- Homologous series
chemistry, a homologous series is a series of organic compounds with a similar general formula, possessing similar chemical properties due to the presence of the same functional group, and shows a gradation in physical properties as a result of increase in molecular size and mass (see relative molecular mass). For example, ethane has a higher boiling point than methane since it has more Van der Waals forces(intermolecular forces) with neighbouring molecules. This is due to the increase in the number of atoms making up the molecule. Organic compounds in the same homologous series vary by a CH2. Alkanes ( paraffins), alkenes (olefins), methoxyethane( ethers), and alkynes ( acetylenes) form such series in which members differ in mass by 14 atomic mass units. For example, the alkane homologous series begins with methane(CH4), ethane(C2H6), propane(C3H8), butane(C4H10), and pentane(C5H12), each member differing from the previous one by a CH2 group (or 14 atomic massunits).
Similarly, there is the
alcoholhomologous series that starts with methanol(CH4O), ethanol(C2H6O), as primary alcohols, isopropanol(C3H8O) as a simple secondary alcohol, and a simple tertiary alcohol is tert- butanol(C4H10O).
Even while the "general formula" are the same, they have different structures that can lead the exact same compound to different properties, although they will always present the same chemical properties while as a homologous compound.
Compounds in each set have the same little group of atoms called the
functional group. Most chemical properties of organic compoundsare due to the presence of the functional group. Homologous series General formula Example Functional group Straight Chain Alkanes C"n"H2"n" + 2 ("n" ≥ 1) CH4, "n" = 1 Alkyl C"n"H2"n" + 1 ("n" ≥ 1) CH3, "n" = 1 Alkenesand Cyclic Alkanes C"n"H2"n" ("n" ≥ 2) C2H4, "n" = 2 C = C Alkynes C"n"H2"n" − 2 (n ≥ 2) C2H2, "n" = 2 C ≡ C Alcohols C"n"H2"n" + 1OH ("n" ≥ 1) CH4O, "n" = 1 - OH Carboxylic acids C"n"H2"n"O2 ("n" ≥ 1) CH2O2, "n" = 1 - COOH Carbohydrates C"n"(H"2"O)"n" ("n" ≥ 1) C6H12O6
Where "n" represents the number of carbon atoms present.
Homologation reactionis any chemical processconverting one member of a homologues series to the next.
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