MCAT Organic Chemistry Review

Alcohols

5.1 Description and Properties

Alcohols have the general formula ROH, with the functional group –OH referred to as a hydroxyl group.

MCAT EXPERTISE

Alcohols are an important group of compounds. They will be seen on the MCAT as protic solvents, reactants, products, and prime examples of hydrogen bonding.

NOMENCLATURE

Alcohols are named in the IUPAC system by replacing the e ending of the root alkane with the ending ol. If the alcohol is the highest-priority functional group, the carbon atom attached to it receives the lowest possible number. Some examples are shown in Figure 5.1.

Figure 5.1. IUPAC Names of Alcohols

Alternatively, the common naming practice is to name the alkyl group as a derivative, followed by alcohol, as shown in Figure 5.2.

Figure 5.2. Common Names of Alcohols

When the alcohol is not the highest-priority group, it is named as a substituent, with the prefix hydroxy–.

Finally, we will also see that hydroxyl groups can be attached to aromatic rings, as in Figure 5.3. These compounds are called phenols. The hydroxyl hydrogens of phenols are particularly acidic due to resonance within the phenol ring. When benzene rings contain two substituents, their relative positions must be indicated. Two groups on adjacent carbons are called ortho–, or simply o–. Two groups separated by a carbon are called meta–, or m–. Two groups on opposite sides of the ring are called para–, or p–.

Figure 5.3. Phenols: Aromatic Alcohols

KEY CONCEPT

Aromatic alcohols are called phenols. The possible resonance between the ring and the lone pairs of the oxygen atom of the hydroxyl group make the hydrogen of the alcohol more acidic than other alcohols.

PHYSICAL PROPERTIES

One of the prominent properties of alcohols is that they are capable of intermolecular hydrogen bonding, which results in significantly higher melting and boiling points than those of analogous hydrocarbons, as shown in Figure 5.4.

Figure 5.4. Intermolecular Hydrogen Bonding in Alcohols

MCAT EXPERTISE

Hydrogen bonding causes increased melting points, boiling points, and solubility in water.

Molecules with more than one hydroxyl group show greater degrees of hydrogen bonding. This is evident from the boiling points shown in Figure 5.5.

Figure 5.5. Boiling Points for Various Alcohols Boiling point increases significantly with additional hydroxyl groups, which permit more hydrogen bonding.

Hydrogen bonding occurs when hydrogen atoms are attached to highly electronegative atoms like nitrogen, oxygen, or fluorine. Hydrogen bonding is the result of the extreme polarity of these bonds. In the case of a hydroxyl group, the electronegative oxygen atom pulls electron density away from the less electronegative hydrogen atom. This generates a slightly positive charge on the hydrogen and slightly negative charge on the oxygen. Then, the slightly positive hydrogen of one molecule electrostatically attract the slightly negative oxygen of another molecule, generating a noncovalent bonding force known as a hydrogen bond.

The hydroxyl hydrogen is weakly acidic, and alcohols can dissociate into protons and alkoxide ions in the same way that water dissociates into protons and hydroxide ions. Table 5.1 gives pKa values of several hydroxyl-containing compounds.

Dissociation

pKa

H2O

HO + H+

15.7

CH3OH

CH3O + H+

15.5

C2H5OH

C2H5O + H+

15.9

i-PrOH

i-PrO + H+

16.5

t-BuOH

t-BuO + H+

17.0

CF3CH2OH

CF3CH2O + H+

12.5

PhOH

PhO + H+

∼10.0

Table 5.1. pKa Values of Hydroxyl-Containing Compounds

BRIDGE

Remember from Chapter 10 of MCAT General Chemistry Review that pKa = –log Ka. Strong acids have high Ka values and low pKa values. Thus, phenol, which has the smallest pKa, is the most acidic of the alcohols listed in Table 5.1.

Looking at Table 5.1, we can see that the hydroxyl hydrogens of phenols are more acidic than those of other alcohols. This is due to the aromatic nature of the ring, which allows for the resonance stabilization of the negative charge on oxygen, stabilizing the anion. Like other alcohols, phenols form intermolecular hydrogen bonds and have relatively high melting and boiling points. Phenol is slightly soluble in water, owing to hydrogen bonding, as are some of its derivatives. Because phenols are much more acidic than nonaromatic alcohols, they can form salts with inorganic bases such as NaOH.

The presence of other substituents on the ring has significant effects on the acidity, boiling points, and melting points of phenols. As with other compounds, electron-withdrawing substituents increase acidity, and electron-donating groups decrease acidity.

KEY CONCEPT

Charges like to be spread out as much as possible. Acidity decreases as more alkyl groups are attached because they are electron-donating, which destabilizes the alkoxide anion. Resonance or electron-withdrawing groups stabilize the alkoxide anion, making the alcohol more acidic.

Another trend seen in Table 5.1 is that the presence of more alkyl groups in nonaromatic alcohols produces less acidic molecules. Because alkyl groups donate electron density, they destabilize a negative charge. Additionally, alkyl groups help stabilize positive charges, explaining why more substituted carbocations have higher stability than less substituted carbocations.

MCAT Concept Check 5.1:

Before you move on, assess your understanding of the material with these questions.

1.    Which has a lower pKa: ethanol or p-ethylphenol Why?

2.    Rank the following by decreasing boiling point: 1-pentanol, 1-hexanol, 1,6-hexanediol

1.     

2.     

3.