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25.2: Functional Groups

Created by: CK-12

Lesson Objectives

  • Describe the importance of functional groups in organic reactions.
  • Identify and name functional groups in compounds.
  • Draw structures of functional groups.

Lesson Vocabulary

  • functional group: An atom or group of atoms within a molecule that has similar chemical properties whenever it appears in various compounds.
  • alcohol: A functional group that involves an oxygen atom that is bonded to one hydrogen atom and one carbon atom.
  • ether: A functional group that consists of an oxygen atom that forms single bonds with two carbon atoms.
  • amine: A nitrogen atom bonded to some combination of carbons and hydrogens.
  • carbonyl: A carbon atom and an oxygen atom connected by a double bond.
  • aldehyde: A carbonyl in which the carbon atom is most commonly bonded to one carbon atom and one hydrogen atom.
  • ketone: A carbonyl in which the carbon atom makes single bonds with two other carbon atoms.
  • carboxylic acid: A functional group in which the carbon atom is bonded to an OH group on one side and either a carbon or hydrogen atom on the other.
  • ester: A functional group in which the carbon is bonded to one additional oxygen atom and one carbon or hydrogen atom, with the second oxygen atom bonded to another carbon atom.
  • amide: A carbonyl which is attached to one nitrogen atom and one carbon or hydrogen atom.

Check Your Understanding

  • What is organic chemistry?
  • What serves as the framework for all organic compounds?

Introduction

With over twenty million known organic compounds in existence, it would be very challenging to memorize chemical reactions for each one. Fortunately, molecules with similar functional groups tend to undergo similar reactions. A functional group is defined as an atom or group of atoms within a molecule that has similar chemical properties whenever it appears in various compounds. Even if other parts of the molecule are quite different, certain functional groups tend to react in certain ways.

In the previous lesson, we already looked at two common functional groups: alkenes and alkynes. Although C-C and C-H single bonds are relatively unreactive, the pi bonds in alkenes and alkynes undergo a variety of characteristic reactions. We will look at some of these reactions in the next lesson. The majority of functional groups involve atoms other than carbon and hydrogen. Some of the most common functional groups are presented in the following sections.

Alcohols

The alcohol functional group involves an oxygen atom that is bonded to one hydrogen atom and one carbon atom. The carbon atom will be part of a larger organic structure. One way to indicate a generic alcohol would be with the formula R-OH. R represents any organic fragment in which a carbon atom is directly bonded to the explicitly indicated functional group (in this case, OH).

Alcohols can be classified as primary, secondary, or tertiary based on the characteristics of the carbon to which it is attached. In a primary alcohol, the carbon bonded directly to the oxygen atom is also bonded to exactly one carbon atom, with the other bonds generally going to hydrogen atoms. In a secondary alcohol, the carbon is attached to two other carbon atoms, and in a tertiary alcohol, the carbon is bonded to three other carbon atoms. The type of alcohol being used will determine the product of certain reactions. Note the naming of alcohols as illustrated above. The location of the –OH group is indicated with the number of the carbon to which it is attached.

We are already familiar with several common alcohols. For example, ethanol (CH3CH2OH) is the alcohol present in alcoholic beverages. It is also widely used in the industrial manufacture of other chemicals. Methanol (CH3OH) is used as a gasoline additive or alternative. Additionally, methanol can be used to manufacture formaldehyde, which is employed in the production of plastics, paints, and other useful substances. Isopropanol is commonly known as rubbing alcohol. In addition to its industrial uses, isopropanol is used to clean various surfaces, including computer monitors, whiteboards, and even skin (e.g., before getting blood drawn).

Ethers

The ether functional group consists of an oxygen atom that forms single bonds with two carbon atoms.

Ethers are good solvents for other organic compounds because of their low reactivity. They readily dissolve non-polar molecules. Diethyl ether is perhaps the best known ether. It is widely used as a solvent and has been used as an inhalable anesthetic.

Although ethers themselves are relatively unreactive, they can be converted to peroxides after prolonged exposure to oxygen. Peroxides are very reactive and are often explosive at elevated temperatures. Many commercially available ethers come with a small amount of a peroxide scavenger dissolved in them to help prevent this type of safety hazard.

Amine

An amine consists of a nitrogen atom bonded to some combination of carbons and hydrogens.

Like alcohols, amines can be classified as primary, secondary, or tertiary. However, the rules for assigning these categories are slightly different. In an alcohol, the oxygen atom is always bonded to exactly one carbon atom, so we look at the branching on the adjacent carbon, not the oxygen atom itself. In a neutral amine, the nitrogen can be bonded to one, two, or three carbon atoms, and this is how we decide whether it is called a primary, secondary, or tertiary amine.

Neutral amines are weak bases, because the lone pair on nitrogen can act as a proton acceptor. Many smaller amines have very strong and offensive odors. For example, the aptly-named compounds cadaverine and putrescine are foul-smelling amines, formed as a part of the decay process after death.

Amines serve a wide variety of uses. Diphenylamine acts as a stabilizer for certain types of explosives. Amines are found as components in some lubricating materials, in developers, and are a part of waterproofing textiles. Some amines, such as novocaine, are used as anesthetics. Many pharmaceutical compounds contain amines, including 8 of the 10 most prescribed medications in 2012.

Aldehydes

A very common structural component of organic structures is the carbonyl, which is simply a carbon atom and an oxygen atom connected by a double bond. The reactivity of carbonyls is primarily dictated by the polarization of the C=O bond, but the surrounding atoms also play a role in its specific reaction pathways. All of the remaining functional groups that we will be discussing contain a carbonyl, but they have different properties based on the other atoms that are also connected to the central carbon.

An aldehyde is a carbonyl in which the carbon atom is bonded to one carbon atom and one hydrogen atom (or two hydrogen atoms, see below). Because the hydrogen atom is so small, the partial positive charge on the carbonyl carbon is very easy for other molecules to approach, making aldehydes a particularly reactive type of carbonyl. Aldehydes are versatile reactants for a wide variety of organic syntheses. Many aldehydes also have distinctive flavors and aromas. For example, the flavor of cinnamon is primarily due to the molecule cinnamaldehyde, and vanillin is the aldehyde most responsible for the smell and taste of vanilla extract.

A special aldehyde is the molecule in which the carbonyl carbon is bonded to two hydrogen atoms. This molecule, called formaldehyde, has a wide variety of uses. By itself, it can be used as a tissue preservative or as a very harsh disinfectant. It is also used as a precursor to various materials, including plastics, resins, and other polymers.

Ketones

A ketone involves a carbonyl in which the carbon atom makes single bonds with two other carbon atoms. Ketones undergo most of the same reactions as aldehydes, but they tend to be slightly less reactive. The simplest ketone is acetone, in which the carbonyl carbon is bonded to two CH3 groups. This ketone is commonly used to remove fingernail polish and serves as an industrial solvent. Methyl ethyl ketone is used as a paint stripper and a solvent. Ketones are also used in the production of various polymers, either as a building block or as a solvent.

Carboxylic Acids

Carboxylic acids are another carbonyl-containing functional group, in which the carbon atom is bonded to an OH group on one side and either a carbon or hydrogen atom on the other.

The OH group in an alcohol is not a very good acid; simple alcohols are similar in acidity to water. However, as the name implies, carboxylic acids are weak acids. An OH group that is directly connected to a carbonyl will ionize to a small extent when dissolved in water. The reason for this is the relative stability of the resulting anion. A carboxylate ion (Figure below), in which the negative charge is spread over two different oxygen atoms through resonance structures, is more stable than an isolated oxygen-centered anion.

Carboxylic acids are used in a variety of environments. Formic acid acts as a protective chemical for many stinging insects and plants. Acetic acid gives vinegar its characteristic smell and flavor and is a fundamental biological and industrial building block. Carboxylic acids with longer carbon chains (fatty acids) are used by animals as a way of storing energy and are widely used in the manufacture of soaps. Some compounds contain multiple carboxylic acids within a single molecule. For example, citric acid (three carboxyl groups) is especially abundant in citrus fruits and is used as a flavoring and preservative in many foods and beverages.

Ester

An ester is similar to a carboxylic acid, in that it contains a carbonyl where the carbon is bonded to one additional oxygen atom and one carbon or hydrogen atom. However, the second oxygen atom is bonded to another carbon instead of to an acidic hydrogen atom. Structurally, carboxylic acids and esters are related to one another in the same way as alcohols and ethers.

Esters can be formed by heating carboxylic acids and alcohols in the presence of an acid catalyst. This process is reversible, and the starting materials can be regenerated by reacting an ester with water in the presence of a weak base.

Some esters have very pleasant odors, so they are used in the manufacture of many perfumes. Propyl acetate contributes to the odor of pears, while isoamyl acetate gives bananas their smell. This ester also serves as an alarm signal for honeybees. Esters are employed in the manufacture of fabrics (polyesters) and Plexiglass®. Anesthetics such as procaine and benzocaine also contain esters.

Amide

An amide is a carbonyl in which the carbonyl is attached to one nitrogen atom and one carbon or hydrogen atom. Alternatively, we could define an amide as an amine in which one of the carbon atoms attached to the nitrogen is part of a carbonyl.

An amide can be formed by combining a carboxylic acid and an amine. Only primary and secondary amines can be used to form amides, since they have a hydrogen that can be replaced with the carbonyl carbon; tertiary amines will not form amides. The amide shown above was formed from a carboxylic acid and a primary amine.

Amides are used as coloring agents in crayons, pencils, and ink. They are employed in the paper, plastic, and rubber industries. Polyacrylamide is a very widely used amide; it is involved in the treatment of drinking water and sewage, and in plastics manufacture. The amide Kevlar® is widely employed for the production of body armor, and nylon is another type of amide-based polymer.

Lesson Summary

  • Functional groups retain similar chemical properties in a variety of compounds. Functional groups help to distinguish organic molecules from one another.
  • Alcohols consist of an -OH group and commonly appear in the form of ethanol (in alcoholic beverages) and methanol (used in plastic and paint ingredients).
  • Ethers are composed of an oxygen atom that forms single bonds with two carbon atoms. Ethers are relatively unreactive making them good solvents for other organic compounds, such as in the formation of anesthetics.
  • Amines are composed of a nitrogen atom bonded to some combination of carbons and hydrogens. Amines are weak bases commonly used in pharmaceuticals.
  • Carbonyl is composed of a carbon atom double bonded with an oxygen atom, and is a common structural component of many other organic molecules. The reactivity of carbonyl derivatives is largely due to the polarity of the carbon-oxygen double bond.
  • Aldehydes are commonly composed of a carbon bonded to one carbon atom and one hydrogen atom. Aldehydes are particularly reactive due to their high polarity, and are commonly associated with strong smells and tastes.
  • Ketones are carbonyls in which the carbon atom makes single bonds with two other carbon atoms. Ketones are less reactive than aldehydes and are a common component in nail polish removers and paint strippers.
  • Carboxylic acid is a carbonyl in which in which the carbon atom is bonded to an OH group on one side and either a carbon or hydrogen atom on the other. Carboxylic acids are weak acids, for example, vinegar.
  • Esters are similar to carboxylic acids but contain a second oxygen rather than the acidic hydrogen that is in a carboxylic acid. Esters create the pleasant aroma associated with many fruits and flowers.
  • Amides are carbonyls attached to one nitrogen atom and one carbon or hydrogen atom. Amides are used in materials such as crayons and inks, as well as in paper rubber products.

Lesson Review Questions

  1. Draw structures for the following functional groups:
    1. ether
    2. ester
    3. ketone
    4. alkene
  2. Name the following functional groups (in color):

  1. Designate the following alcohols as primary, secondary, or tertiary:

  1. Designate the following amines as primary, secondary, or tertiary:

Further Reading/Supplementary Tasks

Points to Consider

How to different functional groups react in organic molecules?

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Date Created:

Sep 09, 2013

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Aug 01, 2014
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