a peak cluster. In fact, it is a it more complicated (see next ullet). Multiplicity is the numer of peaks in a peak cluster (also called splitting or proton-proton coupling). It tells you the numer of nonequivalent neighor s within three onds. For example, a doulet (two peaks) tells you there is exactly one non-equivalent within three onds of the responsile for this signal. Equivalent s do NT split each other. opyright oughton Mifflin arcourt ompany. All rights reserved.