Hydroxyl Group: The -OH functional group found in alcohols
What Exactly is a Functional Group?
In organic chemistry, a functional group is a specific cluster of atoms within a molecule that determines its characteristic chemical reactions and many of its physical properties. Think of it as the "personality" of the molecule. While the long carbon chains in molecules like octane (in gasoline) are relatively unreactive, attaching a functional group like -OH completely transforms the molecule, making it soluble in water and reactive. The hydroxyl group is one of the most common and important functional groups.
The Structure and Bonding of the -OH Group
The hydroxyl group is not just a random mix of oxygen and hydrogen. It has a specific structure. The oxygen atom is bonded to a carbon atom of an organic group (represented as 'R') with a single covalent bond, and to a hydrogen atom with another single covalent bond. The oxygen atom has two lone pairs of electrons. This structure, with polar bonds and lone pairs, is the source of its power.
The bond between the oxygen and hydrogen (O-H) is highly polar because oxygen is much more electronegative than hydrogen. This means the oxygen atom pulls the shared electrons closer to itself, giving it a partial negative charge ($\delta^-$), and the hydrogen gets a partial positive charge ($\delta^+$). This polarity is crucial for the group's behavior.
Classifying Alcohols by Their -OH Group
Not all alcohols are the same. They are classified based on the number of other carbon atoms attached to the carbon that bears the hydroxyl group. This classification affects the alcohol's properties and reactivity.
| Classification | Description | Example (Name & Structure) |
|---|---|---|
| Primary (1°) | The carbon with the -OH group is attached to only one other carbon atom. | Ethanol $CH_3-CH_2-OH$ |
| Secondary (2°) | The carbon with the -OH group is attached to two other carbon atoms. | Isopropanol (Rubbing Alcohol) $(CH_3)_2CH-OH$ |
| Tertiary (3°) | The carbon with the -OH group is attached to three other carbon atoms. | tert-Butyl Alcohol $(CH_3)_3C-OH$ |
How the Hydroxyl Group Influences Physical Properties
The hydroxyl group dramatically changes how a molecule interacts with its environment. Let's compare an alkane, like hexane ($C_6H_{14}$), with its alcohol counterpart, hexanol ($C_6H_{13}OH$). Hexane is a liquid that floats on water and doesn't mix with it, while hexanol, though not infinitely soluble, is much more miscible with water. The key to this difference is hydrogen bonding.
In alcohols, the $\delta^+$ hydrogen of the -OH group in one molecule is attracted to the $\delta^-$ oxygen of the -OH group in another molecule. This creates a network of strong attractions between molecules.
Boiling Point: More energy (heat) is required to separate molecules that are held together by hydrogen bonds. This is why methanol ($CH_3OH$) has a boiling point of 65°C, while its alkane counterpart, methane ($CH_4$), which only has weak London forces, is a gas at room temperature with a boiling point of -162°C.
Solubility in Water: Water is a pro at hydrogen bonding. For a substance to dissolve in water, it must be able to "plug into" water's hydrogen-bonding network. Alcohols can do this because their -OH groups can form hydrogen bonds with water molecules. Smaller alcohols like methanol and ethanol are completely miscible with water. As the carbon chain gets longer, the non-polar hydrocarbon part starts to dominate, and the solubility decreases.
Common Chemical Reactions of Alcohols
The hydroxyl group makes alcohols participants in several important chemical reactions. Here are a few key ones:
Combustion: Like many organic compounds, alcohols burn in the presence of oxygen, producing carbon dioxide and water, along with a lot of heat. This is the reaction that occurs in an alcohol burner. The general equation is:
$2C_2H_5OH + 7O_2 \rightarrow 4CO_2 + 6H_2O + Heat$
Oxidation: Primary alcohols can be oxidized to form aldehydes, which can then be further oxidized to carboxylic acids. Secondary alcohols are oxidized to ketones. Tertiary alcohols are generally resistant to oxidation. This is the principle behind the breathalyzer test, where ethanol in a person's breath is oxidized to acetic acid.
Dehydration: This is an elimination reaction where an alcohol loses a water molecule to form an alkene. This requires a catalyst, like concentrated sulfuric acid. For example, ethanol can be dehydrated to ethene:
$CH_3-CH_2-OH \rightarrow CH_2=CH_2 + H_2O$
Hydroxyl Groups in Action: From Hand Sanitizer to Sugar
The hydroxyl group is not just a laboratory curiosity; it's all around us. Let's look at some concrete examples:
Ethanol in Hand Sanitizer and Beverages: Ethanol ($CH_3CH_2OH$) is effective in hand sanitizer because it can denature proteins in bacterial and viral cell membranes, killing them. Its ability to mix with both water and oils helps it penetrate. The same molecule, when diluted and flavored, is the alcohol in beer, wine, and spirits. The body metabolizes it using oxidation reactions.
Glycerol in Skincare: Glycerol (or glycerin) is a triol, meaning it has three hydroxyl groups ($C_3H_5(OH)_3$). All these -OH groups make it extremely good at forming hydrogen bonds with water, which is why it is hygroscopic—it attracts and holds water from the air. This property makes it a fantastic humectant in lotions and creams, helping to keep skin hydrated.
Sugars and Carbohydrates: Table sugar (sucrose) and other carbohydrates like glucose are loaded with hydroxyl groups. These groups allow sugar to dissolve easily in your tea or coffee through hydrogen bonding. The many -OH groups also allow for the complex structures of starch and cellulose in plants, which are essential for energy storage and building cell walls.
Antifreeze in Cars: Ethylene glycol, with the formula $(CH_2OH)_2$, has two hydroxyl groups. This structure allows it to form strong hydrogen bonds with water, which lowers the freezing point of the water mixture (preventing the radiator from freezing in winter) and raises its boiling point (preventing it from boiling over in summer).
Important Questions
No, this is a very important distinction. The hydroxyl group (-OH) is a covalent part of an organic molecule. The hydroxide ion ($OH^-$) is a negatively charged ion, consisting of an oxygen and hydrogen atom, that exists in ionic compounds like sodium hydroxide ($NaOH$). Alcohols are generally neutral, while solutions containing hydroxide ions are basic.
Ethanol has a short carbon chain and a strong hydroxyl group. The -OH group forms hydrogen bonds with water molecules, allowing it to mix freely. Oil molecules are large hydrocarbons with no polar groups. They are held together only by weak London dispersion forces and cannot form hydrogen bonds with water, so they separate.
Absolutely! Molecules with one -OH group are simply called alcohols. Molecules with two -OH groups are called diols or glycols (e.g., ethylene glycol in antifreeze). Molecules with three -OH groups are called triols (e.g., glycerol). Sugars have many -OH groups, which is why they are called polyhydroxy compounds.
Footnote
1 Functional Group: A specific grouping of atoms within a molecule that determines its characteristic chemical reactions.
2 Alkyl Group: A group of atoms derived from an alkane by removing one hydrogen atom (general formula $C_nH_{2n+1}$-).
3 Polar Bond: A covalent bond between two atoms where the electrons are unequally shared, due to a difference in electronegativity.
4 Hydrogen Bond: A strong type of intermolecular attraction between a hydrogen atom bonded to a highly electronegative atom (O, N, F) and a lone pair on another electronegative atom.
5 Hygroscopic: The ability of a substance to attract and hold water molecules from the surrounding environment.