OChem Radical Halogenation Concepts

[LuminousTruth]

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I was reading TBR OChem and I had a few concepts I hope you all can clarify:

1) "The halogen will not stabilize the free radical since halogens are electron withdrawing by the inductive effect."

How is it not stabilized? I thought electron withdrawing atoms like halogens distribute electron charge, which stabilize the alkyl halide.

2) "It is safe to assume that the reaction will choose a pathway of lowest energy, so breaking the C-H bond of the alkane is preferred over breaking the C-H of the alkyl halide. It can be concluded that the bond broken is of lower energy."

So does that mean the C-H bond on alkane has lower energy than C-H bond on alkyl halide?

3) "Alkanes are more susceptible to free radical chlorination than monochlorinated alkyl chlorides since alkanes are either more reactive than the alkyl halide or a C-H bond is the weakest bond in a monohalogenated hydrocarbon and thus the Cl is removed rather than a second hydrogen."

If C-H bond is the weakest bond, wouldn't that mean it break more easily than a C-Cl bond? And the second part about the Cl being removed rather than the hydrogen: Wouldn't that make the C-Cl bond weaker than C-H bond? Since it is supposedely 'easier' to break the C-Cl bond? These two points just seem to contradict each other.

 

[TieuBachHo]

I was reading TBR OChem and I had a few concepts I hope you all can clarify:

1) "The halogen will not stabilize the free radical since halogens are electron withdrawing by the inductive effect."

How is it not stabilized? I thought electron withdrawing atoms like halogens distribute electron charge, which stabilize the alkyl halide.

2) "It is safe to assume that the reaction will choose a pathway of lowest energy, so breaking the C-H bond of the alkane is preferred over breaking the C-H of the alkyl halide. It can be concluded that the bond broken is of lower energy."

So does that mean the C-H bond on alkane has lower energy than C-H bond on alkyl halide?

3) "Alkanes are more susceptible to free radical chlorination than monochlorinated alkyl chlorides since alkanes are either more reactive than the alkyl halide or a C-H bond is the weakest bond in a monohalogenated hydrocarbon and thus the Cl is removed rather than a second hydrogen."

If C-H bond is the weakest bond, wouldn't that mean it break more easily than a C-Cl bond? And the second part about the Cl being removed rather than the hydrogen: Wouldn't that make the C-Cl bond weaker than C-H bond? Since it is supposedely 'easier' to break the C-Cl bond? These two points just seem to contradict each other.

You have to put them in illustrated rxns to clarify. However, all organic reactions follow the same rules with few exceptions to bear in mind. Here are a few pointers might help you out.

1) How is it not stabilized? I thought electron withdrawing atoms like halogens distribute electron charge, which stabilize the alkyl halide.

--- Electron withdrawing halogens consume electrons, which destablize the free radicals.

2) ... so breaking the C-H bond of the alkane is preferred over breaking the C-H of the alkyl halide ...
So does that mean the C-H bond on alkane has lower energy than C-H bond on alkyl halide?

--- This part is not correct and ONLY makes sense under certain circumstances. C-H bond of the alkane is more stable than C-H bond of the alkyl halide because the C-H bond of the alkyl halide is more acidic which makes it more susceptible to deprotonation. But which C-H bond are you talking about. It depends on other factors as well such as relative locations, solvents, etc.

3) What removes what?
C-H bond is the weakest bond in a monohalogenated hydrocarbon. ... removed rather than a second hydrogen.
--- Be careful with the underlined words. They makes big differences.