Why are tertiary halides > 2ndary halides > Primary halides in E2 rxns?

[FROGGBUSTER]

Why are tertiary halides most reactive in E2 rxns?

Is it because there are simply more Hydrogens to de-protonate in a tertiary halide compared to a 2nd or 1st?

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[loveoforganic]

By reactive, do you mean higher rate of reaction? Number of protons would make sense for that. Can't think of an activation energy explanation. As far as equilibrium, more substituted alkene product

 

[FROGGBUSTER]

By reactive, do you mean higher rate of reaction? Number of protons would make sense for that. Can't think of an activation energy explanation. As far as equilibrium, more substituted alkene product

I guess that's what I mean, lol. One of the question in my workbook states this, but I had sworn that I had learned in undergrad that they were all "equally reactive." I suppose that means equal activation energy like you stated.

Thanks.

 

[salim271]

I believe it has to do with a more stable transition state and a more stable final product. The more substituted a alkene is, the more stable it is. I do not think more hydrogens to deprotonate is right, especially since tertiary carbons have only 1 hydrogen to deprotonate, remember, the carbon attacked by the must be alpha to the carbon that is bonded to the halogen.

 

[FROGGBUSTER]

I believe it has to do with a more stable transition state and a more stable final product. The more substituted a alkene is, the more stable it is. I do not think more hydrogens to deprotonate is right, especially since tertiary carbons have only 1 hydrogen to deprotonate, remember, the carbon attacked by the must be alpha to the carbon that is bonded to the halogen.

i think you're confusing something perhaps? tertiary halides have a lot of hydrogens. ignore the reaction, but here's an example of a tertiary halide:

i also think you're confusing kinetics with thermodynamics. a more reactive reaction isn't necessarily always more stable than a less reactive one. a lot of times sterics may interfere and you'll end up with a greater yield of the less stable product as a result.

 

[flodhi1]

i think you're confusing something perhaps? tertiary halides have a lot of hydrogens. ignore the reaction, but here's an example of a tertiary halide:

i also think you're confusing kinetics with thermodynamics. a more reactive reaction isn't necessarily always more stable than a less reactive one. a lot of times sterics may interfere and you'll end up with a greater yield of the less stable product as a result.

The last guy (salim) had a point. Look at the mechanism for the reaction E2. A strong base (e.g ethoxide ion) removes a proton, a halide ion anti to the proton leaves, resulting in the formation of a double bond. Now the thing is the key to the whole E2 reaction is the formation of the double bond. The reason why highly substituted like tertiary halides form E2 easily is because these highly substituted carcon chains, form the most stable ALKENES. The ideology that solely it has more hydrogens to deprotanate is wrong.

 

[FROGGBUSTER]

The last guy (salim) had a point. Look at the mechanism for the reaction E2. A strong base (e.g ethoxide ion) removes a proton, a halide ion anti to the proton leaves, resulting in the formation of a double bond. Now the thing is the key to the whole E2 reaction is the formation of the double bond. The reason why highly substituted like tertiary halides form E2 easily is because these highly substituted carcon chains, form the most stable ALKENES. The ideology that solely it has more hydrogens to deprotanate is wrong.

Okay I see now. Thanks for clearing that up, thank you too Salim.

I have one more question. My workbook makes a blanket statement that tertiary halides undergo E2 the quickest. Is this always true though?

What if you carried out an E2 rxn with tert-butyl bromide (tertiary halide) vs. a secondary halide like 3-Bromo-2,2,4-trimethylpentane though? The secondary halide would yield the more stable alkene.

 

[flodhi1]

Okay I see now. Thanks for clearing that up, thank you too Salim.

I have one more question. My workbook makes a blanket statement that tertiary halides undergo E2 the quickest. Is this always true though?

What if you carried out an E2 rxn with tert-butyl bromide (tertiary halide) vs. a secondary halide like 3-Bromo-2,2,4-trimethylpentane though? The secondary halide would yield the more stable alkene.

Listen man don't take the statement to the extreme level, the reason why I'm mentioning that is because there are several factors that dictate the reactivity for example steric hindrance and the strength of the base. So even though the secondary halide will form a more stable alkene IN THIS case you also have to account in the other factors. So if I were you I'd stick to the tertiary being more reactive unless you blatantly see a case such as this. Any reaction that you have in the Biological Sciences or Physical Sciences always has exceptions. Don't go into too much minute detail to the point where you lose sight of the big picture.

 

[salim271]

i think you're confusing something perhaps? tertiary halides have a lot of hydrogens. ignore the reaction, but here's an example of a tertiary halide:

i also think you're confusing kinetics with thermodynamics. a more reactive reaction isn't necessarily always more stable than a less reactive one. a lot of times sterics may interfere and you'll end up with a greater yield of the less stable product as a result.

Ah... not sure why I said only one hydrogen to deprotonate. Guess I was thinking of the wrong carbon. Orgo review I shall get to you eventually... I can see your point about more hydrogens, although, unless they're equivalent alpha carbons like in your example, you can end up with a mixture of products so its not a very good yielding reaction.

 

[plzNOCarribbean]

By reactive, do you mean higher rate of reaction? Number of protons would make sense for that. Can't think of an activation energy explanation. As far as equilibrium, more substituted alkene product

Tertiary halides ARE more reactive than secondary haildes to E2, because in an E2 reaction, bond breaking (the halide leaving) and bond making occur simultaneously. The reason that 3>2 is because tertiary halides have more alky groups, WHICH ARE ELECTRON DONATING. This is favorable becase it adds to the stability of the transition state of that step, because as you recall TS consist of of partial bonds. Because the Pi bond is not fully formed, the added stability of the electron donating alkyl groups causes the reaction to proceed more rapidly (because the Ea would be lower---and thus its easier to get over that "hill")

So, the tertiary halides are infact more reactive because of the added stability to f the TS, because the partial pi bond benefits from the added electron density, because it is initially losing some electron denisty as the halide leaves , since the halide is more E.N and takes all of the electrons that were initially in that C-Halogen bond

 

[plzNOCarribbean]

By reactive, do you mean higher rate of reaction? Number of protons would make sense for that. Can't think of an activation energy explanation. As far as equilibrium, more substituted alkene product

Does my explanation above make sense? do you follow my logic?

 

[loveoforganic]

Sounds plausible to me

 

[plzNOCarribbean]

Sounds plausible to me

OP, not to be a dick to the poster LOVEFORORGANIC, but that IS the reason why. It is not a plausible assumption, but a fact. Trust me, I tutored ochem 1.5 yrs and almost went on the get a master's in it and this was the explanation my professor gave anytime anyone asked this question. If you really think about it, it really makes sense. The reason they are more reactiive is because they are more stable, and the reason they are more stable is because the electron density donated by the additional alkyl group helps stabilize the partial double bond character in the TS.

 

[loveoforganic]

Does my explanation above make sense? do you follow my logic?

made it seem to me like you were proposing something to me as a possibility. Wasn't sure what the transition state would be, but if that's the transition state, should also toss in leaving group stability and steric factors for ea