18. Consider the following reaction. Which statement best describes the species

Question

18. Consider the following reaction. Which statement best describes the species involved? HBr 0 4 A) 0 is the nucleophile and the 1,4-product is not shown B) O is the nucleophile and e is the 1.4-product C) O is the nucleophile and O is the 1,4-product is the nucleophile and O is the 1,4-product is the nucleophile and O is the 1,4-product _p 19. Which of the following conjugated dienes represent two conformations? A) Only 1 and I1 B) Only I and I11 Only II and III ) None of the choices 20. When does conjugation occur? A) whenever hybrid sp-orbitals can overlap on three or more adjacent atoms B) whenever atomic p-orbitals can overlap on two adjacent atoms whenever atomic p-orbitals can overlap on three or more adj whenever hybrid sp'-orbitals can overlap on two adjacent atoms whenever atomic s-orbitals can overlap on four or more adjacent atoms acent atoms 21. Which of the following statements correctly describes the effect of electron density upon a nucleus in the presence of a magnetic field, Bo A) Electron density decreases the magnetic field strength experienced by a nucleus. x B) A nucleus with high electron density requires less energy to spin flip, y Electrons are said to shield a nucleus from B0- D) The more shielded a nucleus, the more upfield its signal. E) All of the above statements are true

Explanation / Answer

18) B

In halogenation of alkenes, pi bond of alkene act as nucleophile to form carbocation intermediate and also in conjugated dienes 1,4-addition products are also formed.

19) C

20) C

In conjugated compounds pi bonds are not only restricted to two adjacent atoms rather the pi electrons are delocalised over whole system.

21) E

All of the statements are true. Electrons do shield the nucleus and reduce the effect of external magnetic field to nucleus. And as the electron density increases, more is the shielding effect expereinced on the nucleus and hence chemical shifts shifts to upfield region.

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