74) Mesodibromobutane on debromination gives:

75) Octane number can be changed by:

76) A fuel has the same knocking property as a mixture of 70% isooctane (2,2,4−trimethylpentane) and 30% n−heptane by volume. The octane number of the fuel is:

77) To a mixture of conc. HNO₃ and conc. H₂SO₄, benzene was added. This mixture was heated for 30 minutes at 100^oC. The main product is:

78) An unknown compound A has the formula C₄H₆. When A is treated with excess of Br₂, a new compound B is formed with formula C₄H₆Br₄. A also gives white ppt. with Tollen′s reagent. The compound A may be: