[Bassel]

being used in SN1 with tertiary alkyl-halides, this means that the H2O is not affected by steric hindrance due to the tertiary carbon. is that right ? if it is , why is it so? why isn't OH- the same ?

[Babcock_Hall]

A tertiary carbocation has a different shape from the starting material, a tertiary alkyl halide.  I am not sure why you think that water will behave differently from hydroxide ion.  Can you explain further?

[spirochete]

Assuming there are beta hydrogens, hydroxide will act as a strong base and do an E2 reaction with a tertiary halide.

[orgopete]

Assuming there are beta hydrogens, hydroxide will act as a strong base and do an E2 reaction with a tertiary halide.

That could be the case, but it is a fairly common lab reaction of t-butyl halides go by first order kinetics. Undoubtedly, it can go by second order kinetics if base concentration is increased.

[TwistedConf]

That could be the case, but it is a fairly common lab reaction of t-butyl halides go by first order kinetics.

Only in the presence of weak nucleophiles.

Undoubtedly, it can go by second order kinetics if base concentration is increased.

Base strength, not concentration, causes the shift from unimolecular to bimolecular reactions.

[orgopete]

That could be the case, but it is a fairly common lab reaction of t-butyl halides go by first order kinetics.

Only in the presence of weak nucleophiles.

The reaction is performed with NaOH. The reaction goes by first order kinetics It isn't the pKa of the base, but the concentration.

Undoubtedly, it can go by second order kinetics if base concentration is increased.

Base strength, not concentration, causes the shift from unimolecular to bimolecular reactions.

At low concentrations, the solvent causes first order bond cleavage faster than a second order E2 elimination. If the base concentration is increased, at some point, the second order reaction become predominant.

This can be the case for many second order reactions in protic solvents. Since you know t-butyl bromide has first order kinetics with dilute NaOH, the NaOH is simply not showing its presence in the reaction kinetics. If the concentration is doubled, the reaction rate doubles. If the NaOH concentration is continuously increased, at some point it will become predominant. You can think of it like this. Any of those secondary or tertiary halides should have second order kinetics. If the reaction is diluted, at some point the solvent effect can become greater than the nucleophile/base. This should be consistent with reaction kinetics. Second order reactions only take place by the molecular collision. The number of collisions depends upon concentration. At some point, the solvent can compete with the base/nucleophile.

[spirochete]

That could be the case, but it is a fairly common lab reaction of t-butyl halides go by first order kinetics.

Only in the presence of weak nucleophiles.

The reaction is performed with NaOH. The reaction goes by first order kinetics It isn't the pKa of the base, but the concentration.

Undoubtedly, it can go by second order kinetics if base concentration is increased.

Base strength, not concentration, causes the shift from unimolecular to bimolecular reactions.

At low concentrations, the solvent causes first order bond cleavage faster than a second order E2 elimination. If the base concentration is increased, at some point, the second order reaction become predominant.

This can be the case for many second order reactions in protic solvents. Since you know t-butyl bromide has first order kinetics with dilute NaOH, the NaOH is simply not showing its presence in the reaction kinetics. If the concentration is doubled, the reaction rate doubles. If the NaOH concentration is continuously increased, at some point it will become predominant. You can think of it like this. Any of those secondary or tertiary halides should have second order kinetics. If the reaction is diluted, at some point the solvent effect can become greater than the nucleophile/base. This should be consistent with reaction kinetics. Second order reactions only take place by the molecular collision. The number of collisions depends upon concentration. At some point, the solvent can compete with the base/nucleophile.

Obviously concentration will influence kinetics and I'll take your word for it that it's possible in the lab to make NaOH with t-butyl bromide go by first order kinetics.

But shouldn't you at least acknowledge that 95% of organic professors will teach this as being a second order (E2) type reaction?

[orgopete]

Obviously concentration will influence kinetics and I'll take your word for it that it's possible in the lab to make NaOH with t-butyl bromide go by first order kinetics.

But shouldn't you at least acknowledge that 95% of organic professors will teach this as being a second order (E2) type reaction?

Honestly, I had to go back to read the original poster's question (which I don't actually understand). I'm probably answering Babcock's comment. Can water have a different effect on a reaction than hydroxide? My answer is still yes, it can change reaction kinetics from SN2 to SN1.

If the reagents listed with a reaction include hydroxide, I assume by listing it as a reagent means it should be kinetically important. That is what I advise students. That means the reaction must be E2 or SN2. I would consider it disingenuous to list reagents, but to say they do not have a role in a reaction.   

When I attended a workshop and saw the reaction written on the board, I thought it was going to be second order kinetics. It wasn't. How do you explain this or can water have a different effect on a reaction than hydroxide?