Grinards Question

[hkmedbound]

Hi all,

Quick question for any orgo people...

 

I was doing the extra Kaplan questions in their 'Q Bank' and one question asked for the starting materials in order to produce a straight chain alkene... one was a Grinard, and the other was an alcohol.

 

Do primary alcohols and Grinards produce straight chain alkanes after work up? I was under the impression that Grinards only produced alcohols after nucleophilic attack with a carbonyl.

 

Cheers.

[yeoli]

Hi,

I'm a chemist so I can help you!

I won't give you the answer right away (it would be too easy). Just keep in mind that a Grignard (not Grinard) can act as a very strong base (pKa~40) and an alcohol is slightly acidic (pKa~15). Does that help?? Let me know if you need more help.

[hkmedbound]

Hi,

I'm a chemist so I can help you!

I won't give you the answer right away (it would be too easy). Just keep in mind that a Grignard (not Grinard) can act as a very strong base (pKa~40) and an alcohol is slightly acidic (pKa~15). Does that help?? Let me know if you need more help.

 

 

So... then yes? the Grignard could attack forming the alkane???

[yeoli]

Well, if you just want a yes/no answer, the answer is : YES.

 

Now, if you want to understand what you're doing, I'd say the alkane you're going to get is definitely NOT the one I think you have in mind. The Grignard will NOT kick the alcohol's butt off in a nucleophilic substition fashion! It'll rather do a simple ACID-BASE reaction. (Same as mixing vinegar with baking soda:D ) Here's the equation:

R1-MgBr + R2-OH --> R1-H + R2-OMgBr,

and upon aqueous workup, you get R1-H (your alkane) + R2-OH (your starting alcohol) + MgBrOH (salts that precipitate off and get stuck everywhere in your glassware).

 

Voilà!

[hkmedbound]

cheers. thanks a lot