rmm30

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i need some clarification on a topic thats been driving me crazy of late. Normally when a species is more stable it is less reactive. For example I- is more stale than F- because the size of Iodine ion is larger and can handle negative charge better stabilizing it and making it less reactive. However in the case of carbocations and radicals, tertiary radicals are more stable than methyl radicals due to hyperconjugation making them MORE reactive. why does stability correlate with reactivity differently in this case?
 
Sep 21, 2010
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Tertiary radicals have methyl groups that act as electron donors to somewhat stabilize it where as methyl radical does not. Therefore methyl radical is less stable and more reactive than tertiary radicals.
 

chemtopper

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Carbocation and radicals are electron deficient species and anything which can donate electrons can stabilize it .Like in carbocations ,the positive charge on C BECOMES MORE STABLE if some electron donating group is bonded to that C and it becomes more reactive or less stable if some elctron withdrawing group is present next to positive carbon.
Alkayls groups are electron donating groups ,I am sure you know the reason .Now you can correlate how primary,sec,tert radicals and carbocations differ in stability and reactivity
 

Isoprop

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When an intermediate is more stable, it makes the overall reaction more reactive because it is easier to get to the intermediate. Strictly speaking, a tertiary radical is more stable and less reactive than a primary radical. But a reaction that has a tertiary radical intermediate is more likely to proceed than a reaction with a primary radical intermediate. This is best illustrated with energy diagrams.

You can think of the stability as "bait" in that it "lures" the reactant to proceed to product.