Question

Model 7: Which H Will Be Replaced by X? • In a radical halogenation reaction a halogen (F, CI, Br or I) replaces an H. • In most cases there is more than one type of H to choose from. For example the hydrocarbon below has two different types of H's: CHE CHE Сн bv axor H3C CH3 X-F. CI. Br or 1 3 H₂C CH₂ X2 сн. CH На THS Three factors determine which I on a hydrocarbon will be replaced: i. The type of H. (benzyl, allyl, 3, 2, 1° or methyl) ii. The number of H's of a given type. iii. The identity of X. (We will discuss this on the next page.) First Propagation Step First Propagation Step (H Abstraction by -X) (H Abstraction by -X) Abstraction of a lº H leads Abstraction of a 3°H X: to a higher H gives a lower V.E. V.E. radical H H radical intermediate. HH intermediate. But there is only ONE 3° H to choose from.. But there are NINE 1° H's to choose from! Critical Thinking Questions 12. Explain why, for the example above, you would expect a 1:9 ratio of 3 to 1°H substitution products if a 3º radical had about the same potential energy as a 1° radical XII HC 13. But a 3º radical is LOWER in potential energy than a lº radical. When X = C1, a 3º radical intermediate is 5 times more favorable than a lº radical intermediate. Given this, explain why, for the example above, a 5:9 ratio of 3 to 1°H substitution products is expected. (Consider both the type of H and the number of identical H's.)

only questions 12 and 13

Answer 1

12 . The halogen radicals are of big size. So when it will attack the molecule a steric hindrance factor will be a decider of the product when the potential energy of 3^° radical is almost equal to potential of 1^° radical. Thus the for the above given reaction the expectation of products ratio will be 1:9 for 3^° to 1° radical product as 3^° radical is more sterically hindered than 1^° radical and there 9 one degree hydrogen available for the halogen to attack. A schematic diagram is given below.13. When Cl is the halogen molecule then 3° product is 5 time favorable because it this intermediate radical is stabilized by combined inductive effect of three methyl groups and as intermediate is stabilized then product will also be stable

H3 the y rets steric hindrance play a role No steric Hindrance

Inductive effect is pushing and pulling of electrons through the. More electronegative atoms will pull the electrons and otherwise electrons will be pushed.

chilla : Keth more stable as electroni deficiency in the central Carbon will be filled by other three methyl groups y No indiretive . effect so there is no ose to fill up electron deficiency of carbon