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2.4: Functional Groups: Centers of Reactivity

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  • Objectives

    After completing this section, you should be able to

    1. explain why the properties of a given organic compound are largely dependent on the functional group or groups present in the compound.
    2. identify the functional groups present in each of the following compound types: alkenes, alkynes, arenes, (alkyl and aryl) halides, alcohols, ethers, aldehydes, ketones, esters, carboxylic acids, (carboxylic) acid chlorides, amides, amines, nitriles, nitro compounds, sulfides and sulfoxides.
    3. identify the functional groups present in an organic compound, given its structure.
    4. Given the structure of an organic compound containing a single functional group, identify which of the compound types listed under Objective 2, above, it belongs to.
    5. draw the structure of a simple example of each of the compound types listed in Objective 2.

    Key Terms

    Make certain that you can define, and use in context, the key term below.

    • functional group

    Study Notes

    The concept of functional groups is a very important one. We expect that you will need to refer back to tables at the end of Section 3.1 quite frequently at first, as it is not really feasible to learn the names and structures of all the functional groups and compound types at one sitting. Gradually they will become familiar, and eventually you will recognize them automatically.

    Functional groups are small groups of atoms that exhibit a characteristic reactivity. A particular functional group will almost always display its distinctive chemical behavior when it is present in a compound. Because of their importance in understanding organic chemistry, functional groups have specific names that often carry over in the naming of individual compounds incorporating the groups.

    As we progress in our study of organic chemistry, it will become extremely important to be able to quickly recognize the most common functional groups, because they are the key structural elements that define how organic molecules react. For now, we will only worry about drawing and recognizing each functional group, as depicted by Lewis and line structures. Much of the remainder of your study of organic chemistry will be taken up with learning about how the different functional groups tend to behave in organic reactions.

    Drawing abbreviated organic structures

    Often when drawing organic structures, chemists find it convenient to use the letter 'R' to designate part of a molecule outside of the region of interest. If we just want to refer in general to a functional group without drawing a specific molecule, for example, we can use 'R groups' to focus attention on the group of interest:

    Lewis structures for a generic primary alcohol, secondary alcohol, aldehyde, and ketone.svg

    The 'R' group is a convenient way to abbreviate the structures of large biological molecules, especially when we are interested in something that is occurring specifically at one location on the molecule.

    Common Functional Groups

    In the following sections, many of the common functional groups found in organic chemistry will be described. Tables of these functional groups can be found at the bottom of the page.


    The simplest functional group in organic chemistry (which is often ignored when listing functional groups) is called an alkane, characterized by single bonds between two carbons and between carbon and hydrogen. Some examples of alkanes include methane, CH4, is the natural gas you may burn in your furnace or on a stove. Octane, C8H18, is a component of gasoline.


    Lewis structure of methane, and Lewis and line structures of octane.svg

    Alkenes (sometimes called olefins) have carbon-carbon double bonds, and alkynes have carbon-carbon triple bonds. Ethene, the simplest alkene example, is a gas that serves as a cellular signal in fruits to stimulate ripening. (If you want bananas to ripen quickly, put them in a paper bag along with an apple - the apple emits ethene gas, setting off the ripening process in the bananas). Ethyne, commonly called acetylene, is used as a fuel in welding blow torches.

    Alkenes and alkynes

    Lewis structures of ethene, an alkene, and ethyne, an alkyne.svg

    Alkenes have trigonal planar electron geometry (due to sp2 hybrid orbitals at the alkene carbons) while alkynes have linear geometry (due to sp hybrid orbitals at the alkyne carbons). Furthermore, many alkenes can take two geometric forms: cis or trans (or Z and E which will be explained in detail in Chapter 7). The cis and trans forms of a given alkene are different molecules with different physical properties there is a very high energy barrier to rotation about a double bond. In the example below, the difference between cis and trans alkenes is readily apparent.

    Lewis and line structures for a cis alkene, with methyl groups on the same side of the double bond.svg                            Lewis and line structures for a trans alkene, with methyl groups on opposite sides of the double bond.svg

    Alkanes, alkenes, and alkynes are all classified as hydrocarbons, because they are composed solely of carbon and hydrogen atoms. Alkanes are said to be saturated hydrocarbons, because the carbons are bonded to the maximum possible number of hydrogens - in other words, they are saturated with hydrogen atoms. The double and triple-bonded carbons in alkenes and alkynes have fewer hydrogen atoms bonded to them - they are thus referred to as unsaturated hydrocarbons. As we will see in Chapter 7, hydrogen can be added to double and triple bonds, in a type of reaction called 'hydrogenation'.

    The aromatic group is exemplified by benzene (which used to be a commonly used solvent on the organic lab, but which was shown to be carcinogenic), and naphthalene, a compound with a distinctive 'mothball' smell. Aromatic groups are planar (flat) ring structures, and are widespread in nature. We will learn more about the structure and reactions of aromatic groups in Chapter 15.



                                                Lewis and line structures of benzene.svg                line structure of naphthalene.svg

    Functional Groups with Carbon Single Bonds to other Atoms


    When the carbon of an alkane is bonded to one or more halogens, the group is referred to as a alkyl halide or haloalkane. The presence of a halogen atom (F, Cl, Br, or I), is often represented by X due to the similar chemistry of halogens. Chloroform is a useful solvent in the laboratory, and was one of the earlier anesthetic drugs used in surgery. Chlorodifluoromethane was used as a refrigerant and in aerosol sprays until the late twentieth century, but its use was discontinued after it was found to have harmful effects on the ozone layer. Bromoethane is a simple alkyl halide often used in organic synthesis. Alkyl halides groups are quite rare in biomolecules.

    Lewis structures of chloroform, Freon-12, and bromoethane.svg

    Alcohols and Thiols

    In the alcohol functional group, a carbon is single-bonded to an OH group (the OH group, by itself, is referred to as a hydroxyl). Except for methanol, all alcohols can be classified as primary, secondary, or tertiary. In a primary alcohol, the carbon bonded to the OH group is also bonded to only one other carbon. In a secondary alcohol and tertiary alcohol, the carbon is bonded to two or three other carbons, respectively. When the hydroxyl group is directly attached to an aromatic ring, the resulting group is called a phenol.

    Lewis structures of methanol, ethanol, isopropanol, tert-butanol, and phenol.svg

    The sulfur analog of an alcohol is called a thiol (the prefix thio, derived from the Greek, refers to sulfur).

    Lewis structures of a primary, secondary, and tertiary thiol.svg

    Ethers and sulfides

    In an ether functional group, a central oxygen is bonded to two carbons. Below are the line and Lewis structures of diethyl ether, a common laboratory solvent and also one of the first medical anaesthesia agents.

    line and Lewis structures of diethyl ether.svg

    In sulfides, the oxygen atom of an ether has been replaced by a sulfur atom.

    Lewis structures of diethylsulfide and tetrahydrothiopyran.svg


    Amines are characterized by nitrogen atoms with single bonds to hydrogen and carbon. Just as there are primary, secondary, and tertiary alcohols, there are primary, secondary, and tertiary amines. Ammonia is a special case with no carbon atoms.

    Lewis structures of ammonia, methylamine, dimethylamine, and trimethylamine.svg

    One of the most important properties of amines is that they are basic, and are readily protonated to form ammonium cations. In the case where a nitrogen has four bonds to carbon (which is somewhat unusual in biomolecules), it is called a quaternary ammonium ion.

    Lewis structures of ammonia, methylamine, dimethylamine, and trimethylamine with color.svg

    Lewis structures of ammonium ion, methylammonium ion, and tetramethylammonium ion.svg

    Note: Do not be confused by how the terms 'primary', 'secondary', and 'tertiary' are applied to alcohols and amines - the definitions are different. In alcohols, what matters is how many other carbons the alcohol carbon is bonded to, while in amines, what matters is how many carbons the nitrogen is bonded to.

    comparison of a tertiary alcohol and a primary amine.svg


    Carbonyl Containing Functional Groups

    Aldehydes and Ketones

    There are a number of functional groups that contain a carbon-oxygen double bond, which is commonly referred to as a carbonyl. Ketones and aldehydes are two closely related carbonyl-based functional groups that react in very similar ways. In a ketone, the carbon atom of a carbonyl is bonded to two other carbons. In an aldehyde, the carbonyl carbon is bonded on one side to a hydrogen, and on the other side to a carbon. The exception to this definition is formaldehyde, in which the carbonyl carbon has bonds to two hydrogens.

    Lewis structures of formaldehyde, acetaldehyde, and acetone.svg

    Carboxylic acids and acid derivatives

    If a carbonyl carbon is bonded on one side to a carbon (or hydrogen) and on the other side to a heteroatom (in organic chemistry, this term generally refers to oxygen, nitrogen, sulfur, or one of the halogens), the functional group is considered to be one of the ‘carboxylic acid derivatives’, a designation that describes a grouping of several functional groups. The eponymous member of this grouping is the carboxylic acid functional group, in which the carbonyl is bonded to a hydroxyl (OH) group.

    Lewis structures of formic acid and acetic acid.svg

    As the name implies, carboxylic acids are acidic, meaning that they are readily deprotonated to form the conjugate base form, called a carboxylate (much more about carboxylic acids in Chapter 20).

    Lewis structures of formate and acetate.svg

    In amides, the carbonyl carbon is bonded to a nitrogen. The nitrogen in an amide can be bonded either to hydrogens, to carbons, or to both. Another way of thinking of an amide is that it is a carbonyl bonded to an amine.

    Lewis structures of amides.svg

    In esters, the carbonyl carbon is bonded to an oxygen which is itself bonded to another carbon. Another way of thinking of an ester is that it is a carbonyl bonded to an alcohol. Thioesters are similar to esters, except a sulfur is in place of the oxygen.

    Lewis structure of an ester and a thioester.svg

    In an acid anhydride, there are two carbonyl carbons with an oxygen in between. An acid anhydride is formed from combination of two carboxylic acids with the loss of water (anhydride).

    Lewis structure for an anhydride.svg

    In an acyl phosphate, the carbonyl carbon is bonded to the oxygen of a phosphate, and in an acid chloride, the carbonyl carbon is bonded to a chlorine.

    Lewis structure for an acyl phosphate and an acid chloride.svg


    Nitriles and Imines

    In a nitrile group, a carbon is triple-bonded to a nitrogen. Nitriles are also often referred to as cyano groups.

    Lewis structure for acetonitrile.svg

    Molecules with carbon-nitrogen double bonds are called imines, or Schiff bases.

    Lewis structure for propan-2-imine and N-methylpropan-2-imine.svg


    Phosphorus is a very important element in biological organic chemistry, and is found as the central atom in the phosphate group. Many biological organic molecules contain phosphate, diphosphate, and triphosphate groups, which are linked to a carbon atom by the phosphate ester functionality.

    structures of organic phosphate, an organic phosphate ester, and an organic diphosphate ester.svg

    Because phosphates are so abundant in biological organic chemistry, it is convenient to depict them with the abbreviation 'P'. Notice that this 'P' abbreviation includes the oxygen atoms and negative charges associated with the phosphate groups.

    structure of dihydroxyacetone phosphate.svg

    structure of isopentenyl diphosphate.svg

    Molecules with Multiple Functional Groups

    A single compound may contain several different functional groups. The six-carbon sugar molecules glucose and fructose, for example, contain aldehyde and ketone groups, respectively, and both contain five alcohol groups (a compound with several alcohol groups is often referred to as a ‘polyol’).

    Lewis structure of glucose.svg            Lewis structure of fructose.svg

    Capsaicin, the compound responsible for the heat in hot peppers, contains phenol, ether, amide, and alkene functional groups.

    structure of capsaicin.svg

    The male sex hormone testosterone contains ketone, alkene, and secondary alcohol groups, while acetylsalicylic acid (aspirin) contains aromatic, carboxylic acid, and ester groups.

    structure of testosterone.svg                structure of acetylsalicylic acid.svg

    While not in any way a complete list, this section has covered most of the important functional groups that we will encounter in biological and laboratory organic chemistry. The table found below provides a summary of all of the groups listed in this section, plus a few more that will be introduced later in the text.


    Exercise \(\PageIndex{1}\)

    Identify the functional groups in the following organic compounds. State whether alcohols and amines are primary, secondary, or tertiary.



    a) carboxylate, sulfide, aromatic, two amide groups (one of which is cyclic)

    b) tertiary alcohol, thioester

    c) carboxylate, ketone

    d) ether, primary amine, alkene

    2: Draw one example each (there are many possible correct answers) of compounds fitting the descriptions below, using line structures. Be sure to designate the location of all non-zero formal charges. All atoms should have complete octets (phosphorus may exceed the octet rule).

    a) a compound with molecular formula C6H11NO that includes alkene, secondary amine, and primary alcohol functional groups

    b) an ion with molecular formula C3H5O6P 2- that includes aldehyde, secondary alcohol, and phosphate functional groups.

    c) A compound with molecular formula C6H9NO that has an amide functional group, and does not have an alkene group.


    Functional Group Tables

    Exclusively Carbon Functional Groups

    Group Formula Class Name Specific Example IUPAC Name Common Name
    generic alkene structure.svg alkene H2C=CH2 ethene ethylene
    generic alkyne structure.svg alkyne HC≡CH ethyne acetylene
    generic arene structure.svg arene C6H6 benzene benzene

    Functional Groups with Single Bonds to Heteroatoms

    Group Formula Class Name Specific Example IUPAC Name Common Name
    generic halide structure.svg halide H3C-I iodomethane methyl iodide
    generic alcohol structure.svg alcohol CH3CH2OH ethanol ethyl alcohol
    generic ether structure.svg ether CH3CH2OCH2CH3 diethyl ether ether
    generic amine structure.svg amine H3C-NH2 aminomethane methylamine
    generic structure for nitro compound.svg  nitro compound H3C-NO2 nitromethane  
    generic thiol structure.svg thiol H3C-SH methanethiol methyl mercaptan
    generic sulfide structure.svg sulfide H3C-S-CH3 dimethyl sulfide  

    Functional Groups with Multiple Bonds to Heteroatoms

    Group Formula Class Name Specific Example IUPAC Name Common Name
    generic nitrile structure.svg nitrile H3C-CN ethanenitrile acetonitrile
    generic aldehyde structure.svg aldehyde H3CCHO ethanal acetaldehyde
    generic ketone structure.svg ketone H3CCOCH3 propanone acetone
    generic carboxylic acid structure.svg carboxylic acid H3CCO2H ethanoic Acid acetic acid
    generic ester structure.svg ester H3CCO2CH2CH3 ethyl ethanoate ethyl acetate
    generic acid halide structure.svg acid halide H3CCOCl ethanoyl chloride acetyl chloride
    generic amide structure.svg amide H3CCON(CH3)2 N,N-dimethylethanamide N,N-dimethylacetamide
    generic acid anhydride structure.svg acid Anhydride (H3CCO)2O ethanoic anhydride acetic anhydride




    The following is the molecule for ATP, or the molecule responsible for energy in human cells. Identify the functional groups for ATP.



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