Why do ethanol and phenol differ in acidity

As a result the phenoxide ion has less tendency than an alkoxide ion to combine with a proton. If groups that stabilize negative charge are attached to the benzene ring, the charge may be further delocalized, and the phenol becomes even more acidic. Groups that are meta directing in aromatic substitution because they repel an adjacent positive charge will increase the acidity of phenol because they stabilize an adjacent negative charge. Consider ethanol as a typical small alcohol.

In both pure water and pure ethanol the main intermolecular attractions are hydrogen bonds. In order to mix the two, the hydrogen bonds between water molecules and the hydrogen bonds between ethanol molecules must be broken.

Energy is required for both of these processes. However, when the molecules are mixed, new hydrogen bonds are formed between water molecules and ethanol molecules. The energy released when these new hydrogen bonds form approximately compensates for the energy needed to break the original interactions.

In addition, there is an increase in the disorder of the system, an increase in entropy. This is another factor in deciding whether chemical processes occur. Consider a hypothetical situation involving 5-carbon alcohol molecules. The hydrocarbon chains are forced between water molecules, breaking hydrogen bonds between those water molecules.

The -OH ends of the alcohol molecules can form new hydrogen bonds with water molecules, but the hydrocarbon "tail" does not form hydrogen bonds. This means that many of the original hydrogen bonds being broken are never replaced by new ones. In place of those original hydrogen bonds are merely van der Waals dispersion forces between the water and the hydrocarbon "tails.

Even allowing for the increase in disorder, the process becomes less feasible. As the length of the alcohol increases, this situation becomes more pronounced, and thus the solubility decreases. Several important chemical reactions of alcohols involving the O-H bond or oxygen-hydrogen bond only and leave the carbon-oxygen bond intact. An important example is salt formation with acids and bases. Alcohols, like water, are both weak bases and weak acids. It is convenient to employ sodium metal or sodium hydride, which react vigorously but controllably with alcohols:.

By this we mean that the equilibrium position for the proton-transfer reaction Equation lies more on the side of ROH and OHe as R is changed from primary to secondary to tertiary; therefore, tert-butyl alcohol is considered less acidic than ethanol:.

However, in the gas phase the order of acidity is reversed, and the equilibrium position for Equation lies increasingly on the side of ROGas R is changed from primary to secondary to tertiary, terf-Butyl alcohol is therefore more acidic than ethanol in the gas phase. This seeming contradiction appears more reasonable when one considers what effect solvation or the lack of it has on equilibria expressed by Equation In solution, the larger anions of alcohols, known as alkoxide ions, probably are less well solvated than the smaller ions, because fewer solvent molecules can be accommodated around the negatively charged oxygen in the larger ions:.

Acidity of alcohols therefore decreases as the size of the conjugate base increases. They do this by polarization of their bonding electrons, and the bigger the group, the more polarizable it is. Also see Section A, which deals with the somewhat similar situation encountered with respect to the relative acidities of ethyne and water.

Alcohols are bases similar in strength to water and accept protons from strong acids. An example is the reaction of methanol with hydrogen bromide to give methyloxonium bromide, which is analogous to the formation of hydroxonium bromide with hydrogen bromide and water:.

Compounds like alcohols and phenol which contain an -OH group attached to a hydrocarbon are very weak acids. This seeming contradiction appears more reasonable when one considers what effect solvation or the lack of it has on equilibria.

In solution, the larger alkoxide ions, probably are less well solvated than the smaller ions, because fewer solvent molecules can be accommodated around the negatively charged oxygen in the larger ions:.

Acidity of alcohols therefore decreases as the size of the conjugate base increases. They do this by polarization of their bonding electrons, and the bigger the group, the more polarizable it is. Alcohols are bases similar in strength to water and accept protons from strong acids.

An example is the reaction of methanol with hydrogen bromide to give methyloxonium bromide, which is analogous to the formation of hydroxonium bromide with hydrogen bromide and water:. Compounds like alcohols and phenol which contain an -OH group attached to a hydrocarbon are very weak acids. Alcohols are so weakly acidic that, for normal lab purposes, their acidity can be virtually ignored.

However, phenol is sufficiently acidic for it to have recognizably acidic properties - even if it is still a very weak acid. A hydrogen ion can break away from the -OH group and transfer to a base. For example, in solution in water:. Phenol is a very weak acid and the position of equilibrium lies well to the left. Phenol can lose a hydrogen ion because the phenoxide ion formed is stabilised to some extent. The negative charge on the oxygen atom is delocalised around the ring.

The more stable the ion is, the more likely it is to form. One of the lone pairs on the oxygen atom overlaps with the delocalised electrons on the benzene ring. This overlap leads to a delocalization which extends from the ring out over the oxygen atom.

As a result, the negative charge is no longer entirely localized on the oxygen, but is spread out around the whole ion. Spreading the charge around makes the ion more stable than it would be if all the charge remained on the oxygen. However, oxygen is the most electronegative element in the ion and the delocalized electrons will be drawn towards it. Note: We are writing the acid as AH rather than HA, because, in all the cases we shall be looking at, the hydrogen we are interested in is at the right-hand end of a molecule.

This equilibrium is sometimes simplified by leaving out the water to emphasise the ionisation of the acid. If you write it like this, you must include the state symbols - " aq ". Hydrogen ions are always attached to something during chemical reactions.

The organic acids are weak in the sense that this ionisation is very incomplete. At any one time, most of the acid will be present in the solution as un-ionised molecules.

The position of equilibrium therefore lies well to the left. The strengths of weak acids are measured on the pK a scale. The smaller the number on this scale, the stronger the acid is. Remember - the smaller the number the stronger the acid. Comparing the other two to ethanoic acid, you will see that phenol is very much weaker with a pK a of In each case, the same bond gets broken - the bond between the hydrogen and oxygen in an -OH group.

Writing the rest of the molecule as "X":. Note: If you aren't sure about coordinate covalent dative covalent bonding , you might like to follow this link. It isn't, however, particularly important to the rest of the current page. In these cases, you seem to be breaking the same oxygen-hydrogen bond each time, and so you might expect the strengths to be similar. Note: You've got to be a bit careful about this. The bonds won't be identically strong, because what's around them in the molecule isn't the same in each case.

The most important factor in determining the relative acid strengths of these molecules is the nature of the ions formed. You always get a hydroxonium ion - so that's constant - but the nature of the anion the negative ion varies markedly from case to case. Ethanoic acid. The acidic hydrogen is the one attached to the oxygen.

You might reasonably suppose that the structure of the ethanoate ion was as below, but measurements of bond lengths show that the two carbon-oxygen bonds are identical and somewhere in length between a single and a double bond. If you don't already understand about the bonding in the carbon-oxygen double bond , you would be well advised to skip this next bit - all the way down to the simplified structure of the ethanoate ion towards the end of it.

It goes beyond anything that you are likely to want for UK A level purposes. If you do choose to follow this link, it will probably take you to several other pages before you are ready to come back here again. Like any other double bond, a carbon-oxygen double bond is made up of two different parts. One electron pair is found on the line between the two nuclei - this is known as a sigma bond.