What is the OH in alcohol?

What is the $\ce{OH}$ functional group in alcohol (ethanol)? My teacher told me it isn't the same as the $\ce{OH-}$ ion or hydroxide.
But it does donate one electron to the respective carbon chain and it's very tempting to say it's hydroxide as it fits perfectly into the role. What exactly is it?

• Hydroxyl group, not hydroxide, though I'm not the one that gave -1. Jan 12 '15 at 17:00

You could call ethyl alcohol "ethyl hydroxide" analogously to "sodium hydroxide". But it is a poor analogy, since (unlike NaOH) ethyl alcohol is not an ionic compound, does not dissociate in water to form ethyl cation and hydroxide anion, and does not raise the pH when dissolved in water.

Many people confuse between the three forms of the OH group.

When the OH has a negative charge, it is called hydroxide, and it forms ionic compounds with cations. An example is sodium hydroxide, which contains discrete Na+ ions and OH- ions. When dissolved in water, it separates into Na+ ions and OH- ions (solvation not mentioned for simplicity)

When the OH forms a covalent bond (different from the ionic bond above) with any other group, it is called a hydroxy group. An example is ethanol, H3C-CH2-OH, the oxygen in the OH forming a covalent bond with the carbon in the ethyl group. When you dissolve ethanol in water, it does not separate into C2H5+ and OH- ions like the NaOH would.

The final form of the OH is called a hydroxyl radical. Imagine removing an electron from a negatively charged hydroxide ion to make it charge-neutral. In this form it is very, very reactive, as it lacks an electron to achieve the stable form of hydroxide (like how a halogen behaves).

Well, I don’t understand what you are trying to ask.

But the $\ce{OH}$ you are bothered about can be thought exactly as that of $\ce{OH}$ of water ($\ce{H2O}$)

$\ce{C2H5-OH}$ in the same way as $\ce{H-OH}$

As other answers point out, the hydroxyl group in an alcohol is covalently bonded to carbon and thus is not the same as a hydroxide ion.

But don't assume that the hydroxyl group lacks any basic character. It can still act as a Lewis base towards strong acids like $$\ce{HCl}$$, facilitating nucleophilic substitution reactions between alcohols and these strong acids. The hydroxyl group in an alcohol can also act as a Lewis base towards the $$\ce{-COOH}$$ function in a carboxylic acid, enabling the formation of esters from such acids. You should encounter these properties when you study nucleophilic reactions, including ester formation, in organic chemistry.

Alcohols contain a hydroxyl functional group. This is different from the hydroxide ion, $$\ce{OH-}$$.

By way of example, the formula for the alcohol present in alcoholic drinks, ethanol or ethyl alcohol, is $$\ce{C2H5OH}$$. If you want to emphasize the hydroxyl functional group, you could also write $$\ce{C2H5-OH}$$. To symbolize any alcohol, you could use $$\ce{R-OH}$$. As you can see, this is different from $$\ce{OH-}$$ because there is no formal charge on the hydroxyl group and it is connected to the remainder of the molecule with a covalent bond.

In terms of chemical reactions, you can find similarities in the reactivity of water and alcohols. Many reactions of water and of alcohols involve proton (hydrogen ion, $$\ce{H+}$$) transfer. Water can lose a proton (to make hydroxide, $$\ce{OH-}$$), and alcohols can as well (to make an alkoxide, $$\ce{R-O-}$$). Both can also gain a proton (to make hydronium $$\ce{H3O+}$$ or protonated alcohol, $$\ce{R-OH2+}$$, respectively). So the alcohol is analogous to water, and the alkoxide is analogous to hydroxides in some ways.

This is illustrated below (top, alcohol; bottom, water; left, deprotonated; center, as is; right, protonated):