Leaving Group Conversions - Using HX - Online Tutor, Practice Problems & Exam Prep

Alcohols are crucial in organic chemistry but are poor leaving groups due to their instability after leaving as hydroxide ions (OH^-). To enhance their reactivity, alcohols can be converted into alkyl halides or sulfonate esters. Secondary and tertiary alcohols typically undergo an SN1 mechanism, forming stable carbocations, while primary alcohols follow an SN2 mechanism, allowing for backside attack. Using strong acids like HX facilitates these conversions, making alkyl halides more functional in subsequent reactions.

Why do we need to convert Alcohol into a good leaving group?

As you guys might have figured out by now, alcohols are a pretty important functional group for organic chemistry, but there's one major limitation of alcohols and that's that they make terrible leaving groups. Okay? Remember that the definition of a good leaving group is something that once it leaves, it's stable. Well, alcohol after it leaves, it becomes OH^-. OH^- is the same as a hydroxide base, which is a very unstable molecule. It's a very strong base. So that means that whenever we have an alcohol, we're a little bit stuck. We don't know exactly what to do with it because a lot of reactions in organic chemistry require leaving groups and alcohol isn't a good option. But wait, there is a solution. It turns out that a major topic that we're going to discuss in this section is how to turn alcohol into a good leaving group.

It turns out that there are 2 major options like a fork in the road. We can take 2 major pathways, and they're both going to lead to awesome outcomes. They're both going to lead to alcohol being a much better leaving group. Let's go ahead and talk about the first one. The first option that we have is to convert alcohol simply into an alkyl halide. Now remember that alkyl halides have the molecular formula RX, and the reason that they're such good leaving groups is because X^-, once it takes off, is very stable. X could stand for iodine or bromine or chlorine. These are very electronegative atoms that don't mind having a negative charge. So alkyl halides are an awesome option. But another option that we also have that we'll discuss in a little bit is sulfonate esters. Now you might have learned already that sulfonate esters make great leaving groups. I actually have talked about that before, but now what we're going to learn is how to actually turn an alcohol into a sulfonate ester so that it can become a great leaving group. Okay?

Using HX acids via SN2 reaction.

About a primary alcohol? Well, if we have a primary alcohol, then the only thing that really changes is the mechanism. Primary alcohols are really good at having a backside and they're really bad at making carbocations. That means if I use HX, once again, I'm going to protonate in my first step. So what I'm going to wind up getting is O H 2 +. But if this is left by itself, and then obviously I would get the X leaving by itself, so I'd get X - . But the problem is that this can't just leave and make a stable carbocation. So instead what we're going to do is we're going to do a straight-up backside attack where the hits the backside and kicks out the water. Now the reason this is possible here, but it wasn't possible with the other situation is that the primary alcohol has a much better backside. It has a good backside. Since it has a good backside, it's easy for my X to just kick out the water all in one step. So what we're going to get here is an alkyl halide once again. But in this case, my mechanism was different. I used SN2 instead of SN1. The same exact thing would apply if I used HCl and the Lucas reagent. What I would wind up getting is O H 2 ZnCl 2 +. And what you would get in the second step is that my chlorine would do a backside attack and kick out that entire complex. So then in this case, if I was reacting specifically with those reagents, I would get a chlorine. Alright? So really this isn't that bad. I just went through the mechanisms so you guys will understand it, but really all you need to know is HX. It's really that easy. You just take HX and you can convert an alcohol to an alkyl halide. All right. Awesome. So let's go ahead and move on.

Here’s what students ask on this topic:

Why are alcohols considered poor leaving groups in organic chemistry?

Alcohols are considered poor leaving groups because, upon leaving, they form hydroxide ions (OH^-), which are highly unstable and strong bases. A good leaving group should be stable after departure, but OH^- is not. This instability makes alcohols less reactive in many organic reactions that require a good leaving group.

How can alcohols be converted into better leaving groups?

Alcohols can be converted into better leaving groups by transforming them into alkyl halides or sulfonate esters. This is typically done using strong acids like HX (where X can be Br or I). The conversion process involves protonating the alcohol to form water, which is a much better leaving group, or forming a sulfonate ester, which is also a good leaving group.

What is the mechanism for converting secondary and tertiary alcohols into alkyl halides using HX?

Secondary and tertiary alcohols are converted into alkyl halides via an SN1 mechanism. The process involves protonating the alcohol with HX to form water, which then leaves, creating a stable carbocation. This carbocation is then nucleophilically attacked by the halide ion (X^-), resulting in the formation of an alkyl halide.

What is the Lucas reagent and how is it used in converting alcohols to alkyl halides?

The Lucas reagent is a mixture of HCl and zinc chloride (ZnCl₂). It is used to convert alcohols, particularly primary and secondary alcohols, into alkyl halides. The ZnCl₂ acts as a Lewis acid, enhancing the leaving ability of the hydroxyl group by forming a complex with it, making it easier for the chloride ion to perform a nucleophilic attack and displace the complex, resulting in the formation of an alkyl halide.

What is the difference between SN1 and SN2 mechanisms in the context of alcohol conversion to alkyl halides?

In the context of alcohol conversion to alkyl halides, the SN1 mechanism is used for secondary and tertiary alcohols and involves a two-step process: formation of a carbocation intermediate followed by nucleophilic attack. The SN2 mechanism, used for primary alcohols, involves a single-step backside attack where the nucleophile displaces the leaving group simultaneously. The choice of mechanism depends on the stability of the carbocation and the steric hindrance around the alcohol.