Okay...so I\'m working on some lab discussion questions after completing a radic

Question

Okay...so I'm working on some lab discussion questions after completing a radical chlorination lab. Basically, the reagents 1-chlorobutane, Sulfuryl Chloride, and the chemical initiator AIBN were synthesized, and produced 1,1-dichlorobutane, 1,2-dichlorobutane, 1,3-dichlorobutane, and 1,4-dichlorobutane. It was predicted that 1,1 through 1,3-dichlorobutane would have distribution percentages of 22%, and 1,4-dichlorobutane would be 33.3%. Actual results revealed the following: 1,1-dichlorobutane: 5.66% 1,2-dichlorobutane: 21.8% 1,3-dichlorobutane: 45.0% 1,4-dichlorobutane: 27.5% I understand that relative reactivity increases as carbons get further away from the Cl atom on Carbon 1, but why does 1,4-dichlorobutane have a lower product distribution than 1,3-dichlorobutane? Is it because of the extra hydrogen? AND, why does preexisting chlorine affect this radical reaction? Anything helps...thanks!

Explanation / Answer

I think its on basis of Cl presnt on primary or secondary carbon atom because a secondary C-H bond is weaker than a primary C-H bond (95 versus 98 kcal/mol), we might expect more 1,3 dichlorobutane.

The relatively weak tertiary C-H bond (92 kcal/mol) gives rise to 22 % of product in contrast to the 8% expected on the basis of a random attack of .Cl on the starting material.

The molecule has 9 primary hydrogens, chlorination of which accounts for 48% of

the products. This means that each primary hydrogen is responsible for 48 %/9 = 5.3 % of the

product. Similarly, the 2 secondary hydrogens account for 33 % of the product, so each

secondary hydrogen is responsible for 16.5 % of the product. Finally, the one tertiary hydrogen

is responsible for 22 % of the product. These results mean that tertiary hydrogens are 22/5.3 =

4.2 times as reactive as primary hydrogens toward chlorination and secondary hydrogens are

16.5/5.3 = 3.1 times as reactive toward chlorination as primary hydrogens.

the preexisting chlorine initates the rxn faster i tj=hink making it more reactive to react faster

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