Alcohol Reactions

As a result of the presence of their highly polarized hydroxyl (OH) group, alcohols can be involved in many organic reactions.

Types of Alcohol Reactions

Concept: Understanding Alcohol Reactions

6m
Video Transcript

So, here weÕre dealing with alcohols and their reactions. We are going to say alcohols can be easily identified by the presence of the hydroxyl group, which is just an OH group. Now weÕre going to say, because of the presence of this highly polarized group alcohols can be involved in many organic reactions, so what reactions are there involved in? Is substitution reactions. So here, under this reaction alcohols react with hydrohalic acids, X here represents a halogen, and the two hydrohalic acids that we look at are HCl and HBr, both of them are being strong assets, here they react with the alcohol to form and alkyl halid, so basically our carbon will attached to a halogen. Here fluoride is too reactive to basically be used, so thatÕs why we donÕt use Hf and Hi, iodide is so large and so slow that we donÕt worry about that either, so thatÕs why we are looking for a chlorine and bromine. Now, the easiest way to look at this reaction is the alcohol, the OH group, will combine with a hydrogen from the asset and weÕre going to lose water. As a result the Br will be where the OH used to be, so we rewrite this formula, we put that halid in there plus the water we lost. Now again, when we get to actually get to organic chemistry youÕll learn something called mechanism and see how electrons move around in order to create this new alkyl halid, since this is still chemistry and this is just an introduction to organic, we donÕt worry about those mechanisms, because at this point it would just highly confuse you, thereÕs a few things you have to learn beforehand before you get into that step, what you learn in organic chemistry. So, the same thing would have happened if I use HCl except a Cl would have been there instead of a Br.

Now, another reaction that alcohols can under go, is elimination reactions. Under this reaction, under the presence of concentrated acids, such as phosphoric acid or sulfuric acid, an alcohol undergoes dehydration to form an alkene. So, two carbons will be double bonded to each other. So, what ends up happening here is we use H2SO4 and we use some heat which can be presented by a triangle, this causes us to lose water, so weÕre going to lose OH and then one of the neighboring carbons will lose a hydrogen as well, so we can lose an H here or even from one of these other two. So, we lose an H and an OH and a carbon that loses the H and the carbon that loses an OH they double bond to each other in the process. So, now we only have two hydrogens on this carbon and it double bonds this carbon here in the middle, and again it could have happened to one of the other two carbons, they could have lost an agent double bond instead a carbon, and we get an alkene in the process, where two carbons are double bond to each other. Now, another reaction that can occur is oxidation reactions. Here an alcohol can be seen as an alkene that has undergone in initial oxidation. So, here we have a carbon structure with only carbons and hydrogens to form an alcohol and an OH group is added, which is a partial oxidation, so the alcohol can then be oxidized to form either a ketone or a carboxilyc acid, remember, carboxylic acid is when you have carbon double bonded to an O, single bonded to an OH and a ketone is when you have a C double bond O connected to a carbon on each side. Now, here weÕre going to say this is done with the strong oxidizing agent sodium dichromate, Na2CrO7 and the strong acid sulfuric acid. So, If we take a look, weÕre going to have here initially a primary alcohol, why is it primary? We look at the carbon and the OH connected to this one group and we see that that carbon is connected to only one other carbon, thatÕs why itÕs primary and here as a result of this reaction this is going to change into a carboxylic acid, so becomes CH3, CH2 and that group becomes a carboxylic acid portion. So, a primary alcohol will always change into a carboxylic acid under this process. Now here, this is a secondary alcohol, why? Because the carbon with the OH, that carbon is connected to two other carbons, secondary alcohols become ketones. So, instead of making carboxylic acid, what happens here, is the carbon connected to the OH, it just become C double bond O, everything else stays the same. And then finally we have here one a tertiary alcohol. Tertiary because that carbon with the OH is connected to three other carbons, and we need to realize here is that tertiary alcohols cannot be oxidized, so it say here simply no reaction. So we get the correct answer here you have to first identify the type of alcohol you have, primary, secondary or tertiary, once you do that you know what your product would be at the end.

In Substitution Reactions we react an alcohol with either HCl or HBr to change the OH group into a halogen. 

In Elimination Reactions we react an alcohol with concentrated H2SO4 or H3PO4 to create an alkene. 

In Oxidation Reactions an alcohol reacts with an oxidizing agent and is transformed into carboxylic acid or a ketone