Chapter 23 – Summary

23.1 Alcohols – Structure, Naming and Classification

The –OH group is the functional group of an alcohol. Ethanol is a commonly known alcohol.  Its common name is ethyl alcohol and has the formula CH₃CH₂OH. In the IUPAC system, alcohols are named by changing the ending of the parent alkane name to -ol. Alcohols are classified as primary, secondary or tertiary according to the number of carbon atoms attached to the carbon atom that is attached to the OH group. More than one -OH group can be in a molecule but typically only one -OH exists per carbon atom.

23.2 Physical Properties of Alcohols

Alcohols are bent molecules similar to water.  The presence of the oxygen-hydrogen bond results in a polar molecule. Alcohols have higher boiling points than do ethers and alkanes of similar molar masses because the OH group allows alcohol molecules to engage in hydrogen bonding. Alcohols of four or fewer carbon atoms are soluble in water because the alcohol molecules engage in hydrogen bonding with water molecules; comparable alkane molecules cannot engage in hydrogen bonding. Adding more -OH groups to a molecule increases its boiling point and solubility.

23.3 Formation of Alcohols

Many alcohols are made by the hydration of alkenes. Ethanol can be made by the fermentation of sugars or starch from various sources. Alcohols can also be produced by the alkyl halide substitution, ester hydrolysis and reduction of aldehydes and ketones.

23.4 Reactions of Alcohols

Alcohols are a highly versatile functional group.  Many other functional groups can be formed from alcohol molecules. Primary alcohols are oxidized to form aldehydes. Secondary alcohols are oxidized to form ketones. Tertiary alcohols are not readily oxidized. Alcohols can be dehydrated to form either alkenes (higher temperature, excess acid) or ethers (lower temperature, excess alcohol). Unsymmetrical alcohols will dehydrate to alkenes following Zaitsev’s rule which says that the more highly substituted double bond isomer is favoured. Alcohols combine with carboxylic acids to form esters through esterification. In some situations, alcohols can be substituted to form alkyl halides.

23.5 Phenols

Phenols are compounds in which an OH group is attached directly to an aromatic ring. Many phenols are used as antiseptics. For the IUPAC nomenclature of phenols, the carbon bonded to the OH group is C1 and all substituents are named according to their location relative to the C1 carbon.

23.6 Ethers

The –R–O–R’– group is the functional group of an ether. It is a derivative of the alcohol and has similar shape to the water molecule. To name ethers, simply name the groups attached to the oxygen with the longest continuous carbon chain as the parent chain and the shorter carbon chain as an alkoxy group. If both groups are the same, it does not matter which side you use as the parent and alkoxy group. Ether molecules have no OH group and thus no intermolecular hydrogen bonding. Ethers therefore have quite low boiling points for a given molar mass. Ether molecules have an oxygen atom and can engage in hydrogen bonding with water molecules. An ether molecule has about the same solubility in water as the alcohol that is isomeric with it.

23.7 Thiols

Thiols, sulfides (thioethers), and disulfides are common in biological compounds that contain carbon. Thiols have a -SH group and disulfides have a -S-S- group. Thiols have strong odours like those of skunk scent and natural gas. To name thiols, the longest continuous chain is located, and the parent chain named as an alkane.  The thiol suffix is added with its location.