IR Spectroscopy CH12

7/29/2019 IR Spectroscopy CH12

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Organic Chemistry II


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Outline

Electromagnetic Radiation

Molecular Spectroscopy

Infrared Spectroscopy Interpreting Infrared Spectra

Solving Infrared Spectral

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Objectives

Understand the concept of electromagneticradiation, and other forms of radiant energy and the

relationship to wavelength and frequency.

Understand the features of an IR spectrum. Understand and learn the characteristic absorption

patterns to identify various functional groups such as

alkanes, alkenes, alkynes, alcohols, ethers, amines,

aldehydes and ketones and carboxylic acids.

Learn to solve infrared spectral problems.

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Introduction

Spectroscopy is an analytical technique

which helps determine structure.

It destroys little or no sample.

The amount of light absorbed by the

sample is measured as wavelength is

varied.

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IR Spectroscopy

measures the bond vibration frequencies

in a molecule and is used to determine

the functional group.

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Electromagnetic Spectrum

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V I B G Y O R

the higher the frequency, the greater the energy of the radiation


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Electromagnetic Spectrum

Frequency(n) and wavelength (l) are inverselyproportional.

c= nl, where cis the speed of light. Energy per photon, E = hn, where his Plancks

constant.

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n - a Greek symbol called nul - a Greek symbol called lamda


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The IR Region

Just below red in the visible region. Wavelengths usually 2.5-25 mm. More common units are wavenumbers, or cm-1,

the reciprocal of the wavelength in centimeters. Wavenumbers are proportional to frequency and

energy.

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cm-1 called reciprocal centimetre


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The Spectrum and

Molecular Effects

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Molecular Vibrations

Covalent bonds vibrate at only certain

allowable frequencies.

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Stretching Frequencies

Frequency decreases with increasing atomic weight.

Frequency increases with increasing bond energy.

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Vibrational Modes

Nonlinear molecule with n atoms usually has 3n - 6

fundamental vibrational modes.

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For linear molecules with n atoms, there are 3n 5 allowed

fundamental vibrations


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Fingerprint of Molecule

Whole-molecule vibrations and bendingvibrations are also quantized.

No two molecules will give exactly the same

IR spectrum (except enantiomers). Simple stretching: 1600-3500 cm-1.

Complex vibrations: 600-1400 cm-1, called

the fingerprint region.

N.B.: Quantized- only specific vibrational energy levels are allowed


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IR-Active and Inactive

A polar bond is usually IR-active.

A nonpolar bond in a symmetrical molecule

will absorb weakly or not at all.

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An Infrared Spectrometer

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FT-IR Spectrometer

Uses an interferometer.

Has better sensitivity.

Less energy is needed from source.

Completes a scan in 1-2 seconds.

Takes several scans and averages them.

Has a laser beam that keeps the instrumentaccurately calibrated. =>

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Carbon-Carbon Bond Stretching

Stronger bonds absorb at higher frequencies:

C-C 1200 cm-1

C=C 1660 cm-1

CC 2200 cm-1 (weak or absent if internal) Conjugation lowers the frequency:

isolated C=C 1640-1680 cm-1

conjugated C=C 1620-1640 cm-1 aromatic C=C approx. 1600 cm-1

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Carbon-Hydrogen Stretching

Bonds with more s character absorb at a higher

frequency.

sp3

C-H, just below 3000 cm-1

(to the right) sp2 C-H, just above 3000 cm-1 (to the left)

sp C-H, at 3300 cm-1

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An Alkane IR Spectrum

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An Alkene IR Spectrum

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An Alkyne IR Spectrum

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O-H and N-H Stretching

Both of these occur around 3300 cm-1, but

they look different.

Alcohol O-H, broad with rounded tip.

Primary amine (RNH2), broad with two sharp

spikes.

Secondary amine (R2NH), broad with one sharpspike.

No signal for a tertiary amine (R3N) =>

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An Alcohol IR Spectrum

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broad with rounded tip


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A 1 AmineIR Spectrum

broad with two sharp spikes

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A 2 AmineIR Spectrum

broad with one sharp spike

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Carbonyl Stretching

The C=O bond of simple ketones,aldehydes, and carboxylic acids absorb

around 1710 cm-1

. Usually, its the strongest IR signal.

Carboxylic acids will have O-H also.

Aldehydes have two C-H signals around2700 and 2800 cm-1.=>

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A Ketone

IR Spectrum

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An Aldehyde

IR Spectrum

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O-H Stretch of a

Carboxylic AcidThis O-H absorbs broadly, 2500-3500 cm-1, due to

strong hydrogen bonding.

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Variations in

C=O Absorption

Conjugation of C=O with C=C lowers the

stretching frequency to ~1680 cm-1.

The C=O group of an amide absorbs at an evenlower frequency, 1640-1680 cm-1.

The C=O of an ester absorbs at a higher

frequency, ~1730-1740 cm

-1

. Carbonyl groups in small rings (5 Cs or less)

absorb at an even higher frequency. =>

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An Amide

IR Spectrum

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Carbon - Nitrogen Stretching

C - N absorbs around 1200 cm-1.

C = N absorbs around 1660 cm-1and is

much stronger than the C = C absorption inthe same region.

C N absorbs strongly just above2200 cm-1. The alkyne C C signal is much weakerand is just below2200 cm-1 .

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A Nitrile

IR Spectrum

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Summary of IR

Absorptions

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Strengths and Limitations

IR alone cannot determine a structure.

Some signals may be ambiguous.

The functional group is usually indicated. The absence of a signal is definite proof that

the functional group is absent.

Correspondence with a known samples IRspectrum confirms the identity of the

compound.

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Solving Infrared Spectral Problems

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IR Spectroscopy CH12