CHE 240Unit II

Chemical Reactions, Infrared and Mass

Spectroscopy

Terrence P. SherlockBurlington County College

2004

Chapter 4 2

Tools for Study• To determine a reaction’s

mechanism, look at:Equilibrium constantFree energy changeEnthalpyEntropyBond dissociation energyKineticsActivation energy =>

Chapter 4 3

Chlorination of Methane

• Requires heat or light for initiation.• The most effective wavelength is blue, which

is absorbed by chlorine gas.• Lots of product formed from absorption of only

one photon of light (chain reaction). =>

C

H

H

H

H + Cl2heat or light

C

H

H

H

Cl + HCl

Chapter 4 4

Overall Reaction

C

H

H

H

H Cl+ C

H

H

H

+ H Cl

C

H

H

H

+ Cl Cl C

H

H

H

Cl + Cl

C

H

H

H

H + Cl Cl C

H

H

H

Cl + H Cl =>

Cl Cl + photon (h) Cl + Cl

Chapter 4 5

Termination Steps• Collision of any two free radicals

• Combination of free radical with contaminant or collision with wall.

C

H

H

H

Cl+ C

H

H

H

Cl

Can you suggest others? =>

Chapter 4 6

Equilibrium constant

• Keq = [products] [reactants]

• For chlorination Keq = 1.1 x 1019

• Large value indicates reaction “goes to completion.”

=>

Chapter 4 7

Free Energy Change• G = free energy of (products - reactants),

amount of energy available to do work.• Negative values indicate spontaneity.• Go = -RT(lnKeq)

where R = 1.987 cal/K-moland T = temperature in kelvins

• Since chlorination has a large Keq, the free energy change is large and negative. =>

Chapter 4 8

Factors Determining G• Free energy change depends on

enthalpyentropy

H = (enthalpy of products) - (enthalpy of reactants)

S = (entropy of products) - (entropy of reactants)

G = H - TS =>

Chapter 4 9

Enthalpy

• Ho = heat released or absorbed during a chemical reaction at standard conditions.

• Exothermic, (-H), heat is released.• Endothermic, (+H), heat is absorbed.• Reactions favor products with lowest

enthalpy (strongest bonds). =>

Chapter 4 10

Entropy• So = change in randomness, disorder,

freedom of movement.

• Increasing heat, volume, or number of particles increases entropy.

• Spontaneous reactions maximize disorder and minimize enthalpy.

• In the equation Go = Ho - TSo the entropy value is often small. =>

Chapter 4 11

Bond Dissociation Energy

• Bond breaking requires energy (+BDE)

• Bond formation releases energy (-BDE)

• Table 4.2 gives BDE for homolytic cleavage of bonds in a gaseous molecule.

A B A + B

We can use BDE to estimate H for a reaction. =>

Chapter 4 12

Which is more likely?

Estimate H for each step using BDE.

CH4 HCl+ +Cl CH3

CH3 + Cl2 CH3Cl + Cl

or

Cl+CH4 CH3Cl + H

H Cl2+ HCl Cl+

104 103

58 84

=>

104 84

58 103

Chapter 4 13

Kinetics

• Answers question, “How fast?”

• Rate is proportional to the concentration of reactants raised to a power.

• Rate law is experimentally determined.

=>

Chapter 4 14

Reaction Order• For A + B C + D, rate = k[A]a[B]b

a is the order with respect to Aa + b is the overall order

• Order is the number of molecules of that reactant which is present in the rate-determining step of the mechanism.

• The value of k depends on temperature as given by Arrhenius: ln k = -Ea + lnA RT =>

Chapter 4 15

Activation Energy• Minimum energy required to reach

the transition state.

• At higher temperatures, more molecules have the required energy.

=>

C

H

H

H

H Cl

Chapter 4 16

Reaction-Energy Diagrams• For a one-step reaction:

reactants transition state products

• A catalyst lowers the energy of the transition state.

=>

Chapter 4 17

Energy Diagram for a Two-Step Reaction

• Reactants transition state intermediate• Intermediate transition state product

=>

Chapter 4 18

Rate-Determining Step

• Reaction intermediates are stable as long as they don’t collide with another molecule or atom, but they are very reactive.

• Transition states are at energy maximums.

• Intermediates are at energy minimums.

• The reaction step with highest Ea will be the slowest, therefore rate-determining for the entire reaction. =>

Chapter 4 19

Bromination vs. Chlorination

=>

Chapter 4 20

Endothermic and Exothermic Diagrams

=>

Chapter 4 21

Hammond Postulate• Related species that are similar in energy are

also similar in structure. The structure of a transition state resembles the structure of the closest stable species.

• Transition state structure for endothermic reactions resemble the product.

• Transition state structure for exothermic reactions resemble the reactants. =>

Chapter 4 22

Radical Inhibitors

• Often added to food to retard spoilage.• Without an inhibitor, each initiation step

will cause a chain reaction so that many molecules will react.

• An inhibitor combines with the free radical to form a stable molecule.

• Vitamin E and vitamin C are thought to protect living cells from free radicals. =>

Chapter 4 23

Reactive Intermediates

• Carbocations (or carbonium ions)

• Free radicals

• Carbanions• Carbene

=>

Chapter 4 24

Carbocation Structure

• Carbon has 6 electrons, positive charge.

• Carbon is sp2 hybridized with vacant p orbital. =>

Chapter 4 25

Carbocation Stability• Stabilized by alkyl

substituents 2 ways:• (1) Inductive effect:

donation of electron density along the sigma bonds.

• (2) Hyperconjugation: overlap of sigma bonding orbitals with empty p orbital. =>

Chapter 4 26

Free Radicals

• Also electron-deficient

• Stabilized by alkyl substituents

• Order of stability:3 > 2 > 1 > methyl

=>

Chapter 4 27

Carbanions

• Eight electrons on C:6 bonding + lone pair

• Carbon has a negative charge.

• Destabilized by alkyl substituents.

• Methyl >1 > 2 > 3

=>

Chapter 4 28

Carbenes

• Carbon is neutral.• Vacant p orbital, so

can be electrophilic.• Lone pair of

electrons, so can be nucleophilic.

=>

Chapter 4 29

Types of Spectroscopy

• Infrared (IR) spectroscopy measures the bond vibration frequencies in a molecule and is used to determine the functional group.

• Mass spectrometry (MS) fragments the molecule and measures the masses.

• Nuclear magnetic resonance (NMR) spectroscopy detects signals from hydrogen atoms and can be used to distinguish isomers.

• Ultraviolet (UV) spectroscopy uses electron transitions to determine bonding patterns. =>

Chapter 4 30

The Spectrum and Molecular Effects

=>

=>

Chapter 4 31

Molecular Vibrations

Covalent bonds vibrate at only certain allowable frequencies.

=>

Chapter 4 32

Stretching Frequencies

• Frequency decreases with increasing atomic weight.

• Frequency increases with increasing bond energy. =>

Chapter 4 33

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 =>

Chapter 4 34

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

=>

Chapter 4 35

An Alkane IR Spectrum

=>

Chapter 4 36

An Alkene IR Spectrum

=>

Chapter 4 37

An Alkyne IR Spectrum

=>

Chapter 4 38

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.Secondary amine (R2NH), broad with one

sharp spike.Primary amine (RNH2), broad with two

sharp spikes.No signal for a tertiary amine (R3N) =>

Chapter 4 39

An Alcohol IR Spectrum

=>

Chapter 4 40

An Amine IR Spectrum

=>

Chapter 4 41

Carbonyl Stretching

• The C=O bond of simple ketones, aldehydes, and carboxylic acids absorb around 1710 cm-1.

• Usually, it’s the strongest IR signal.

• Carboxylic acids will have O-H also.• Aldehydes have two C-H signals around

2700 and 2800 cm-1. =>

Chapter 4 42

A Ketone IR Spectrum

=>

Chapter 4 43

An Aldehyde IR Spectrum

=>

Chapter 4 44

O-H Stretch of a Carboxylic Acid

This O-H absorbs broadly, 2500-3500 cm-1, due to strong hydrogen bonding.

=>

Chapter 4 45

Summary of IR Absorptions

=>=>

Chapter 4 46

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 sample’s IR spectrum confirms the identity of the compound. =>

Chapter 4 47

Mass Spectrometry• Molecular weight can be obtained from a

very small sample.• It does not involve the absorption or

emission of light.• A beam of high-energy electrons breaks

the molecule apart.• The masses of the fragments and their

relative abundance reveal information about the structure of the molecule. =>

Chapter 4 48

Electron Impact Ionization

A high-energy electron can dislodge an electron from a bond, creating a radical cation (a positive ion with an unpaired e-).

e- + H C

H

H

C

H

H

H

H C

H

H

C

H

H

H

H C

H

H

C

H

H

+ H

H C

H

H

C

H

H

H

+=>

Chapter 4 49

Separation of Ions

• Only the cations are deflected by the magnetic field.

• Amount of deflection depends on m/z.

• The detector signal is proportional to the number of ions hitting it.

• By varying the magnetic field, ions of all masses are collected and counted. =>

Chapter 4 50

Mass Spectrometer

=>

Chapter 4 51

The Mass Spectrum

Masses are graphed or tabulated according to their relative abundance.

=>

Chapter 4 52

The GC-MS

=>

A mixture of compounds is separatedby gas chromatography, then identifiedby mass spectrometry.

Chapter 4 53

Molecules with Heteroatoms

• Isotopes: present in their usual abundance.

• Hydrocarbons contain 1.1% C-13, so there will be a small M+1 peak.

• If Br is present, M+2 is equal to M+.

• If Cl is present, M+2 is one-third of M+.

• If iodine is present, peak at 127, large gap.

• If N is present, M+ will be an odd number.• If S is present, M+2 will be 4% of M+. =>

Chapter 4 54

Isotopic Abundance

=>

81Br

Chapter 4 55

Mass Spectrum with Sulfur

=>

Chapter 4 56

Mass Spectrum with Chlorine

=>

Chapter 4 57

Mass Spectrum with Bromine

=>

Chapter 4 58

Mass Spectra of Alkanes

More stable carbocations will be more abundant.

=>

Chapter 4 59

Mass Spectra of Alkenes

Resonance-stabilized cations favored.

=>

Chapter 4 60

Mass Spectra of Alcohols

• Alcohols usually lose a water molecule.

• M+ may not be visible.

=>

Chapter 4 61

Chapter 4 62

POWER POINT IMAGES FROM “ORGANIC CHEMISTRY, 5TH EDITION”

L.G. WADEALL MATERIALS USED WITH PERMISSION OF AUTHOR

PRESENTATION ADAPTED FOR BURLINGTON COUNTY COLLEGEORGANIC CHEMISTRY COURSE

BY:ANNALICIA POEHLER STEFANIE LAYMAN CALY MARTIN