 In this video we are going to look at another interesting way by which alcohols can be synthesized and that is using Grignard's reagent. We are already familiar with Grignard's reagent. We know that the general formula of Grignard's reagent looks something like this, rMGX. Grignard's reagents fall under the bigger umbrella of organometallic compounds where a carbon atom is bonded to a metal. In this case it is magnesium. We have organolithium compounds where the carbon atom is bonded to the lithium atom. Now the usability of Grignard's reagent or any of these organometallic reagents comes from the electronegativity difference between carbon and the metal. Now electronegativity of carbon is about 2.55 and that of magnesium is about 1.31. As a result carbon gets a partial negative charge and magnesium of course being more electropositive gets a partial positive charge. This partial negative charge on the carbon atom comes in very handy because now it can act as a very strong nucleophile as well as a strong base. And one of the most important reactions of Grignard's reagents is this addition to carbonyl compounds. For example let's look at the same carbonyl compound here which is the ketone and if you react it with a Grignard's reagent let's say RMGX then R being nucleophilic the carbon here being nucleophilic will attack the electrophilic carbon here because CO bond as we know is polar the carbonyl carbon has a partial positive charge as electrons are drawn towards the oxygen atom and oxygen gets a partial negative charge. Now as a result of this addition we get an intermediate alkoxide ion. So the intermediate looks something like this O minus and MGX becomes the counter ion here. Now once this addition is complete in the second step we perform a hydrolysis. So this is the first step and this is the second step. We add water or mild acid in order to protonate this alkoxide ion. So the oxygen atom abstracts a proton from water molecule and gives us the final alcohol. Now it is important to remember that we add water or we perform the hydrolysis step only after the first step is completely done because you see the carbon here is not just a strong nucleophile but also a strong base that means if there is any presence of water or any compound that has an acidic hydrogen like alcohols or even amines you know NH2 or NH2. In all of these cases where we have acidic proton the Grignard region gets deactivated because the R will simply abstract this acidic proton and become a simple hydrocarbon. So basically our Grignard region gets destroyed if we have the presence of acidic hydrogen in our first step and this is why we carry out this step in the presence of dry ether. This is something that you always need to remember when we talk about Grignard reagents. Alright so now that you understood the basic mechanism behind the addition of Grignard reagents to obtain alcohols let's look at a couple of questions. Let's look at the first question. Here we need to identify the product that is formed in the given reaction. So here we are reacting a benzaldehyde with ethyl magnesium bromide. So the first step is obviously the nucleophilic addition. Now in RMGX the nucleophile is the R minus group right and R minus here is the ethyl group here. In the first step the nucleophilic addition occurs. Remember this step occurs in dry ether. Okay the solvent is dry ether and this would give us an alkoxide ion. Now the structure of alkoxide ion would look like this. In the second step we add water or mild acid and in this step hydrolysis takes place or our alkoxide ion gets protonated. So the final alcohol looks like this. So this is our final alcohol and this is a secondary alcohol. So as you can see here addition of Grignard reagents to aldehydes will give us a secondary alcohol. Okay so let's now look at another question. Here we need to identify the starting carbonyl compound and Grignard reagent that are required to prepare the given alcohol. So here we have a primary alcohol and we need to identify the starting reactants. So we know that the alcohol synthesis using Grignard reagents takes place in two steps. The first one is the addition or nucleophilic addition followed by hydrolysis. So this is basically the steps involved and in order to figure out the starting reactants we simply have to go backwards. Okay so what would be the immediate previous step of this alcohol synthesis? It is deprotonation right because the last step is addition of water. So we deprotonate it we get the intermediate which is the alkoxide ion correct. And if I draw the structure completely here is the hydrogen atom and here is the hydrogen atom. This is the primary alcohol. Now from this intermediate how can we figure out the structures of our carbonyl compound and Grignard reagent? Let's see. Now a Grignard reagent must have an alkyl group because its structure is RMGX. And the CO bond in our intermediate should belong to the carbonyl compound. So from this intermediate you can see that the only alkyl group here in this intermediate is this one and that means this group has to come from the Grignard reagent and this becomes a part of our carbonyl compound. So if I draw the structures it would be MGBR and our carbonyl compound would have structure HC double bond OH. So HC double bond OH and H. So these are the starting reagents that are required to prepare the primary alcohol. The carbonyl from the Grignard reagent attacks the carbonyl group of the formaldehyde and this addition gives us an intermediate alkoxide ion which on protonation would give us a primary alcohol. Now the important thing to note here is that only addition to formaldehyde will give us a primary alcohol. The addition of Grignard reagent to any other alcohols would give us a secondary alcohol as we saw in the previous example. So let me write it down here. Grignard reagent plus formaldehyde will give us a primary alcohol. With any other aldehyde we would get a secondary alcohol and with ketones we would get a tertiary alcohol.