[pfnm]

We are measuring the reaction times of t-butyl chloride with NaOH under various conditions.

We have 3 mmoles of t-butyl chloride and 0.3mmoles of NaOH. The solvent composition is 3mL acetone/7mL H2O

We put bromophenol blue into the NaOH flask, mix the NaOH with the t-butyl, and measure the time it takes to react (once the indicator changes colour, all the OH- has reacted).

My question is:

What's to stop H2O reacting with the t-butyl carbocation?

Does the OH- ion preferentially attack the carbocation, rather than a water molecule? If so, why?

Thanks

[Honclbrif]

It may be possible that since water is present in such a great excess that it is the species that is reacting with the t-butyl cation to initially produce tBuOH + HCl. Then the OH^- mops up the acid

tBu-Cl  tBu⁺ + Cl^-

tBu⁺ + H₂O  tBuOH₂⁺

tBuOH₂⁺ + Cl^-  tBuOH + HCl

HCl + OH^-  H₂O + Cl^-

[pfnm]

I talked to the lecturer today. I asked him this question.

He said OH- reacts preferentially to H2O because OH- is a stronger nucleophile. That once all the OH- is used up, bromophenol blue reacts with H+ ions (are they present because of the self-ionization of water?) and a colour change is observed.

Then, my question is, once the OH- ions are used up (their amount is 10% of the molar amount of t-butyl chloride), would water react as the nucleophile, to continue to produce alcohols, as you show in your reaction scheme?

Until all the t-butyl chloride is converted to the alcohol (or the alkene by E1 elimination)

[orgopete]

I am inclined to agree with Honlbrif. Here is why. This reaction could have been an E2 elimination. However, in this experiment, we know the rates are unimolecular. The rate depends on the concentration of the t-butyl bromide. If the concentration of hydroxide is so low to not alter the rate of elimination, I don't see why water could not outcompete hydroxide in the addition/elimination reaction.

Also in the way in which the reaction is carried out, the hydroxide concentration is falling until it becomes acidic. If the reaction depended on hydroxide, the reaction should slow. I think this all points to the unimolecular aspect of the bond breaking step. Further reaction of the carbocation should continue in the absence of hydroxide.