Justify that alkanes give substitution reactions.


  1. No.12. Justify that alkanes give substitution reactions.

Answer. Alkanes give only substitution reactions. A reaction in which one or more hydrogen atoms of a saturated compound are replaced with some other atoms like halogen is called a substitution reaction. For example in diffused sunlight one hydrogen atom of CH4 is replaced by halogen atom to produce chloromethane.

diffused sunlight

CH4 + C12 ——————> CH3C1 + HC1

  1. No.13. Both, alkenes and alkynes are unsaturated hydrocarbons. State the one most significant difference between them.

Answer. Although both of the compounds are unsaturated, they decolorize pink colour of acidified dilute solution of potassium permanganate. Alkenes (ethene) produce ethyl glycol (1,2-ethenediol) by addition of two hydroxyl groups at the double bond. Such as:

3CH2=CH2   +   2KMn04   +   4H2O ——>   3H2C—CH2 + 2MnO2 + 2KOH


While alkynes (ethene) adds four hydroxyl groups at the triple bonds. Such as:



HC=CH   +     2KMnO4   +   2H2O —————>     H-C—C-H     +   2MnO2   +   2KOH

\                                      \


This intermediate product eliminates water molecules to from glyoxal, which further oxidize to form oxalic acid.

OH OH       2H2O        O O        2[O]              O O

│  │                      ║ ║        KMnO4            ║ ║

H-C—C-H                 H-C-C-H                     HO-C-C-OH

│  │                                                   oxalic acid


  1. No.14. Write the molecular, dot and cross and structural formula of ethyne.


Name Molecular


Condensed formula Structural formula Cross and dot formula


C2H2 HC ≡ CH H―C≡C―H H..C⁞⁞C..H


  1. No.15. Why hydrocarbons are soluble in organic solvents?

Answer. Hydrocarbons are soluble in organic solvents according to like dissolve  like”  principle. Hydrocarbons are non-polar and are soluble in organic solvents which are also non-polar.


  1. No.16. Give the physical properties of alkanes. Answer.
  2. Alkanes form a homologous series of compounds. First four members of the series are gases. The alkanes consisting of C5 to C10 are liquids while higher members of the series are solids.
  3. They are non polar, therefore, they are insoluble in water but soluble in organic solvents.
  4. The density of alkanes increases gradually with the increase of molecular size.
  5. The melting and boiling points of alkanes increase regularly with the increase of molecular sizes. This is because of increase of attractive forces between the molecules of alkanes.
  6. Viscosity of alkanes increases with the increase of molecular size.
  7. Alkanes are less flammable, i.e., large size molecules produces difficulty in burning.
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