Elimination and temperature

[chiddler]

What I thought: Higher temperatures increase likelyhood of SN1 and E1 reactions because the higher temperature helps stabilize the carbocation.

A passage i'm doing says that higher temps increase likelihood of elimination reactions.

?

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[MedPR]

What I thought: Higher temperatures increase likelyhood of SN1 and E1 reactions because the higher temperature helps stabilize the carbocation.

A passage i'm doing says that higher temps increase likelihood of elimination reactions.

?

Elimination reactions break more bonds, thus requiring more energy input. For me it's easier to think of it not as high temp increases elimination (though it does), but as low temp hinders elmination since there isn't enough energy.

 

[chiddler]

Elimination reactions break more bonds, thus requiring more energy input. For me it's easier to think of it not as high temp increases elimination (though it does), but as low temp hinders elmination since there isn't enough energy.

What are physical (temp, pH, etc) conditions that are favorable for SN1 then?

 

[chiddler]

here is the question. I'm not sure about pH either :x

 

[mcloaf]

Well, for SN1 or E1 to happen at all you need to be in acidic conditions, so even though you're raising the pH, it's hardly like you're in 50% NaOH.

The first step of SN1 and E1 is protonation of the leaving group. I believe raising the pH simply slows down both reactions at this step, allowing the temperature effects to better exert thermodynamic control. Ultimately, you cannot get one without getting the other to some degree.

 

[chiddler]

Well, for SN1 or E1 to happen at all you need to be in acidic conditions, so even though you're raising the pH, it's hardly like you're in 50% NaOH.

The first step of SN1 and E1 is protonation of the leaving group. I believe raising the pH simply slows down both reactions at this step, allowing the temperature effects to better exert thermodynamic control. Ultimately, you cannot get one without getting the other to some degree.

I'm not sure that's true. Br is the leaving group and does not need to be protonated.

 

[mcloaf]

I'm not sure that's true. Br is the leaving group and does not need to be protonated.

True. That one was sort of a shot in the dark.

 

[MedPR]

Ok, now I can see the question (not at work anymore).

The reason you must raise the pH is because, in acid the OCH3 will get protonated and will become a good leaving group. In base, it continues to be a worse leaving group than Br, thus allowing you to keep your product.

Remember, in acidic solution and water/alcohol: protonate, break, make, deprotonate!

 

[mcloaf]

Ok, now I can see the question (not at work anymore).

The reason you must raise the pH is because, in acid the OCH3 will get protonated and will become a good leaving group. In base, it continues to be a worse leaving group than Br, thus allowing you to keep your product.

Remember, in acidic solution and water/alcohol: protonate, break, make, deprotonate!

Aha! I was like, less acid...less protonated Br?? Obviously you're right though. Interesting, I haven't actually seen many MCAT questions that make you analyze product stability. This was a good one.

 

[MT Headed]

Just to reiterate:

Lower temp to avoid the elimination side reaction
Raise pH to shift the equlibrium to the right (it is an equilibrium after all)

 

[chiddler]

What are temperature and pH conditions that favor SN1 conditions?

 

[SaintJude]

This is from Wiki, but it should answer your question

If the reaction is performed under warm or hot conditions (which favor an increase in entropy), E1 elimination is likely to predominate, leading to formation of an alkene. At lower temperatures, SN1 and E1 reactions are competitive reactions and it becomes difficult to favor one over the other. Even if the reaction is performed cold, some alkene may be formed. If an attempt is made to perform an SN1 reaction using a strongly basic nucleophile such as hydroxide or methoxide ion, the alkene will again be formed, this time via an E2 elimination. This will be especially true if the reaction is heated. Finally, if the carbocation intermediate can rearrange to a more stable carbocation, it will give a product derived from the more stable carbocation rather than the simple substitution product.

In terms of pH, I don't know if a general rule can be said. Looks like both acidic and basic!

Also from Wiki:
The reaction involves a carbocation intermediate and is commonly seen in reactions of secondary or tertiary alkyl halides under strongly basic conditions or, under strongly acidic conditions, with secondary or tertiary alcohols.

 

[MedPR]

What are temperature and pH conditions that favor SN1 conditions?

Low temp favors substitution over elminitation. pH will depend on the nucleophile, leaving group, and possibly electrophile.

 

[chiddler]

thanks very much for the help