Lecture 11

IR Theory

Next Class:• Lecture Problem 4 due• Thin-Layer Chromatography

This Week In Lab:Ch 6: Procedures 2 & 3 Procedure 4 (outside of lab)

Next Week in Lab:• Ch 7: PreLab Due• Quiz 4• Ch 5 Final Report Due

Spectroscopy

1H NMR:Determine bond connectivities/pieces of a structure, whole structure

IR Use:Determine the functional groups present in a structure:

-OH, C=O, C-O, NH2, C=C, CC, C=N, CN

NMR (Nuclear Magnetic Resonance Spectroscopy):•Uses radio waves (electromagnetic radiation)•Interacts with sample’s nuclei in the presence of a magnet•Effect: nuclei flip and relax (known as resonance)

IR (Infrared Spectroscopy)•IR radiation•Interacts with molecule as a whole•Effect: bond vibrations within molecule

An IR Spectrum of Hexanol

IR Spectroscopy

Main Use: To detect the presence or absence of a functional group (specific bonds) in a molecule

How It Works:1. Bonds vibrate freely at specific wavelengths (wavenumbers)2. Want to cause the bonds to increase the magnitude of this vibrational

frequency3. Subject compound to IR radiation, 4000-625 cm-1

cm-1 is the unit for wavenumber () (The numbers of waves within 1 cm) is directly proportional to energy (unlike wavelength)

4. Bonds absorb energy equal to their natural vibrational energy - it is quantized. This absorption of energy causes a change in dipole moment

for the bond.5. Upon absorption, bonds stretch and/or bend; the IR measures this

absorption.

Vibrational Modes of Bonds

Correlation Chart Specific bonds absorb specific IR radiation and signals will appear within certainwavenumber ranges (similar to NMR).

Your lab notebook also has an IR correlation chart.

Correlation Chart Specific bonds absorb specific IR radiation and signals will appear within certainwavenumber ranges (similar to NMR).

Correlation of Bond Stretching and IR Absorption (See also Correlation Chart & Table in LabGuide)

Type of Bond Group Family of Compounds Wavenumber Range (cm-1)

Single Bonds —C—H Alkanes 2850-3300

=C—H Alkenes, aromatics 3000-3100

≡ —C H Alkynes 3300-3320

O—H Alcohols 3200-3600

N—H Amines 3300-3500

—C O Ethers, Ester , s AlcoholsCarboxyl icAcids

1330-1000

Doubl e Bonds =C C Alkene, s aromatics 1600-1680

=C O Carbonyls 1680-1750

Aldehyde , s ketones 1710-1750

Carboxyl icacids 1700-1725

Esters, amides 1680-1750

=C N Imines 1500-1650

Trip leBonds C≡C Alkynes 2100-2200

C≡N Nitriles 2200-2300

Functional group region: Fingerprint region:1550-4000 cm-1 400-1550 cm-1

Most useful portion of More diff icult to interpret; mayIR spectrum contain useful informationEvaluate t his portion foryour spectral unknown

IR spectrum of hexanoic acid

OH

O

Different bond stretches & what their signals look like in IR:

A: O-H stretch (strong, broad)C: C-H stretch (strong, sharp)E: CC or CN stretch (sharp)F: C=O stretch (strong, medium to sharp)G: C=C stretch (sharp)J: C-O stretch (strong, medium)K: C-X stretch (sharp)

Lab Guide Problem 16.1

A sample is known to have the molecular formula C4H10O and to be one of two constitutional isomers, either t-butyl alcohol or isopropyl methylether. What is the structure of the unknown?

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Lab Guide Problem 16.2(a)

Indicate how the following pairs of compounds could be distinguished usingcharacteristic IR peaks:

(a) Benzaldehyde (C6H5O) and benzoic acid (C6H5COOH)

1. Consider each structure:

2. Determine the main differences that would be seen in IR.

Lab Guide Problem 21.20

An unknown oxygen-containing compound is suspected of being an alcohol,a ketone, or a carboxylic acid. Its IR spectrum shows a broad strong peakat 3100-3400 cm-1 and a strong, sharp peak at 1700 cm-1. What kind ofcompound is it?

Consider what type of bonds appear in the ranges given. Refer to correlation chart.

Broad peak at 3100-3400 cm-1 Strong, sharp peak at 1700 cm-1

Lab Guide Problem 21.21

A compound with molecular formula C4H8O shows no absorption in the IRregion near 1700 cm-1 or 3400 cm-1. What can you deduce about its structure? Propose one possible structure.

What can you deduce about its structure?

Propose one possible structure.1. Calculate HDI.

2. Give possible structure(s):