Halogenation of Alkanes - Relative reactivies of halogens?

SORRY I MEANT ALKENES IN THE HEADING

Addition of hydrogen halides to alkenes, the rate of reaction depends on the bond energy of the hydrogen halide, so HI is the fastest reaction due to the lowest bond energy.

So during halogenation of alkenes, does the rate depend on the bond energy of the halogens? in that case I2 is the fastest one, but we all know Flourine is alot more reactive despite having high bond energy? so is F2 faster or I2?

(edited 9 years ago)

Reply 1

Jooooshy

Original post by nonipaify

Addition of hydrogen halides to alkanes, the rate of reaction depends on the bond energy of the hydrogen halide, so HI is the fastest reaction due to the lowest bond energy.

So during halogenation of alkanes, does the rate depend on the bond energy of the halogens? in that case I2 is the fastest one, but we all know Flourine is alot more reactive despite having high bond energy? so is F2 faster or I2?

Do you mean hydrogen halides and alkenes? If so, it reactivity increases from HF to HI, for the exact reason in your first paragraph: bond energies.

F-F isn't a high bond energy. The bond length is small so there's some repulsion there. But generally, bond energies decrease as you go down group 7.

(edited 9 years ago)

Reply 2

nonipaify

Original post by Jooooshy

So will it be flourine or iodine that reacts faster with an alkene?

Reply 3

Marcum

Original post by nonipaify

So will it be flourine or iodine that reacts faster with an alkene?

I think fluorine will react faster than iodine.

Reply 4

Jooooshy

Original post by nonipaify

So will it be flourine or iodine that reacts faster with an alkene?

Original post by Marcum

I think fluorine will react faster than iodine.

HF will not react faster than HI. The bond energies between F-F and H-F are different, so don't confuse yourself!

For a further reference:

http://www.chemguide.co.uk/organicprops/alkenes/hhal.html

Reply 5

Marcum

Original post by Jooooshy

HF will not react faster than HI. The bond energies between F-F and H-F are different, so don't confuse yourself!

For a further reference:

http://www.chemguide.co.uk/organicprops/alkenes/hhal.html

Derp! Thanks! :tongue:

These weeks off have turned my brain to mush.

Reply 6

nonipaify

Original post by Marcum

Derp! Thanks! :tongue:

These weeks off have turned my brain to mush.

H-I will certainly react faster andI know that already. My question was whether we use the same reasoning (bond energy) in determining if it is F-F or I-I that would react faster with an alkene.

Reply 7

Tytrox

Original post by nonipaify

H-I will certainly react faster andI know that already. My question was whether we use the same reasoning (bond energy) in determining if it is F-F or I-I that would react faster with an alkene.

Short answer: yes
Long answer: yes, so as I₂ bonds are weaker than F₂ bonds, due to electronegitivity, the iodine would react faster than the fluorine.

Hope this helps :smile:

Reply 8

charco

Study Forum Helper

Original post by Tytrox

Short answer: yes
Long answer: yes, so as I₂ bonds are weaker than F₂ bonds, due to electronegitivity, the iodine would react faster than the fluorine.

Hope this helps :smile:

Post incorrect...

Reply 9

pj80602

Original post by nonipaify

So will it be flourine or iodine that reacts faster with an alkene?

During this addition, generally, a carbocation is formed(when the alkene gets attacked by H+ from HX) whose stability decides the rate of reaction. This carbocation is then attacked by X- as a nucleophile which results in the formation of the end product. Since I- is a better nucleophile than F- it will be easier to proceed with HI. But this step is not the rate-determining step, it is the one where H+ attacks the alkene and forms carbocation, so this step will not have much effect on the rate of reaction. Since H+ is being generated by breaking of HX (X= I or F)using HI will give a faster reaction as compared to HF. Because HI bond is too weak(because of the size difference of both atoms) as compared to HF.