write the molecular formulae of two homologous of hexane.
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The homologous series of straight-chained alkanes begins methane (CH4), ethane (C2H6), propane (C3H8), butane (C4H10), and pentane (C5H12). In that series, successive members differ in mass by an extra methylene bridge (-CH2- unit) inserted in the chain. Thus the molecular mass of each member differs by 14 atomic mass units. Adjacent members in such a series, such as methane and ethane, are known as "adjacent homologues."[4]
Within that series, many physical properties such as boiling point gradually increase with increasing mass. For example, ethane (C2H6), has a higher boiling point than methane (CH4). This is because an ethane molecule experiences greater dipole moments, as in a large molecule, the electron cloud tends to distort at random to a greater extent. Thus, the London dispersion forces between ethane molecules are higher than that between methane molecules, resulting in stronger forces of intermolecular attraction, raising the boiling point.
Some important classes of organic molecules are derivatives of alkanes, such as the primary alcohols, aldehydes, and (mono)carboxylic acids form analogous series to the alkanes. The corresponding homologous series of primary straight-chained alcohols comprises methanol (CH4O), ethanol (C2H6O), 1-propanol (C3H8O), 1-butanol, and so on. The single-ring cycloalkanes form another such series, starting with cyclopropane.
Homologous series
General formula
Repeating unit
Functional group(s)
Straight-chain alkanes
CnH2n + 2 (n ≥ 1)
−CH2−
H3C− ... −CH3
Straight-chain perfluoroalkanes
CnF2n + 2 (n ≥ 1)
−CF2−
F3C− ... −CF3
Straight-chain alkyl
CnH2n + 1 (n ≥ 1)
−CH2−
H3C− ... −CH2−
Straight-chain 1-alkenes
CnH2n (n ≥ 2)
−CH2−
H2C=C− ... −CH3
Cycloalkanes
CnH2n (n ≥ 2)
−CH2−
Singly-bonded ring
Straight-chain 1-alkynes
CnH2n − 2 (n ≥ 2)
−CH2−
HC≡C− ... −CH3
polyacetylenes
C2nH2n + 2 (n ≥ 2)
−CH=CH−
H3C− ... −CH3
Straight-chain primary alcohols
CnH2n + 1OH (n ≥ 1)
−CH2−
H3C− ... −OH
Straight-chain primary monocarboxylic acids
CnH2n + 1COOH (n ≥ 0)
−CH2−
H3C− ... −COOH
Straight-chain azanes
NnHn + 2 (n ≥ 1)
−NH−
H2N− ... −NH2
Within that series, many physical properties such as boiling point gradually increase with increasing mass. For example, ethane (C2H6), has a higher boiling point than methane (CH4). This is because an ethane molecule experiences greater dipole moments, as in a large molecule, the electron cloud tends to distort at random to a greater extent. Thus, the London dispersion forces between ethane molecules are higher than that between methane molecules, resulting in stronger forces of intermolecular attraction, raising the boiling point.
Some important classes of organic molecules are derivatives of alkanes, such as the primary alcohols, aldehydes, and (mono)carboxylic acids form analogous series to the alkanes. The corresponding homologous series of primary straight-chained alcohols comprises methanol (CH4O), ethanol (C2H6O), 1-propanol (C3H8O), 1-butanol, and so on. The single-ring cycloalkanes form another such series, starting with cyclopropane.
Homologous series
General formula
Repeating unit
Functional group(s)
Straight-chain alkanes
CnH2n + 2 (n ≥ 1)
−CH2−
H3C− ... −CH3
Straight-chain perfluoroalkanes
CnF2n + 2 (n ≥ 1)
−CF2−
F3C− ... −CF3
Straight-chain alkyl
CnH2n + 1 (n ≥ 1)
−CH2−
H3C− ... −CH2−
Straight-chain 1-alkenes
CnH2n (n ≥ 2)
−CH2−
H2C=C− ... −CH3
Cycloalkanes
CnH2n (n ≥ 2)
−CH2−
Singly-bonded ring
Straight-chain 1-alkynes
CnH2n − 2 (n ≥ 2)
−CH2−
HC≡C− ... −CH3
polyacetylenes
C2nH2n + 2 (n ≥ 2)
−CH=CH−
H3C− ... −CH3
Straight-chain primary alcohols
CnH2n + 1OH (n ≥ 1)
−CH2−
H3C− ... −OH
Straight-chain primary monocarboxylic acids
CnH2n + 1COOH (n ≥ 0)
−CH2−
H3C− ... −COOH
Straight-chain azanes
NnHn + 2 (n ≥ 1)
−NH−
H2N− ... −NH2
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