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25.4: Functional Groups

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Lesson Objectives

The student will:

  • define and give examples of functional groups.
  • identify alcohols, aldehydes, ketones, ethers, organic acids, and esters based on their functional groups.
  • name and draw simple alcohols, aldehydes, ketones, ethers, organic acids, and esters.

Vocabulary

  • alcohol
  • aldehyde
  • carbonyl group
  • ester
  • ether
  • hydroxyl group
  • ketone
  • organic acid

Introduction

Earlier in this chapter, we were introduced to several functional groups, including alkenes and alkynes. Remember that a functional group is an atom or a group of atoms that replaces hydrogen in an organic compound and is responsible for the characteristic properties of the compound. Functional groups can be used to identify different categories of organic compounds. In this lesson, six of these categories will be studied in terms of their functional groups: the alcohols, aldehydes and ketones, ethers, organic acids, and esters.

Alcohols

Alcohols have the same general formula as an alkane, but alchols also have the functional group -\mathrm{OH}, called the hydroxyl group. In terms of solubility, alcohols are soluble in water if the number of carbon atoms is low (three or less). If, however, the number of carbon atoms increases, the solubility decreases accordingly.

The most common alcohol, known as ethanol, is used in alcoholic drinks, as a fuel (gasohol), in thermometers, as a preservative for biological specimens, and as a solvent for paints and drugs. The structure for ethanol is shown below. Notice the hydroxyl group on the end of the two carbon chain.

\mathrm{CH}_3-\mathrm{CH}_2-\mathrm{OH}

Butanol, an alcohol with four carbon atoms, is used in paint thinners and in the cosmetic industry. The four different isomers of butanol are shown in the figure below.

Naming and Drawing Simple Alcohols

In naming alcohols, the suffix “-ol” is added to the parent chain of the alkane name. The position of the -\mathrm{OH} functional group is indicated in the name. Remember to start numbering the parent chain on the end closest to where the -\mathrm{OH} is located.

Example:

Name the following:

\mathrm{CH}_3-\mathrm{CH}_2-\mathrm{CH}_2-\mathrm{OH}

Solution:

The carbon chain contains 3 carbon atoms and an -\mathrm{OH} group at one end of the chain.

Therefore, the molecule is 1-propanol.

Alcohols can be classified as primary (1^\circ), secondary (2^\circ) and tertiary (3^\circ) alcohols, as shown below.

1-propanol is a primary alcohol because the carbon atom that the hydroxyl group is attached to is connected to only one alkyl group. In other words, carbon 1 in 1-propanol is attached to -\mathrm{OH}, two hydrogen atoms, and an ethyl group. Therefore, 1-propanol is a 1^\circ alcohol.

An example of a secondary (2^\circ) alcohol is 2-butanol. Notice in 2-butanol that the carbon atom to which the hydroxyl group is bounded to is also connected to a methyl group on the right and an ethyl group on the left.

In comparison, 2-methyl-2-propanol is a tertiary (3^\circ) alcohol. If you look at this structure, the carbon atom bounded to the hydroxyl group is also bounded to three methyl groups.

As with the other organic compounds introduced in this chapter, alcohols can be drawn by looking at the name of the compound and analyzing what the name tells about the structure of the compound.

Example:

Draw the structure of 2-pentanol.

Solution:

Aldehydes and Ketones

The aldehydes and ketones contain what is known as the carbonyl group. A carbonyl group is a carbon atom double bonded to an oxygen atom (\mathrm{C}=\mathrm{O}), and the carbon is also bounded to two other atoms or groups. In an aldehyde, the carbonyl group is always on an end carbon. In a ketone, the carbonyl group is never on an end carbon. Look at the diagram below to compare the location of the carbonyl group in ethanal (an aldehyde) and 2-butanone (a ketone).

Ethanal

2 - butanone

Aldehydes

Aldehydes play an important role in our everyday lives. Those that have higher molecular masses are responsible for some very pleasant smells that you may have experienced in your own home. Benzaldehyde is an aldehyde responsible for the baking ingredient almond extract, while the smell of vanilla is due partially to an aldehyde component in the structure methoxybenzaldehyde (structure of vanilla is shown below).

Of course, other aldehydes, such as formaldehyde, do not smell as nice. Formaldehyde is used as a preservative for organs and as an embalming fluid. Acetaldehyde is the oldest known aldehyde (more than 300 years old) and has its use in the preparation of ethanol.

Naming and Drawing Aldehydes

In order to name aldehydes, use the parent chain name of the alkane and add the suffix “–al.” Remember that all aldehydes have the general formula shown below.

In the general formula above, R represents a hydrogen atom or any alkyl group. The simplest aldehyde is methanal, where \mathrm{R}=\mathrm{H}.

Example:

Name the following structure:

Solution:

The parent chain contains 3 carbon atoms, and it has a carbonyl group on the end of the chain. Therefore, the name is propanal.

Example:

Draw the structure of pentanal.

Solution:

Pentanal has the prefix “pent-,” meaning it has five carbon atoms.

Pentanal has the suffix “–al,” meaning it has the aldehyde functional group

Therefore, the structure is:

Ketones

Ketones are equally as important as aldehydes in our daily lives. Ketones are present in the body when fat is broken down for energy. A buildup of ketones leads to ketoacidosis, a potentially dangerous condition. Acetone is also a ketone and is the main component in finger nail polish. Carvone is a ketone that is found in many naturally occurring products, such the oils from mandarin oranges, dill seeds, and spearmint. Another example of a useful ketone is isophorone (3,5,5-trimethyl-2-cyclohene-1-one), which is used in some paints to improve the flow and increase the glossiness. Isophorone has the structure shown below.

Naming and Drawing Ketones

In order to name compounds that are ketones, use the parent chain name of the alkane and add the suffix “–one." For parent chains more than four carbon atoms, the position of the carbonyl group must be indicated. Remember that all ketones have the general formula shown below.

In the general formula above, R and R' are both alkyl groups, which can be the same or different. The simplest ketone is propanone (acetone), where R and R' are both methyl groups.

Example:

Name the following structure.

Solution:

There are alkyl groups attached on both sides of the carbonyl group, therefore the molecule is a ketone. The name of this compound is 2-heptanone.

Example:

Draw the structure of 3-pentanone.

Solution:

The carbon chain will be 5 carbon atoms long, and it will have a carbonyl group at carbon 3. Therefore, the structure is:

Ethers

All ether compounds have the general formula \mathrm{R}-\mathrm{O}-\mathrm{R'}, where \mathrm{R} and \mathrm{R'} are both alkyl groups, which could either be the same or different. The most common ether is a compound also known as diethyl ether. The structure of diethyl ether is shown below:

\mathrm{CH}_3-\mathrm{CH}_2-\mathrm{O}-\mathrm{CH}_2-\mathrm{CH}_3

In this structure the alkyl groups on either side of the oxygen atom are ethyl groups, hence the name diethyl ether. Diethyl ether was first publicly demonstrated as an anesthetic in 1846 at Massachusetts General Hospital. It was considered at the time to be a great breakthrough because it produced the ability for physicians to provide “painless” surgery. Today it also has other uses, such as a solvent for fats and oils. It is also sometimes used to anesthetize ticks before removing them from the skin. The harmful side effects and highly flammable nature of the compound, however, makes its use less common today than in past eras.

Naming and Drawing Ethers

There are two possible ways to to name ethers. One way is to name the alkyl groups on either side of the ether functional group and then adding the word “ether” on the end. The other possibility is to name the smaller alkyl group, add the suffix “–oxy,” and then give the alkane name to the larger alkyl group. The alkyl groups are named in alphabetical order.

Example:

Draw the structure for ethyl methyl ether.

Solution:

The general structure is \mathrm{R}-\mathrm{O}-\mathrm{R'}. Looking at the name of the structure, we can determine what alkyl groups will be the \mathrm{R} and the \mathrm{R'}.

Let \mathrm{R}=\mathrm{CH}_3\mathrm{CH}_2 and let \mathrm{R'}=\mathrm{CH}_3, then draw the structure.

\mathrm{CH}_3-\mathrm{CH}_2-\mathrm{O}-\mathrm{CH}_3

Note this structure is also called methoxy ethane.

Organic Acids

The organic acids (or carboxylic acids) contain the carboxyl group. A carboxyl group is a carbon atom double bonded to an oxygen atom (\mathrm{C}=\mathrm{O}), a hydroxyl group (-\mathrm{OH}), and either a hydrogen atom or an alkyl group. Carboxylic acids are often found in nature and often combined with other functional groups. Many of these compounds are liquids or solids with low melting points. Carboxylic acids are also highly soluble in water and have a relatively low pH. Vinegar is a water solution containing 5% of the common carboxylic acid acetic acid (shown below).

Another common carboxylic acid is niacin (Vitamin B3). Niacin (shown below) is found in carrots, green leafy vegetables, milk, eggs, and some fish. It is used by the body to aid metabolism, DNA repair, and the functioning of the adrenal gland.

The formula for the carboxyl group is written as \mathrm{R}-\mathrm{COOH}, where R is a hydrogen atom or an alkyl group. The general formula for the organic acid is found in the diagram below.

Organic acids play a key role in many aspects of our lives. For example, citric acid is the taste you experience when you drink citrus drinks such as orange juice or put lemon juice on fish. Aspirin (acetylsalicylic acid or ASA) is a widely used pain killer. Oxalic acid is a naturally occurring organic acid found in the leaves of, among other plants, rhubarb, star fruit, black pepper, and parsley. The structures of these organic acids are shown below.

Naming and Drawing Organic Acids

The simplest of organic acids is one where the \mathrm{R} group is hydrogen.

When \mathrm{R}=\mathrm{H} in the general formula, the organic acid is methanoic acid. Methanoic acid (or formic acid) is commonly used in the recycling industry, especially for recycling rubber. It is found in the venom of bees and ants and is released when an insect bites.

When naming organic acids, use the parent chain name of the alkane and add the suffix “–oic acid.”

Example:

Name the following structure.

Solution:

This molecule is a 4-carbon chain ending with a carboxylic acid group. Therefore, the name is butanoic acid.

Example:

Draw the structure of 2-methyl propanoic acid.

Solution:

Methyl (\mathrm{CH}_3) is a branch found on carbon 2.

Propanoic has the prefix “prop-,” meaning it has three carbon atoms in the parent chain. The suffix “–oic acid” means that it has the organic acid functional group. Therefore, the structure is:

Esters

Have you ever wondered why an orange has such a wonderful aroma, or where the smell from a freshly cut pineapple comes from? Most fruits get their rich aroma from molecules known as esters. Octyl ethanoate is responsible for the smell for oranges, while butyl butanoate is best known for the smell of pineapple.

Octyl ethanoate (orange flavor)

Butylbutanoate (pineapple flavor)

Actually, quite a few of the scents that we are familiar with can be traced back to an ester. Look at Table below at some of the common scents and the ester for which the scent can be attributed to.

Scents and Their Esters
Scent Ester
Apples Methylbutanoate
Pears Propylethanoate
Raspberries 2-methylpropylethanoate
Peach Ethylbutanoate
Banana 3-methylbutylethanoate
Orange Octylacetate
Jasmine Benzylethanoate

Naming and Drawing Esters

The general formula for esters is found below.

Notice that the general formula looks similar to the organic acid, but in the case of the ester, the carboxylic acid hydrogen has been replaced with a second alkyl group. This is because esters can be produced by a reaction between an organic acid and an alcohol. This general reaction can be represented as:

A specific example of this type of reaction is:

Since esters have an alkyl group on either side of the functional group, the name of the ester will be in two pieces. As was described earlier, esters are formed from organic acids and alcohols. The alkyl group from the organic acid is \mathrm{R}. The alkyl group from the alcohol is \mathrm{R'}. The name of an ester is composed of the name of the alkyl group from the alcohol, then of the alkyl group from the organic acid, and finally the suffix “–oate.” A sample reaction for a general reaction and then a specific reaction to demonstrate this principal is shown below.

To draw an ester, remember that the first alkyl group is attached to the oxygen and the alkyl group with the suffix “–oate” is part of the \mathrm{C}=\mathrm{O} chain.

Example:

Name the following structure:

Solution:

The name of this compound is methyl pentanoate (the odor of apple).

Example:

Draw the structure of ethyl heptanoate (the odor of red grapes).

Solution:

Ethyl (\mathrm{CH}_3\mathrm{CH}_2) is listed first and is therefore from the alcohol, so it will be attached to the oxygen.

Heptanoate is named second and is therefore from the organic acid. Heptanoate has the prefix “hept-,” meaning that it has seven carbon atoms in the parent chain. Heptanoate also has the suffix “–oate,” meaning it has the ester functional group

Therefore, the structure is:

This video shows an example of naming some compounds containing functional groups (10e), see http://www.youtube.com/watch?feature=player_profilepage&v=nuLxS4SZ4zU (4:38).

Categories of Organic Compounds

In the table below is a summary of the organic compounds studied in this chapter. Table below lists the general formula and the functional groups for the alkanes, the alkenes, the alkynes, and the substituted halogens along with the aromatics, as well as the six new categories introduced in this lesson. An example from each category is also provided.

Summary of the Categories of Organic Compounds
Category General Formula Distinguishing Feature Example Formula Name
Alkane NA All single \mathrm{C}-\mathrm{C} bonds \mathrm{CH}_3\mathrm{CH}_2\mathrm{CH}_3 propane
Alkenes NA One \mathrm{C}=\mathrm{C} bonds \mathrm{CH}_2=\mathrm{CH}_2 Ethene
Alkynes NA One \mathrm{C}\equiv\mathrm{C} bond \mathrm{HC}\equiv\mathrm{CH} Ethyne
Aromatic NA Benzene ring part of structure, Benzene-like structure Nitrobenzene (floor polish)
Substituted Halogens \mathrm{R}-\mathrm{X} One or more of halogens attached to organic compound \mathrm{CH}_2\mathrm{FCl} Chlorofluoromethane (Freon)
Alcohol \mathrm{R}-\mathrm{OH} Hydroxyl group \mathrm{CH}_3\mathrm{CH}_2-\mathrm{OH} Ethanol
Ether \mathrm{R}-\mathrm{O}-\mathrm{R'} \mathrm{C}-\mathrm{O}-\mathrm{C} \mathrm{CH}_3-\mathrm{O}-\mathrm{C}_2\mathrm{H}_5 Methyl ethyl ether
Aldehydes Ethanal
Ketone Butanone
Organic Acids Ethanoic acid
Ester Butyl butyrate (pineapple)

Lesson Summary

  • Alcohols have the same general formula as an alkane, except alcohols have the functional group -\mathrm{OH}, called the hydroxyl group.
  • Primary (1^\circ)alcohols have a carbon atom attached to a hydroxyl group and one alkyl group. Secondary (2^\circ) alcohols have a carbon atom that is attached to a hydroxyl group and two alkyl groups. Tertiary (3^\circ) alcohols have a carbon atom that is attached to a hydroxyl group and three alkyl groups.
  • All aldehydes have the general formula below:
  • In order to name aldehydes use the parent chain name of the alkane and add the suffix “–al.”
  • All ketones have the general formula below:
  • To name ketones, use the parent chain name of the hydrocarbon and add the suffix “–one.”
  • A carbonyl group is a carbon atom double bonded to an oxygen atom (\mathrm{C}=\mathrm{O}) and to two other atoms or groups.
  • All ether compounds have the general formula \mathrm{R}-\mathrm{O}-\mathrm{R'}, where \mathrm{R} and \mathrm{R'} are alkyl groups.
  • Ethers can either be named by naming the alkyl groups on both sides of the ether functional group and then adding the word “ether” on the end. The other possibility is to name the smaller alkyl group, add the suffix “–oxy,” and then giving the alkane name to the larger alkyl group.
  • The organic acids (or carboxylic acids) contain the carboxyl group (\mathrm{C}=\mathrm{O}), a hydroxyl group (-\mathrm{OH}), and either a hydrogen atom or an alkyl group. The formula for the carboxyl group is written as \mathrm{R}-\mathrm{COOH} , where R is a hydrogen atom or an alkyl group.
  • The general formula for the organic acid is below:
  • When naming organic acids use the parent chain name of the alkane and add the suffix “–oic acid.”
  • The general formula for an ester is shown below:
  • When naming an ester, the name is composed of the alkyl group from the alcohol, then the alkyl group from the organic acid, and then the suffix “–oate.”

Further Reading / Supplemental Links

The learner.org website allows users to view streaming videos of the Annenberg series of chemistry videos. You are required to register before you can watch the videos but there is no charge. The website has one video that relates to this lesson called “Carbon.” The versatility of carbon's molecular structures and the enormous range of properties of its compounds are presented.

Review Questions

  1. Complete the following chart (Table below).
Group Distinguishing Feature Draw Example (with Name)
Alcohol
Aldehyde
Ketone
Ether
Organic Acid
Ester
  1. What is the difference between the carbonyl group in the aldehydes and the carbonyl group in the ketones? Give an example to illustrate your answer.
  2. Which of the following compounds is an alcohol?
    1. \mathrm{CH}_3\mathrm{COOCH}_3
    2. \mathrm{CH}_3\mathrm{CH}_2\mathrm{OH}
    3. \mathrm{CH}_3\mathrm{COOH}
    4. \mathrm{CH}_3\mathrm{COCH}_3
  3. To which family of organic compounds does \mathrm{CH}_3\mathrm{COCH}_2\mathrm{CH}_3 belong?
    1. alcohol
    2. aldehyde
    3. ketone
    4. carboxylic acid
  4. Which of the following classes of organic compounds contain a carbon-oxygen double bond: i. alcohols, ii. aldehydes, iii. ketones, iv. ethers, v. organic acids, vi. esters?
    1. i, iii, and iv only
    2. ii, iv, and vi only
    3. i, iii, iv, and v only
    4. ii, iii, v, and vi only
  5. What is the name of the compound represented below?
    1. heptanone
    2. 2,3-dimethyl-3-pentanone
    3. 2,3-dimethylpentanone
    4. diisopropyl ketone
  6. Name the following compounds
  7. Draw the following compounds.
    1. 3-ethyl–2-hexanol
    2. 2,2-dimethylpropanal
    3. 2-propanone
    4. dibutyl ether
    5. methanoic acid
    6. methyl butanoate

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