Answer: HBr.
- A HF
- B HCl
- C HI
- D HBr
Correct answer: D. HBr
Explanation: Only HBr has the right bond energies for the free-radical chain; HCl and HF bonds are too strong and HI too weak for the propagation steps to succeed.
Going from a single to a triple bond between carbons increases the s-character of hybridisation (sp³→sp²→sp), shortening the bond and locking the geometry, while increasing π-bond reactivity (alkenes and alkynes readily undergo addition reactions that alkanes don't).
Concept context
The simplest organic compounds made only of carbon and hydrogen. Covers alkanes, alkenes, alkynes, and aromatic compounds like benzene. Learn preparation methods, key reactions, and physical properties.