Organic Chemistry: Concepts and Applications - Headley Allan D. 2020

Heteroatomic Functional Groups and Organic Nomenclature
3.1 Properties and Structure of Alcohols, Phenols, and Thiols

Alcohols have the ─OH functional group, and alcohols are very important compounds in organic chemistry and today’s economy. Alcohols that have three or less carbons are some of the most common alcohols. Methanol, which has one carbon, is often used as an alternate fuel for the internal combustion engines. It is a desirable alternative fuel since its combustion is very efficient. It is typically combined directly with gasoline but can be used directly as is the case with some racing cars. Methanol can typically be oxidized to form formaldehyde and formic acid, which are compounds that are used widely in industry. Ethanol has two carbons and is also used as a blend in combination with gasoline for fuel. It is also the alcohol of alcoholic beverages. Since pure ethanol is a strong dehydrating agent, it cannot be consumed directly. Alcoholic liquids, such as whiskey, are mixtures of ethanol and water, and the typical mixture is in the range of 45—50% in ethanol/water by volume. The term proof is an old terminology, which refers to the alcoholic content of an alcoholic mixture and is defined as twice the percentage of alcohol by volume; thus, for example, a 50% alcohol solution, is also referred to as 100 proof. A major health disadvantage in the use of ethanol is that it is oxidized rapidly and can result in cirrhosis of the liver. On the other hand, methanol, which has a very similar structure to ethanol, is toxic to the body; it damages the optic nerve, which results in blindness, so it should never be consumed. Ethylene glycol, another alcohol that has two ─OH groups, is the main component of antifreeze/coolant. Owing to the low vapor pressure, very high boiling point (197 °C), and very low freezing point (−13 °C) of ethylene glycol, it remains a liquid even in the most severe winter and summer weather. Interestingly, propylene glycol, another alcohol that has two ─OH groups, is used in small amounts in the food and cosmetic industries, whereas ethylene glycol is poisonous. Another important alcohol is isopropanol, which is also known as rubbing alcohol. Isopropanol has many household and personal care uses, and it is also used in industry as a solvent.

Phenolic compounds are a unique category of alcohols and will be studied in greater details in Chapter 17, aromatic compounds. Phenolic compounds were first used as antiseptics for surgery, but they were determined to be poisonous because these compounds also kill cells and are easily absorbed through the skin and cause damage to key organs including the kidney, spleen, and pancreas. As a result, extreme care must be exercised in the use of phenolic compounds. Today, most phenols and derivatives of phenol are used as antiseptics and disinfectants; o-phenylphenol and hexachlorophene are two that are commonly used, which are shown below along with other examples of phenolic compounds.

Phenol and phenol derivatives are also found in natural products. For example, thymol and vanillin are the constituents of thyme and vanilla beans are phenolic compounds. BHT (butylated hydroxytoluene) is a food additive that prolongs shelf life and protects against oxidation, and it is also a phenolic compound.

Alcohols are often converted, through different types of reactions, to other important compounds that are essential everyday compounds, such as polymers and pharmaceutical compounds. The alcohol functionality is also found in many biological compounds, such as glucose and even an amino acid, serine. The alcohol functionality is found in many pharmaceutical products, such as formoterol, which is one of the drugs used in the management of asthma and chronic obstructive pulmonary disease (COPD).

Thiols, also known as mercaptans, contain the ─SH functional group, and low molecular weight thiols, such as methanethiol, which has one carbon, have a distinct pungent, very disagreeable characteristic odor, often described as that of rotten eggs. As a result, lower molecular weights thiols are used as additive odorants to natural gas. Since natural gas in its pure form is odorless, the addition of thiols assists in the detection of natural gas, especially in situations where there are leakages. The SH functionality is also found in one of the naturally occurring amino acids, cysteine.

Thiols have a common feature in that a hydrogen atom is bonded to the fairly electronegative atom, sulfur, resulting in the formation of hydrogen bonds among thiol molecules. Similar hydrogen bonds are possible for alcohols, but for alcohols, the hydrogen atom is bonded to the oxygen atom, which is more electronegative, compared to sulfur. As a result, hydrogen bonds that are formed among alcohol molecules are stronger that those formed with thiol molecules. Hence, a thiol, with a comparable molecular weight as an alcohol, has a lower boiling point than the corresponding alcohol.

3.1.1 Types of Alcohols

There are a large number of alcohols. As a result, alcohols are often classified based on the type of carbon that contains the OH functionality. If the ─OH functionality is bonded to a carbon that has only one other alkyl group (R), the alcohol is classified as a primary alcohol (1°). If the ─OH is bonded to a carbon that has two alkyl groups, the alcohol is a secondary alcohol (2°), and if the ─OH group is bonded to a carbon that has three alkyl groups, the alcohol is classified as a tertiary alcohol (3°).

Isopropanol, also known as rubbing alcohol, has two methyl groups bonded to the carbon that has the ─OH functionality, and as a result, it is classified as a secondary alcohol. A similar classification as that described above for alcohols can be carried out for thiols.

Problem 3.1

Classify the following alcohols as primary (1°), secondary (2°), or tertiary (3°).