2.2: Functional group
- Recognize the two basic mechanisms of bond breaking and making and draw curly arrows to show the electron movement in these steps.
- 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., sp 3 -hybridized halogen, \(\ce{O}\), \(\ce{N}\), \(\ce{S}\), \(\ce{P}\), etc., listed in Table 2; a double bonded, i.e., sp 2 -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 sp 3 -, sp 2 -, and sp-hybridized heteroatoms in functional groups. (R-, R 1 -, R 2 , R 3 , and R 4 - 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- | - |