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The Main Types
of Organic Reactions
2.1
Figure 2.1(A) shows raw fruit. The crisp, sharp-tasting fruit becomes soft
and sweet when it is cooked. Figure 2.1(B) shows a chemist accelerating
the tranformation of ethanol into ethanoic acid, by adding potassium
dichromate and sulfuric acid.
What do these reactions have in common? They are both examples of
organic reactions. In this section, you will take a quick look at the main
types of organic reactions. You will concentrate on simply recognizing
these types of organic reactions. In the next section, you will examine the
reactions of specific functional groups and learn how to predict the products of organic reactions.
Section Preview/
Specific Expectations
A
Figure 2.1
In this section, you will
■
describe different types of
organic reactions, including
addition, substitution,
elimination, oxidation, and
reduction
■
predict the products of
organic reactions, including
addition, substitution,
elimination, oxidation, and
reduction
■
communicate your understanding of the following
terms: addition reaction,
substitution reaction,
elimination reaction,
oxidation, reduction,
condensation reaction,
hydrolysis reaction
B
Organic reactions take place all around you, not only in a science lab.
Addition, Substitution, and Elimination Reactions
Addition reactions, substitution reactions, and elimination reactions are
the three main types of organic reactions. Most organic reactions can be
classified as one of these three types.
Addition Reactions
In an addition reaction, atoms are added to a double or triple bond. One
bond of the multiple bond breaks so that two new bonds can form. To
recognize an addition reaction, remember that two compounds usually
react to form one major product. (Sometimes two isomers are formed.)
The product has more atoms bonded to carbon atoms than the organic
reactant did. A general example of addition to an alkene is given below.
CH
CH
+ XY →
X
Y
CH
CH
Addition reactions are common for alkenes and alkynes. Addition
reactions can also occur at a CO bond. Some examples of addition
reactions are shown on the next page.
Chapter 2 Reactions of Organic Compounds • MHR
57
Example 1
H
H
C
+
C
→
HBr
ethene
hydrobromic acid
C
C
CH2
+
CH3
Br2
→
C
H
H
Br
Br
C
C
H
bromine
butyne
C
H
bromoethane
Example 2
H
H
H
H
H
Br
CH2
CH3
1,2-dibromobutene
Substitution Reactions
You can express an addition
reaction algebraically, using
the equation a + b → ab.
Come up with similar
equations for substitution
and elimination reactions.
In a substitution reaction, a hydrogen atom or a functional group is
replaced by a different functional group. To help you recognize this
type of reaction, remember that two compounds usually react to form
two different products. The organic reactant(s) and the organic product(s)
have the same number of atoms bonded to carbon.
X
C
+ AY →
+ AX
Y
C
Alcohols, alkyl halides, and aromatic compounds commonly undergo
substitution reactions, as shown in these examples.
Example 1
CH3
CH2
+
OH
ethanol
HI
→
CH3
hydroiodic acid
CH2
I
+
iodoethane
HOH
water
Example 2
Br
CH3
CH
NH2
CH2
CH3 + NH3 → CH3
2-bromobutane
ammonia
CH
CH2
CH3
2-butanamine
+ HBr
hydrobromic
acid
Example 3
H
H
NO2
H
H
+
HNO2
H
58
MHR • Unit 1 Organic Chemistry
H
H
H
H
benzene
→
+
HOH
H
H
nitrous acid
nitrobenzene
water
Elimination Reactions
In an elimination reaction, atoms are removed from a molecule to form a
double bond. This type of reaction is the reverse of an addition reaction.
One reactant usually breaks up to give two products. The organic product
typically has fewer atoms bonded to carbon atoms than the organic
reactant did.
X
Y
C
C
→
C
+
C
XY
Alcohols often undergo elimination reactions when they are heated in
the presence of strong acids, such as sulfuric acid, H2SO4 , which acts
as a catalyst. (See the first example below.) Alkyl halides also undergo
elimination reactions to produce alkenes. (See the second example.)
Example 1
H
H
H
OH
H
C
C
C
H
H
H
H
H2SO4
H
C
H
H C
H
2-propanol
+
C
HOH
H
propene
water
In the first example of an
elimination reaction, the strong
acid, H2SO4, does not count
as a reactant. It is not directly
involved in the reaction. It is
a catalyst : a compound that
speeds up a reaction but is not
consumed by it.
Example 2
H
H
Br
C
C
H
H
H
H
H
→
bromoethane
C
+
C
H
HBr
H
ethene
hydrobromic acid
Oxidation and Reduction
An important type of organic reaction occurs when there is a change in
the number of hydrogen or oxygen atoms that are bonded to carbon. In
Unit 5, you will take a close look at oxidation-reduction reactions in terms
of the transfer of electrons. As you will learn, oxidation and reduction
always occur together. One reactant is oxidized while the other reactant
is reduced. In this unit, however, you will focus on the organic reactant
only. Therefore, you will deal with oxidation and reduction separately, as
they apply to organic compounds. In organic chemistry, oxidation and
reduction are defined by the changes of the bonds to carbon atoms in the
organic reactant.
Chapter 2 Reactions of Organic Compounds • MHR
59
Oxidation
In organic chemistry, oxidation is defined as a reaction in which a carbon
atom forms more bonds to oxygen, O, or less bonds to hydrogen, H. An
oxidation that involves the formation of double CO bonds may also be
classified as an elimination reaction.
For example, alcohols can be oxidized to produce aldehydes and
ketones. Oxidation occurs when an organic compound reacts with an
oxidizing agent. Common oxidizing agents include acidified potassium
permanganate, KMnO4 , acidified potassium dichromate, K2Cr2O7 , and
ozone, O3 . The symbol [O] is used to symbolize an oxidizing agent, as
shown below. Note that equations for the oxidation of organic compounds
are often left unbalanced. The purpose of the equation is to show the
changes in the organic reactant only.
O
OH
C
+
H
alcohol
→
[O]
oxidizing agent
C
aldehyde or ketone
O
O
+
C
→
[O]
C
H
OH
aldehyde
oxidizing agent
carboxylic acid
To identify an oxidation, count and compare the number of CH
and CO bonds in both the reactant and product. Try it for the
following example.
H
H
OH
C
C
H
H
ethanol
H
+
[O]
→
H
H
O
C
C
H
H
ethanal
Reduction
In organic chemistry, reduction is defined as a reaction in which a
carbon atom forms fewer bonds to oxygen, O, or more bonds to hydrogen,
H. Often, a CO bond or CC bond is reduced to a single bond by
reduction. A reduction that transforms double CC or CO bonds to
single bonds may also be classified as an addition reaction. Aldehydes,
ketones, and carboxylic acids can be reduced to become alcohols. Alkenes
and alkynes can be reduced by the addition of H2 to become alkanes.
Reduction occurs when an organic compound reacts with a reducing
agent. Common reducing agents are lithium aluminum hydride, LiAlH4 ,
and hydrogen gas over a platinum catalyst, H2/Pt . The symbol [H] is used
to symbolize a reducing agent. As is the case for oxidation, equations
showing the reduction of organic compounds are often left unbalanced.
60
MHR • Unit 1 Organic Chemistry
O
OH
+
C
aldehyde or ketone
C
C
reducing agent
+
C
→
[H]
alcohol
H
H
C
C
→
[H]
alkene
H
reducing agent
alkane
To identify a reduction, count and compare the number of CH and
CO bonds in both the reactant and the product. Try it for the following
example.
O
H3C
C
OH
CH3
+
→
[H]
H3C
CH3
C
H
propanone
2-propanol
CHEM
FA C T
Other Important Organic Reactions
In this chapter, you will also encounter the following classes of organic
reactions: condensation reactions and hydrolysis reactions. Condensation
and hydrolysis reactions are both types of substitution reactions.
Condensation Reactions
In a condensation reaction, two organic molecules combine to form a
single organic molecule. A small molecule, usually water, is produced
during the reaction. For example, a carboxylic acid and an alcohol can
condense to form an ester.
CH3
O
H3C
C
OH + HO
CH2
carboxylic acid
CH2
CH
When you see two arrows
that point in opposite directions
in a chemical equation, the
reaction can proceed in both
directions. This type of reaction
is called an equilibrium
reaction. You will learn more
about equilibrium reactions
in Unit 4.
CH3
O
CH3
H2SO4
H3C
C
O
alcohol
CH2
CH2
CH
CH3 + H2O
ester
water
Hydrolysis Reactions
In a hydrolysis reaction, water adds to a bond, splitting it in two. This
reaction is the reverse of a condensation reaction. For example, water
can add to an ester or amide bond. A carboxylic acid and an alcohol are
produced if an ester bond is hydrolyzed, as shown in the example below.
A carboxylic acid and an amine are produced if an amide bond
is hydrolyzed.
CH3
O
H3C
C
O
CH2
ester
CH2
CH
CH3
O
CH3 + H2O
water
H2SO4
H3C
C
OH + HO
carboxylic acid
CH2
CH2
CH
CH3
alcohol
The following Sample Problems show how to identify different types of
organic reactions.
Chapter 2 Reactions of Organic Compounds • MHR
61
Sample Problem
Identifying Addition, Substitution, and Elimination Reactions
Problem
Identify each reaction as an addition, substitution, or elimination reaction.
(a) HOCH2CH2CH3 → CH2CHCH3 + H2O
(b) H2CCHCH2CH3 + H2 → CH3CH2CH2CH3
(c) CH3CH(CH3)CH2CH2Br + NaOH →CH3CH(CH3)CH2CH2OH + NaBr
Solution
(a) A double bond is formed. One reactant becomes two products.
The organic product has fewer atoms bonded to carbon. Thus,
this reaction is an elimination reaction.
(b) A double bond becomes a single bond. Two reactants become one
product. The organic product has more atoms bonded to carbon.
Thus, this reaction is an addition reaction.
(c) No double bond is broken or formed. Two reactants form two
products. An atom in the organic reactant (Br) is replaced by a
different group of atoms (OH). Thus, this is a substitution reaction.
Sample Problem
Identifying Oxidation and Reduction
Problem
Identify each reaction as an oxidation or a reduction. (The oxidizing and
reducing agents are not shown.)
OH
O
(a) CH3CH2CH2CH
→ CH3CH2CH2CH2
OH
(b)
O
→
Solution
(a) Count the number of CH bonds and CO bonds in the reactant
and in the product.
Reactant: 8 CH bonds, 2 CO bonds (or 1 CO bond)
Product: 9 CH bonds, 1 CO bond
The product has gained a CH bond and lost a CO bond. Thus,
this is a reduction.
(b) Count the number of CH bonds and CO bonds in the reactant
and in the product. Although the CH bonds are not shown, you
know that a carbon atom forms a total of four bonds.
Reactant: 11 CH bonds, 1 CO bond
Product: 10 CH bonds, 2 CO bonds (or 1 CO bond)
The product has lost a CH bond and gained a CO bond.
Thus, this is an oxidation.
62
MHR • Unit 1 Organic Chemistry
Practice Problems
1. Identify each reaction as an addition, substitution, or elimination
reaction.
OH
(a) CH3
CH2
Br
CH3 + HBr → CH3
CH
(b)
CH3
CH
CH
CH
Cl
Cl
CH3
CH
CH
CH
CH3 + Cl2 → CH3
CH
CH3 + HOH
CH2
CH3
CH3
Cl
→
(c)
+ HCl
2. Identify each reaction as an oxidation or a reduction. The oxidizing
and reducing agents are not shown.
(a)
CH3
CH3
CH
CH2
CH
O
CH3
CH → CH3
CH
O
CH2CH3
CH3CH2
(b)
OH
CH3
→ CH3CH2CH2CHCH3
C
H
C
CH2CH3
CH3
C
CH2
CH
CH3
O
OH
→
(c)
O
OH
3. Classify each reaction in two different ways: for example, as
oxidation and as an elimination reaction.
H
CH3
C
(a)
+ H2 → CH3CH2CH2CH2CH3
C
H
CH2CH3
O
O
(b) CH3
C
OH + HO
CH3
CH3
C
O
CH3 + HOH
4. Identify the type of reaction.
OH
(a) CH3
CH
CH2 + HOH → CH3
CH
CH3
OH
(b) CH3
CH2
CH
O
CH2
CH3 → CH3
CH2
C
CH2
CH3
Chapter 2 Reactions of Organic Compounds • MHR
63
Section Summary
In this section, you were introduced to some of the main types of organic
reactions: addition, substitution, and elimination reactions; oxidation and
reduction; and condensation and hydrolysis reactions. In the next section,
you will take a close look at each type of reaction. You will find out how
organic compounds, such as alcohols and carboxylic acids, can react in
several different ways.
Section Review
1
K/U Identify each reaction as an addition, substitution, or
elimination reaction.
Br Br
CH2 + Br2 → H2C
(a) H2C
CH2
OH
(b) CH3CH2CHCH3 → CH3CH
FeBr3
+ Cl2
(c)
CHCH3 + H2O
+ HCl
Cl
(d) CH3CH2CH2Br + H2NCH2CH3 → CH3CH2CH2NHCH2CH3 + HBr
2
K/U Identify each reaction as an oxidation or a reduction.
(Oxidizing and reducing agents are not shown.)
O
(a) CH3CH2CH2OH → CH3CH2CH
O
OH
(b) CH3CH2CCH3 → CH3CH2CHCH3
CHCH3 → CH3CH2CH3
(c) H2C
O
O
(d)
H →
(e) CH3C
CCH3 → CH3CH2CH2CH3
3 (a)
OH
Draw a complete structural diagram for each compound that
is involved in the following reaction.
K/U
N-ethylpentanamide + water → pentanoic acid + ethanamine
(b)
4
Is this reaction a condensation reaction or a hydrolysis
reaction? Explain your reasoning.
K/U
In your own words, describe each type of organic reaction.
Include an example for each type.
C
(a) addition
5
MHR • Unit 1 Organic Chemistry
(c) elimination
C The following reaction can be classified as an addition reaction.
It can also be classified as a reduction. Explain why this reaction fits
into both classes.
CH3CH
64
(b) substitution
CHCH3 + H2 → CH3CH2CH2CH3