Chapter 14 Mass Spectrometry Infrared Spectroscopy and UltravioletVisible

Classes of Organic Compounds [Insert Table 14. 1] © 2014 Pearson Education, Inc.

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

A Mass Spectrometer Only positively charged species reach the recorder. © 2014 Pearson Education,

The Mass Spectrum of Pentane m/z = mass-to-charge ratio of the fragment because z

The Molecular Ion Pentane forms a molecular ion with m/z = 72. © 2014

Fragmentation of the Molecular Ion The more stable the fragments, the more abundant they

Loss of H 2 From a Fragment © 2014 Pearson Education, Inc.

More Stable Fragments are More Abundant The peak at m/z = 57 is more abundant for isopentane than for pentane because a secondary carbocation is more stable than a primary carbocation. © 2014 Pearson Education, Inc.

Secondary Carbocations are More Stable Than Primary Carbocations © 2014 Pearson Education, Inc.

Natural Abundance of Isotopes © 2014 Pearson Education, Inc.

High Resolution Mass Spectrometry Can Distinguish Between Compound with the Same Molecular Mass Exact

The Carbon—Bromine Bond Breaks Heterolytically © 2014 Pearson Education, Inc.

The Carbon—Chlorine Bond Breaks Heterolytically The Carbon—Carbon Bond Breaks Homolytically © 2014 Pearson Education,

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. © 2014 Pearson Education, Inc.

The Mass Spectrum of 2 -Chloropentane © 2014 Pearson Education, Inc.

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. © 2014 Pearson Education, Inc.

The Carbon—Oxygen Bond Breaks Heterolytically © 2014 Pearson Education, Inc.

α-Cleavage in an Ether © 2014 Pearson Education, Inc.

α-Cleavage in an Alcohol © 2014 Pearson Education, Inc.

Loss of a Hydrogen from a γ-Carbon © 2014 Pearson Education, Inc.

α-Cleavage in a Ketone © 2014 Pearson Education, Inc.

Common Fragmentation Behavior in Alkyl Halides, Ethers, Alcohols, and Ketones 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. © 2014 Pearson Education, Inc.

The Electromagnetic Spectrum high energy high frequency short wavelengths © 2014 Pearson Education, Inc.

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

Wavelength © 2014 Pearson Education, Inc.

Wavenumber © 2014 Pearson Education, Inc.

A Stretching Vibration A stretching vibration occurs along the line of the bond. ©

Stretching and Bending Vibrations © 2014 Pearson Education, Inc.

Each Stretching and Bending Vibration Occurs at a Characteristic Wavenumber © 2014 Pearson Education,

The Functional Group Region (4000– 1400 cm– 1) The Fingerprint Region (1400– 600 cm– 1) Functional group regions: Both compounds are alcohols © 2014 Pearson Education, Inc. Fingerprint regions: Compounds are different alcohols

The More Polar the Bond, the More Intense the Absorption © 2014 Pearson Education,

Hooke’s Law © 2014 Pearson Education, Inc.

The Greater the Bond Order, the Larger the Wavenumber © 2014 Pearson Education, Inc.

Electron Delocalization (Resonance) Affects the Frequency of the Absorption The more double bond character,

This C═O Bond Is Essentially a Pure Double Bond © 2014 Pearson Education, Inc.

This C═O Bond Has Significant Single Bond Character The less double bond character, the

Resonance Electron Donation Decreases the Frequency Inductive Electron Withdrawal Increases the Frequency © 2014

The IR Spectrum of an Ester © 2014 Pearson Education, Inc.

The IR Spectrum of an Amide © 2014 Pearson Education, Inc.

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. © 2014 Pearson Education, Inc.

The IR Spectrum of an Alcohol © 2014 Pearson Education, Inc.

The IR Spectrum of a Carboxylic Acid © 2014 Pearson Education, Inc.

Hydrogen Bonded OH Groups Stretch at a Lower Frequency It is easier to stretch

The Strength of a Carbon—Hydrogen Bond Depends on the Hybridization of the Carbon An sp 3 -carbon—hydrogen bond is the weakest, so its stretch occurs at the shortest wavenumber (< 3000 cm– 1). © 2014 Pearson Education, Inc.

Where Carbon—Hydrogen Bonds Stretch and Bend Stretching vibrations require more energy than bending vibrations. An sp 3 -carbon—hydrogen stretch occurs at < 3000 cm– 1. An sp 2 -carbon—hydrogen stretch occurs at > 3000 cm– 1. © 2014 Pearson Education, Inc.

Where Carbon—Hydrogen Bonds Bend An sp 3 -carbon—hydrogen bend of a methyl occurs at < 1400 cm– 1. An sp 2 -carbon—hydrogen bend of a methyl and/or a methylene occurs at > 1400 cm– 1. © 2014 Pearson Education, Inc.

The IR Spectrum of an Aldehyde The carbon—hydrogen stretch of an aldehyde hydrogen occurs

The IR Spectrum of an Amine © 2014 Pearson Education, Inc.

The IR Spectrum of Diethyl Ether © 2014 Pearson Education, Inc.

Some Vibrations are Infrared Inactive A bond absorbs IR radiation only if its dipole

The IR Spectrum of 2 -Methyl-1 -pentene Wavenumber (cm– 1) Assignment 3075 2950 1650 and 890 absence of bands 1500– 1430 and 720 sp 2 CH sp 3 CH A terminal alkene with two substituents has less than four adjacent CH 2 groups. © 2014 Pearson Education, Inc.

The IR Spectrum of Benzaldehyde Wavenumber (cm– 1) Assignment 3050 2810 and 2730 1600 and 1460 1700 sp 2 CH an aldehyde benzene ring a partial single-bond character carbonyl © 2014 Pearson Education, Inc.

The IR Spectrum of 2 -Propyn-1 -ol Wavenumber (cm– 1) 3300 2950 2100 ©

The IR spectrum of N-Methylethanamide Wavenumber (cm– 1) 3300 2950 1660 1560 © 2014

IR Spectrum of Ethyl Benzyl Ketone Wavenumber (cm– 1) >3000 <3000 1605 and 1500

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 © 2014 Pearson Education, Inc.

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. © 2014 Pearson Education, Inc.

A UV Spectrum © 2014 Pearson Education, Inc.

UV/Vis Absorption Bands are Broad UV/Vis absorption bands are broad because an electronic state

Chromophore A chromophore is that part of a molecule that is responsible for a

The Beer–Lambert Law A=ecl A = absorbance of the sample c = concentration of substance in solution l = length of the cell in cm e = molar absorptivity of the sample (a measure of the probability of the transition) © 2014 Pearson Education, Inc.