Alcohols and Phenols

Alcohols are organic compounds which contain a hydroxyl group $(-\text{OH})$ attached to a saturated carbon atom. Generally they can be represented as $\text{R}-\text{OH},$ where $\text{R}$ is a substituent (which generally refers to alkyl groups). The following are some examples of alcohols:

• Methanol $(\text{CH}_3\text{OH})$
  
• Ethanol $(\text{C}_2\text{H}_5\text{OH})$
  
• 1-propanol $(\text{C}_3\text{H}_7\text{OH})$
  
• 1-butanol $(\text{C}_4\text{H}_9\text{OH})$
  
• 1-pentanol $(\text{C}_5\text{H}_{11}\text{OH})$
  
• 1-hexanol $(\text{C}_6\text{H}_{13}\text{OH})$
  

Like acids, one property of alcohols $(\text{ROH})$ is that they react with alkaline metals $(\text{M})$ to produce hydrogen:

$$2\text{ROH}+2\text{M}\rightarrow2\text{ROM}+\text{H}_2.$$

The most famous one of alcohol is, as we know, ethanol also called as ethyl alcohol. It has got many special characteristics of its own and it undergoes a number of reactions to give different useful products. Pure alcohol $C_2H_5OH$ (100%) is called as absolute alcohol.

Another famous characteristic of alcohols is that they easily undergo oxidation. Primary alcohols oxidize to form aldehydes, and aldehydes again oxidize to produce carboxylic acids. Secondary alcohols can only oxidize once, which results in ketones. Tertiary alcohols cannot be oxidized, as the carbon atom has no orbitals left for forming a double bond with the oxygen atom.

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Phenol is a type of alcohol where the hydroxyl group is bound to an aromatic ring. It is an exception to the rule that the hydroxyl group must be attached to a saturated carbon. Phenol and its chemical derivatives are the key ingredient to many compounds such as nylon, detergents, herbs, and pharmaceutical drugs. The following is the structural formula of phenol:

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However, acetic acid $(\text{CH}_3\text{COOH})$ is not an alcohol despite having a hydroxyl group. This is because the carbon atom to which the hydroxyl group is attached is unsaturated, as it is double bonded with an oxygen atom. Instead, acetic acid is a carboxylic acid.

Since alcohols contain the hydroxyl group, they are capable of forming hydrogen bonds with other compounds like water. Since alkyl groups are considered to be non-polar, alcohols tend to have a higher solubility in water when the carbon chain of the substituent $(\text{R})$ is small. Thus alcohols like methanol or ethanol tend to mix well with water, whereas alcohols with more than four carbons (e.g. pentanol, hexanol) are considered insoluble in water.

Most alcohols are very weak acids. Though the pH of an alcohol solution is almost neutral, they are still able to react with strong bases. Phenol is relatively strong an acid compared to alkyl alcohols.

Which of the following compounds is NOT an alcohol?

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(a) Note that the hydroxyl group is attached to an unsaturated carbon atom. This compound is called benzoic acid, which is a carboxylic acid.

(b) Observe that a hydroxyl group is bound to an aromatic ring, which implies that this compound is a derivative of phenol. This compound is known as salicylic acid, which reacts with acetic acid to produce aspirin.

(c) There are many hydroxyl groups attached to saturated carbon atoms, and thus this compound is an alcohol. This is the structural formula of glucose, which is the main carbohydrate our brain uses.

(d) Observe the hydroxyl group in the bottom-left corner. Indeed it is attached to a saturated carbon atom, and therefore this compound is an alcohol. It is the structural formula of cholesterol.

Therefore, only (a) is not an alcohol. $_\square$