Phenol: Physical properties of phenol

Hydrogen bonding in phenol

Phenol is often compared to alcohols since both contain a hydroxyl group (-OH), however the reactions and properties of phenol are generally quite different and unique from alcohols. One way in which they are both similar is that both phenol and alcohols contain intermolecular hydrogen bonding between neighbouring molecules. hydrogen bonding in phenol and alcohols

The hydroxyl bond (O-H) in phenol is a polar one with a δ+ hydrogen atom and a δ- oxygen atom. Therefore the hydrogen atom on one phenol molecule will form hydrogen bonds with the lone pair of electrons on an oxygen atom in a neighbouring molecule as shown in the diagram opposite. The presence of this hydrogen bonding in phenol will result in an elevated melting and boiling point. Phenol melts at 410C and has a boiling point of 1820C, much higher than would be expected from a small molecule with a low molecular mass. Alcohols like phenol contain a polar hydroxyl group and will therefore exhibit similar hydrogen bonding between different alcohol molecules.

Solubility of phenol

When a substance dissolves there must be some sort of interaction between the solute and the solvent molecules, this interaction is likely to be one or more types of intermolecular bonding. Since phenol molecules will engage in hydrogen bonding it will show some solubility in water at room temperature. The polar hydrogen atom in phenol will form hydrogen bonding with the lone pairs of electrons on a water molecule.

However a phenol molecule also contains a large covalent phenyl group (-C6H5) attached to the hydroxyl group (-OH) and since the bonding here is covalent you would expect this group to be insoluble in water. So as you might expect phenol shows some solubility in water due to the presence of hydrogen bonding with the water molecules, despite the presence of the large covalent aromatic ring on the molecule. Approximately 8.3g of phenol will dissolve in 100ml of water at room temperature. If you add more phenol a rather unusual phenomenon occurs. Two liquid layers will form in the flask, one layer is a solution of phenol in water and the other is a solution of water in phenol. However if these two layers are heated to around 700C a single layer forms which is a solution phenol in water. The phenol dissolves in water mainly due to its ability to form hydrogen bonds with water molecules, as shown below.

hydrogen bonding between water molecules and phenol molecule

Acidity of phenol

The reactions of phenol as we have mentioned are often compared to that of alcohols, since they both contain the hydroxyl group (R-OH). However there are many differences in the reactions of alcohols and phenols, one area where they differ greatly is in their acidity. Phenol and many of its compounds are weak acids, while alcohols are very very poor acids, for example ethanol is less acidic than water. The table below gives an indication of the relative strengths of several acids. Recall that the larger the Ka and the smaller the value of pKa the stronger the acid.

acid formula Ka pKa
ethanoic CH3COOH 1.76 x 10-5 4.75
carbonic H2CO3 4.3 x 10-7 6.37
phenol C6H5OH 1.6 x -10 9.8
water H2O 1 x 10-14 14
ethanol C2H5OH 1.3-16 15.9

Comparing the acidity of phenol and the alcohol ethanol

Using the pKa values in the table above we can clearly see that phenol is much more acidic than a typical alcohol such as ethanol. The hydrogen atom in the O-H bond in phenol has a much larger δ+ charge than the hydrogen atom in the hydroxyl group in ethanol due to the fact that the electrons in the lone pair on the oxygen atom in phenol are partly withdrawn into the aromatic electron delocalisation in the ring. This means that the hydrogen atom is more acidic, that is it is easier to be removed as a hydrogen ion (H+). This is outlined in the diagram below:

comapring the acidity of phenol and ethanol

Recall that acidity is a property of water, so when phenol is added to water it partly dissolves to form a weak acid, often called carbolic acid. Phenol partly dissolves in water to form a hydrogen ion (H+) and a phenoxide ion (C6H5O-):

phenol dissolving in water equation

The resonance stabilised phenoxide ion

The image below shows each of the resonance stabilised structures for the phenoxide ion

resonance stabilised hybrids for the phenoxide ion

Reaction with alkalis

Although phenol dissolves in water to form a weak acid, it is acidic enough to undergo acid base reactions to form a salt and water. So while phenol is only slightly soluble in water it reacts with alkalis such as sodium hydroxide to form soluble salts e.g.

reaction of phenol and sodium hydroxide

Acids and metal carbonates

A reaction you have seen before is when an acid reacts with a metal carbonate such as sodium carbonate or calcium carbonate to form a salt, water and carbon dioxide gas, an equation for this reaction is shown below:

metal carbonate(s) + acid(aq) → salt(aq) + water(l) + carbon dioxide(g)

Indeed this reaction is often used as a test for acidity. This simple test is used with both mineral acids such as hydrochloric and sulfuric acids as well as carboxylic acids. However while minerals acids (pKa < 1) and carboxylic acids such as ethanoic (pKa =4.75) are sufficiently acidic to react with carbonates and solutions of carbonates to release carbon dioxide phenol is a much weaker acid (pKa= 9.8) and it is not able to produce carbon dioxide from metal carbonates.

Comparing the reactions of alcohols and phenol with metals

As an example consider the reaction of sodium with alcohols. This reaction is very similar to that of sodium with water. At first glance this may seem odd as alcohol and water appear to have very different structures. However all the parts of the alcohols circled in the image below do not take part in any of the chemical reactions of the alcohol molecules, only the hydroxyl functional group (-OH) group takes part in its reactions.

reaction of alcohol and sodium

So if you swap all these circles groups and simply replace them with an alkyl group (-R) label to represent them, then we have:

Recall the reaction of sodium with water:

sodium(s) + water(l) sodium hydroxide (aq) + hydrogen(g)
2Na(s) + 2H2O(l) 2NaOH(aq) + H2(g)

Well the hydroxide ion (OH-) in the sodium hydroxide is simply a water molecule (H2O), or H-OH which has lost a hydrogen, this lost hydrogen is given off as hydrogen gas during the reaction.

When sodium reacts with alcohols the same reaction occurs. The sodium removes the hydrogen attached to the hydroxyl functional group in the alcohol. The lost hydrogen is given off as hydrogen gas.

2R-OH + 2Na 2RO-Na+ + H2
The RO- ion which forms (compare with the hydroxide ion, OH-, which forms when sodium and water react) is called the alkoxide ion e.g.
sodium(s) + methanol(l) → sodium methoxide (aq) + hydrogen(g)
2Na(s) + 2CH3OH(l) 2CH3ONa (aq) + H2(g)

now an almost identical equation, but time the alcohol reacting is ethanol:
sodium(s) + ethanol(l) sodium ethoxide (aq) + hydrogen(g)
2Na(s) + 2C2H5OH(l) → 2C2H5ONa (aq) + H2(g)
and finally sodium reacting with the alcohol propanol

sodium(s) + propanol(l) → sodium propoxide (aq) + hydrogen(g)
2Na(s) + 2C3H7OH(l) → 2C3H7ONa (aq) + H2(g)
These equations are all very similar the alcohol loses its hydrogen from the -OH group and it is replaced by sodium- that’s all there is to it!

Trends in the reactions of alcohols with sodium metal

As mentioned above the structure of an alcohol can be considered in some ways similar to that of a water molecule. However as the chain length of the alcohol grows then the rate of reaction of the alcohols with sodium begin to slow down. Methanol, CH3OH is the smallest alcohol and it reacts the fastest with sodium. Ethanol C2H5OH, the next alcohol reacts more slowly with sodium and propanol the next alcohol in the homologous series reacts even more slowly. This is shown in the image below where the rate of release of hydrogen gas gives an excellent indicator of the speed of the reaction taking place.

sodium reacting with methanol, ethanol and propanol

Phenol and alkali metals

Phenol being a weak acid will react with alkali metals to give, as you might expect a salt and hydrogen gas. Since phenol is a weak acid this reaction is slow. It is usually carrried out by placing a small amount of solid phenol in a boiling tube and warming gently until the phenol melts forming a liquid. A small piece of sodium or potassium can then be added to the molten phenol. The salt formed will be a phenoxide e.g.

sodium(s) + phenol(l) → sodium phenoxide (s) + hydrogen(g)
Na(s) + C6H5OH(l) → C6H5O-Na+ (s) + 1/2H2(g)
equation for sodium and phenol reacting

Key Points