IR Spect: Extra Practice

Problem: Based on IR data given determine the structure of the unknown. Unknown compound A has molecular formula C4H11N. It shows a peak at 2900 cm-1 and peaks in the fingerprint region.

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Problem: Based on IR data given determine the structure of the unknown. Unknown compound B has molecular formula C4H11N. It shows a single peak at approximately 3400 cm-1 as well as peaks at 2900 cm-1 and in the fingerprint region. Compound B also possesses a branched alkyl group.

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Problem: Based on IR data given determine the structure of the unknown. Unknown compound C has molecular formula C6H10O3. It shows peaks at 2900, 1850 , 1740 cm-1 and in the fingerprint region.

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Problem: Match the following functional group choices with the supplied infrared spectra data 

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Problem: Match the following functional group choices with the supplied infrared spectra data

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Problem: Match the following functional group choices with the supplied infrared spectra data. 

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Problem: Match the following functional group choices with the supplied infrared spectra data. 

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IR Spect: Extra Practice Additional Practice Problems

A C-D(carbon–deuterium) bond is electronically much like a C-H bond, and it has a similar stiffness, measured by the spring constant, k. The deuterium atom has twice the mass (m) of a hydrogen atom, however.

(a) The infrared absorption frequency is approximately proportional to  when one of the bonded atoms is much heavier than the other, and m is the lighter of the two atoms (H or D in this case). Use this relationship to calculate the IR absorption frequency of a typical C-D bond. Use 3000 cm-1 as a typical C-H absorption frequency.

(b) A chemist dissolves a sample in deuterochloroform (CDCl3), then decides to take the IR spectrum and simply evaporates most of the CDCl3.What functional group will appear to be present in this IR spectrum as a result of the CDCl3 impurity?

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Convert the following infrared wavelengths to cm-1.

(a) 6.24 typical for an aromatic 

(b) 3.38  typical for an aromatic C-H bond

(c) 5.58  typical for a ketone carbonyl

(d) 5.75  typical for an ester carbonyl

(e) 4.52  typical for a nitrile

(f) 3.03  typical for an alcohol O-H

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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.

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For each hydrocarbon spectrum, determine whether the compound is an alkane, an alkene, an alkyne, or an aromatic hydrocarbon, and assign the major peaks above (to the left of) 1600 cm-1. More than one unsaturated group may be present.

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a. Which compound will have the stretching vibration for its carbonyl group at the highest frequency: acetyl chloride, methyl acetate, or acetamide?

b. Which one will have the stretching vibration for its carbonyl group at the lowest frequency?

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Match the compound to the appropriate carbonyl IR absorption band:

acyl chloride 1800 cm-1

ester 1640 cm-1

amide 1730 cm-1

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There are three carbon–oxygen bonds in methyl acetate.

a. What are their relative lengths?

b. What are the relative infrared (IR) stretching frequencies of these bonds?

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How could IR spectroscopy be used to distinguish between the following compounds?

a. a ketone and an aldehyde d. cis-2-hexene and trans-2-hexene

b. a cyclic ketone and an open-chain ketone e. cyclohexene and cyclohexane

c. benzene and cyclohexene f. a primary amine and a tertiary amine

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a. An oxygen-containing compound shows an absorption band at ~1700 cm−1 and no ab­sorption bands at ~3300 cm−1, ~2700 cm−1, or ~1100 cm−1. What class of compound is it?

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

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What changes occur in molecules after being irradiated with IR absorption?

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Match each compound to the correct IR spectrum, and give a brief reason for your selection.

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For the following reaction, which of the following is consistent with the IR spectrum of the product? 

A) Absorption at 2250 cm –1 should disappear and absorption around 3400 cm –1 should appear.

B) Absorption at 2250 cm –1 should disappear.

C) None of these.

D) Absorption at 3200-3400 cm –1 and 1720cm –1 should appear

E) Absorption at 2250 cm –1 should disappear, new absorption at 2600-2800 cm –1 and 1720 cm –1 should appear. 

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Write structures for all compounds with molecular formula C 4H6O that would not be expected to exhibit infrared absorption in the 3200–3550-cm -1 and 1620–1780-cm -1 regions.

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There are four amides with the formula C3H7NO. (a) Write their structures. ( b) One of these amides has a melting and a boiling point that are substantially lower than those of the other three. Which amide is this? Explain your answer. (c) Explain how these amides could be differentiated on the basis of their IR spectra.

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Write structural formulas for four compounds with the formula C 3H6O and classify each according to its functional group. Predict IR absorption frequencies for the functional groups you have drawn.

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Circle the molecule within each given set that most likely produces the given Infrared (IR) spectrum.

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PRACTICE: State how the following pairs of molecules can be differentiated using Infrared (IR) spectroscopy.

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