The hydroxyl group (OH) is polar. Oxygen has an electronegativity of 3.44, while hydrogen’s is 2.20, creating an electronegativity difference of about 1.4. That’s one of the largest differences found in common organic bonds, making the O-H bond strongly polar with a partial negative charge on oxygen and a partial positive charge on hydrogen.
Why the O-H Bond Is Polar
Polarity comes down to how unevenly two atoms share electrons. Oxygen pulls electrons toward itself much more strongly than hydrogen does. For comparison, a C-H bond has an electronegativity difference of only 0.4 and is considered nearly nonpolar. The O-H bond’s difference of 1.4 puts it firmly in the polar covalent category, well short of the 1.7+ threshold where a bond starts to become ionic, but far enough from zero to create a meaningful charge separation.
This uneven sharing means the oxygen end of the hydroxyl group carries a partial negative charge (written as δ-), while the hydrogen end carries a partial positive charge (δ+). That charge imbalance is what gives the hydroxyl group its distinctive chemical behavior.
How Hydroxyl Groups Interact With Water
The partial charges on a hydroxyl group allow it to form hydrogen bonds, which are the same type of attraction that holds water molecules together. Oxygen has two lone pairs of electrons, so a single hydroxyl group can act as both a hydrogen bond donor (through its partially positive hydrogen) and a hydrogen bond acceptor (through oxygen’s lone pairs). This dual capability makes the hydroxyl group exceptionally good at interacting with water.
Adding even one hydroxyl group to a molecule dramatically increases its hydrophilicity, meaning its tendency to dissolve in or mix with water. This is why ethanol (which is just ethane with a hydroxyl group attached) mixes freely with water, while ethane itself is a gas that barely dissolves at all. The solubility of alcohols in water can be quantitatively predicted based on the ratio of hydrocarbon surface area to hydroxyl surface area in the molecule. As the carbon chain grows longer, the nonpolar portion eventually overwhelms the polar hydroxyl group, and solubility drops. Butanol dissolves reasonably well in water, but octanol barely does.
Hydroxyl Groups in Everyday Molecules
Hydroxyl groups show up in an enormous range of molecules, and their polarity is often the reason those molecules behave the way they do.
- Sugars: Glucose has five hydroxyl groups, which is why it dissolves so readily in water and why your blood can carry it efficiently. The properties of carbohydrates are determined in large part by the number and arrangement of their hydroxyl groups.
- Alcohols: Methanol, ethanol, and isopropanol all owe their ability to mix with water to their hydroxyl group. Branched alcohols tend to be slightly more soluble than their straight-chain counterparts because their shape exposes the hydroxyl group more effectively.
- Cellulose and wood: The hydroxyl groups on cellulose fibers form hydrogen bonds with each other, creating the rigid structure that gives wood its strength.
- Skin and hair: Proteins in your body contain amino acids with hydroxyl-bearing side chains (serine and threonine), which help proteins fold correctly and interact with the watery environment inside cells.
How Hydroxyl Compares to Other Polar Groups
The hydroxyl group is one of the most polar functional groups in organic chemistry, but it’s not the only polar one. The N-H bond has an electronegativity difference of 0.9, making amine groups polar but less so than hydroxyl. The C-O bond sits at 1.0. So oxygen-hydrogen bonds are more polar than both nitrogen-hydrogen and carbon-oxygen bonds.
This stronger polarity is why alcohols generally form stronger hydrogen bonds than amines do, and why replacing an amine group with a hydroxyl group on a molecule typically increases its water solubility. It’s also why water (two O-H bonds) is such an effective solvent for polar and ionic substances.
Polarity and Acid-Base Behavior
The polarity of the O-H bond also explains why hydroxyl groups can lose their hydrogen under the right conditions. In a typical alcohol, the pKa is around 16 to 18, meaning it takes a very strong base to pull that hydrogen off. Water itself has a pKa of 15.7, which is similar. Phenols, where the hydroxyl group is attached to a ring of carbon atoms, are more acidic with a pKa around 10, because the ring helps stabilize the negative charge left behind when hydrogen leaves.
In strongly acidic conditions, the oxygen in a hydroxyl group can actually pick up an extra hydrogen, becoming protonated. This happens at a pKa of roughly -2 to -3, which means it only occurs in very acidic environments. The ability to both donate and accept protons ties directly back to the polar nature of the O-H bond and oxygen’s lone electron pairs.