Functional Groups

Bromine reacts with 2-butene to form 2,3-dibromobutane.

It also reacts with 3-methyl-2-pentene to form 2,3-dibromopentane.

Instead of trying to memorize both equations, we can build a general rule that bromine reacts with compounds that contain a C=C double bond to give the product expected from addition across the double bond. This approach to understanding the chemistry of organic compounds presumes that certain atoms or groups of atoms known as functional groups give these compounds their characteristic properties.
Functional groups focus attention on the important aspects of the structure of a molecule. We don't have to worry about the differences between the structures of 1-butene and 2-methyl-2-hexene, for example, when these compounds react with hydrogen bromide. We can focus on the fact that both compounds are alkenes that add HBr across the C=C double bond in the direction predicted by Markovnikov's rule.

Some common functional groups are given in the table below.

Common Functional Groups

Functional Group		Name		Example
		Alkane		CH3CH2CH3 (propane)
		Alkene		CH3CH=CH2 (propene)
		Alkyne		CH3CCH (propyne)
F, Cl, Br, or I		Alkyl halide		CH3Br (methyl bromide)
		Alcohol		CH3CH2OH (ethanol)
		Ether		CH3OCH3 (dimethyl ether)
		Amine		CH3NH2 (methyl amine)

The C=O group plays a particularly important role in organic chemistry. This group is called a carbonyl and some of the functional groups based on a carbonyl are shown in the table below.

Functional Groups That Contain a Carbonyl

Functional Group		Name		Example
		Aldehyde		CH3CHO (acetaldehyde)
		Ketone		CH3COCH3 (acetone)
		Acyl chloride		CH3COCl (acetyl chloride)
		Carboxylic acid		CH3CO2H (acetic acid)
		Ester		CH3CO2CH3 (methyl acetate)
		Amide		CH3NH2 (acetamide)

Practice Problem 1:Root beer hasn't tasted the same since the use of sassafras oil as a food additive was outlawed because sassafras oil is 80% safrole, which has been shown to cause cancer in rats and mice. Identify the functional groups in the structure of safrole. Click here to check your answer to practice problem 1

Practice Problem 2:The following compounds are the active ingredients in over-the-counter drugs used as analgesics (to relieve pain without decreasing sensibility or consciousness), antipyretics (to reduce the body temperature when it is elevated), and/or anti-inflammatory agents (to counteract swelling or inflammation of the joints, skin, and eyes). Identify the functional groups in each molecule. Click here to check your answer to practice problem 2

Practice Problem 3:The discovery of penicillin in 1928 marked the beginning of what has been called the "golden age of chemotherapy," in which previously life-threatening bacterial infections were transformed into little more than a source of discomfort. For those who are allergic to penicillin, a variety of antibiotics, including tetracycline, are available. Identify the numerous functional groups in the tetracycline molecule. Click here to check your answer to practice problem 3

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Oxidation-Reduction Reactions

Focusing on the functional groups in a molecule allows us to recognize patterns in the behavior of related compounds. Consider what we know about the reaction between sodium metal and water, for example.

2 Na(s) + 2 H₂O(l) H₂(g) + 2 Na⁺(aq) + 2 OH^-(aq)

We can divide this reaction into two half-reactions. One involves the oxidation of sodium metal to form sodium ions.

Oxidation:

Na

Na+ + e-

The other involves the reduction of an H⁺ ion in water to form a neutral hydrogen atom that combines with another hydrogen atom to form an H₂ molecule.

Reduction:

Once we recognize that water contains anOH 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 CH₃O^- ions dissolved in this alcohol.

2 Na(s) + 2 CH₃OH(l) H₂(g) + 2 Na⁺(alc) + 2 CH₃O^-(alc)

Because they involve the transfer of electrons, the reaction 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 this reaction. Both before and after the reaction, each carbon atom shares a total of eight valence electrons and each hydrogen atom shares two electrons. Instead of electrons, this reaction involves the transfer of atomsin this case, hydrogen atoms. There are so many atom-transfer reactions that chemists developed the concept of oxidation number to extend the idea of oxidation and reduction to reactions in which electrons aren't necessarily gained or lost.

Oxidation involves an increase in the oxidation number of an atom.

Reduction occurs when the oxidation number of an atom decreases.

During the transformation of ethene into ethane, there is a decrease in the oxidation number of the carbon atom. This 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.

Or the reaction between an alcohol and hydrogen bromide.

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 the table below.

Typical Oxidation Numbers of Carbon

Functional Group		Example		Oxidation Number of Carbon in the Example
Alkane		CH4		-4
Alkyllithium		CH3Li		-4
Alkene		H2C=CH2		-2
Alcohol		CH3OH		-2
Ether		CH3OCH3		-2
Alkyl halide		CH3Cl		-2
Amine		CH3NH2		-2
Alkyne		HCCH		-1
Aldehyde		H2CO		0
Carboxylic acid		HCO2H		2
		CO2		4

These oxidation numbers can be used to classify organic reactions as either oxidation-reduction reactions or metathesis reactions.