Why are the tetrahedral intermediates unstable

The alkoxide formed by addition of a Grignard reagent to an aldehyde or ketone is stable, lasting long enough to be protonated on work-up in acid to give an alcohol as product. 

You probably eat several grams of Cl every day but you would be unwise to eat EtO or MeLi. So neither of these reactions occurs  

We don t expect you to be satisfied with the bland statement that tetrahedral intermediates are formed in these reactions of course, you wonder how we know that this is true. The first evidence for tetrahedral intermediates in the substitution reactions of carboxylic acid derivatives was provided by Bender in 1951. He made carboxylic acid derivatives RCOX that had been labelled with an isotope of oxygen, 0. This is a non-radioactive isotope that is detected by mass spectrometry. He then reacted these derivatives with water to make labelled carboxylic acids. By any reasonable mechanism, the products would have one i 0 atom from the labelled starting material. Because the proton on a carboxylic acid migrates rapidly from one oxygen to another, both oxygens are labelled equally. 

He then reacted these derivatives with insufficient water for complete consumption of the starting material. At the end of the reaction, he found that the proportion of labelled molecules in the remaining starting material had decreased significantly in other words, it was no longer completely labelled with i 0 some contained normal i 0. The formation of the tetrahedral intermediate would be as before but rapid proton transfer would also mean that the two oxygen atoms would be the same. Now you may see the next step in the argument. 

This result cannot be explained by direct substitution of X by H2O, but is consistent with the existence of an intermediate in which the unlabelled and labelled i 0 can change places. This intermediate is the tetrahedral intermediate for this reaction. Either isomer can lose X and in each case labelled carboxylic acid is formed. 

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You can see why hemiacetals are unstable they are essentially tetrahedral intermediates containing a leaving group and, just as acid or base catalyses the formation of hemiacetals, acid or base also catalyses their decomposition back to starting aldehyde or ketone and alcohol. That s why the title of this section indicated that acid or base catalysts increase the rate of equilibration of hemiacetals with their aldehyde and alcohol components—the catalysts do not change the position of that equilibrium ... 

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