Focusing on the functional groups in a molecule allows us to recognize patterns in the be-havior of related compounds. Consider what we know about the reaction between sodium metal and water, for example.
We can divide the reaction into two half-reactions. One involves the oxidation of sodium metal to form sodium ions.
The other involves the reduction of an Hion in water to form a neutral hydrogen atom that combines with another hydrogen atom to form an H2molecule.
Once we recognize that water contains an OOH functional group, we can predict what might happen when sodium metal reacts with an alcohol that contains the same functional group. Sodium metal should react with methanol (CH3OH), for example, to give H2 gas and a solution of the Na and CH3Oions dissolved in the alcohol.
2Na(s)2 CH3OH(l ) H2(g)2 Na(alc)2 CH3O(alc) H
2 H
Reduction
2 H 2 OSOQ H OG
DO S 2 e
2 H 2 HT H2
O Na
Oxidation Nae
2Na(s)2 H2O(l ) H2(g)2 Na(aq)2 OH(aq)
OH OH
HO
OH
OH
O
CPO H N(CH3)2 CH3
NHf 2
/∑
Tetracycline
B
O B
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FUNCTIONAL GROUP CHEMISTRY 5
Because they involve the transfer of electrons, the reactions between sodium metal and either water or an alcohol are examples of oxidation–reduction reactions. But what about the following reaction, in which hydrogen gas reacts with an alkene in the presence of a transition metal catalyst to form an alkane?
There is no change in the number of valence electrons on any of the atoms in the reaction.
Both before and after the reaction, each carbon atom shares a total of eight valence elec-trons and each hydrogen atom shares two elecelec-trons. Instead of elecelec-trons, the reaction involves the transfer of atoms—in this case, hydrogen atoms. There are so many atom-transfer reactions that chemists developed the concept of oxidation number (see Chapter 5) to extend the idea of oxidation and reduction to reactions in which electrons aren’t neces-sarily gained or lost.
Oxidation involves an increase in the oxidation number of an atom. Reduction oc-curs when the oxidation number of an atom decreases.
During the transformation of ethene into ethane, there is a decrease in the oxidation num-ber of the carbon atom. The reaction therefore involves the reduction of ethene to ethane.
Reactions in which none of the atoms undergo a change in oxidation number are called metathesis reactions. Consider the reaction between a carboxylic acid and an amine, for example,
CH3CO2H CH3NH2 88n CH3CO2 CH3NH3
or the reaction between an alcohol and hydrogen bromide.
CH3CH2OH HBr 88n CH3CH2Br H2O
These are metathesis reactions because there is no change in the oxidation number of any atom in either reaction.
The oxidation numbers of the carbon atoms in a variety of compounds are given in Table O2.3. The oxidation numbers can be used to classify organic reactions as either oxidation–reduction reactions or metathesis reactions.
H
H2 C C H
H
H H H
H
H H
H C
GC G D
D P Ni
2
3
H
H2 C C H
H
H H H
H
H H
H C
GC G D
D P Ni
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6 FUNCTIONAL GROUP CHEMISTRY
Exercise O2.4
Classify the following as either oxidation–reduction or metathesis reactions.
H
(a) 2 CH3OH 8n CH3OCH3 H2O O
H
B OB
(b) HCOH CH3OH 8n HCOCH3 H2O (c) CO 2 H2 n CH3OH
(d) CH3Br 2 Li n CH3Li LiBr Solution
(a) This is a metathesis reaction because there is no change in the oxidation number of the carbon atoms when an alcohol is converted to an ether.
2 CH3OH 88n CH3OCH3 H2O
2 2
(b) This is a metathesis reaction because there is no change in the oxidation number of any of the carbon atoms when a carboxylic acid reacts with an alcohol to form an ester.
HCO2H CH3OH 88n HCO2CH3 H2O
2 2 2 2
(c) This is an oxidation–reduction reaction because the carbon atom is reduced from the
2 to the 2 oxidation state when CO combines with H2to form methanol.
CO 2 H2 88n CH3OH
2 2
(d) This is an oxidation–reduction reaction because the carbon atom is reduced from the
2 to the 4 oxidation state when CH3Br reacts with lithium metal to form CH3Li.
CH3Br 2 Li 88n CH3Li LiBr
2 4
TABLE O2.3 Typical Oxidation Numbers of Carbon
Oxidation Number of the Classes of Compounds Example Carbon
Alkane CH4 4
Alkyllithium CH3Li 4
Alkene H2CPCH2 2
Alcohol CH3OH 2
Ether CH3OCH3 2
Alkyl halide CH3Cl 2
Amine CH3NH2 2
Alkyne HCqCH 1
Aldehyde H2CO 0
Carboxylic acid HCO2H 2
— CO2 4
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FUNCTIONAL GROUP CHEMISTRY 7
Because electrons are neither created nor destroyed, oxidation can’t occur in the absence of reduction, and vice versa. It is often useful, however, to focus attention on one compo-nent of the reaction and ask, is that substance oxidized or reduced?
Exercise O2.5
Determine whether the following transformations involve the oxidation or the reduction of the carbon atom.
Solution
Each of the transformations involves the oxidation of the carbon atom. The first reaction involves oxidation of the carbon atom from the 2 to the 0 oxidation state. In the second reaction the carbon atom is oxidized to the 2 state, and the third reaction involves oxi-dation of the carbon atom to the 4 oxioxi-dation state.
Assigning oxidation numbers to the individual carbon atoms in a complex molecule can be difficult. Fortunately, there is another way to recognize oxidation–reduction reac-tions in organic chemistry:
Oxidation occurs when hydrogen atoms are removed from a carbon atom or when an oxygen atom is added to a carbon atom.
Reduction occurs when hydrogen atoms are added to a carbon atom or when an oxygen atom is removed from a carbon atom.
H C OH
H
H
H C H
OB
H C OH
O
H C H
OB B
H C OH
O
O C O
B
P P
2
0
0
2
2 4
H C OH
H
H
H C H
OB
H C OH
O
H C H
OB B
H C OH
O
O C O
B
P P
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8 FUNCTIONAL GROUP CHEMISTRY
The first reaction in Exercise O2.5 involves oxidation of the carbon atom because a hydrogen atom is removed from that atom when the alcohol is oxidized to an aldehyde.
The second reaction in the exercise is an example of oxidation because an oxygen atom is added to the carbon atom when an aldehyde is oxidized to a carboxylic acid.
Reduction, on the other hand, occurs when hydrogen atoms are added to a carbon atom or when an oxygen atom is removed from a carbon atom. An alkene is reduced, for ex-ample, when it reacts with H2to form the corresponding alkane.
CH2PCHCH3 H2 88n CHNi 3CH2CH3
Figure O2.1 provides a useful guide to the oxidation–reduction reactions of organic compounds. Each of the arrows in this figure involves a two-electron oxidation of a car-bon atom along the path toward carcar-bon dioxide. A line is drawn through the first arrow because it is impossible to achieve this transformation in a single step.
HC OB
OB
H HCOH
CH3OH 88n HC OB
H
FIGURE O2.1 The stepwise oxidation of carbon.