Alkenes - Structure and Nomenclature - UNSOLVED PRACTICE SET

Q7. Write the IUPAC names of the following alkenes:

(a) CH₂=CH–CH₂–CH₃

(b) CH₃–CH=C(CH₃)–CH₂–CH₃

(c) (CH₃)₂C=CH–CH₃

Q8. Draw the structures of:

(a) cis-But-2-ene

(b) trans-But-2-ene

(c) (E)-3-Methylhex-3-ene

Q9. Why do alkenes have lower boiling points than alkanes of comparable molecular mass? Explain with an example.

Q10. What is the difference between cis-trans nomenclature and E-Z nomenclature? When is E-Z nomenclature necessary?

Q11. Your mother uses a paper bag with ripe bananas to ripen raw mangoes faster. What alkene is responsible for this, and how does it work?

Q12. Why is ethene (C₂H₄) planar, while ethane (C₂H₆) is not? Explain using hybridisation.

Q13. Discuss the structure and bonding in alkenes:

(a) sp² hybridisation and formation of σ and π bonds

(b) The planar geometry around the double bond

(c) Restricted rotation around the C=C bond and its consequences

(d) Bond lengths and bond energies compared to alkanes

Draw orbital overlap diagrams showing the formation of the double bond in ethene.

Q14. Explain geometrical isomerism in alkenes:

(a) Conditions necessary for geometrical isomerism

(b) cis and trans isomers — definition and examples

(c) E-Z nomenclature — Cahn-Ingold-Prelog priority rules

(d) Physical and chemical differences between cis and trans isomers

Illustrate with but-2-ene and 2-butenoic acid.

Q15. Alkenes are fundamental to India's petrochemical industry. Discuss:

(a) How ethene and propene are produced by steam cracking in Indian refineries

(b) The use of ethene in producing polyethylene for packaging materials (milk pouches, shopping bags)

(c) Why polypropylene (from propene) is preferred for making chairs, buckets, and other household items

(d) The environmental challenge of plastic waste from alkene-derived polymers and India's ban on single-use plastics

Q16. An alkene has the molecular formula C₅H₁₀.

(a) Draw all possible structural isomers (including stereoisomers) and name them using IUPAC nomenclature.

(b) Identify which isomers show geometrical isomerism and draw their cis/trans (or E/Z) forms.

(c) Calculate the degree of unsaturation for C₅H₁₀ and verify your answer.

Q17. The heats of hydrogenation for three isomeric butenes are:

cis-But-2-ene: -119.7 kJ/mol

trans-But-2-ene: -115.5 kJ/mol

But-1-ene: -126.8 kJ/mol

(a) Arrange these alkenes in order of increasing stability.

(b) Calculate the energy difference between cis and trans but-2-ene.

(c) Explain why trans-but-2-ene is more stable than cis-but-2-ene using steric strain.

Q18. A polymer plant in Vadodara produces polyethylene from ethene. The plant processes 1000 kg of ethene daily.

(a) Write the polymerisation reaction for the formation of polyethylene.

(b) Calculate the theoretical yield of polyethylene (assuming no mass loss in polymerisation).

(c) If the plant produces high-density polyethylene (HDPE) with an average molecular mass of 200,000 g/mol, calculate how many polymer chains are formed from 1000 kg of ethene.

[Given: Molar mass of ethene = 28 g/mol, Avogadro's number = 6.022 × 10²³]