306

Unit 3
Elements of Organic Chemistry

3-6. ALCOHOLS

Alcohols are hydroxyl (-OH) derivatives of hydrocarbons formed by replacing a hydrogen with the hydroxyl radical and are of the general form R-OH where R represents the hydrocarbon. There are three classes of alcohols: primary, secondary, and tertiary. A primary alcohol is one in which the hydroxyl group is attached to a carbon atom which, in turn, is attached to not more than one other carbon atom. A secondary alcohol is one in which the hydroxyl group is attached to a carbon atom which in turn, is connected to two carbon atoms. A tertiary alcohol is one in which the hydroxyl group is attached to a carbon atom which in turn, is attached to three other carbon atoms.

803fig0310.jpg (2677 bytes)

Primary Alcohol
Ethanol
(ethyl alcohol)

803fig0311.jpg (3315 bytes)

Secondary Alcohol
2-propanol
(isopropyl alcohol)

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Tertiary Alcohol

Alcohols that contain two or more hydroxyl groups are referred to as polyhydroxy alcohols. An example you will encounter frequently in this course and on the job is glycerin, which is:

803fig0313.jpg (4342 bytes)

Properties of Alcohols. The low-molecular-eight alcohols are volatile liquids, and the high-molecular-weight alcohols (more than 13 carbons) are solids. The first three alcohols (C1 to C3) are completely miscible (mix in any proportion) with water. The water solubility decreases as the number of carbons increases, and the largemolecular-eight alcohols are insoluble in water. Alcohols have higher boiling points and melting points than alkanes with the same or similar molecular weights (MW). For example:

MW MELTING POINT BOILING POINT
CH3-CH2-CH2-CH2-OH 74 – 90C 118C
CH3-CH2-CH2-CH2-CH3 72 –-1 30C 36C

The water solubility and the high melting and boiling points of alcohols result from their ability to form hydrogen bonds with water and to form hydrogen bonds intermolecularly (between themselves).

Reactions of Alcohols. Chemically, the alcohols can be considered to be neutral (in terms of acids and bases) even though they can act as very weak acids or bases as water does. They undergo several kinds of chemical reactions, the most important of which is oxidation. Oxidation in organic chemistry is defined as the elimination of hydrogen from or the addition of oxygen to a compound.

(1) The oxidation of a primary alcohol can be expressed by the following example:

KMnO4 O   O2 O ll

CH3-OH ----------------------- > HCH -- > H—C—OH '

H    O

NOTE: C=O and -C-OH are two more functional groups, indicating aldehydes and carboxylic acids, respectively.

The first step in this oxidation is the removal of two hydrogen atoms from the alcohol to form an aldehyde, and the second step is the addition of one oxygen atom to the aldehyde to form a carboxylic acid.

(2) Secondary alcohols undergo only the first step. For example, a three-carbon alcohol is oxidized to form CH3 –C –CH3 , which is an example of a new class of compounds called ketones. ll

O

(3) Tertiary alcohols are not oxidized.

 

(4) One reason the oxidation reaction is important is that it is the means the body uses to eliminate the popular liquid, ethyl alcohol, or ethanol (CH3-CH2-OH).

Uses of Alcohols. Alcohols are most commonly used as solvents in the pharmacy. They are also used as disinfectants and antiseptics.