Grignard with acid?

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I was doing Chad's quiz and I got a little confused.

[IMG]http://www.***********.com/images/quiz/4/5/5/7/ochem%20013_1282429127.png[/IMG]
In this question, the product is the same as the reactant.

"This question is a little tricky.* A Grignard reagent shouldn’t be in any kind of protic solution as it will be protonated by anything protic in an acid-base reaction which effectively destroys the Grignard reagent as is the case here.* In the first step, the Grignard reagent deprotonates the alcohol.* The addition of H3O+ simply reprotonates the alkoxide converting it back into an alcohol and so the product is the same as the original reactant (choice A)."

Vs. this question: [IMG]http://www.***********.com/images/quiz/4/5/5/7/ochem%20000_1282428390.png[/IMG]

you actually get a different product- [IMG]http://www.***********.com/images/quiz/4/5/5/7/ochem%20001_1282428403.png[/IMG]

Can someone explain this to me? How do I know when Grignard and acid will not work? I know that if you add a grignard and then add acid in the second step, it should be ok. But using these two examples, I don't understand how the first one is different than the second one.

Not sure why the image is not working.If you replace the ***** with c o u r s e s a v e r, it should work

If you have access to Chad's quizzes, 643 and 641

Weird. I feel like number one should definitely be reduced to an alcohol because the h30+ is added on the second step, so the grignard should work....

Originally Posted by diene

I was doing Chad's quiz and I got a little confused.

[IMG]http://www.***********.com/images/quiz/4/5/5/7/ochem%20013_1282429127.png[/IMG]
In this question, the product is the same as the reactant.

"This question is a little tricky.* A Grignard reagent shouldn’t be in any kind of protic solution as it will be protonated by anything protic in an acid-base reaction which effectively destroys the Grignard reagent as is the case here.* In the first step, the Grignard reagent deprotonates the alcohol.* The addition of H3O+ simply reprotonates the alkoxide converting it back into an alcohol and so the product is the same as the original reactant (choice A)."

Vs. this question: [IMG]http://www.***********.com/images/quiz/4/5/5/7/ochem%20000_1282428390.png[/IMG]

you actually get a different product- [IMG]http://www.***********.com/images/quiz/4/5/5/7/ochem%20001_1282428403.png[/IMG]

Can someone explain this to me? How do I know when Grignard and acid will not work? I know that if you add a grignard and then add acid in the second step, it should be ok. But using these two examples, I don't understand how the first one is different than the second one.

As far as I understand the topic, 2. H3O is for the protonation of O- after phenyl group is attached (did I word it right? lol) so it'll be added after the phenyl group is attached to the tertiary carbon?

If there's a protic (-OH group in this case) then it'll destroy grignard and it what you wrote explains it. hope this makes sense lol

So basically H3O+ is the protic solution they mean, they meant the OH group

I'm not an expert in orgo :/ sorry if it was confusing

Grignard is a strong base, therefore will react with anything that has an acidic proton such as alcohols or acids. If you use Grignard together with acid or alcohol or water, not in a second step, then those solutions will simply react with grignard and would not allow grignard to do its job. Hope this helps.

Originally Posted by diene

Not sure why the image is not working.If you replace the ***** with c o u r s e s a v e r, it should work

If you have access to Chad's quizzes, 643 and 641

Chad's website is blocked through our text filter.

Originally Posted by biomolecmed

Grignard is a strong base, therefore will react with anything that has an acidic proton such as alcohols or acids. If you use Grignard together with acid or alcohol or water, not in a second step, then those solutions will simply react with grignard and would not allow grignard to do its job. Hope this helps.

This is correct.