Alcohol Nomenclature - Online Tutor, Practice Problems & Exam Prep

Alcohols are organic compounds characterized by the presence of one or more hydroxyl (–OH) groups. When a molecule contains multiple hydroxyl groups, it is referred to as a glycol. Specific naming conventions apply: a compound with two hydroxyls is called a diol, while three hydroxyls yield a triol. In IUPAC nomenclature, the hydroxyl group takes priority in numbering the carbon chain, often summarized by the phrase "alcohol beats all." This prioritization is crucial when other functional groups, such as alkenes or alkyl halides, are present.

How to name polyols.

A functional group that we definitely need to know how to name is alcohols. And they're really not so hard, so let's just jump straight into it. So a word that we use to describe a molecule that has more than 1 OH or more than 1 hydroxyl on the carbon chain is a glycol. Glycol is a very unspecific word because it just refers to any molecule that has more than 1 hydroxyl on it. So instead of using the term glycol, a lot of times we're going to use a more specific term. In fact, we're going to use prefixes to indicate exactly how many OHs are on that chain. So as you can imagine, we're just going to use the same prefixes that we use for all IUPAC nomenclature. If you have an alcohol with 2 hydroxyls, that's going to be called a diol. Okay? And if you have an alcohol with 3 hydroxyls, you can imagine that's going to be called a triol and that would keep going to tetra, etcetera. Okay? Now keep in mind that something that's unique about alcohol groups is that you always give the most priority or the highest priority in terms of the way that you number the chain or the way that you number the ring to the OH group. Okay? So there's actually this phrase that we use in Organic Chemistry 1 that just says alcohol beats all. Okay? And that just means going to be all the other functional groups that you're really exposed to in Organic Chemistry 1. If you have a double bond, triple bond, alkyl halide, anything else present, you're going to give your priority in terms of prioritizing numbering to the alcohol. Okay? So let's just go ahead and jump into these examples. I want you guys to try to solve it on your own and then I'll go ahead and jump in and give you guys the answer.

Provide the correct common and IUPAC name.

All right. So let's go ahead and start off with the root name. The root for this compound would be cyclohexane. So let's go ahead and write that down. Great. So we've got cyclohexane, but obviously, I have some substituents coming off of this. In fact, these are functional groups. I have 2 alcohols, so I can imagine that my modifier is going to be diol. So I'm just going to put here mod is diol. So I could put these two words together. And I know that this is going to be a cyclohexanediol. Okay. So I just put the modifier at the very end of the root. Now, we need locations, right? Because I don't know exactly where these alcohols are. What we would do is we would just give top priority to one of the alcohols. It doesn't matter which one you choose since they're both the same thing. But we do know that if one had been an alcohol and the other one was, for example, a halogen, I would give my highest priority to the alcohol, right? Because alcohol beats all. So let's just pick this as my one. That means that my other alcohol is going to be at the 3. So this is going to be a 1,3-cyclohexanediol. So are we done? We are so close, but there's one more thing we have to add and that's going to be the relationship of the OHs to each other in terms of where they are in space or their stereochemistry. So remember that if you have 2 groups on the same side of the ring or different sides of the ring, they get different names, right? So if they're on different sides of the ring, that would be considered trans. This would be the trans stereo isomer of 1,3-cyclohexanediol. All right? So not so bad. Let's go ahead and try to name this other example. Go ahead and try to do it on your own, and then I'll give you guys the answer.

Here’s what students ask on this topic:

What is the IUPAC naming convention for alcohols with multiple hydroxyl groups?

In IUPAC nomenclature, alcohols with multiple hydroxyl groups are named using specific prefixes to indicate the number of hydroxyl groups. A compound with two hydroxyl groups is called a diol, while one with three hydroxyl groups is called a triol. The prefixes continue as tetra, penta, etc., for four, five, and more hydroxyl groups. Additionally, the hydroxyl group takes priority in numbering the carbon chain, meaning the carbon attached to the hydroxyl group gets the lowest possible number. This rule is often summarized by the phrase 'alcohol beats all,' indicating that the hydroxyl group takes precedence over other functional groups like alkenes or alkyl halides in the numbering process.

How do you prioritize numbering in a carbon chain with an alcohol group?

When numbering a carbon chain that contains an alcohol group, the hydroxyl group takes the highest priority. This means that the carbon atom bonded to the hydroxyl group should receive the lowest possible number. This rule is crucial when other functional groups, such as double bonds, triple bonds, or alkyl halides, are present. The phrase 'alcohol beats all' is often used to remember this rule, emphasizing that the hydroxyl group outranks other functional groups in the numbering process.

What is a glycol in organic chemistry?

A glycol in organic chemistry refers to any molecule that contains more than one hydroxyl (–OH) group. The term is quite unspecific and can apply to a wide range of compounds. For more precise naming, specific prefixes are used: a compound with two hydroxyl groups is called a diol, while one with three hydroxyl groups is called a triol. These prefixes help to clearly indicate the number of hydroxyl groups present in the molecule.

Why is the phrase 'alcohol beats all' important in organic chemistry nomenclature?

The phrase 'alcohol beats all' is important in organic chemistry nomenclature because it highlights the priority of the hydroxyl group in numbering the carbon chain. When naming a compound, the carbon atom attached to the hydroxyl group should receive the lowest possible number, even if other functional groups like double bonds, triple bonds, or alkyl halides are present. This rule ensures consistency and clarity in the naming of alcohols and compounds containing alcohol groups.

How do you name an alcohol with three hydroxyl groups?

An alcohol with three hydroxyl groups is named using the prefix 'triol.' In IUPAC nomenclature, the base name of the compound is derived from the longest carbon chain containing the hydroxyl groups, and the suffix '-triol' is added to indicate the presence of three hydroxyl groups. Additionally, the carbon atoms bonded to the hydroxyl groups should be numbered to give the lowest possible numbers to these carbons. For example, propane-1,2,3-triol is a triol with hydroxyl groups on the first, second, and third carbon atoms of a propane chain.