Why is grignard reagent sensitive to water

Jun 28, Explanation: The products of the reaction between a Grignard and water are a magnesium hydroxide salt, and a hydrocarbon. Related questions What functional groups are found in proteins? What functional groups are found in the structure of melatonin? The reactions are essentially identical to the reaction with carbon dioxide - all that differs is the nature of the organic product. 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. Assuming that you are starting with CH 3 CH 2 MgBr and using the general equation above, the alcohol you get always has the form:. 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. A secondary alcohol has two alkyl groups the same or different attached to the carbon with the -OH group on it. Ketones have two alkyl groups attached to the carbon-oxygen double bond. The simplest one is propanone. This time when you replace the R groups in the general formula for the alcohol produced you get a tertiary alcohol.

A tertiary alcohol has three alkyl groups attached to the carbon with the -OH attached. The alkyl groups can be any combination of same or different. The mechanisms for these reactions aren't required by any UK A level syllabuses, but you might need to know a little about the nature of Grignard reagents.

The bond between the carbon atom and the magnesium is polar. Carbon is more electronegative than magnesium, and so the bonding pair of electrons is pulled towards the carbon. Note: If you aren't sure about electronegativity , you can read about it in an organic context by following this link. The carbon-oxygen double bond is also highly polar with a significant amount of positive charge on the carbon atom.

Think of these reagents as lit candles — potentially very powerful, but also vulnerable to being extinguished.

For example, substituting D 2 O for water provides us with an excellent method for incorporating deuterium labels into molecules:. Note that each of these molecules has an acidic proton! So as soon as any Grignard forms, it would immediately be neutralized by the acidic functional group to give us the conjugate acid of our organometallic which is useless to us and the conjugate base of whatever the acidic functional group is.

We cover that in the next post. Stay tuned. As you might have suspected, there is a way around the problem of acidic functional groups in formation of Grignard reagents.

This would form a hydrocarbon. But Grignard reagents are stable in ethers. The formation of ions in very nonpolar solvents, where they would not be effectively solvated is very difficult.