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2.2: Functional group

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    Learning Objectives
    • 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., 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.

    Table 1: Classes of hydrocarbons based on their 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
    Alkane Alkyl R R alkyl- -ane clipboard_e32b1f3e01d95eb033db35d1cd57c4110.png
    Alkene Alkenyl clipboard_e6da5b33775d36ad775074efabad1ce5e.png \(\ce{R2C=CR2}\) alkenyl- -ene clipboard_e4276d5febdfb880d024c2b6b714bb7df.png
    Alkyne Alkynyl clipboard_ea3831a752cf209b5c46589e57fbe29eb.png \(\ce{RC≡CR2}\) alkynyl- -yne clipboard_e4fad3ad3830e8bd4a95df50a5cb57079.png
    Aromatic (benzene derivatives) Phenyl clipboard_ec768d43d0fbeabc462ebe2bda38b6e3f.png \(\ce{R-C6H5}\) phenyl- -benzene clipboard_ef4149332a22f6ce8d3a9d8c5d7f2347a.png
    Table 2: Classes of organic compounds containing an sp3-hybridized heteroatom in functional groups. (R-, R1-, R2, R3, and R4- represent a general alkyl group or hydrogen.)
    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 clipboard_e39bd658979f9a3f25ab881354e46c0a1.png \(\ce{C6H5OH}\) - -phenol clipboard_ed9a5e03aa28c802079f0dbf5f3cf26fd.png
    Ether Ether \(\ce{R-O-R'}\) \(\ce{ROR'}\) Alkoxy - \(\ce{CH3-CH2-O-CH2-CH3}\) (ethoxyethane)
    Epoxide Epoxide clipboard_e0d0b05eddc09802880631877f70ff91a.png clipboard_ed67a22f212a40639b638e09bc39eba0f.png epoxy- -oxirane clipboard_eb7c93e3830fada02684d732f7653619c.png
    Peroxide Peroxy \(\ce{R-O-O-R'}\) \(\ce{ROOR'}\) peroxy- alkyl peroxide clipboard_e35b8a4a2c3175bcfbeb2ed215ea8a501.png
    Thiol Sulfhydryl \(\ce{R-S-H}\) \(\ce{RSH}\) sulfanyl- -thiol clipboard_e4390ef0e4dca0f9e08d7ebe263452582.png
    Sulfide (Thioether) Sulfide \(\ce{R-S-R'}\) \(\ce{RSR'}\) sulfanyl- -sulfide clipboard_e8d7a80e422d715a4010635c139b316db.png
    Disulfide Disulfide \(\ce{R-S-S-R'}\) \(\ce{RSSR'}\) disulfanyl- -disulfide clipboard_ebc68e66dae1acfff2ac26f88076ba835.png
    Amine Amine clipboard_e4f862ed62da2d8f94094cd9c58741218.png \(\ce{R1R2R3N}\) amino- -amine clipboard_e2eb24c1f57f8f621484668a6f05a2f7c.png
    Aniline Aniline clipboard_e1475833e4dbf57f69041a9fcd6b3628d.png \(\ce{C6H5NH2}\) - -aniline clipboard_e834aa22e788d82dd4bb3e98f817b54bc.png
    Table 3: Classes of organic compounds containing an sp2-hybridized heteroatom 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
    Aldehyde Aldehyde clipboard_e0cf270ba48d9db1f93d6a4c40532a554.png \(\ce{RCHO}\) oxo- -al clipboard_ed8638516b637348798a79d0b31dffd66.png
    Benzaldehyde Benzaldehyde clipboard_ea73bb7f1e9991d2b871f207cbc6e840a.png \(\ce{C6H5COH}\) - -benzaldehyde clipboard_e09c1c49394a91e45f4d3919feaf3f48b.png
    Ketone Ketone clipboard_e20fd24f94b9dd0186acf82365e9c0cbc.png \(\ce{RCOR'}\) oxo- -one clipboard_ef6563f776f19d93af918a4ba18ecf554.png
    Imine Imine clipboard_e29b5416fcffe85e1de75dec161d19e0a.png \(\ce{R1C(=NR3)R2}\) imino- -imine clipboard_eacbdaf9cc1d34b3895b98fb48fb31646.png

    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 clipboard_edc35c3f02f14964c205aa1672407690e.png \(\ce{R-COOH}\) carboxy- oic acid clipboard_e8455ee13e52aa59264dd361ba772ceba.png
    Acyl halide Acyl halide clipboard_ea633f9b27046683b24d9bf897eebaf2a.png \(\ce{R-COX}\) - oil halide clipboard_e9d06ba4935e9258f34063a9774064d59.png
    Acid anhydride Acid anhydride clipboard_e33743b8ae5e4dadc4bff91945d87204b.png \(\ce{R1-(CO)O(CO)R2}\) - R1-oil R2-oat, or R1-oic R2-oic anhydride clipboard_e0a81ec950c5b89af95deb3828549aa61.png
    Easter Easter clipboard_ecc1e0e43d0aacb5e504bb559db8fc8e9.png \(\ce{R1COORR'}\) - alkyl -alkanoate clipboard_e8885053e837c95dc79b70a95e6794311.png
    Amide Amide clipboard_e3357420d518de658da6234b62a0722d6.png \(\ce{RCONR'R"}\) alkyl carbamoyl -amide clipboard_e63a10ca3fdabcef837e7da836c50b04c.png
    Carboxylate Carboxylate clipboard_ec5ca978b3e58b4c6da48c6d18ee7a84b.png \(\ce{RCOO^{-}}\) -carboxy -oate clipboard_ef79f58845241c44efc16fa864780acfe.png
    Nitrile Nitrile \(\ce{R-C≡N}\) \(\ce{RCN}\) cyano- -nitrile clipboard_ee6cfbe44dd87c9b25c5e4cd9450a16d2.png
    Phosphate Phosphate \(\ce{ROP(=O)(OH)2}\) clipboard_ea7f128ddd68904a78d2aa4ae0ed622bf.png - phosphate clipboard_e20994ad99b209a3c6d3a15e989356d3d.png
    Nitro Nitro \(\ce{RNO2}\) clipboard_e07aaa4ff5c7ef92be5c2e7cacb67d73b.png nitro- - clipboard_e9663cc8d8ed09336efc5f856658d593b.png

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