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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: kapital on June 08, 2012, 05:57:44 AM

Title: hydride anion
Post by: kapital on June 08, 2012, 05:57:44 AM

Why is hydride anion base?(by which theory)

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Title: Re: hydride anion
Post by: sjb on June 08, 2012, 07:06:35 AM

Quote from: kapital on June 08, 2012, 05:57:44 AM

Why is hydride anion base?(by which theory)

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Lewis acid/base

Title: Re: hydride anion
Post by: kapital on June 08, 2012, 07:26:52 AM

Is there any consant, that woud tell how strong this base is?

Title: Re: hydride anion
Post by: james_a on June 08, 2012, 09:53:48 AM

The pKa of hydrogen (H2) is about 42, which makes H- a stronger base than H2N(-) but a weaker base than alkyl anions.

Title: Re: hydride anion
Post by: kapital on June 08, 2012, 12:08:38 PM

Is it posible than to calculate the higest pKa, that hydride ion can deprotonate?

Title: Re: hydride anion
Post by: orgopete on June 08, 2012, 05:25:22 PM

Quote from: kapital on June 08, 2012, 12:08:38 PM

Is it posible than to calculate the higest pKa, that hydride ion can deprotonate?

James_a already gave an approximate pKa. That could tell you something, but what can hydride deprotonate becomes a lot more complicated. It is not like you can pour a bottle of hydride into a reaction. It needs a counter ion, solubility, etc. What can something deprotonate? Ethanol has a pKa of about 16 and it can catalyze a Claisen condensation of an ester with a pKa of approximately 26. That tells you the reaction will be retarded, but it still overcomes the barrier. Just tell us the real question.

Title: Re: hydride anion
Post by: kapital on June 09, 2012, 04:56:04 AM

Can it deprotane acetylene?

Title: Re: hydride anion
Post by: PhDoc on June 26, 2012, 03:24:49 PM

pKa of H2 is approximately 42; pKa of acetylene is approximately 25. Which is the stronger base, hydride or acetylide? When combining hydride with 1-propyne, what product distribution is favored?

I've had over 1200 students of organic chemistry, and only about 10% of them had a functional working knowledge of pKa. This is one of the most important things you need to know to work through organic chemistry problems you've never seen before. Additionally, it's crucial in knowing which species serves as the base in any mechanism requiring a deprotonation.

Download the Evans pKa table, learn some basic pKa's, however more importantly, know what those pKa's mean and how to use them in relative comparisons.

http://evans.harvard.edu/pdf/evans_pka_table.pdf

Should you need more organic chemistry self-help, please go to the following page.

http://lennoxtutoring.com/organic-chemistry-tutor-student-resources/

The best favor you can do for yourself is to give up memorization in favor of rationalization.

The best of luck to you!