Another organic question..

[Avery07]

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From Ochem Odyssey:

Which halogen has the most endothermic first propagation step during alkane halogenation?

A) Cl2
B) I2
C) Br2
D) F2
E) At2


Romano says that the answer is B and justifies his answer with the following halogen reactivity with alkanes:

F2 > Cl2 > Br2 > I2

He stops there.. not going on to include At2 and doesn't bother saying why. I can only assume that At2 should be next in this series but evidently not??

Anyone know what's going on here?

 

[Chemwiz]

is At even a halogen?

 

[Chemwiz]

is At even a halogen?

lawl it is
hmm., i don't know, i haven't seen At being used in halogenation, so I'm guessing we just ignore it. Anyone got any idea?

 

[Blarelli]

My guess would be that since homolytic cleavage of the alkane C-H bond takes about 100 kcal/mol, and the formation of the I-H bond (first propagation step) releases about 71, that means that the propagation step has a net of +29 kcal/mol. The Br-H bond on the other hand releases about 88 kcal/mol making the net propagation step +12 kcal/mol. +29 is more endothermic than +12.

 

[UndergradGuy7]

At is radioactive and never used.

 

[orgohacks]

At is radioactive and never used.

Exactly.

From Wikipedia: "Astatine is currently the rarest naturally occuring element, with less than 30g estimated to be contained in the entire earth's crust"

safe bet you won't see it 🙂 - but you are right in theory!