2.2: Functional group
- Page ID
- 405993
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- Define organic functional group and recognize it in an organic compound.
What is a chemical reaction?
A chemical reaction involves making and breaking chemical bonds. Often the elemental composition of the compound changes during a chemical reaction. Sometimes the composition remains the same; atoms rearrange, resulting in a new combination that is an isomer of the initial compound. Often making and breaking of a covalent bond is involved in organic reactions. Recall that a covalent bond is a shared pair of electrons. The bond breaking or making can happen in one of two modes: homolytic or heterolytic, as described below.
Homolytic breaking and bond-making
In homolytic bond breaking, each bonded atom receives one of the two electrons in a covalent bond, e.g.:
, where half-headed arrows show the movement of a single electron. The above reaction produces two atoms with one unpaired electron, i.e., free radicals. The reverse of it is hemolytic bond making, e.g.:
Heterolytic bond-making and breaking
In heterolytic bond breaking, the bonding electrons move to one of the bonded atoms, usually to a more electronegative atom, making a lone pair, e.g.:
, where the regular double-headed arrows show the movement of an electron pair. The formal charge changes in heterolytic bond-making and breaking. The bonded atom that receives the electrons usually becomes an anion, and the other becomes a cation in the above reaction. Revers of it is heterolytic bond making, e.g.:
What is a functional group?
A functional group is an atom or a group of atoms that imparts a characteristic set of physical and chemical properties to the compound.
For example, alkanes with no functional group are the least reactive classes of organic compounds. Alkanes have only \(\ce{C-C}\) and \(\ce{C-H}\) single bonds, which are nonpolar and strong bonds. Alkenes, alkynes, and aromatic compounds are relatively reactive classes of organic compounds because they have a \(\ce{C=C}\) bond, \(\ce{C≡C}\) bond, or a benzene ring as functional groups. Although alkenes, alkynes, and aromatic hydrocarbons are nonpolar, there is a partial negative (\(\delta^{-}\)) character in the \(\pi\)-bond region that attracts electrophilic reagents having \(\delta^{+}\) regions. Further, the \(\pi\)-bond is weaker than a \(\sigma\)-bond making it easier to break during chemical reactions.
Classification of organic compounds based on the functional groups
One primary class of organic compounds is hydrocarbons that contain only \(\ce{C's}\) and \(\ce{H's}\), listed in Table 1, and the others are organic compounds with heteroatom/s, in their functional group/s. Hydrocarbons have been introduced in the previous section. Other classes of organic compounds containing heteratom\s in their functional groups are described next.
The heteroatom/s in the functional group can be a single bonded, i.e., sp3-hybridized halogen, \(\ce{O}\), \(\ce{N}\), \(\ce{S}\), \(\ce{P}\), etc., listed in Table 2; a double bonded, i.e., sp2-hybridized \(\ce{O}\), \(\ce{N}\), \(\ce{S}\), \(\ce{P}\), etc., listed in Table 3; or a combination of these, listed in Table 4.
| Class name | Group name | General structural formula | General condensed formula | Prefix | Suffix | Example |
|---|---|---|---|---|---|---|
| Alkane | Alkyl | R | R | alkyl- | -ane |  |
| Alkene | Alkenyl |  |
\(\ce{R2C=CR2}\) | alkenyl- | -ene |  |
| Alkyne | Alkynyl |  |
\(\ce{RC≡CR2}\) | alkynyl- | -yne |  |
| Aromatic (benzene derivatives) | Phenyl |  |
\(\ce{R-C6H5}\) | phenyl- | -benzene |  |
| Class name | Group name | General structural formula | General condensed formula | Prefix | Suffix | Example |
|---|---|---|---|---|---|---|
| Haloalkane | Halo | \(\ce{R-X}\), i.e., \(\ce{R-F}\), \(\ce{R-Cl}\), \(\ce{R-Br}\), or \(\ce{R-I}\) | \(\ce{R-X}\) i.e., \(\ce{R-F}\), \(\ce{R-Cl}\), \(\ce{R-Br}\), or \(\ce{R-I}\) | halo- i.e., fluoro-, chloro-, bromo-, or iodo-) | - | \(\ce{CH3-CH2-Cl}\) (chloroethane) |
| Alcohol | Alcohol | \(\ce{R-OH}\) | \(\ce{ROH}\) | hydroxy- | -ol | \(\ce{CH3-CH2-OH}\) (ethanol) |
| Phenol | Phenol |  |
\(\ce{C6H5OH}\) | - | -phenol |  |
| Ether | Ether | \(\ce{R-O-R'}\) | \(\ce{ROR'}\) | Alkoxy | - | \(\ce{CH3-CH2-O-CH2-CH3}\) (ethoxyethane) |
| Epoxide | Epoxide |  |
 |
epoxy- | -oxirane |  |
| Peroxide | Peroxy | \(\ce{R-O-O-R'}\) | \(\ce{ROOR'}\) | peroxy- | alkyl peroxide |  |
| Thiol | Sulfhydryl | \(\ce{R-S-H}\) | \(\ce{RSH}\) | sulfanyl- | -thiol |  |
| Sulfide (Thioether) | Sulfide | \(\ce{R-S-R'}\) | \(\ce{RSR'}\) | sulfanyl- | -sulfide |  |
| Disulfide | Disulfide | \(\ce{R-S-S-R'}\) | \(\ce{RSSR'}\) | disulfanyl- | -disulfide |  |
| Amine | Amine |  |
\(\ce{R1R2R3N}\) | amino- | -amine |  |
| Aniline | Aniline |  |
\(\ce{C6H5NH2}\) | - | -aniline |  |
| Class name | Group name | General structural formula | General condensed formula | Prefix | Suffix | Example |
|---|---|---|---|---|---|---|
| Aldehyde | Aldehyde |  |
\(\ce{RCHO}\) | oxo- | -al |  |
| Benzaldehyde | Benzaldehyde |  |
\(\ce{C6H5COH}\) | - | -benzaldehyde |  |
| Ketone | Ketone |  |
\(\ce{RCOR'}\) | oxo- | -one |  |
| Imine | Imine |  |
\(\ce{R1C(=NR3)R2}\) | imino- | -imine |  |
Table 4: Classes of organic compounds containing a mix of sp3-, sp2-, and sp-hybridized heteroatoms in functional groups. (R-, R1-, R2, R3, and R4- represents a general alkyl group or hydrogen)
| Class name | Group name | General structural formula | General condensed formula | Prefix | Suffix | Example |
|---|---|---|---|---|---|---|
| Carboxylic acid | Carboxyl |  |
\(\ce{R-COOH}\) | carboxy- | oic acid |  |
| Acyl halide | Acyl halide |  |
\(\ce{R-COX}\) | - | oil halide |  |
| Acid anhydride | Acid anhydride |  |
\(\ce{R1-(CO)O(CO)R2}\) | - | R1-oil R2-oat, or R1-oic R2-oic anhydride |  |
| Easter | Easter |  |
\(\ce{R1COORR'}\) | - | alkyl -alkanoate |  |
| Amide | Amide |  |
\(\ce{RCONR'R"}\) | alkyl carbamoyl | -amide |  |
| Carboxylate | Carboxylate |  |
\(\ce{RCOO^{-}}\) | -carboxy | -oate |  |
| Nitrile | Nitrile | \(\ce{R-C≡N}\) | \(\ce{RCN}\) | cyano- | -nitrile |  |
| Phosphate | Phosphate | \(\ce{ROP(=O)(OH)2}\) |  |
- | phosphate |  |
| Nitro | Nitro | \(\ce{RNO2}\) |  |
nitro- | - |  |