Chapter 10

Question 1

What is Homolysis?

Answer 1

Homolysis is the separation of a bond into two radicals A:B-> A(radical) + B(radical)

Question 2

What are radicals and what are their properties?

Answer 2

Radicals are species with unpaired e- and are highly reactive, short-lived. They are formed by bond homolysis.

Question 3

How does Homolysis of covalent bonds occur?

Answer 3

Heat (delta) or light (hv)

Ex: R-O-O-R –(delta)—> 2R-O(radical)

Question 4

Is the bond formation between two radicals an endothermic or exothermic process?

Answer 4

Exothermic (energy must be supplied to break covalent bonds)

Question 5

What are the energies required to break covalent bonds homolytically called?

Answer 5

Homolytic bond dissociation energies (DH°)

Question 6

What are the relative stabilities of different radicals? (Allyl, Vinyl, 1°, 2°, 3°)

Answer 6

Allyl radical is the most stable(resonance). 3°, 2°, and 1° are the second, third, and fourth most stable. A vinyl radical is the least stable.

Question 7

What are allyl and vinyl radicals?

Answer 7

Allyl radical is one bond away from a double bond (=-.) , Vinyl is a radical directly next to a double bond(=.)

Question 8

Describe the geometry and hybridization of radicals

Answer 8

They are planar and sp2 hybridized (similar to carbocation)

Question 9

Give an overview of the reaction of RH with X2 with heat or light. Include the effects of concentration and identity of the X2, as well as stereochemistry.

Answer 9

RH + X2 -(heat or light)-> RX + HX

The more X2 you add, the more halogens will be on the final product (Keep replacing H’s)

Adding Br2 gives the most stable product (most substituted alkyl halide)
Adding Cl2 gives all products

Alkyl radicals are planar, so stereoisomers/enantiomers may be created

Question 10

Describe chain initiation

Answer 10

Covalently bonded compound + heat/light or ROOR -> radicals

Question 11

Describe chain propagation

Answer 11

Radical + Non-radical -> New compound + New radical

Question 12

Describe chain termination

Answer 12

Radicals combine in all possible combinations

Question 13

Describe the mechanism of the addition of X2 to an RH, including which steps are initiation, propagation, and termination.

Answer 13

Initiation
Formation of X* via homolytic cleavage

Propagation
X* + RH -> HX + R*
R* + X2 -> RX + X*

Termination
R+X -> RX
R* + R* -> R2
X* + X* -> X2

Question 14

Describe the reaction of alkenes with HBr in the presence of ROOR (a peroxide)

Answer 14

In the presence of ROOR, an anti-Markovnikov reaction will occur

Br attaches to the least substituted carbon, H attaches to the most substituted carbon

Question 15

Describe the mechanism for allylic chlorination, using propene as an example (CH3-CH=CH2 + Cl2)

Answer 15

Initiation: Cl2 -> Cl* and Cl*

Propagation: CH3-CH=CH2 + Cl* -> *CH2-CH=CH2 + HCl

Second Propagation: *CH2-CH=CH2 + Cl2 -> Cl-CH2-CH=CH2

Question 16

Describe the mechanism for allylic bromination with NBS, using propene as an example (CH3-CH=CH2 + NBS)

Answer 16

Initiation: NBS produces a Br*

Propagation: CH3-CH=CH2 + Br* -> *CH2-CH=CH2 + HBr

Propagation: *CH2-CH=CH2 + Br2 -> Br-CH2-CH=CH2

Br2 came from the reaction of HBr with NBS

Question 17

What factors determine the number of possible products in a radical reaction?

Answer 17

When there is a double bond or a benzene, there may be more than one allylic or benzylic position. Each of those may also have resonance.