Question

How many of the following alcohols are secondary alcohols? a. ethanol b. isopropyl alcohol c. tert-butyl alcohol d. methanol

Short answer

Only 1 of the given alcohols, isopropyl alcohol, is classified as a secondary alcohol.

Step-by-step solution

a. Ethanol

Ethanol has the molecular formula C2H5OH. The hydroxyl group (-OH) is bonded to a carbon atom that is bound to only one other carbon atom and two hydrogen atoms. Since the carbon atom bonded to the hydroxyl group is bound to only one other carbon atom, ethanol is classified as a primary alcohol and not a secondary alcohol.

b. Isopropyl alcohol

Isopropyl alcohol has the molecular formula (CH3)2CHOH. The hydroxyl group (-OH) is bonded to a carbon atom that is bound to two other carbon atoms and one hydrogen atom. Since the carbon atom bonded to the hydroxyl group is bound to two other carbon atoms, isopropyl alcohol is classified as a secondary alcohol.

c. tert-Butyl alcohol

tert-Butyl alcohol has the molecular formula (CH3)3COH. The hydroxyl group (-OH) is bonded to a carbon atom that is bound to three other carbon atoms and no hydrogen atoms. Since the carbon atom bonded to the hydroxyl group is bound to three other carbon atoms, tert-butyl alcohol is classified as a tertiary alcohol and not a secondary alcohol.

d. Methanol

Methanol has the molecular formula CH3OH. The hydroxyl group (-OH) is bonded to a carbon atom that is bound to no other carbon atom and three hydrogen atoms. Since the carbon atom bonded to the hydroxyl group is bound to no other carbon atom, methanol is classified as a primary alcohol and not a secondary alcohol.

Conclusion

Among the given alcohols, only isopropyl alcohol (b) is a secondary alcohol. Therefore, there is only 1 secondary alcohol in the given list.

Key concepts

classification of alcohols

Alcohols can be classified based on the type of carbon atom that is bonded to the hydroxyl group (-OH). There are three main categories:

• Primary (1°) Alcohols
• Secondary (2°) Alcohols
• Tertiary (3°) Alcohols

These classifications depend on how many carbon atoms are attached to the carbon carrying the hydroxyl group. This determines the physical and chemical properties of the alcohols. Understanding these categories is crucial for identifying how an alcohol might behave in different chemical reactions.

hydroxyl group

The hydroxyl group (-OH) is the defining feature of alcohols, consisting of an oxygen and a hydrogen atom. This group is highly polar, allowing alcohols to engage in hydrogen bonding with water and other molecules.
As a result, many alcohols are soluble in water, and this solubility decreases as the carbon chain length increases. In alcohols, the hydroxyl group is bonded to a carbon atom, and the nature of this carbon atom (whether it is primary, secondary, or tertiary) plays a significant role in determining the type of alcohol.

primary alcohols

Primary alcohols have their hydroxyl group connected to a carbon atom that is bonded to only one other carbon atom. This makes them distinctive because they have two hydrogen atoms on the carbon with the hydroxyl group (unless it's methanol, where no carbon atoms are attached).
Examples include:

• Ethanol (C2H5OH)
• Methanol (CH3OH)

Primary alcohols often participate in oxidation reactions, which may form aldehydes or further oxidize to carboxylic acids.

tertiary alcohols

In tertiary alcohols, the carbon atom that holds the hydroxyl group is connected to three other carbon atoms, leaving no hydrogen atoms on this carbon. This makes tertiary alcohols rather stable and less likely to oxidize compared to primary and secondary alcohols.
A well-known example would be tert-butyl alcohol \((CH_3)_3COH\). Due to the crowded nature of the carbon atom, tertiary alcohols generally resist reactions that involve breaking its carbon-carbon bonds, such as oxidation. Their structure lends them unique structural and reactivity characteristics, setting them apart from other alcohol classes.