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25.3: Functional Groups

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Lesson Objectives

  • Define a functional group and explain the importance of functional groups to organic chemistry.
  • Be able to identify the following functional groups in an organic molecule: alkyl halide, alcohol, ether, aldehyde, ketone, carboxylic acid, ester, and amine.
  • Know how to name simple molecules containing one of the basic functional groups listed above.
  • Relate the properties of organic compounds to their structures and the functional groups that they contain, and describe the uses of some of these compounds.

Lesson Vocabulary

  • alcohol
  • aldehyde
  • alkyl halide
  • amine
  • carboxylic acid
  • ester
  • ether
  • functional group
  • ketone

Check Your Understanding

Recalling Prior Knowledge

  • What is hydrogen bonding, and how does it relate to the properties of compounds that are able to hydrogen bond?
  • What is the Brønsted-Lowry definition of an acid and a base?

A functional group is an atom or group of atoms that is primarily responsible for the properties and reactivity of a given organic compound. A given functional group undergoes the same type of reactions in every molecule in which it is found. In this lesson, we will give an overview of several major functional groups, concentrating on the structure, nomenclature, and general properties of compounds that contain each group. In the following lesson, we will study organic reactions in more detail.

Alkyl Halides

An alkyl halide is an organic compound in which one or more halogen atoms are substituted for one or more hydrogen atoms in a hydrocarbon. The general formulas for organic molecules with functional groups use the letter R to stand for the rest of the molecule outside of the functional group. Because there are four possible halogen atoms (fluorine, chlorine, bromine, or iodine) that can act as the functional group, we can use the general formula R−X to represent an alkyl halide. The rules for naming simple alkyl halides are listed below.

  1. Name the parent compound by finding the longest continuous carbon atom chain that also contains the halogen. Add a prefix for the particular halogen atom. The prefixes for each of the four halogens are fluoro-, chloro-, bromo-, and iodo-. If more than one kind of halogen atom is present, put them in alphabetical order. If there is more than one of the same halogen atom in a compound, use the prefixes di-, tri-, or tetra- before the prefix for the halogen.
  2. As with branched hydrocarbons, start numbering the parent carbon chain on the end that will give the substituents the lowest possible numbers. If there are multiple substituents, it is the sum of the numbers that should be considered. If different halogens are present in equivalent positions, give the lower number to the one that comes first in alphabetical order.
  3. Add the numerical prefix into the name before the halogen prefix.
  4. Separate numbers with commas and separate numbers from names or prefixes with a hyphen. There are no spaces in the name.

Figure below shows some examples of names and structural formulas for a few alkyl halides.

Note that for the structure based on methane, no number needs to be used, since there is only one carbon atom. In the third example, the chloro substituent is listed first alphabetically, and the chain is numbered so that the sum of the numbers is as low as possible.

Properties of Alkyl Halides

Alkyl halides are seldom found in nature but can be prepared from their parent hydrocarbons. A class of simple alkyl halides called chlorofluorocarbons (CFCs) was once very widely used in aerosol sprays and as refrigerants. Once such example is CCl3F, called trichlorofluoromethane. Unfortunately, CFCs are harmful to the ozone layer of our upper atmosphere. Ozone is critical in limiting the amount of damaging ultraviolet radiation that reaches the Earth. A CFC molecule breaks down in the atmosphere when exposed to solar radiation.

CCl3F(g) \mathrm{\overset{solar \ radiation}{\longrightarrow}} CCl2F(g) + Cl(g)

The free chlorine atoms react readily with molecules of ozone, creating chlorine monoxide and molecular oxygen

Cl(g) + O3(g) → ClO(g) + O2(g)

The ClO is then able to react with isolated oxygen atoms that are also present in the atmosphere, regenerating the free chlorine atoms.

ClO(g) + O(g) → Cl(g) + O2(g)

If these two reactions are summed together, we can see that chlorine atoms act as a catalyst for the decomposition of ozone to oxygen gas. Since it is regenerated at the end of this pair of reactions, one single chlorine atom can catalyze the destruction of thousands of ozone molecules. Beginning in the late 1970s, ozone depletion was recognized as a significant phenomenon. The most dramatic decrease in ozone occurs seasonally over the continent of Antarctica. The size and duration of the ozone hole steadily increased, with the largest hole recorded in 2006 (Figure below). Fortunately, most countries have recognized the danger of CFCs and dramatically curtailed their use in recent years. It is hoped that ozone depletion will slow and that the ozone layer may eventually return to its earlier levels.

A type of alkyl halide called a chlorofluorocarbon (CFC) was once used nearly exclusively as a propellant for aerosol sprays. Since CFCs destroy the important ozone layer in Earth’s atmosphere, their use has been largely banned. The image on the right shows the seasonal hole in the ozone layer above Antarctica as of 2006.

Alcohols

An alcohol is an organic compound that contains one or more hydroxyl (−OH) groups. The general formula for alcohols is R−OH. Do not confuse alcohols with inorganic bases that contain the hydroxide ion (OH). The –OH group in an alcohol is covalently bonded to a carbon atom and does not ionize in solution. The steps for naming alcohols are listed below.

  1. Name the parent compound by finding the longest continuous carbon atom chain that also contains the hydroxyl group. If there is one hydroxyl group in the molecule, change the final –e in the name of the alkane to –ol. If there is more than one hydroxyl group, use the full name of the alkane and add a suffix to indicate the number of hydroxyl groups. For example, two hydroxyl groups is –diol, three is –triol, etc.
  2. Number the parent carbon chain in a way that makes the sum of hydroxyl numbers as low as possible.
  3. Add the numerical prefix into the name before the name of the alcohol.
  4. Separate numbers with commas and separate numbers from names or prefixes with hyphens. There are no spaces in the name.

Three examples of alcohols and their IUPAC names are shown below.

Aliphatic alcohols can be classified according to the number of R groups attached to the carbon with the hydroxyl group. If one R group is attached to that carbon, the alcohol is a primary alcohol. If two R groups are attached, the alcohol is a secondary alcohol. If three R groups are attached, the alcohol is a tertiary alcohol. Shown below is an example of each. The primary alcohol is 1-propanol, the secondary alcohol is 2-butanol, and the tertiary alcohol is 2-methyl-2-propanol.

The simplest aromatic alcohol consists of a benzene ring in which one of the hydrogen atoms has been replaced by the hydroxyl group. The name of this compound is phenol.

Properties of Alcohols

The smallest and lightest alcohols (methanol, ethanol, and propanol) are completely soluble in water in all proportions. Recall that water molecules interact with one another through hydrogen bonding, which is an attraction between the lone pair electrons on the oxygen of one water molecule and an electron-deficient hydrogen atom on another molecule. The hydroxyl group of the alcohol also undergoes hydrogen bonding. In solution, the alcohol molecules and the water molecules form hydrogen bonds with each other, resulting in complete miscibility. However, as the length of the nonpolar hydrocarbon chain increases, the aqueous solubility decreases. The solubilities of 1-butanol, 1-pentanol, and 1-octanol per 100 g of water are 7.4 g, 2.7 g, and 0.06 g, respectively.

Hydrogen bonding also explains the relatively high boiling points of alcohols compared to alkanes of similar molar mass (Table below).

Compound Formula Molar Mass (g/mol) Boiling Point (°C)
ethane CH3CH3 30 −88
methanol CH3OH 32 64.7
propane CH3CH2CH3 44 −42.1
ethanol CH3CH2OH 46 78.3

Only weak London dispersion forces hold molecules of nonpolar alkanes together in the liquid phase. Consequently, less energy is required to break these molecules away from the surface of the liquid and turn them into a vapor. The boiling points of small alkanes are low. The stronger hydrogen bonding between alcohol molecules means that more energy is required to convert the liquid to vapor, resulting in significantly higher boiling points. Because all molecules display London dispersion forces, the total strength of the intermolecular forces, and therefore the boiling point, increases along with molar mass for groups of similar compounds.

Alcohols are commonly found in many everyday materials. Hand sanitizers typically contain ethanol or isopropanol (2-propanol). Isopropanol (rubbing alcohol) is also used as a common antiseptic. Ethylene glycol (1,2-ethanediol) is the active ingredient in most commercial antifreezes. A 50% by volume solution of ethylene glycol freezes at −36°C. Glycerol (1,2,3-propanetriol) is a very viscous alcohol that is frequently used as a moistening agent in cosmetics.

Some of the many household products that contain one or more alcohols include hand sanitizer and cosmetics such as lipstick.

Fermentation is a process in which sugars such as glucose or fructose are converted into ethanol and carbon dioxide. Fermentation is used in the production of wine, beer, and other spirits. The ethanol in these beverages, like all simple alcohols, is toxic to some extent. Ethanol is metabolized to acetaldehyde within the body. These compounds are fatal in large enough doses. Methanol is lethal in even smaller doses than ethanol, because the body converts it into formaldehyde and formic acid, which are extremely toxic. Ethylene glycol is also quite toxic. Its sweet smell and taste makes it very dangerous to dogs and other animals that may drink spilled antifreeze. Many antifreezes now use the much safer propylene glycol (1,2-propanediol) instead of ethylene glycol.

Ethers

An ether is an organic compound in which two hydrocarbon groups are bonded to the same oxygen atom. An ether is represented by the general formula R−O−R’. The R’ in the formula means that the second hydrocarbon group (R') may be different from the first one (R). The steps for naming ethers are listed below.

  1. The word ether appears at the end of the name.
  2. The two hydrocarbon groups are named using the rules for alkyl substituents, and these names come before the word ether. If the two alkyl groups are the same, the prefix –di is used. If the two alkyl groups are different, they are listed in alphabetical order.
  3. Spaces are left between the names of the alkyl groups (if different) and before the word ether.

Shown below are two examples of ethers with their IUPAC names.

Properties of Ethers

Ethers are somewhat soluble in water, but less so than alcohols. The lone pairs of electrons on the oxygen atom of the ether can hydrogen bond with the hydrogen atoms of water molecules. As in the case of alcohols, the solubility is greater for ethers that have shorter R groups. The boiling points of ethers are much lower than the boiling points of alcohols. Ether molecules do not have hydrogen atoms that are covalently bonded to a highly electronegative atom, and so ether molecules cannot hydrogen bond with each other. The weaker intermolecular forces acting between ether molecules result in boiling points that are much closer to alkanes of similar molar mass than to alcohols.

The anesthetic effects of ether were first discovered in the 1840s. Diethyl ether was used as a general anesthetic for patients undergoing surgery for many years. However, ethers are very flammable and have undesirable side effects, such as nausea and vomiting. Safer alternatives to ether are now used in anesthesia, and the primary use of ethers today is as a solvent for other organic compounds.

Aldehydes and Ketones

Aldehydes and ketones are two related categories of organic compounds that both contain the carbonyl group, shown below.

The difference between aldehydes and ketones is the placement of the carbonyl group within the molecule. An aldehyde is an organic compound in which the carbonyl group is attached to a carbon atom at the end of a carbon chain. A ketone is an organic compound in which the carbonyl group is attached to a carbon atom within the carbon chain. The general formulas for each are shown below.

For aldehydes, the R group may be a hydrogen atom or a carbon chain of any length. To name an aldehyde, first name the parent compound by finding the longest continuous chain that contains the carbonyl group. Then, change the –e at the end of the name of the alkane to –al. Because an aldehyde only occurs at the end of the chain, no numbers are needed to indicate its position. If other substituents are present, begin numbering the parent chain on the end where the aldehyde is found. Two examples of aldehydes are shown below.

For ketones, R and R’ are both carbon chains, which can be the same or different. The steps for naming ketones, followed by two examples, are shown below.

  1. Name the parent compound by finding the longest continuous chain that contains the carbonyl group. Change the –e at the end of the name of the alkane to –one.
  2. Number the carbon atoms in the chain so that the carbonyl group has the lowest possible number.
  3. Add the numerical prefix into the name before the name of the ketone.
  4. Use a hyphen between the number and the name of the ketone.

Properties of Aldehydes and Ketones

Aldehydes and ketones can participate in weak hydrogen bonds with water through the carbonyl oxygen atom. The smallest aldehydes and ketones (3 carbons or fewer) are soluble in water in all proportions. As the length of the carbon chain increases, water solubility decreases. Like ethers, aldehydes and ketones cannot hydrogen bond with themselves, because they do not contain any hydrogen atoms in sufficiently polar bonds. As a result, their boiling points are generally lower than those of alcohols. However, aldehydes and ketones are polar molecules due to the polarized carbonyl group. The dipole-dipole interactions are stronger than the dispersion forces present in alkanes of a similar size. The boiling points of aldehydes and ketones are intermediate between those of alkanes and alcohols. For example, the boiling points of propane, ethanal, and ethanol (all of which have molar masses in the range of 44-46 g/mol) are −42°C, 20°C, and 78°C, respectively.

Methanal, commonly known as formaldehyde, was once commonly used as a biological preservative for dead animals. In recent years, formaldehyde has been shown to be a carcinogen, so safer alternatives are often used instead. Aldehydes are also used in the production of resins and plastics. The simplest ketone, propanone, is commonly called acetone. Acetone is a common organic solvent that was once used in most nail polish removers, but it has largely been replaced by other solvents. Aldehydes and ketones are fairly volatile compounds and are commonly found in perfumes and flavorings. For example, the odor and flavor of cinnamon is a result of an aldehyde called cinnamaldehyde. The aroma of raspberries is provided by a ketone simply called raspberry ketone.

Carboxylic Acids

Organic acids such as acetic acid all contain a functional group called a carboxyl group.

The carboxyl group consists of a carbonyl group (C=O) in which the carbon is also bonded to a hydroxyl (−OH) group. A carboxylic acid is an organic compound that contains the carboxyl functional group. The general formula for a carboxylic acid can be abbreviated as –COOH. In addition to the carbonyl oxygen and the hydroxyl group, the carbon atom of the carboxyl group may be attached to a hydrogen atom or to a carbon chain. To name a simple carboxylic acid, find the longest continuous chain that contains the carboxyl group and name the parent chain. Then, change the –e at the end of the name of the alkane to –oic acid. Because a carboxyl group must be at the end of a chain, no numbers are needed to indicate its position. If other substituents are present, number the parent chain so that the carboxyl carbon atom is first. Two examples are shown below.

Properties of Carboxylic Acids

Carboxylic acids are all weak acids. In aqueous solution, the O−H bond of the hydroxyl group can break, yielding a negative carboxylate ion and the hydrogen ion.

The smaller members of the aliphatic carboxylic acid series are colorless, volatile liquids with strong odors. Ethanoic acid is commonly known as acetic acid, and common household vinegar is a 5% solution of acetic acid. Larger carboxylic acids are solids with low melting points. There are a great many aromatic carboxylic acids, which are generally crystalline solids. Carboxylic acids can form intermolecular hydrogen bonds, so they have relatively high melting and boiling points compared to other organic compounds that cannot hydrogen bond. Carboxylic acids with shorter carbon chains are very soluble in water, while those with longer carbon chains are less soluble.

Many carboxylic acids occur naturally in plants and animals. Citrus fruits such as oranges and lemons contain citric acid (Figure below).

Citric acid is a large carboxylic acid with three ionizable hydrogen atoms. It is found in citrus fruits and gives them their sour or tart flavor.

Ethanoic and citric acids are frequently added to foods to give them a tart flavor. Benzoic, propanoic, and sorbic acids are used as food preservatives because of their ability to kill microorganisms that can lead to spoilage. Methanoic and ethanoic acids are widely used as starting points for the industrial manufacture of paints, adhesives, and coatings.

Esters

An ester is an organic compound that is a derivative of a carboxylic acid in which the hydrogen atom of the hydroxyl group has been replaced with an alkyl group. The general formula for an ester is shown below.

The R group can either be a hydrogen atom or a carbon chain. The R’ group must be a carbon chain since a hydrogen atom would make the molecule a carboxylic acid. The steps for naming esters are shown below along with two examples.

  1. Identify and name the alkyl group (R’) that has replaced the hydrogen of the hydroxyl group. This is the first part of the ester name.
  2. Name the carboxylic acid portion of the molecule (R-COO), but change the ending of the name from –oic acid to –oate. This is the second part of the ester name.
  3. Leave a space between the alkyl group name and the name of the carboxylic acid derivative.

In cases where the R' group is relatively small, esters have lower boiling points than the carboxylic acids from which they were derived. This is due to the fact that esters cannot form hydrogen bonds with each other, since they no longer possess the O-H bond that is present in the acid form. Esters with very short carbon chains are somewhat soluble in water, while those with longer chains are less soluble.

Esters are very commonly found in plants and are responsible for many distinctive odors and flavors. For example, methyl salicylate has the odor and flavor of oil of wintergreen, while propyl ethanoate smells and tastes like a pear (see Figure below).

Esters are responsible for many aromas and flavors in natural products. Methyl salicylate (IUPAC: methyl 2-hydroxybenzoate) is found in the plant that provides oil of wintergreen, which is used in mints and candies. Propyl ethanoate is an ester found in pears.

At one time, these compounds had to be isolated from their natural sources in order to be used as food additives, but chemists are now able to synthesize these and many other naturally occurring compounds in the laboratory.

Amines

An amine is an organic compound in which one or more of the hydrogen atoms of ammonia (NH3) is replaced by an alkyl group. The general structure of an amine can be abbreviated as R−NH2, where R is a carbon chain. Like alcohols, amines are often referred to as primary, secondary, or tertiary. However, in the case of amines, this classification tells you how many alkyl chains are attached to the nitrogen atom. The nitrogen atom of a primary amine is bonded to two hydrogen atoms and one carbon, the nitrogen atom of a secondary amine is bonded to one hydrogen and two carbons, and the nitrogen atom of a tertiary amine is bonded to three carbon atoms. Recall that alcohols are labelled as primary, secondary, or tertiary based on how many alkyl chains are attached to the carbon that bears the hydroxyl group.

Although there are IUPAC rules for naming amines, many simple amines are generally referred to by a naming system that is more closely analogous to the way that ethers are named. This common system for naming amines is shown below along with several examples.

  1. Name the alkyl groups that are attached to the nitrogen atom of the amine. If there is more than one different alkyl group, put them in alphabetical order. If there are two or three of the same alkyl group, use the di− or tri− prefix.
  2. Follow the alkyl group name with the suffix –amine, with no spaces.

Properties of Amines

Amines are weak bases due to the presence of a lone pair of electrons on the nitrogen atom. This lone pair can attract the hydrogen atom from a water molecule, causing the bond between it and the oxygen atom to break. The resultant products are the conjugate acid of the amine and the hydroxide ion.

Amines are capable of hydrogen bonding, though their boiling points are generally a bit lower than the corresponding alcohol. Methylamine and ethylamine are gases at room temperature, while larger amines are liquids. As with other organic compounds that hydrogen bond, aqueous solubility is affected by the lengths of the carbon chains. Smaller amines are soluble in water, while larger ones are less soluble.

Amines generally have rather pungent or noxious odors. Ammonia can be considered the simplest amine and has a very distinctive smell. Methylamine has an unpleasant odor that is often associated with dead fish. Amines are frequently formed when the proteins in animal cells begin to decompose, so many of them have scents that are associated with death and decay. The toxins that many animals use as a form of defense are often amines as well. Amines are used industrially as dyes and are found in many drugs.

Table below summarizes the main organic functional groups along with their general formulas.

Functional Groups
Functional Group Structure Compound Classification

Halogen

-X (X is F, Cl, Br, I)

Alkyl halide

Hydroxyl

-OH

Alcohol

Carboxyl

Carboxylic acid

Carbonyl

Aldehyde

Ketone

Ester

Ester

Ether

Ether

Amine

Primary Amine

Secondary Amine

Tertiary Amine

Lesson Summary

  • A functional group is an atom or group of atoms that is primarily responsible for the properties of the organic compound in which it is present. Organic molecules are given systematic names to identify the identities and locations of functional groups within the molecule.
  • Alkyl halides are hydrocarbons in which one or more hydrogen atoms has been replaced by a halogen atom.
  • Alcohols contain the hydroxyl functional group and can be primary, secondary, or tertiary.
  • Ethers are compounds with an oxygen atom bonded to two alkyl groups.
  • Aldehydes and ketones contain the carbonyl functional group. In an aldehyde, the carbonyl is at the end of a carbon chain, while in a ketone, it is in the middle.
  • A carboxylic acid contains the carboxyl functional group. They are weak acids, because the hydrogen of the hydroxyl group is ionizable.
  • In an ester, the hydrogen of a carboxylic acid group is replaced by an alkyl group.
  • Amines are derivatives of ammonia that are weak bases due to the lone pair of electrons on the nitrogen atom.

Lesson Review Questions

Reviewing Concepts

  1. Why are alcohols not considered bases even though they contain an –OH substituent?
  2. Which types of organic compounds from this lesson are capable of intermolecular hydrogen bonding?
  3. Which types of organic compounds from this lesson are capable of hydrogen bonding with water?
  4. Why do amines act as bases?

Problems

  1. Identify each of the following as an alcohol, aldehyde, ketone, carboxylic acid, ether, ester, amine, or alkyl halide.
  2. Name each of the compounds from question 5.
  3. Name the following alcohols and identify each as a primary, secondary, or tertiary alcohol.
  4. Glycerol is an important alcohol used in the cosmetic industry. Its IUPAC name is 1,2,3-propanetriol. Draw the structural formula of glycerol.
  5. Draw and name all of the structural isomers of an alkyl halide with the formula C3H6Cl2.
  6. Draw condensed structural formulas for each of the following.
    1. pentyl methanoate
    2. ethylpropylamine
    3. 3-heptanone
    4. 2-methylphenol
    5. octanoic acid
    6. butanal

Further Reading / Supplemental Links

Points to Consider

Common organic chemical reactions can be classified into several broad categories.

  • What are some of the most common types of organic reactions?
  • How do functional groups give organic compounds their chemical properties?

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