© 2014 Pearson Education, Inc. Mass Spectrometry, Infrared Spectroscopy, and Ultraviolet/Visible Spectroscopy Paula Yurkanis Bruice University of California,

© 2014 Pearson Education, Inc.

Mass Spectrometry, Infrared

Spectroscopy, and

Ultraviolet/Visible Spectroscopy

Paula Yurkanis BruiceUniversity of California,

Santa Barbara

Chapter 14


Classes of Organic Compounds

[Insert Table 14.1]


Mass Spectrometry

An electron is ejected from the compound, thereby forming a molecular ion.


A Mass Spectrometer

Only positively charged species reach the recorder.


The Mass Spectrum of Pentane

m/z = mass-to-charge ratio of the fragment because z = 1


The Molecular Ion

Pentane forms a molecular ion with m/z = 72.


Fragmentation of the Molecular Ion

The more stable the fragments, the more abundant they will be.

C-2—C-3 fragmentation forms more stable fragments.


Loss of H2 From a Fragment


More Stable Fragments are More Abundant

The peak at m/z = 57 is more abundant for isopentane than for pentanebecause a secondary carbocation is more stable than a primary carbocation.


Secondary Carbocations are More Stable Than Primary Carbocations


Natural Abundance of Isotopes


High Resolution Mass Spectrometry Can DistinguishBetween Compound with the Same Molecular Mass

Exact Masses of Isotopes


The Carbon—Bromine Bond Breaks Heterolytically


The Carbon—Chlorine Bond Breaks Heterolytically The Carbon—Carbon Bond Breaks Homolytically


α-Cleavage in an Alkyl Chloride

The homolytic cleavage of the carbon—carbon bond is called α-cleavage.

The bonds that break are• the weakest bonds, and• the bonds that form the most stable fragments.


The Mass Spectrum of 2-Chloropentane


α-Cleavage Occurs in Alkyl Chlorides but is Less Likely to Occur in Alkyl Bromides

The carbon—carbon bond and the carbon—chlorine bond have similar strengths.

The carbon—carbon bond is much stronger than the carbon—bromine bond.


The Carbon—Oxygen Bond Breaks Heterolytically


α-Cleavage in an Ether


α-Cleavage in an Alcohol


Loss of a Hydrogen from a γ-Carbon


α-Cleavage in a Ketone


Loss of a Hydrogen from a γ-Carbon


1. A bond between carbon and a more electronegative atom breaks heterolytically.

2. A bond between carbon and an atom of similar electronegativity breaks homolytically.

3. The bonds most likely to break are the weakest bonds and those that lead to formation of the most stable cation.

Common Fragmentation Behavior in Alkyl Halides, Ethers, Alcohols, and Ketones


The Electromagnetic Spectrum

high energy low energy

high frequency low frequency

short wavelengths long wavelengths


The Greater the Energy, the Greater the Frequency The Greater the Energy, the Shorter the Wavelength


Wavelength


Wavenumber


A Stretching Vibration

A stretching vibration occurs along the line of the bond.


Stretching and Bending Vibrations


Each Stretching and Bending Vibration Occurs at a Characteristic Wavenumber


The Functional Group Region (4000–1400 cm–1)The Fingerprint Region (1400–600 cm–1)

Functional group regions: Both compounds are alcohols

Fingerprint regions: Compounds are different alcohols



The More Polar the Bond, the More Intense the Absorption


Hooke’s Law


The Greater the Bond Order, the Larger the Wavenumber


Electron Delocalization (Resonance) Affects the Frequency of the Absorption

The more double bond character, the greater the frequency (wavenumber).


This C═O Bond Is Essentially a Pure Double Bond


This C═O Bond Has Significant Single Bond Character

The less double bond character, the lower the frequency.


Resonance Electron Donation Decreases the Frequency Inductive Electron Withdrawal Increases the Frequency


The IR Spectrum of an Ester


The IR Spectrum of an Amide


Carbon—Oxygen Bonds

The carbon—oxygen bond in an alcohol is a pure single bond.

The carbon—oxygen bond in an ether is a pure single bond.

The carbon—oxygen single bond in a carboxylic acid has partial double bond character.

One carbon—oxygen single bond in an ester is a pure single bond and one has partial double bond character.


The IR Spectrum of an Alcohol


The IR Spectrum of a Carboxylic Acid


Hydrogen Bonded OH Groups Stretch at a Lower Frequency

It is easier to stretch a hydrogen bonded OH group.



The Strength of a Carbon—Hydrogen Bond Depends on the Hybridization of the Carbon

An sp3-carbon—hydrogen bond is the weakest, so its stretch occurs at the shortest wavenumber (< 3000 cm–1).


An sp3-carbon—hydrogen stretch occurs at < 3000 cm–1.An sp2-carbon—hydrogen stretch occurs at > 3000 cm–1.

Where Carbon—Hydrogen Bonds Stretch and Bend

Stretching vibrations require more energy than bending vibrations.


Where Carbon—Hydrogen Bonds Bend

An sp3-carbon—hydrogen bend of a methyl occurs at < 1400 cm–1.

An sp2-carbon—hydrogen bend of a methyl and/or a methylene occurs at > 1400 cm–1.


The IR Spectrum of an Aldehyde

The carbon—hydrogen stretch of an aldehyde hydrogen occursat 2820 cm–1 and at 2720 cm–1.


The IR Spectrum of an Amine


The IR Spectrum of Diethyl Ether


Some Vibrations are Infrared Inactive

A bond absorbs IR radiation only if its dipole moment changes when it vibrates.


The IR Spectrum of 2-Methyl-1-pentene

Wavenumber (cm–1) Assignment

307529501650 and 890absence of bands 1500–1430 and 720

sp2 CHsp3 CHA terminal alkene with two substituentshas less than four adjacent CH2 groups.


The IR Spectrum of Benzaldehyde


30502810 and 27301600 and 14601700

sp2 CHan aldehydebenzene ringa partial single-bondcharacter carbonyl


The IR Spectrum of 2-Propyn-1-ol


330029502100

OH group sp3 CHalkyne


The IR spectrum of N-Methylethanamide


3300295016601560

N—Hsp3 CHamide carbonylN—H bend


IR Spectrum of Ethyl Benzyl Ketone


>3000<30001605 and 150017201380

sp2 CHsp3 CHa benzene ringa ketone carbonyla methyl group


Ultraviolet and Visible Spectroscopy

• Spectroscopy is the study of the interaction between matter and electromagnetic radiation

• UV/Vis spectroscopy provides information about compounds with conjugated double bonds


An Electronic Transition

Only organic compounds with electrons can produce UV/Vis spectra.

A UV spectrum is obtained if UV light is absorbed.

A visible spectrum is obtained if visible light is absorbed.


A UV Spectrum


UV/Vis Absorption Bands are Broad

UV/Vis absorption bands are broadbecause an electronic state has vibrational sublevels.


Chromophore

A chromophore is that part of a molecule that is responsible for a UV/Vis spectrum.


The Beer–Lambert Law

A = c l

A = absorbance of the sample

c = concentration of substance in solution

l = length of the cell in cm

= molar absorptivity of the sample (a measure of the probability of the transition)


Cells Used for Taking UV/Vis Spectra


The More Conjugated Double Bonds,the Longer the Wavelength


Conjugation Makes theElectronic Transition Easier


Conjugation Makes the Electronic Transition Easier


Colored Compounds Absorb Visible Light (> 400 nm)


Auxochrome

An auxochrome is a substituent that altersthe position and the intensity of the absorption.


The Color Observed Depends on the Color Absorbed


Common Dyes


Anthocyanins


UV/Vis Spectroscopy Can Be Used to Measure the Rate of a Reaction


UV/Vis Spectroscopy Can Be Used to Measure the Rate of a Reaction


UV/Vis Spectroscopy Can Be Used to Determine a pKa Value


UV/Vis Spectroscopy Can Be Used to Determine the Melting Temperature of DNA

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© 2014 Pearson Education, Inc. Mass Spectrometry, Infrared Spectroscopy, and Ultraviolet/Visible Spectroscopy Paula Yurkanis Bruice University of California,