Why do Organolithium or Grignard reagents act as nucleophiles and not as bases with aldehydes and ketones

Question

I've read entire Chapter 14: Organometallic Compounds of Francis Carey's "Organic Chemistry" but I still didn't get an answer to my question.

Quote from the book:

Because of their basicity organolithium compounds and Grignard reagents cannot be prepared or used in the presence of any material that bears a $\ce{-OH}$ hydroxyl group. Nor are these reagents compatible with $\ce{-NH}$ or $\ce{-SH}$ groups, which can also convert an organolithium or organomagnesium compound to a hydrocarbon by proton transfer.

Fine. No arguments with that.

Terminal alkynes also give alkanes with Organolithium and Organomagnesium compounds.

Must be true if he says so. Now coming to the point,

1. Ammonia and Amines have $\mathrm{p}K_\mathrm{a}$ around 36.
2. Terminal alkynes have $\mathrm{p}K_\mathrm{a}$ of 25.
3. Aldehyde $\mathrm{p}K_\mathrm{a} = 17$, Ketone $\mathrm{p}K_\mathrm{a} = 19$ and an Ester $\mathrm{p}K_\mathrm{a} = 25$.
4. Water & Alcohols have $\mathrm{p}K_\mathrm{a}$ of 15–18.

If organolithium compounds and Grignard reagents act like bases with alcohols, terminal alkynes, and amines/ammonia, why do they act like nucleophlies and give addition reaction with carbonyl compounds? Why don't they act as bases and give alkanes instead? What am I missing here?

Answer 1

In fact, both reagents you noted here have quite complex structure and are not nucleophiles at all: both are electrophiles, because metallic atom here has too little neighbors to draw electrons from.

Let me explain, using Grignard reagent $\ce{EtMgCl}$ . In reality it has complex structure with $\ce{Mg}$ atom coordinated to alkyl fragment and two diethyl ether molecules. When reacts, it coordinates first with most nucleophilic atom. In case of alcohol it is oxygen of $\ce{OH}$ fragment, and of course it reacts with closest easily migrating atom: hydrogen, producing alkane. In case of ketone it coordinates with $\ce{CO}$ group, and alkyl group attacks closest electrophile : carbon from $\ce{CO}$ group, whose electrophility was just boosted.

If right compound, effectively bounding to metal atom, is added and no other way of reactions is readily available, both reagents can work as bases. For example, organolithium compounds can replace hydrogen atoms in alkynes, alkenes and arenes as you noted earlier.

Answer 2

Technically, you are comparing apples and oranges. Grignards and organolithium compounds are good nucleophiles (with the caveat permeakra highlighted) towards carbonyl groups. But they cannot attack alcohols, water, amines or alkynes nucleophilicly. There is simply no attack mechanism possible that would result in a more stable addition product. In fact, alcohols amines and water are pretty much entirely unimpressed by nucleophilic attacks. All of these can react as bases, though, so when presented with one of the two compounds mentioned, they will gladly give up their proton. (Remember that butane has a $\mathrm{p}K_\mathrm{a}$ value of $\approx 40$, so n-butyllithium is a very strong base.)

Carbonyls simply react much faster under nucleophilic attack than under deprotonation.

Answer 3

Well, in fact, the alpha-proton of aldehyde and other carbonyl/carboxylic acids are acidic, across a large range. The alpha-proton has a chance to be deprotonated by organolithium and magnesium reagents and forms a so called "enolate", which undergoes "aldol condensation" reaction with residual aldehyde.

Some knowledge derived from practice but not covered in text book is, if alkyl lithium reagent is added to an enolizable aldehyde, the nucleophilic additon and enolization-aldol condensation of aldehyde would take place together, and the product will be a mixture of alcohol and aldol condensation in various ratio depending on substrates. Some aldehydes, e.g. phenylacetaldehyde, provides aldol condensation product in major due to their high enolizabization property.

Mg/Na/Li alkoxides and amides also can deprotonate the alpha-proton of carbonyl as well as attacking certain certain carbonyls, like carboxylic ester/amide and nitrile. The carbonyl of aldehydes and ketone are not attacked.

Answer 4

Butyllithium is a powerful nucleophile much faster than it is a strong base. Add two equivalents of dry N,N,N',N'-tetramethylethylenediamine to coordinate the lithium. Now you have an extraordinary base (it will deprotonate THF at room temp) but a very poor nucleophile. One then presumes that nucleophilic attack upon a carbonyl may be a four-center intermediate in which C-O-Li coordination sets up and directs attack of the alkyl anion.

BuLi plus Li tert-butoxide is a strong enough base to initiate anionic polymerizations. The alkyllithium cage polymer is disaggregated.