Transcript The Study of Chemical Reactions - chemistry

CHE 240
Unit II
Chemical Reactions,
Infrared and Mass
Spectroscopy
Terrence P. Sherlock
Burlington County College
2004
Tools for Study
• To determine a reaction’s
mechanism, look at:
Equilibrium constant
Free energy change
Enthalpy
Entropy
Bond dissociation energy
Kinetics
Activation energy
Chapter 4
=>
2
Chlorination of Methane
H
H
heat or light
H
C
H
+
Cl2
H
H
C
Cl
+
HCl
H
• 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).
=>
Chapter 4
3
Overall Reaction
Cl
Cl
+
photon (
h)
H
H
Cl
H
C
H
+
H
Cl
H
C
+
H
Cl
+
Cl
H
H
H
+
Cl
H
+
C
Cl
Cl
H
H
C
Cl
H
H
H
C
H
H
+
Cl
Cl
H
H
C
Cl
+
H
Cl
=>
H
Chapter 4
4
Termination Steps
• Collision of any two free radicals
• Combination of free radical with
contaminant or collision with wall.
H
H
C
H
+ Cl
H
H
C
Cl
H
Can you suggest others?
=>
Chapter 4
5
Equilibrium constant
• Keq = [products]
[reactants]
• For chlorination Keq = 1.1 x 1019
• Large value indicates reaction “goes to
completion.”
=>
Chapter 4
6
Free Energy Change
• DG = free energy of (products - reactants),
amount of energy available to do work.
• Negative values indicate spontaneity.
• DGo = -RT(lnKeq)
where R = 1.987 cal/K-mol
and T = temperature in kelvins
• Since chlorination has a large Keq, the free
energy change is large and negative.
=>
Chapter 4
7
Factors Determining DG
• Free energy change depends on
enthalpy
entropy
• DH = (enthalpy of products) - (enthalpy
of reactants)
• DS = (entropy of products) - (entropy of
reactants)
• DG = DH - TDS
=>
Chapter 4
8
Enthalpy
• DHo = heat released or absorbed during
a chemical reaction at standard
conditions.
• Exothermic, (-DH), heat is released.
• Endothermic, (+DH), heat is absorbed.
• Reactions favor products with lowest
enthalpy (strongest bonds).
=>
Chapter 4
9
Entropy
• DSo = 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 DGo = DHo - TDSo the
entropy value is often small.
=>10
Chapter 4
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 DH for a reaction.
=>
Chapter 4
11
Which is more likely?
Estimate DH for each step using BDE.
CH 4 +
Cl
104
CH 3
+
CH 3
H Cl
103
+ Cl2
58
CH 3 Cl
+ Cl
CH 3 Cl
+ H
84
or
CH 4 +
Cl
104
H
84
+ Cl2
H Cl
58
103
Chapter 4
+ Cl
=>
12
Kinetics
• Answers question, “How fast?”
• Rate is proportional to the concentration
of reactants raised to a power.
• Rate law is experimentally determined.
=>
Chapter 4
13
Reaction Order
• For A + B  C + D, rate = k[A]a[B]b
 a is the order with respect to A
 a + b is the overall order
• Order is the number of molecules of that
reactant which is present in the ratedetermining step of the mechanism.
• The value of k depends on temperature as
given by Arrhenius: ln k = -Ea + lnA
RT
=>
Chapter 4
14
Activation Energy
• Minimum energy required to reach
H
the transition state.
H
C
H
Cl
H
• At higher temperatures, more molecules
have the required energy.
=>
Chapter 4
15
Reaction-Energy Diagrams
• For a one-step reaction:
reactants  transition state  products
• A catalyst lowers the energy of the
transition state.
=>
Chapter 4
16
Energy Diagram for a
Two-Step Reaction
• Reactants  transition state  intermediate
• Intermediate  transition state  product
=>
Chapter 4
17
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
18
Bromination vs. Chlorination
Chapter 4
=>
19
Endothermic and
Exothermic Diagrams
Chapter 4
20
=>
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
21
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
22
Reactive Intermediates
•
•
•
•
Carbocations (or carbonium ions)
Free radicals
Carbanions
Carbene
=>
Chapter 4
23
Carbocation Structure
• Carbon has 6 electrons,
positive charge.
• Carbon is sp2 hybridized
with vacant p orbital.
=>
Chapter 4
24
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
25
Free Radicals
• Also electrondeficient
• Stabilized by alkyl
substituents
• Order of stability:
3 > 2 > 1 >
methyl
=>
Chapter 4
26
Carbanions
• Eight electrons on C:
6 bonding + lone pair
• Carbon has a negative
charge.
• Destabilized by alkyl
substituents.
• Methyl >1 > 2  > 3 
=>
Chapter 4
27
Carbenes
• Carbon is neutral.
• Vacant p orbital, so
can be electrophilic.
• Lone pair of
electrons, so can be
nucleophilic.
=>
Chapter 4
28
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
29
The Spectrum and
Molecular Effects
=>
Chapter 4
30
=>
Molecular Vibrations
Covalent bonds vibrate at only certain
allowable frequencies.
=>
Chapter 4
31
Stretching Frequencies
• Frequency decreases with increasing
atomic weight.
• Frequency increases with increasing
bond energy.
=>
Chapter 4
32
Carbon-Carbon
Bond Stretching
• Stronger bonds absorb at higher
frequencies:
C-C
C=C
CC
1200 cm-1
1660 cm-1
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
=>
33
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
34
An Alkane IR Spectrum
=>
Chapter 4
35
An Alkene IR Spectrum
=>
Chapter 4
36
An Alkyne IR Spectrum
Chapter 4
37
=>
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
38
An Alcohol IR Spectrum
=>
Chapter 4
39
An Amine
IR Spectrum
=>
Chapter 4
40
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
41
A Ketone
IR Spectrum
=>
Chapter 4
42
An Aldehyde
IR Spectrum
=>
Chapter 4
43
O-H Stretch of a
Carboxylic Acid
This O-H absorbs broadly, 2500-3500 cm-1,
due to strong hydrogen bonding.
=>
Chapter 4
44
Summary of IR
Absorptions
Chapter 4
45
=>
=>
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
46
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
47
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-).
H
e-
+
H
H
H
C
C
H
H
H
H
C
C
H
H
H
H
H
H
Chapter 4
H
H
C+
C
H
H
H
H
C
C+
H
H
H
H
=>
48
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
49
Mass Spectrometer
Chapter 4
50
=>
The Mass Spectrum
Masses are graphed or tabulated according to
their relative abundance.
Chapter 4
51
=>
The GC-MS
A mixture of compounds is separated
by gas chromatography, then identified
by mass spectrometry.
Chapter 4
52
=>
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
53
Isotopic Abundance
81Br
=>
Chapter 4
54
Mass Spectrum
with Sulfur
=>
Chapter 4
55
Mass Spectrum
with Chlorine
Chapter 4
56
=>
Mass Spectrum
with Bromine
Chapter 4
57
=>
Mass Spectra
of Alkanes
More stable carbocations will be more
abundant.
Chapter 4
58
=>
Mass Spectra
of Alkenes
Resonance-stabilized cations favored.
Chapter 4
59
=>
Mass Spectra
of Alcohols
• Alcohols usually lose a water molecule.
• M+ may not be visible.
Chapter 4
60
=>
Chapter 4
61
POWER POINT IMAGES FROM
“ORGANIC CHEMISTRY, 5TH EDITION”
L.G. WADE
ALL MATERIALS USED WITH PERMISSION OF AUTHOR
PRESENTATION ADAPTED FOR BURLINGTON COUNTY COLLEGE
ORGANIC CHEMISTRY COURSE
BY:
ANNALICIA POEHLER STEFANIE LAYMAN
CALY MARTIN
Chapter 4
62