This paper consists of FOUR (4) questions. Answer ALL questions in the answer sh

Question

This paper consists of FOUR (4) questions. Answer ALL questions in the answer sheets provided. (a) Name the following compounds according to the IUPAC nomenclature: (i) Arrange the following compounds in order of increasing in boiling points. (ii) Explain your answer in (i). Explain why butane is soluble in benzene but insoluble in water. Write a balanced chemical equation for the combustion of propane. A mixture of propane and bromine is exposed to sunlight. the reaction equation is given below. CH_3CH_2CH_3 + Br_2 rightarrow A + B (i) What is the function of sunlight in the reaction? (ii) Draw the structure of the products, A and B. Identify the major product.

Explanation / Answer

a)     IUPAC names of the given compounds are:
i) 3,6-dimethyl-4-(isopropyl)-octane

ii) 5-ethyl-1,2,3-trimethyl-cyclohexane

iii) 1-bromo-3-ethyl-1-methyl-cyclopentane

b)   i) Isobutene < n-butane < Neo-pentane < n-pentane (structures are given in the question, so I just mentioning their names.)

ii) The Boiling points of alkanes increase with increase in the molecular weights in the series. Consider isomers of alkanes; straight chain compounds have higher boiling points than branched compounds. For branched compounds surface area decreases such that inter molecular forces of attraction (weak van der Waal forces) decrease and so boiling points also decrease. That means greater the branching lesser the boiling point.

c) Alkanes are insoluble in water. As alkanes are non polar molecules they dissolve in non polar solvents only. Alkanes are soluble on non-polar solvents like CCl4 and benzene. They obey general principle “like dissolves like”. Therefore butane is soluble in benzene but not in water.

d) Balanced equation for the combustion of propane is:   

Propane ( C3H8 ) will burn completely when it combines with the oxygen (O2) in air to form carbon dioxide (CO2) and water (H2O). The equation looks like this:

C3H8 + 5O2 3CO2 + 4H2O

e) i) Sunlight acts as the radical initiator for the bromine.

ii) CH3-CH2-CH3+ Br2(UV) CH3-CH2-CH2-Br + HBr

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