Classes of Functional Groups

A hydroxyl group is a functional group with the formula ${{-}\rm{OH}}$ that forms ${\rm{C}{-}{OH}}$ fragments and characterizes molecules called alcohols and phenols. An alcohol is a hydrocarbon containing a hydroxyl functional group. A benzene ring in which a ${\rm{C}{-}{H}}$ bond is substituted for a hydroxyl group, producing a ${\rm{C}{-}{OH}}$ unit group, is a phenol. The terms primary, secondary, and tertiary refer to hydroxyl groups bound to a carbon atom. In a primary molecule, a carbon atom is bound to one other carbon atom. In a secondary molecule, a carbon atom is bound to two other carbon atoms. In a tertiary molecule, a carbon atom is bound to three other carbon atoms. Alcohols are named by adding the suffix -ol to the molecular root name and otherwise following the IUPAC rules. The hydroxyl group on alcohols and phenols makes them polar and gives the molecules the ability to form hydrogen bonds with one another and with water. This means that low molecular weight alcohols and phenols are hydrophilic, or water-soluble, and have higher boiling points than analogous alkanes. As the number of carbon atoms increases, however, the polarizing effect of the ${{-}\rm{OH}}$ groups is overcome by the nonpolar carbon-carbon bonds, so that high molecular weight alcohols are hydrophobic. The boiling points and melting points of alcohols also depend on the other functional groups attached to the chain or ring.

Alcohols can be formed by hydration or hydrolysis reactions. Hydrolysis is the addition of a hydroxyl group (${{-}\rm{OH}}$) to a molecule. This be accomplished by a substitution reaction with an alkyl halide: An ether is similar to an alcohol, but instead of a hydroxyl group (${\rm{R}{-}{OH}}$), it is an organic molecule containing an oxygen bound by two alkyl or aryl groups through ${\rm{C}{-}{O}}$ bonds (${\rm{R}{-}{O}{-}{R}}$). Ethers can be named two different ways. If the longest (parent) chain contains a ${\rm{C}{-}{O}{-}{C}}$ unit, the standard rules are followed. The parent chain is given the suffix -yl, and the word ether is added at the end. Alternatively, if there are more functional groups than just one ether, the ether is treated as a functional group with the prefix alkoxy-. The IUPAC rules use alkoxy- for the shorter chain, and the normal -ane for the parent chain. A thiol is an organic compound that is derived from hydrogen sulfide (H₂S). It contains an alkyl or aryl group covalently linked to a sulfhydryl group (${{-}\rm{SH}}$) by ${\rm{C}{-}{S}}$ bonds with ${\rm{R}{-}{SH}}$ stoichiometry. Thiols follow the same nomenclature rules as alcohols but using the suffix -thiol instead of -ol. Ethers can be formed by joining two alcohols by a dehydration reaction. Consider the formation of diethyl ether by the dehydration of two ethyl alcohol molecules: Once formed, ethers are not very reactive because the ${\rm{C}{-}{O}{-}{C}}$ oxygen bonds are relatively stable. Ethers have a bent molecular geometry like water molecules so they are polar molecules, unlike nonpolar alkanes. So like alcohols, ethers have an elevated boiling point compared to the analogous alkane, but the effect is not as dramatic as with alcohols because ethers cannot form hydrogen bonds with each other; they have weaker intermolecular forces known as dipole-dipole interactions. Short-chain ethers are soluble in water because of the polarity imbued by the oxygen linkage. Thiols are also polar and have increased boiling points and solubilities compared to alkanes, but because sulfur is less electronegative than oxygen, thiols are not as soluble as ethers. The defining feature of thiols is their terrible odors. For example, thiols are responsible for skunk odor. Thiols are able to form disulfide molecules, which help stabilize protein structures, such as the disulfide bonds in hair.

The aldehyde and ketone functional groups both contain a ${\rm{C}{=}{O}}$ double bond called the carbonyl group. In an aldehyde, a carbonyl group (${\rm{C}\!=\!\!{O}}$) is bound to one alkyl (${{-}\rm{R}}$) fragment and one hydrogen atom, with ${\rm{RC({=}O)H}}$ or ${\rm{R}{-}{CHO}}$ stoichiometry. In a ketone, both R groups are alkyl, aryl, or a combination of the groups (${\rm{RC({=}O)R^\prime}}$ or ${\rm{R{-}CO{-}R^\prime}}$). Both groups follow the usual IUPAC naming conventions. For an aldehyde, the suffix -al is used, and for a ketone, the suffix -one is used. For naming purposes, the longest chain contains the carbonyl group and the aldehydes only. When an aldehyde is a substituent, it is called a carbaldehyde group. When ketones are functional groups, they are called acyl groups. As usual, low molecular weight aldehydes and ketones have common names still in use. For example, methanal is usually called "formaldehyde," and 2-propanone is known commonly as "acetone." The carbonyl group is planar because of the double bond. The electronegative oxygen atom creates a dipole moment, so the carbonyl group is also polar. This means aldehydes and ketones boil at higher temperatures than their analogous alkanes. Acetone, for example, is a liquid at room temperature, but its alkane analog, propane, is a gas. The polarity of the carbonyl group also increases the solubility of the molecules in water. The oxygen atom can form hydrogen bonds with water molecules. As usual, the effect of polarity is lessened as the number of carbon atoms increases. Aldehydes and ketones are formed from alcohols, either by partial oxidation (the addition of an oxygen atom) or by dehydrogenation (the removal of a hydrogen atom in the form of H₂O). For partial oxidation, a primary alcohol is required to form an aldehyde to get the terminal hydrogen atom. A secondary alcohol is needed to make a ketone. A carboxylic acid is a compound that contains an alkyl or aryl group (R) attached to a carboxyl group (${{-}\rm{COOH}}$). A carboxyl group is a functional group containing a ${\rm{C}{=}{O}}$ unit linked to an ${{-}\rm{OH}}$ (hydroxyl) fragment of ${\rm{R{-}C({=}O)OH}}$ or RCOOH stoichiometry. Carboxylic acids are named like aldehydes and ketones, but with the suffix -oic acid. When the carboxyl group replaces a ${\rm{C}{-}{H}}$ or hydroxyl group, which is a common reaction, the remaining ${\rm{CH_3{-}CO}}$ group is identified by the prefix acetyl-.

Carboxylic acids are able to form dimers with one another because of the hydroxyl group using hydrogen bonds resulting from the polarization of the ${\rm{COO}{-}{H}}$ unit. This greatly increases the intermolecular forces in carboxylic acids, and the boiling points of carboxylic acids are increased dramatically, even compared to alcohols. Compare the boiling points of acetic acid, ethanol, and ethane, each of which contains two carbon atoms.

Carboxylic acids are also more acidic than alcohols because the negative charge is spread out over two oxygen atoms, creating a more stable resonance structure when the molecule donates a proton.

When a carboxylic acid and an alcohol react, they combine to form an ester, an organic compound that contains a carboxyl unit in which a hydroxyl group is replaced by an alkyl or aryl group, giving it ${\rm{R{-}C({=}O)OR^\prime}}$ or ${\rm{R}{-}{COOR}^\prime}$ stoichiometry. The two oxygen atoms in the functional group impose polarity, but because neither oxygen atom is bound to a hydrogen atom, they cannot form hydrogen bonds with each other. Their boiling points are therefore lower than those of the analogous carboxylic acids. The oxygen atoms can form hydrogen bonds with water, however, so they are hydrophilic. As a result, small esters are soluble in water. Fats and oils are made of esters with long-chain R' groups; they are insoluble in water because of the hydrophobicity of the long carbon chains.

Amines and amides are two organic molecule classes that contain a basic nitrogen atom and a lone electron pair. An amine is an organic compound that is a derivative of ammonia (NH₃), in which one or more hydrogen atoms are replaced by alkyl or aryl units (R), forming ${\rm{N{-}R}}$ single bonds. If there is one R group, it is a primary amine (one ${\rm{C}{-}{N}}$ bond). There are two R groups (two ${\rm{C}{-}{N}}$ bonds) in a secondary amine, and three R groups in a tertiary amine (three ${\rm{C}{-}{N}}$ bonds). To name an amine, the suffix -amine is added either to the alkyl substituent or to the name of the base compound. If an amine is itself a substituent, the prefix amino- is added to the name of the base compound. If an amine contains different substituents, the largest alkyl or aryl group attached to the nitrogen atom is considered the base chain name, and the other groups are designated as N-substituents instead by their numbers, to indicate they are attached to the nitrogen atom. In cyclic amines, the nitrogen atom is in position 1 on the ring. An amide is an organic compound with a general ${\rm{RC({=}O)NRR^\prime}}$ stoichiometry that contains a carbonyl (${\rm{C}{=}{O}}$) linked to a nitrogen atom. The nitrogen atom can have between zero and two additional carbon-nitrogen (${\rm{C}{-}{N}}$) bonds. It is similar to an amine, but one of the R groups in the ammonia is replaced by an acyl group (${\rm{RC}{=}{O}{-}}$). The nitrogen atoms are not as basic as amines since the lone pair is delocalized into the ${\rm{N}{-}{C}{=}{O}}$ fragment (resonance). Alternatively, an amide is a carboxyl group with a nitrogen molecule replacing the single-bonded oxygen atom. Thus, an amide is considered to be a derivative of carboxylic acid. Like amines, there are primary, secondary, and tertiary amides, depending on how many hydrogen atoms have been replaced by alkyl groups. Primary amides are named by changing the -oic acid suffix of the corresponding carboxylic acid and replacing it with -amide. Secondary and tertiary amides are named by choosing the base chain to be the longest chain containing the nitrogen atom and adding -amide as a suffix to its name. As with amines, the substituents on the nitrogen atom are identified by N- rather than by number. Amines and amides, like alcohols and carboxylic acids, can form hydrogen bonds and, therefore, have higher boiling points than alkanes with the same number of carbon atoms but not as high as analogous alcohols because nitrogen is less electronegative than oxygen. The nitrogen atom induces polarity on both amine and amide molecules, so low molecular weight amines and amides are miscible in water. An important difference between amides and amines, however, is that in aqueous solutions, amines act as a base, but amides are neutral. Amines are basic because water donates a proton to the nitrogen atom to create an ${{-}{\rm{NH}}_{3}}^+$ substituent, which causes an increase in the concentration of OH^– in solution and, thus, an increase of the pH. In an amide, however, the highly electronegative oxygen atom pulls the nitrogen atom's lone pair of electrons toward it, creating a resonance structure that stabilizes the molecule and makes it a poor proton-acceptor. In addition, a protonated ${{-}{\rm{NH}}_{3}}^+$ substituent is repelled by the induced positive dipole on the carbon atom, which also makes it a poor proton-acceptor. Amides are therefore neutral. Amides can be formed by the condensation of a carboxylic acid reacting with an amine.