# Characteristics of Functional Groups

### Identifying Functional Groups

Functional groups are combinations of atoms that have a predictable chemical behavior. Functional groups can be identified by both their structure and their abbreviation, such as COOH for carboxylic acid.

A functional group is a group of atoms with specific physical, chemical, and reactivity properties. Functional groups in organic chemistry are important for many future topics, including their predictable characteristics, nomenclature, reactivity, synthesis, spectroscopy, and more. The main functional groups are often identified by their stoichiometry, which is the relative quantities of atoms in the formula of the group.

There are 19 main organic functional groups:

• An alkane is a hydrocarbon containing only ${\rm {C{-}C}}$ single bonds and hydrogen with CnH2n+2 stoichiometry.
• An alkene is a hydrocarbon containing at least one carbon-carbon double bond (${\rm {C{=}C}}$) with CnH2n stoichiometry.
• An alkyne is a hydrocarbon that contains at least one carbon-carbon triple bond (${\rm {C{\equiv}C}}$) with CnH2n-2 stoichiometry.
• An alkyl halide is an organic compound that contains a halogen atom. The halogen atom can be classified as primary, secondary, tertiary, vinylic, or aromatic.
• An alcohol is a hydrocarbon containing a hydroxyl functional group (${\rm {R}{-}{OH}}$).
• An ether is an organic molecule containing an oxygen bound by two alkyl or aryl groups through ${\rm {C{-}O}}$ bonds (${\rm {R}{-}O{-}{R}}$).
• A thiol is an organic compound that is derived from H2S. It contains an alkyl or aryl group covalently linked to a sulfhydryl group, ${\rm {-}{SH}}$, through ${\rm {C}{-}{S}}$ bonds, with ${\rm {R}{-}{SH}}$ stoichiometry.
• A sulfide is an organic molecule containing a sulfur bonded to two alkyl or aryl groups through ${\rm {C}{-}{S}}$ bonds.
• An aromatic is an uninterrupted planar ring of sp2 and/or p orbitals whose electron count adds up to $4n+2$ where $n =$ any positive integer. Benzene, C6H6, is the most common example.
• A ketone is an organic compound that contains a carbonyl group (${\rm {C}{=}{O}}$) bound to two alkyl (${-}{\rm {R}}$) fragments (which can be the same, R and R, or different, R and R′) with ${\rm {RC}({=}O){R}}'$ or RCOR′ stoichiometry.
• An aldehyde is an organic compound that contains a carbonyl group (${\rm {C{=}O}}$) bound to one alkyl (${-}{\rm {R}}$) fragment and one hydrogen atom, with ${\rm {RC}({=}O){H}}$ or ${\rm {R{-}CHO}}$ stoichiometry.
• A carboxylic acid is a compound that contains an alkyl or aryl group (R) attached to a carboxyl group (${-}{\rm{COOH}}$).
• An ester is an organic compound that contains a carboxyl unit in which a hydrogen of the hydroxyl group is replaced by an alkyl or aryl group, giving it ${\rm {R{-}C({=}O){OR}}}'$ or ${\rm {R{-}{COOR}}}'$ stoichiometry.
• An acyl halide is an organic compound that contains a carbonyl group (${\rm {C{=}O}}$) bound to one alkyl (${-}{\rm {R}}$) fragment and one halogen atom, with ${\rm {RC({=}O)X}}$ or ${\rm {R{-}COX}}$ stoichiometry.
• An acid anhydride is an organic compound that contains two carbonyl groups (${\rm {C{=}O}}$) bound to the same oxygen atom with (RCO)2O stoichiometry.
• An amide is an organic compound that contains a carbonyl (${\rm {C{=}O}}$) linked to nitrogen atoms through a ${\rm {C{-}N}}$ bond. It has a general ${\rm{RC({=}O)NR}'R''}$ stoichiometry.
• An amine is an organic compound that is a derivative of ammonia (NH3), where one or more hydrogen atoms are replaced by alkyl or aryl units (R), forming ${\rm {N{-}R}}$ single bonds.
• A nitrile is an organic compound that has a carbon triple bonded to a nitrogen with RCN stoichiometry.
• An epoxide is a compound that contains a cyclic ether made of two carbons and an oxygen.
• An aryl is the term that applies when a group is attached to an aromatic compound; ${\rm {C}_6{H}_5-{Br}}$ is an aryl bromide.

In organic chemistry, R is the abbreviation for any alkyl group. So, R can be ${-}{\rm {CH}}_3$, ${-}{\rm {CH}_2{CH}}_3$, and so on. To indicate there are different alkyl groups, a prime (′) or double prime (″) symbol is used. Alternately, R1 or R2 is used to indicate different alkyl groups. For example, an ether with different alkyl groups may be abbreviated with either ${\rm {R{-}O{-}R}}$ or ${\rm {R}_1{-}O{-}{R}}_2$.

### Organic Functional Groups

Functional Group Name Abbreviation General Structure
Alkane ${\rm {C{-}C}}$
Alkene ${\rm {C{=}C}}$
Alkyne ${\rm {C{\equiv}C}}$
Alkyl halide ${\rm {R{-}X}}$
Alcohol ${\rm {R{-}OH}}$
Ether ${\rm {R{-}O{-}R}}$
Thiol ${\rm {R{-}SH}}$
Sulfide $\rm R{-} S{-}{{R}}$
Aromatic ${\rm {R{-}C}_6{H}}_5$ or ${\rm {R{-}{Ar}^{\ast}}}$
Ketone ${\rm{RCOR}}$
Aldehyde ${\rm {RCHO}}$
Carboxylic acid ${\rm{RCO}_2{H}}$ or ${\rm{RCOOH}}$
Ester ${\rm {RCO}_2 R}$
Acyl halide ${\rm {RCOX}}$
Acid anhydrides $({\rm {RCO)_2 O}}$
Amide ${{\rm{RCONR_1R}}_{2}}^{\ast\ast}$
Amine ${{\rm{{RNR}}_1{R}}_2}^{\ast\ast}$
Nitrile (also called cyano) ${\rm{RCN}}$
Epoxide No abbreviation

Functional groups are identified by name, structure, and abbreviation. ${\rm {R{-}C}_6{H}_5}$and ${\rm {R{-}Ph}}$are abbreviations for benzene. ${\rm {R{-}Ar}}$and ${\rm {R}}-\phi$are abbreviations used for aromatics.

For amides and amines, ${\rm {R}}$, ${\rm{ R}}_1$, and ${\rm{R}}_2=2$, alkyl, or aryl.

### Classification

Alcohols, alkyl halides, amines, and amides are classified as primary (1°), secondary (2°), tertiary (3°), vinyl, aryl, or phenyl.
Alcohols and alkyl halides are classified by how many carbons are attached to the carbon to which they are connected. If that carbon is bonded to one other carbon, the group is primary (1°). If that carbon is bonded to two other carbons, the classification is secondary (2°), and if that carbon is bonded to three other carbons, the classification is tertiary (3°). If the alcohol or alkyl halide is bonded to a double bond, the classification is vinyl, and if it is bonded to an aromatic functional group, the classification is aryl (or phenyl if the aromatic is a benzene).

#### Classification of Alkyl Halides and Alcohols

Amines and amides are classified by the number of carbons attached to the nitrogen. If there are three carbons attached to the nitrogen, the classification is tertiary (3°); if there are two carbons attached to the nitrogen, the classification is secondary (2°); if there is one carbon attached to the nitrogen, the classification is primary (1°). Vinyl and aryl amines are often known by other names and are not usually classified as vinyl or aryl.

#### Classification of Amines and Amides

Functional groups are found in all sorts of compounds, including natural products and medicine. Identifying functional groups in larger, more complex molecules is no different from identifying functional groups in smaller molecules. Functional groups are important to the function of many pain relievers, such as acetylsalicylic acid, acetaminophen, and ibuprofen.

#### Examples of Functional Groups in Medicine

Functional Group Primary (1°) Secondary (2°) Tertiary (3°) Vinyl Aryl Phenyl
Alcohols (ROH) ${\rm {R}={CH}_2{R}}$ ${\rm R={\rm{CHR}}}_2$ $\rm R={CR}_3$ ${\rm {R}}={\text {double bond}}$ ${\rm {R}}={\text{aromatic ring}}$ ${\rm {R}}={\text {benzene}}$
Alkyl halides (RX) ${\rm {R}={\rm{CH}_2{R}}}$ ${\rm R={\rm{CHR}}}_2$ $\rm R={CR}_3$ ${\rm {R}}={\text {double bond}}$ ${\rm {R}}={\text{aromatic ring}}$ ${\rm {R}}={\text {benzene}}$
Amines (RNR1R2) ${\rm {R}}_1={\rm {R}}_2={\rm {H}}$ ${\mathrm R}_1=\mathrm H;\;{\mathrm R}_2=\mathrm{alkyl}$ ${\mathrm R}_1={\mathrm R}_2=\mathrm{alkyl}$ ${\rm {R}}={\text {double bond}}$ ${\rm {R}}={\text{aromatic ring}}$ ${\rm {R}}={\text {benzene}}$
Amides (RCONR1R2) ${\rm {R}}_1={\rm {R}}_2={\rm {H}}$ ${\rm {R}}_1=\rm {H};\;{\rm {R}}_2={\text {alkyl}}$ ${\rm {R}}_1={\rm {R}}_2=\text {alkyl}$ ${\rm {R}}_1\;{\text {or}}\;{\rm {R}}_2=\text {double bond}$ ${\rm {R}}_1\;\text {or}\;{\rm {R}}_2=\text {aromatic ring}$ ${\rm {R}}_1\;\text {or}\;{\rm {R}}_2={\text {benzene}}$