Q: Which reagent and conditions would selectively brominate the allylic position of cyclohexene?

$$\mathrm{NBS}$$ with $$h\nu or$$ $$\mathrm{ROOR}$$

Question 1

Which of the following statements best describes the difference between homolytic and heterolytic bond cleavage?

Accepted Answer

Homolytic cleavage splits the bond so each atom gets one electron, forming radicals, while heterolytic cleavage gives both electrons to one atom, forming ions.

Question 2

What is the hybridization and geometry of a carbon-centered radical?

Accepted Answer

sp2-hybridized, trigonal planar

Question 3

Which of the following radicals is the most stable?

Accepted Answer

Tertiary alkyl radical ($$3^\circ)$$

Question 4

Why is the allylic radical more stable than a tertiary alkyl radical?

Accepted Answer

Because of resonance stabilization

Question 5

Which reagent and conditions would selectively brominate the allylic position of cyclohexene?

Accepted Answer

$$\mathrm{NBS}$$ with $$h
u or$$ $$\mathrm{ROOR}$$

Question 6

During the radical halogenation of propane with $$\mathrm{Cl_2$}$$ and light, why is more 2-chloropropane formed than 1-chloropropane, even though there are more primary hydrogens?

Accepted Answer

Because secondary C–H bonds are weaker and form more stable radicals

Question 7

Which of the following best explains why bromination is more selective than chlorination in radical halogenation?

Accepted Answer

Bromination is endothermic and the transition state resembles the products, favoring the more stable radical

Question 8

What is the role of light ($$h
u) or $$heat ($$\Delta) in $$the radical halogenation of alkanes?

Accepted Answer

To initiate the reaction by causing homolytic cleavage of the halogen molecule

Question 9

Which of the following is NOT a propagation step in the radical halogenation of alkanes?

Accepted Answer

Two radicals combine to form a stable bond

Question 10

In the radical bromination of 2-methylpropane, which product will be the major one?

Accepted Answer

2-bromo-2-methylpropane

Question 11

Which of the following statements about the stereochemistry of radical halogenation is correct?

Accepted Answer

If the reaction occurs at a stereocenter, racemization occurs

Question 12

Which of the following best describes the product distribution in the radical chlorination of propane?

Accepted Answer

More 2-chloropropane is formed due to the greater stability of the secondary radical

Question 13

Which of the following is a correct statement about the termination step in a radical chain reaction?

Accepted Answer

It involves the combination of two radicals to form a stable molecule

Question 14

Why does the use of NBS in the presence of light or ROOR selectively brominate the allylic position of alkenes?

Accepted Answer

Because NBS generates a low concentration of Br$$_2$$ and Br$$\cdot$$ radicals, favoring allylic substitution

Question 15

Which of the following best explains why the allylic C–H bond is weaker than a tertiary C–H bond?

Accepted Answer

The allylic radical is stabilized by resonance

Question 16

In the radical halogenation of butane, what is the stereochemical outcome if the reaction occurs at a prochiral center?

Accepted Answer

A racemic mixture of enantiomers is formed

Question 17

Which of the following is true about the use of Br$$_2$$ versus NBS in the bromination of alkenes?

Accepted Answer

NBS leads to allylic bromination via a radical mechanism, while Br$$_2$$ leads to vicinal dibromide via an ionic mechanism

Radical Reactions in Organic Chemistry: Mechanisms, Stability, and Selectivity

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