Chem 150
Unit 4 - Chemical Properties I
Chemical Reactions
The chemical properties of molecules describe processes
that involve the making and breaking the stronger covalent
bonds that hold molecules together. As a consequence,
the compositions of the the molecules participating in a
chemical process change. These processes are called
chemical reactions and can be expressed using balanced
chemical equations.
Introduction
Chemical reactions involve the rearrangement of the atoms
within and between molecules that results in the formation of
new molecules.
• This process involves the making and breaking of covalent
bonds.
• An important concept in these processes is that all of the
atoms present before a reaction are also present after the
reaction
•
This a concept allows us to describe chemical reactions using chemical
equations
2 H2 (g)
2
+
O2 (g)
2H2O (g)
Introduction
If you need to review how to create and balance chemical
equation, take a look at Section 6.1 in Raymond.
We will focus on some reactions that are important in
biological chemistry, including:
•
•
Oxidation/Reduction reactions
Reactions involving water
We will also look at the different forms of free energy that can
be used to predict the directions and rates of chemical
reactions.
3
Question
When you are driving along in your automobile, octane in the
gasoline is reacting with oxygen from the air to produce
carbon dioxide and water. Write a balanced chemical
equation that can be used to describe this reaction
2C8H18 +25 O2 --> 16 CO2 + 18 H2O
4
Oxidation and Reduction
In Unit 1 we discussed some of the strategies that atoms use
to obtain 8 valence electrons.
See Unit I
Elaboration - The Octet Rule
See Unit 1
Elaboration - Compounds
5
Oxidation and Reduction
When metal atoms combine with non-metal atoms, they
transfer electrons from the metal to the non-metal to form
ionic compounds:
2 Na (s)
6
+
Cl2 (g)
2 NaCl (s)
•
Sodium, Na (s), is a soft grey metal.
•
Chlorine, Cl2 (g), is toxic green gas.
•
Sodium chloride, NaCl (s), is a crystalline white solid
comprising sodium ions, Na+, and chloride ions, Cl–.
Oxidation and Reduction
Reactions that involve the transfer of electrons from one
atom to another are called oxidation/reduction reactions.
•
The atom losing the electrons is oxidized.
•
In the previous example, the sodium is oxidized:
2 Na+
2 Na
•
+
2e–
The atom gaining the electrons is reduced.
• In the previous example, the chlorine is reduced:
Cl2
•
7
+
2e–
2 Cl–
While the two processes can be separated, one cannot
occur without the other.
Oxidation and Reduction
Reactions that involve the transfer of electrons from one
atom to another are called oxidation/reduction reactions.
•
The reactant that takes away the electrons is the oxidizing
agent.
•
•
‣
The chlorine took the electrons away from the sodium.
The reactant that donates the electrons is the reducing
agent.
•
8
In the previous example, the chlorine is the oxidizing agent.
In the previous example, the sodium is the reducing agent.
‣
The sodium gave the electrons to the chlorine.
Oxidation and Reduction
In oxidation and reduction, metals can also transfer electrons
between themselves:
Cu (s) + 2 AgNO3 (aq)
9
2 Cu(NO3)2 (aq) + 2 Ag (s)
•
•
Copper, Cu (s), is a reddish metal.
•
copper(II) nitrate, Cu(NO3)2 (aq), a green aqueous solution
containing copper(II) ions, Cu2+, ions and nitrate ions, NO3.
•
Silver, Ag (s), a silvery metal
Silver(I) nitrate, AgNO3 (aq), a colorless aqueous solution
containing silver(I) ions, Ag+ ions and nitrate ions, NO3-.
Oxidation and Reduction
Oxidation and Reduction
• The atom losing the electrons is oxidized.
•
In the previous example, the copper is oxidized:
Cu2+
Cu
•
2e–
The atom gaining the electrons is reduced.
•
In the previous example, the silver(I) ion is reduced:
2 Ag+ +
10
+
2e–
2 Ag
Questions (Clickers)
Zinc reacts with copper(II) sulfate according to the equation:
Zn (s)
+
CuSO4 (aq)
Is the zinc being
11
A)
Oxidized?
B)
Reduced?
ZnSO4 (aq)
+
Cu (s)
Questions (Clickers)
Zinc reacts with copper(II) sulfate according to the equation:
Zn (s)
+
CuSO4 (aq)
Is the copper(II) ion being
12
A)
Oxidized?
B)
Reduced?
ZnSO4 (aq)
+
Cu (s)
Questions (Clickers)
Zinc reacts with copper(II) sulfate according to the equation:
Zn (s)
+
CuSO4 (aq)
ZnSO4 (aq)
Which reactant is the oxidizing agent?
13
A)
Zinc
B)
Copper(II) ion
+
Cu (s)
Questions (Clickers)
Zinc reacts with copper(II) sulfate according to the equation:
Zn (s)
+
CuSO4 (aq)
ZnSO4 (aq)
Which reactant is the reducing agent?
14
A)
Zinc
B)
Copper(II) ion
+
Cu (s)
Oxidation and Reduction
Oxidation/reduction reactions can also occur when no ions or
metals are involved.
• This can occur when molecular compounds composed of
nonmetals react with one another to form other molecular
compounds,
•
And:
• Polar covalent bonds are produced or eliminated
‣Or
•
Double or triple bonds are produced or eliminated
See Unit 1
Elaboration - Polarity
15
Oxidation and Reduction
The combustion of an organic molecule to produce carbon
dioxide and water is an example:
• The products of this reaction contain polar covalent bonds
CH4 + 2 O2
CO2 + 2 H2O
in which the electrons are being drawn away from
•
•
16
• The carbon
atom in CO2
H
• The hydrogen atoms in H2O
H
C
H
+
2
O O
-  O C
O
 - 
+
2 H O
H
The carbons and the hydrogens are being oxidized.
H
The oxygen is being reduced.
Oxidation and Reduction
An easier way to assess whether a reaction is an
oxidation/reduction reaction or not is to look for the following:
Oxidation
Reduction
An atom loses
electrons
An atom gains
electrons
An atom gains a
bond to oxygen
An atom loses a
bond to oxygen
An atom loses a An atom gains a
bond to hydrogen bond to hydrogen
17
Oxidation and Reduction
Applying these rules to the combustion of methane:
-  -
H
H
C
H
+
2
O O
O C
O
 - 
+
2 H O
H
H
18
•
The carbon is being oxidized because it gains bonds to
oxygen.
•
The carbon is also being oxidized because it is losing
bonds to hydrogen.
•
The hydrogens are being oxidized because they gain
bonds to oxygen.
Oxidation and Reduction
Rules for Assigning Oxidation Numbers
* The oxidation number of an atom is zero in a neutral substance that contains atoms of only one
element. Thus, the atoms in O2, O3, P4, S8, and aluminum metal all have an oxidation number of 0.
* The oxidation number of monatomic ions is equal to the charge on the ion. The oxidation number of
sodium in the Na+ ion is +1, for example, and the oxidation number of chlorine in the Cl- ion is -1.
* The oxidation number of hydrogen is +1 when it is combined with a nonmetal. Hydrogen is therefore
in the +1 oxidation state in CH4, NH3, H2O, and HCl.
* The oxidation number of hydrogen is -1 when it is combined with a metal. Hydrogen is therefore in
the -1 oxidation state in LiH, NaH, CaH2, and LiAlH4.
* The metals in Group IA form compounds (such as Li3N and Na2S) in which the metal atom is in the
+1 oxidation state.
* The elements in Group IIA form compounds (such as Mg3N2 and CaCO3) in which the metal atom is
in the +2 oxidation state.
* Oxygen usually has an oxidation number of -2. Exceptions include molecules and polyatomic ions
that contain O-O bonds, such as O2, O3, H2O2, and the O22- ion.
* The nonmetals in Group VIIA often form compounds (such as AlF3, HCl, and ZnBr2) in which the
nonmetal is in the -1 oxidation state.
* The sum of the oxidation numbers of the atoms in a molecule is equal to the charge on the
molecule.
* The most electronegative element in a compound has a negative oxidation number.
http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch19/oxred_1.php#assign
19
Oxidation and Reduction
An easier way to assess whether a reaction is an
oxidation/reduction reaction or not is to look for the following:
Oxidation
Reduction
An atom loses
electrons
An atom gains
electrons
An atom gains a
bond to oxygen
An atom loses a
bond to oxygen
An atom loses a An atom gains a
bond to hydrogen bond to hydrogen
20
Oxidation and Reduction
Hydrogenation
• Another type of oxidation/reduction reaction is the
hydrogenation reaction:
•
In this example, an alkene is reduced to an alkane.
‣
•
21
H
H
H
H
This isCconsidered
reduction,
because the hydrogenHis bringing
in
+
H
C
C
C additional
H
2
platinum
electrons to the molecule.
H
H
catalyst
H
H
The alkane that is produced in this reaction is considered “saturated” because it
can no
longer absorb any more hydrogen atoms.
unsaturated
saturated
Oxidation and Reduction
Often chemist use a shorthand method of writing equations
like these:
• The equation shown on the previous slide can be written
as follows:
H
H
C
H
•
C
H
H2
Pt
H
H
H
C
C
H
H
H
One of the reactants, H2, is placed above the reaction arrow
‣
Technically, this equation is no longer balanced
• The shorthand method of writing a chemical equation is used to emphasize what
happens to a key component of the reaction
‣
22
In this case it is the alkene.
Oxidation and Reduction
Saturated vs Unsaturated Fats
23
Oxidation and Reduction
Saturated vs Unsaturated Fats
24
Oxidation and Reduction
Saturated vs Unsaturated Fats
25
Oxidation and Reduction
Saturated vs Unsaturated Fats
26
Oxidation and Reduction
Saturated vs Unsaturated Fats
saturated fat
O
C
trans unsaturated fat
H
O
CH2
O
C
O CH
H
cis unsaturated fat
H
H
27
O
C
O CH2
Fat (Triacylglyceride)
Oxidation and Reduction
Dehydrogenation
• Oxidation/reduction also occurs when hydrogens are taken
away from a molecule. This is called dehydrogenation.
•
•
The oxidation of succinic acid to fumaric acid:
This reaction takes place in the Citric
Cycle.
FAD Acid
FADH
2
‣
•
O
H
H
O
O
We will discuss
the Citric Acid Cycle in Unit 12.
HO C C C C OH
HO
C
C
C
C
OH
H H
H H called Flavin Adenine
The FAD is an abbreviation
for a large organic molecule
fumaric acid
Dinucleotide. succinic acid
(saturated)
28
O
(unsaturated)
Oxidation and Reduction
The reaction equation on the previous slide also illustrates
another shorthand method of writing equations, which used
multiple reaction arrows.
FAD
FADH2 equation is
• The longhandO form
H H of
O this reaction
O
H O
HO C
C
C
H
H
C
OH
HO C
H
H
O
HO C
C
C
C
H
H
succinic acid
(saturated)
29
C
OH
fumaric acid
(unsaturated)
O
OH
C
H
succinic acid
(saturated)
O
C
+
FAD
HO C
C
H
O
C
C
H
fumaric acid
(unsaturated)
OH + FADH2
Oxidation and Reduction
Dehydration example
•
The oxidation of ethanol to form acetaldehyde:
•
This reaction occurs in liver after
NAD+consuming
NADHalcohol.
+ H+
•
The NAD+ is
abbreviation
for a large organic molecule
H an H
H
H named Nicotinamide
AdenineHDinucleotide.
C
C
O
H C
C
O
H
H
ethanol
30
H
H
acetaldehyde
Questions (Clickers)
In the following reaction, what is the ON# of carbon #1 vs.
carbon #2?
A)
+1
H
H
B)
#1
0+1 and
H C+1C
C)
H+1
+1 H +1
-1 and
D)
31
0 and -1 +1
-3
ethanol
NAD+
NADH + H+
+1
+1
H
H
#2
C
C
H
H
+1
+1
O -2
+1
None, this is not a redox reaction
H
acetaldehyde
O
-2
Questions (Clickers)
In the following reaction, what role is played by the NAD+
A)
reducing agent
B)
oxidizing
agent
H
H
H
C)
H C
neither
H
C
H
ethanol
32
O
NAD+
NADH + H+
H
H
C
C
H
H
acetaldehyde
O
Reactions Involving Water
While the major role for water in biology is a physical one as
the primary solvent in living cell, it also plays a chemical role
as a reactant or product in some chemical reactions.
•
Reactions involving water as a reactant or product
•
Acid-catalzyed hydrolysis
• Base-catalyzed hydrolysis
•
Hydration
• Dehydration
33
Nobel prize in chemistry-2008
Chalfie, Chimomura and Tsien-Green Fluorescent Protein-GFP
GFP mouse
Now: red fluorescent protein cat
34
Nobel prize in chemistry-2008
Chalfie, Chimomura and Tsien-Green Fluorescent Protein-GFP
A whole bunch of
new fluorescent
proteins-Roger
Tsien
Brainbows!
35
Reactions Involving Water
Hydrolysis
• In the hydrolysis reaction, water (hydro) is used to split
(lyse) another molelcule.
O
O
CH3
CH2
CH2
C
O
CH2
ethylbutanoate
(an ester)
•
36
CH3
+
H
OH
acid
catalyzed
CH3
CH2
CH2
C
OH
butanoic acid
(a carboxylic acid)
+
HO
CH2
CH3
ethanol
(an alcohol)
In this case, water is being used to split an ester into a
carboxylic acid plus and alcohol.
Reactions Involving Water
Hydrolysis example
• The hydrolysis of the ester bond in the neurotransmitter
acetylcholine.
O
CH3
O used:
CH3
• Again,
shorthand
notation
is
being
HO
CH3
•
•
C
O
CH2 CH2 N
2
CH3
C
OH
The H2O reactant is placed above
the reaction arrow,
H+
+
CH3
Theacetylcholine
H+ below the arrow indicates an acid acetic
catalyst
acid is used.
(an ester)
37
CH2
(a carboxylic acid)
HO
CH2 CH2 N
CH2
CH3
choline
(an alcohol)
Reactions Involving Water
Hydrolysis
• Hydrolysis can also be catalyzed using a base (OH-):.
O
CH3
CH2 CH2 C
O
O
CH2 CH3
ethylbutanoate
(an ester)
38
H2O
OH-
CH3
CH2
CH2
C
O
butanoate ion
(a carobxylate ion)
+
HO
CH2 CH3
ethanol
(an alcohol)
•
Because one of the products of the hydrolysis is a carboxylic acid, in base
catalyzed hydrolysis the base undergoes a second acid/base reaction with the
carboxylic acid to produce a carboxylate ion.
•
The base catalyzed hydrolysis of esters is also called saponification
•
We will be discussing acids and bases in Unit 6
Reactions With Water
Hydrolysis example:
• The base catalyzed hydrolysis of fats produces soap and
glycerol
O
C
H
O
CH2
O
C
3 H2O
O CH
H
H
H
O
C
Fat
39
O CH2
OH-
Reactions With Water
Hydrolysis example:
• The base catalyzed hydrolysis of fats produces soap and
glycerol
O
C
O
HO
CH2
HO
CH
HO
CH2
O
H
3 H2O
C
O
OH-
+
H
H
O
H
C
O
Soap
40
Glycerol
Reactions Involving Water
Hydration
• In the hydration reaction water is also split, but instead of
being used to split another molecule, it is added to another
molecule to produce a single product.
•
•
The water it is added to either an alkene or alkyne:
H OH
The hydration of an alkene produces an alcohol.
H
C
C
H
H
ethene
(an alkene)
41
H
+
H
OH
acid
catalyst
H
C
C
H
H
ethanol
(an alcohol)
H
Reactions Involving Water
Hydration
• This can also be written in shorthand as:
•
•
The H+ below the reaction arrow is used to indicate that this is an acid-catalyzed
reaction.
The shorthand is used to emphasize what happens
H to
OHthe key reactant.
H
C
C
H
H
ethene
(an alkene)
42
H
H2O
H+
H
C
C
H
H
ethanol
(an alcohol)
H
Reactions Involving Water
Hydration example
• On an earlier slide a reaction from the Citric Acid Cycle
was shown, which involved the dehydrogenation of
succinic acid to produce fumaric acid.
•
The sequent reaction in the Citric Acid Cycle is an example
of a hydration reaction:
O
HO
C
C
C
H
H
fumaric acid
(an alkene)
43
H2O
O
C
OH
HO
O
H
OH O
C
C
C
H
H
malic acid
(an alcohol)
C
OH
Reactions Involving Water
Dehydration
• In the dehydration reaction is the reverse of the hydration
reaction.
•
•
The water it is removed from an alcohol:
H OHof an alcohol produces an alkene.
The dehydration
H
C
C
H
H
ethanol
(an alcohol)
44
H
H+
H
C
C
H
H
ethene
(an alkene)
H
+
H
OH
Reactions Involving Water
Dehydration example
• The Citric Acid Cycle also provides a good example of a
dehydration reaction.
•
A dehydration reaction followed by a hydration reaction is used to move a
hydroxyl group from one carbon to an adjacent carbon in citric acid:
O
OH
O
C
H
HO
CH
O
C
C
H2O
OH
Dehydration
CH
O
C
C
H2O
OH
HO
H
Hydration
citric acid
(both a carboxylic acid
and an alcohol)
O
CH
O
C
C
OH
CH2
C
H
OH
C
CH2
C
45
O
C
CH2
O
OH
C
H
cis aconntic acid
(both a carboxylic acid
and an alkene
O
H
isocitric acid
(both a carboxylic acid
and an alcohol)
Free Energy and Reaction Rates
In Unit 3 we discussed how changes in the free energy can
be used to predict whether a process is spontaneous
(favorable) or nonspontaneous (not favorable)
46
ΔG < 0
ΔG > 0
spontaneous
nonspontaneous
Free Energy and Reaction Rates
The same principles can be applied to chemical reactions to
predict whether they are favorable or not:
Α → B
A
Free
Energy
(G)
Α → B
spontaneous
Β
Progress of
reaction
47
ΔG > 0
ΔG < 0
Free
Energy
(G)
nonspontaneous
A
Progress of
reaction
Β
Free Energy and Reaction Rates
Just because a reaction is
spontaneous, does not
mean that it will occur at
an observable rate.
• For example, diamond
and graphite are two
different forms of pure
carbon. The reaction
that converts diamond
to graphite is actually a
favorable one
Diamond → Graphite
ΔG < 0
Diamond
Free
Energy
(G)
spontaneous
Graphite
• This does not make diamonds
•
48
a bad investment for fear that
they will turn into pencil lead.
Why?
Progress of
reaction
Free Energy and Reaction Rates
There is is a hill that for most reactions the reactants must
climb and go over to before they can go on to become
product.
Α → B
Free
Energy
(G)
A
Β
Progress of
reaction
49
Free Energy and Reaction Rates
The height of this hill is called the activation energy, Eact.
• The activation energy has no effect on the overall change
in the free energy for the reaction.
Α → B
Free
Energy
(G)
Eact > 0
A
ΔG < 0
Β
Progress of
reaction
50
spontaneous
Free Energy and Reaction Rates
Diamonds are still good investment because the activation
energy for the conversion of diamond to graphite is very high.
Diamond → Graphite
Free
Energy
(G)
Eact > 0
Diamond
ΔG < 0
spontaneous
Graphite
Progress of
reaction
51
Free Energy and Reaction Rates
The reaction rate (speed) of a reaction is determined by the
height of the hill.
•
52
The higher the activation energy, the slower the reaction
rate.
Free Energy and Reaction Rates
There are several ways that reactants can be pushed over
the hill to speed up the reaction rate. Two of these include:
•
•
53
Increase the temperature of the reactant molecules.
•
This increases the kinetic energy, which increases the motion of the reactant
molecules. This increases the frequency with which they will collide with one
another to react.
Increase the concentration of the reactant molecules.
•
This increases the number of reactant molecules. This also increases the
frequency with which they will collide with other reactant molecules.
Free Energy and Reaction Rates
There is a third way to speed up the reaction rate and that is
to lower the height of the hill.
•
This is done using catalysts, which provide an alternative
pathway over the hill for the reactants.
Α → B
Free
Energy
(G)
Eact > 0
without catalyst
with catalyst
A
ΔG < 0
Β
Progress of
reaction
54
spontaneous
Free Energy and Reaction Rates
Catalysts speed up a reaction, but are not produced or
consumed in a reaction.
•
In the reaction equation, their presence in indicated above
or below the reaction arrow.
H
H
C
H
•
H
H2
platinum
catalyst
H
H
C
C
H
H
H
They have not effect on the change in free energy for the
reaction, ΔG.
•
55
+
C
H
They cannot be used to make an unfavorable reaction favorable.
Free Energy and Reaction Rates
In biological systems, catalysts are called enzymes.
56
•
•
Most enzymes are proteins.
•
Enzymes bind the reactants, facilitate the reaction, and
then release the products.
Nearly every reaction that takes place in a living cell has an
enzyme associated with.
Free Energy and Reaction Rates
Example
•
Hexokinase
See
Jmol Model
of
Hexokinase
57
The End