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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: xshadow on July 23, 2019, 04:50:40 PM

Title: Grignard and RLi: basicity vs nucleophilicity
Post by: xshadow on July 23, 2019, 04:50:40 PM

Hi...I have a doubt:

I know that RLi( normal butyl lithium) and RMgX are strong bases and also strong nucluophiles

Now If I have some acidic hydrogen usually they act as a base and they deprotonate it

For example
 the -OH of a carboxylic acid RCOOH(pka≈ 4.75) (and not gives nucleophilic substitutio)
 the -NH2 group of an Amide RCONH2 (pka ≈25)  (and not gives nucleophilic substitution!)
 the -OH of a alcohol or also..water!!


Now if I have an aldehyde or a ketone why I'll get, as major product ,the nucleophilic addition product!?


In  ketone and  aldehyde  I have    H-acid(pka ≈20), in the alpha position

So why, here  ,does the nucleophilic  nature  of organometallic compounds prevail over the basic one?


In amides or alcohols I have the deprotonation(over the nucleophilic attack) with an acidic hydrogen having a  pka of about 15/25

Ketones have a SIMILAR acidity of alpha hydrogen but my textbook (and also looking online) says that grignard and RLi compounds usually give a nucleophilic attaco on the carbonyl

Why?
Thanxx!!

Title: Re: Grignard and RLi: basicity vs nucleophilicity
Post by: rolnor on July 24, 2019, 03:43:59 AM

What would be the product from nucleophilic attack on a alcohol?
Are there any difference in electrophilic nature in amides vs ketones/aldehydes?

Title: Re: Grignard and RLi: basicity vs nucleophilicity
Post by: xshadow on July 24, 2019, 11:55:53 AM

Quote from: rolnor on July 24, 2019, 03:43:59 AM

What would be the product from nucleophilic attack on a alcohol?
Are there any difference in electrophilic nature in amides vs ketones/aldehydes?

Hi..

1) the product I think it could be a Sn1/2....where the OH-  leaves ...but is a poor leaving group -OH...it need an acid catalysis

2) amides vs ketones /aldehydes electrophilicity

The C=O of amides is less electrophilic because the -NH2 can donate its lone pair to CO..so the carbon is a bit less electrophilic

But  for the  H-α acidity  of ketones/aldehydes vs amides?
Why RLi attack the -NH2 of amide(acid base reaction) pka=25 and NOT the H , in α to C=O..pka= 20/25?!

Thanks

Title: Re: Grignard and RLi: basicity vs nucleophilicity
Post by: rolnor on July 24, 2019, 04:30:43 PM

Alcohols can not alkylate RLi or RMgX, you need a muck better leavinggroup and even with say R-Halogen, prefferable I, the reaction is very sluggish. If you have activated halides like Allyliodide the reaction can be fast.
The fact is that RLi and RMgX are poor Sn2-nucleophiles but attacks carbonyls very fast, but amides are much less electrophilic then aldehydes or ketones. Even though alpha-protons on aldehydes and ketones are acidic attack on the carbonyl is much faster then acid-base reaction.