Post on 17-Jan-2016
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Mass Spectrometry, Infrared
Spectroscopy, and
Ultraviolet/Visible Spectroscopy
Paula Yurkanis BruiceUniversity of California,
Santa Barbara
Chapter 14
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Classes of Organic Compounds
[Insert Table 14.1]
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Mass Spectrometry
An electron is ejected from the compound, thereby forming a molecular ion.
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A Mass Spectrometer
Only positively charged species reach the recorder.
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The Mass Spectrum of Pentane
m/z = mass-to-charge ratio of the fragment because z = 1
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The Molecular Ion
Pentane forms a molecular ion with m/z = 72.
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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.
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Loss of H2 From a Fragment
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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.
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Secondary Carbocations are More Stable Than Primary Carbocations
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Natural Abundance of Isotopes
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High Resolution Mass Spectrometry Can DistinguishBetween Compound with the Same Molecular Mass
Exact Masses of Isotopes
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The Carbon—Bromine Bond Breaks Heterolytically
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The Carbon—Chlorine Bond Breaks Heterolytically The Carbon—Carbon Bond Breaks Homolytically
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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.
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The Mass Spectrum of 2-Chloropentane
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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.
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The Carbon—Oxygen Bond Breaks Heterolytically
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α-Cleavage in an Ether
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α-Cleavage in an Alcohol
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Loss of a Hydrogen from a γ-Carbon
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α-Cleavage in a Ketone
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Loss of a Hydrogen from a γ-Carbon
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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
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The Electromagnetic Spectrum
high energy low energy
high frequency low frequency
short wavelengths long wavelengths
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The Greater the Energy, the Greater the Frequency The Greater the Energy, the Shorter the Wavelength
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Wavelength
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Wavenumber
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A Stretching Vibration
A stretching vibration occurs along the line of the bond.
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Stretching and Bending Vibrations
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Each Stretching and Bending Vibration Occurs at a Characteristic Wavenumber
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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
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The More Polar the Bond, the More Intense the Absorption
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Hooke’s Law
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The Greater the Bond Order, the Larger the Wavenumber
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Electron Delocalization (Resonance) Affects the Frequency of the Absorption
The more double bond character, the greater the frequency (wavenumber).
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This C═O Bond Is Essentially a Pure Double Bond
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This C═O Bond Has Significant Single Bond Character
The less double bond character, the lower the frequency.
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Resonance Electron Donation Decreases the Frequency Inductive Electron Withdrawal Increases the Frequency
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The IR Spectrum of an Ester
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The IR Spectrum of an Amide
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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.
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The IR Spectrum of an Alcohol
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The IR Spectrum of a Carboxylic Acid
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Hydrogen Bonded OH Groups Stretch at a Lower Frequency
It is easier to stretch a hydrogen bonded OH group.
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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).
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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.
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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.
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The IR Spectrum of an Aldehyde
The carbon—hydrogen stretch of an aldehyde hydrogen occursat 2820 cm–1 and at 2720 cm–1.
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The IR Spectrum of an Amine
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The IR Spectrum of Diethyl Ether
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Some Vibrations are Infrared Inactive
A bond absorbs IR radiation only if its dipole moment changes when it vibrates.
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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.
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The IR Spectrum of Benzaldehyde
Wavenumber (cm–1) Assignment
30502810 and 27301600 and 14601700
sp2 CHan aldehydebenzene ringa partial single-bondcharacter carbonyl
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The IR Spectrum of 2-Propyn-1-ol
Wavenumber (cm–1) Assignment
330029502100
OH group sp3 CHalkyne
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The IR spectrum of N-Methylethanamide
Wavenumber (cm–1) Assignment
3300295016601560
N—Hsp3 CHamide carbonylN—H bend
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IR Spectrum of Ethyl Benzyl Ketone
Wavenumber (cm–1) Assignment
>3000<30001605 and 150017201380
sp2 CHsp3 CHa benzene ringa ketone carbonyla methyl group
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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
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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.
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A UV Spectrum
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UV/Vis Absorption Bands are Broad
UV/Vis absorption bands are broadbecause an electronic state has vibrational sublevels.
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Chromophore
A chromophore is that part of a molecule that is responsible for a UV/Vis spectrum.
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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)
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Cells Used for Taking UV/Vis Spectra
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The More Conjugated Double Bonds,the Longer the Wavelength
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Conjugation Makes theElectronic Transition Easier
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Conjugation Makes the Electronic Transition Easier
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Colored Compounds Absorb Visible Light (> 400 nm)
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Auxochrome
An auxochrome is a substituent that altersthe position and the intensity of the absorption.
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The Color Observed Depends on the Color Absorbed
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Common Dyes
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Anthocyanins
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UV/Vis Spectroscopy Can Be Used to Measure the Rate of a Reaction
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UV/Vis Spectroscopy Can Be Used to Measure the Rate of a Reaction
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UV/Vis Spectroscopy Can Be Used to Determine a pKa Value
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UV/Vis Spectroscopy Can Be Used to Determine the Melting Temperature of DNA