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Organic Letters , 7 9 , Carbinol derivatives via rhodium-catalyzed addition of potassium trifluoro organo borates to aldehydes. Chemical Communications , 37 , Chemical Communications , 35 , Bulletin of the Chemical Society of Japan , 77 11 , Rhodium-catalyzed and sonication-accelerated addition of aryltin and aryllead reagents to imines in air and water. R and R' can be the same or different, and can be an alkyl group or hydrogen. If one or both of the R groups are hydrogens, the compounds are called aldehydes.
If both of the R groups are alkyl groups, the compounds are called ketones. Examples include:. The reactions between the various sorts of carbonyl compounds and Grignard reagents can look quite complicated, but in fact they all react in the same way - all that changes are the groups attached to the carbon-oxygen double bond.
It is much easier to understand what is going on by looking closely at the general case using "R" groups rather than specific groups - and then slotting in the various real groups as and when you need to. The reactions are essentially identical to the reaction with carbon dioxide - all that differs is the nature of the organic product.
In the first stage, the Grignard reagent adds across the carbon-oxygen double bond:. Dilute acid is then added to this to hydrolyse it. I am using the normally accepted equation ignoring the fact that the Mg OH Br will react further with the acid. An alcohol is formed. One of the key uses of Grignard reagents is the ability to make complicated alcohols easily. What sort of alcohol you get depends on the carbonyl compound you started with - in other words, what R and R' are.
A primary alcohol is formed. A primary alcohol has only one alkyl group attached to the carbon atom with the -OH group on it. You could obviously get a different primary alcohol if you started from a different Grignard reagent. The next biggest aldehyde is ethanal. One of the R groups is hydrogen and the other CH 3. Again, think about how that relates to the general case.
The alcohol formed is:. So this time the final product has one CH 3 group and one hydrogen attached:. A secondary alcohol has two alkyl groups the same or different attached to the carbon with the -OH group on it. You could change the nature of the final secondary alcohol by either:.
Ketones have two alkyl groups attached to the carbon-oxygen double bond. The simplest one is propanone. Organometallic reactions in general and lithium reagents reactions in particular usually involve some effort to make the reaction water-free.
Solvents ether and THF are dried over alkali metal, and freshly distilled; glassware is sometimes dried under vacuum with a soft bunsen flame a heady mix where you have ethereal solvents!
Why bother with all these precautions? Well, because water is the natural enemy of the Grignard reagent, and of organometallic reagents in general. Not only will water react irreversibly with your Grignard, the molecular mass of water is so low that a little goes a long way, and ineptly dried solvent or glassware will seriously reduce your yield.
Consider finally! Note that sometimes we can actually exploit this reactivity. The simplest way would be to take the corresponding alkyl halide, make the Grignard, and quench the Grignard carefully!
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