Science
(a) Define a homologous series of carbon compounds.
(b) Why is the melting and boiling points of C4H8 higher than that of C3H6 or C2H4?
(c) Why do we NOT see any gradation in chemical properties of a homologous series compounds?
(d) Write the name and structures of (i) aldehyde and (ii) ketone with molecular form C3H6O.
Organic Chemistry
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Answer
(a) A homologous series is a group of organic compounds having the same functional group, similar chemical properties and a constant difference of CH2 (14 atomic mass units) between successive members. Members show a gradual change in physical properties (like boiling point) but have similar chemical behaviour because of the same functional group.
(b) As carbon chain length increases (C2 → C3 → C4), the molecular mass and surface area increase. Larger molecules have stronger van der Waals (dispersion) forces between molecules, so more heat is needed to separate them. Therefore C4H8 (having a larger size) has higher melting/boiling points than C3H6 or C2H4.
(c) Chemical properties are mainly determined by the functional group, which is the same for all members of a homologous series. Because the reactive part is identical, they undergo similar chemical reactions (no gradual change) but Physical properties (boiling point, melting point) show gradual change due to molecular size.
(d)
- Propanal (also called propionaldehyde)
Structural formula : CH3—CH2—CHO
- Propanone (commonly called acetone)
Structural formula : CH3—CO—CH3
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