---
title: "Hydroxyl Group in Organic Chemistry"
description: "Hydroxyl group is the -OH functional group in organic chemistry, shaping alcohol and phenol structure, polarity, hydrogen bonding, and reactivity."
canonical: "https://fiveable.me/organic-chem/key-terms/hydroxyl-group"
type: "key-term"
subject: "Organic Chemistry"
unit: "Unit 22"
---

# Hydroxyl Group in Organic Chemistry

## Definition

The hydroxyl group is the -OH functional group, an oxygen bonded to hydrogen. In organic chemistry, it is the feature that makes alcohols and phenols polar and reactive.

## What It Is

The hydroxyl group is the functional group made of an oxygen atom covalently bonded to a hydrogen atom, written as -OH. In organic chemistry, you usually see it attached to a carbon chain in alcohols or directly on an aromatic ring in phenols.

What makes it stand out is the oxygen atom. Oxygen pulls electron density toward itself, so the O-H bond is polar and the group has a partial negative charge on oxygen and a partial positive charge on hydrogen. That polarity changes how molecules interact with each other and with water, and it also affects which reactions they can undergo.

A hydroxyl group is not just a label on a structure. It changes the physical behavior of the whole molecule. Molecules with -OH can form hydrogen bonds with each other, which raises boiling points compared with similar-sized hydrocarbons. They can also hydrogen bond with water, which often makes small alcohols more soluble than nonpolar molecules of the same size.

In naming, the hydroxyl group is the part that tells you whether a compound is an alcohol or phenol. For alcohols, the carbon bearing the -OH is part of an alkane chain, so you number the chain to give that group the lowest possible position. For phenols, the -OH is attached directly to a benzene ring, and that placement changes the molecule’s acidity and reactivity because the ring can share electron density with oxygen.

The hydroxyl group also shows up as a product in several reaction types you meet early in organic chemistry. Hydration of alkenes can add -OH to a double bond, halohydrin formation uses water to place -OH next to a halogen, and reduction of carbonyl compounds can create alcohols. So when you spot -OH in a mechanism, think about both structure and reactivity: where it came from, how it changes polarity, and what transformations are possible next.

One common mistake is to treat -OH as always the same thing. In reality, a hydroxyl group on an alcohol behaves differently from one on a phenol because the surrounding carbon framework changes electron distribution, acidity, and substitution patterns. The functional group is the same, but the chemical environment changes the behavior.

## Why It Matters

The hydroxyl group is one of the first functional groups that makes organic chemistry feel less like memorizing names and more like predicting behavior. Once you recognize -OH, you can explain why a molecule has a higher boiling point, why it dissolves in water better than a similar hydrocarbon, or why it reacts in a certain way under acidic or basic conditions.

It also connects several major units. When you study alcohol naming, -OH is the feature you number for. When you study phenols, the same group explains why the ring is activated toward electrophilic aromatic substitution and why phenols are more acidic than ordinary alcohols. When you study synthesis, hydroxyl groups often appear as products of alkene addition or carbonyl reduction.

That makes this term useful far beyond one page of notes. You use it to trace where a molecule came from, decide what class it belongs to, and predict what reactions are likely next. If you can recognize the hydroxyl group quickly, you can move faster through mechanisms, product prediction, and comparison questions.

## Connections

### Alcohol

An alcohol is the most common organic compound built around a hydroxyl group attached to a saturated carbon. The presence of -OH is what gives alcohols their naming rules, polarity, and hydrogen-bonding behavior. When you identify an alcohol, you are really identifying a hydroxyl group in a specific carbon framework.

### Phenol

Phenols contain a hydroxyl group directly bonded to an aromatic ring, and that changes the chemistry a lot. The ring can stabilize the conjugate base after deprotonation, which makes phenols more acidic than typical alcohols. The same -OH group also activates the ring toward electrophilic substitution.

### Hydrogen Bonding

The hydroxyl group is one of the easiest places in organic chemistry to see hydrogen bonding in action. The oxygen can accept hydrogen bonds, and the O-H hydrogen can donate them. That interaction explains why many -OH compounds have higher boiling points and better water solubility than similar nonpolar molecules.

### [2-propanol](/organic-chem/key-terms/2-propanol)

2-propanol is a simple example of a molecule whose properties are dominated by one hydroxyl group. Its -OH makes it polar, gives it hydrogen bonding, and makes it behave very differently from propane. It is a good reference molecule when you want to see how one functional group changes an entire compound.

## On the AP Exam

A quiz item or problem set question usually asks you to identify the hydroxyl group in a structure, name the compound correctly, or predict a property or product based on its presence. If the molecule is an alcohol, you may need to number the chain so the -OH gets the lowest possible position, then classify it as primary, secondary, or tertiary from the carbon attached to oxygen.

In mechanism problems, -OH often tells you where addition, reduction, or substitution has happened. You might be asked whether an alkene hydration gives an alcohol, whether a carbonyl reduction forms a hydroxyl-containing product, or why a phenol reacts faster than benzene. In lab-style questions, expect to compare boiling points, solubility, or acidity using hydrogen bonding and polarity as the reason.

## Hydroxyl Group vs Hydrogen Bonding

Hydroxyl group is a structural feature, the actual -OH group in a molecule. Hydrogen bonding is an intermolecular attraction that can happen because of that group. In other words, the hydroxyl group can cause hydrogen bonding, but it is not the same thing as hydrogen bonding.

## Key Takeaways

- The hydroxyl group is the -OH functional group, made of oxygen bonded to hydrogen.
- In organic chemistry, -OH is the signature feature of alcohols and phenols, but the surrounding structure changes how it behaves.
- Because oxygen is electronegative, hydroxyl groups are polar and can hydrogen bond, which affects boiling point and solubility.
- Phenols and alcohols both contain -OH, but phenols are more acidic because their conjugate base is resonance-stabilized.
- When you see -OH in a reaction, think about whether it was added, preserved, or changed into another functional group.

## FAQs

### What is the hydroxyl group in organic chemistry?

The hydroxyl group is the -OH functional group, with an oxygen atom bonded to a hydrogen atom. In organic chemistry, it appears in alcohols and phenols and strongly affects polarity, hydrogen bonding, and reactivity. It is one of the easiest functional groups to spot in a structure.

### Is a hydroxyl group the same as an alcohol?

Not exactly. The hydroxyl group is the functional group, while an alcohol is the type of compound that contains a hydroxyl group attached to a carbon chain. Phenols also contain hydroxyl groups, but because the -OH is attached to an aromatic ring, they behave differently from alcohols.

### Why does a hydroxyl group raise boiling point?

The O-H bond is polar, so molecules with hydroxyl groups can form hydrogen bonds with each other. Those attractions take extra energy to break, so compounds with -OH usually boil at higher temperatures than similar-sized nonpolar molecules.

### How do I recognize a hydroxyl group in a structure?

Look for an oxygen directly bonded to a hydrogen, written as -OH. If that group is attached to a saturated carbon chain, the compound is usually an alcohol. If it is attached directly to a benzene ring, it is a phenol.

## Related Study Guides

- [22.1 Keto–Enol Tautomerism](/organic-chem/unit-22/keto-enol-tautomerism/study-guide/APlVSAsQffXDBMgc)
- [17.10 Reactions of Phenols](/organic-chem/unit-17/reactions-phenols/study-guide/HZSNBWDBfFd0vNsK)
- [17.2 Properties of Alcohols and Phenols](/organic-chem/unit-17/properties-alcohols-phenols/study-guide/LSaKZDxVQV9pJB0O)
- [8.3 Halohydrins from Alkenes: Addition of HO-X](/organic-chem/unit-8/halohydrins-from-alkenes-addition-ho-x/study-guide/VjR3ZwHbUqX80CPz)
- [17.4 Alcohols from Carbonyl Compounds: Reduction](/organic-chem/unit-17/alcohols-from-carbonyl-compounds-reduction/study-guide/X7RRN1fRDwwNl6rj)
- [8.5 Hydration of Alkenes: Addition of H2O by Hydroboration](/organic-chem/unit-8/hydration-alkenes-addition-h2o-hydroboration/study-guide/XYyEKQ4vV9x5lgUM)
- [17.9 Phenols and Their Uses](/organic-chem/unit-17/phenols-their-uses/study-guide/Y8MTuLwM2VIMgda6)
- [17.1 Naming Alcohols and Phenols](/organic-chem/unit-17/naming-alcohols-phenols/study-guide/o7NEMqk3w8HpPKRz)

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