15. Analytical Techniques:IR, NMR, Mass Spect

Identify which carbonyl group will exhibit a signal at a lower wavenumber

Which molecule would you expect tohave the lowest wavenumber for the C=O π bond IR band?

An IR spectrum has bands at ~ 2100 cm⁻¹ and ~ 3300 cm⁻¹. What functional group does this molecule likely represent?

The infrared spectra for three compounds are provided. Each compound has one or more of the following functional groups: conjugated ketone, ester, amide, nitrile, and alkyne. Determine the functional group(s) in each compound, and assign the major peaks above 1600 cm-1. < of IR spectra>

Spectra are given for three compounds. Each compound has one or more of the following functional groups: alcohol, amine, ketone, aldehyde, and carboxylic acid. Determine the functional group(s) in each compound, and assign the major peaks above 1600 cm-1. < of IR spectra>

A common lab experiment is the dehydration of cyclohexanol to cyclohexene. (a) Explain how you could tell from the IR spectrum whether your product was pure cyclohexene, pure cyclohexanol, or a mixture of cyclohexene and cyclohexanol. Give approximate frequencies for distinctive peaks. (b) Explain why mass spectrometry might not be a good way to distinguish cyclohexene from cyclohexanol.

Four infrared spectra are shown, corresponding to four of the following compounds. For each spectrum, determine the structure and explain how the peaks in the spectrum correspond to the structure you have chosen.

<

of IR spectra>

Predict the characteristic infrared absorptions of the functional groups in the following molecules. (d) pent-1-yne (e) diethylamine (f) pentanoic acid

Predict the characteristic infrared absorptions of the functional groups in the following molecules. (a) cyclohexene (b) pentan-2-ol (c) pentan-2-one

These five structures all have distinguishing absorptions in the IR. Match each structure with its characteristic absorption.

(a) sharp, 2254 cm-1 (b) very broad, centered about 3330 cm-1 (c) strong, slightly broadened, 1645 cm-1 (d) broad with spikes at 3367 and 3292 cm-1 (e) strong, sharp 1717 cm-1

Three common lab experiments are shown. In each case, describe how the IR spectrum of the product would differ from that of the reactant. Give approximate frequencies for distinctive peaks in the IR spectrum of the reactant and also that of the product. <

s of reactions>

Give approximate wavenumbers for the major characteristic IR absorption bands that would be given by each of the following compounds: e. f.

For each of the following pairs of compounds, name one absorption band that can be used to distinguish between them. e. f.

Norlutin and Enovid are ketones that suppress ovulation, so they have been used clinically as contraceptives. For which of these compounds would you expect the infrared carbonyl absorption (C=O stretch) to be at a higher frequency? Explain.

Give approximate wavenumbers for the major characteristic IR absorption bands that would be given by each of the following compounds: a.

b.

Rank the following compounds from highest wavenumber to lowest wavenumber for their C-O absorption band:

Rank the following compounds from highest wavenumber to lowest wavenumber for their C=O absorption band:

How can IR spectroscopy distinguish between 1-hexyne, 2-hexyne, and 3-hexyne?

a. How could you use IR spectroscopy to determine whether the following reaction had occurred? b. After purifying the product, how could you determine whether all the NH2NH2 had been removed?

For each of the following pairs of compounds, identify one IR absorption band that could be used to distinguish between them: i.

j.

For each of the following pairs of compounds, identify one IR absorption band that could be used to distinguish between them: c.

d.

For each of the following pairs of compounds, name one absorption band that can be used to distinguish between them. c.

d.

How can IR spectroscopy be used to distinguish between the following compounds? e. cyclohexene and cyclohexane f. a primary amine and a tertiary amine

How can IR spectroscopy be used to distinguish between the following compounds? a. a ketone and an aldehyde b. a cyclic ketone and an open-chain ketone

b. A nitrogen-containing compound shows no absorption band at ~3400 cm-1 and no absorption bands between ~1700 cm-1 and ~1600 cm-1 or between 2260 cm-1 and 2220 cm-1. What class of compound is it?

The IR spectrum of compound A with a molecular formula of C5H12O is shown below. Compound A is oxidized to give compound B, a ketone with a molecular formula of C5H10O. When compound A is heated with H2SO4, compounds C and D are obtained. Considerably more D is obtained than C. Reaction of compound C with O3, followed by treatment with dimethyl sulfide, gives two products: formaldehyde and compound E, with a molecular formula of C4H8O. Reaction of compound D with O3, followed by treatment with dimethyl sulfide, gives two products: compound F, with a molecular formula of C3H6O, and compound G, with a molecular formula of C2H4O. What are the structures of compounds A through G?

(•••) Identify the important bands you would expect to find in an IR spectrum for the following molecules. (a)

(•••) Identify the important bands you would expect to find in an IR spectrum for the following molecules. (c)

(•••) A carboxylic acid can be converted to an ester using the conditions shown (see Chapter 18). Explain how a comparison of an IR spectrum of the reactant(s) and product can be used to determine whether the reaction was successful or not.

(••••) Under acidic conditions, alkene A can be isomerized to the more stable alkene B. How could IR spectroscopy be used to distinguish between A and B? [There are a few correct answers.]

The IR spectrum for anisole contains two C―O stretching bands in the fingerprint region. Match the band to the bond that gives rise to it. Why are these bands so intense?

How can you distinguish the following compounds using: a. their infrared spectra? <

s>

The acid-catalyzed hydrolysis of an ester (Sections 18.7.2 and 19.7.1) results in the formation of an equal amount of carboxylic acid and alcohol. (b) Once separated, how could you distinguish between the carboxylic acid and alcohol using IR spectroscopy?

The absence of what band in the IR spectrum of the product in Figure 18.59 would be consistent with full conversion of the carboxylic acid to an acid chloride?

The IR spectrum of trans-oct-2-enoic acid is shown. Point out the spectral characteristics that allow you to tell that this is a carboxylic acid, and show which features lead you to conclude that the acid is unsaturated and conjugated.

Following a LiAlH₄ reduction, an IR spectrum suggested that the product was a mixture of the starting amide and the desired amine. (b) Once separated, how could you distinguish between the amide and the amine using IR spectroscopy?

Which shows an O-H stretch at a larger wavenumber: ethanol dissolved in carbon disulfide or an undiluted sample of ethanol?