Chemistry · Ch 14 — Basic Principles of Organic Chemistry
Classification based on functional group
Classification based on functional group
Classification by functional group instead looks at which specific, reactive group of atoms a molecule carries -- and that functional group is defined precisely as the part of an organic molecule that actually undergoes chemical change during a reaction. This definition is illustrated by propanol reacting with sodium metal: CH3-CH2-CH2-OH + Na, comparing the substrate (propanol) with its product (sodium propoxide, CH3-CH2-CH2-ONa) shows that only the -OH part of the molecule was transformed (into -ONa), while the rest of the carbon skeleton was left completely unchanged -- so -OH is identified as propanol's functional group. Sorting organic compounds by which functional group they carry groups them into 'families', each family named after its own defining functional group (the family of alcohols, the family of halogen derivatives, and so on); a large table of the most common functional-group families, their bonding patterns, and one worked example compound each, is given as Table 14.3. Within any one such family, chemists further recognise a HOMOLOGOUS SERIES: a run of compounds that all share the same TYPE of carbon skeleton and the same functional group, where each successive member differs from the next purely by one extra -CH2- (methylene) unit in both its molecular formula and its structural formula. Any single compound belonging to such a series is called a homologue, and every homologue of one series can be described by a single shared general molecular formula (for instance for the alkanes, or for the straight-chain aldehydes shown worked out in Table 14.4). Homologues of the same series behave very similarly to one another CHEMICALLY (since they all share the same functional group) while their PHYSICAL properties -- melting point, boiling point, density, solubility -- change only gradually and predictably as the chain lengthens, exactly as Table 14.4's steadily rising boiling points across the aldehyde series illustrate. As a small piece of etym …
Table 14.3 lists 23 functional-group families with their group structure and one worked example each: (1) Halide, -X, e.g. CH3Br methyl bromide. (2) Cyanide/Nitrile, -C≡N, e.g. CH3CN methyl cyanide (acetonitrile). (3) Isocyanide, -N⊕≡C, e.g. CH3NC methyl isocyanide. (4) Nitro compound, -NO2, e.g. CH3NO2 nitromethane. (5) Alcohol, -OH, e.g. CH3OH methyl alcohol. (6) Phenol, Ar-OH, e.g. C6H5OH phenol. (7) Primary amine, -NH2, e.g. CH3NH2 methyl amine. (8) Secondary amine, -NH-, e.g. CH3-NH-CH3 dimethyl amine. (9) Tertiary amine, -N<, e.g. (CH3)3N trimethyl amine. (10) Ether, -C-O-C-, e.g. CH3-O-CH3 dimethyl ether. (11) Aldehyde, -CHO, e.g. CH3CHO acetaldehyde. (12) Ketone, >C=O, e.g. CH3-CO-CH3 acetone. (13) Carboxylic acid, -COOH, e.g. CH3COOH acetic acid. (14) Ester, -COO-, e.g. CH3COOC2H5 ethyl acetate. (15) Amide, -CONH2, e.g. CH3CONH2 acetamide. (16) Secondary amide, -CO-NH-, e.g. CH3-CO-NH-CH3 N-methyl acetamide. (17) Tertiary amide, -CO-N<, e.g. CH3-CO-N(CH3)2 N,N-dimethyl acetamide. (18) Acid anhydride, -CO-O-CO-, e.g. CH3-CO-O-CO-CH3 acetic anhydride. (19) Acyl chloride, -COCl, e.g. CH3COCl acetyl chloride. (20) Sul …
Table 14.4 shows five successive members of the CnH2nO straight-chain aldehyde homologous series, each differing from the next by one -CH2- unit: Formaldehyde (n=1, CH2O, H-CHO, boiling point -21°C), Acetaldehyde (n=2, C2H4O, CH3-CHO, b.p. 21°C), Propionaldehyde (n=3, C3H6O, CH3-CH2-CHO, b.p. 48°C), Butyraldehyde (n=4, C4H8O, CH3-(CH2)2-CHO, b.p. 75°C), Valeraldehyde (n=5, C5H10O, CH3-(CH2)3-CHO, b.p. 103°C). The steadily rising boiling point with each added -CH2- unit is the table …
Worked out. Worked example generating the first five homologues of the straight-chain saturated alkane series by adding one -CH2- unit at a time, starting from CH4: CH4 (1st) -> CH3-CH3 (2nd) -> CH3-CH2-CH3 (3rd) -> CH3-CH2-CH2-CH3 (4th) -> CH3-CH2-CH2-CH2-CH3 (5th). Counting carbon and hydrogen atoms in these five gives the molecular formulae CH4, C2H6, C3H8, C4H10 and C5H12, from which the general formula CnH2n+2 is deduced for the alkane homologous series. …
Worked out. Worked example starting from CH3-CH2-COOH (propanoic acid). (a) The third higher homologue is obtained by adding three -CH2- units to the chain, giving CH3-CH2-CH2-CH2-CH2-COOH (hexanoic acid). (b) The second lower homologue is obtained by removing two -CH2- units from the chain, giving H-COOH (formic acid, i.e. methanoic acid) -- illustrating that homologues can be generated in either direction, up or down the series, by adding or remo …