Question

Write the formulas of the following compounds and state whether each one is a primary, secondary, or tertiary alcohol or a phenol: (a) 1-chloro-2-hydroxybenzene; (b) 2-methyl-3-pentanol; (c) 2,4-dimethyl-1-hexanol; (d) 2-methyl-2-butanol.

Short answer

a) 1-chloro-2-hydroxybenzene is a phenol. b) 2-Methyl-3-pentanol is a secondary alcohol (C_5H_{11}OH). c) 2,4-Dimethyl-1-hexanol is a primary alcohol (C_8H_{18}O). d) 2-Methyl-2-butanol is a tertiary alcohol (C_5H_{12}O).

Step-by-step solution

Classifying 1-chloro-2-hydroxybenzene

Since 1-chloro-2-hydroxybenzene has the -OH group attached to a benzene ring, it is classified as a phenol, not an alcohol.

Determining the Formula for 2-methyl-3-pentanol

2-Methyl-3-pentanol has its -OH group on the third carbon of a five-carbon chain with a methyl group on the second carbon. This makes it a secondary alcohol because the carbon with the -OH group is attached to two other carbons.

Determining the Formula for 2,4-dimethyl-1-hexanol

2,4-Dimethyl-1-hexanol has its -OH group on the first carbon of a six-carbon chain, with methyl groups on the second and fourth carbons. This is a primary alcohol because the carbon with the -OH group is attached to only one other carbon.

Classifying 2-methyl-2-butanol

2-Methyl-2-butanol has its -OH group attached to the second carbon of a four-carbon chain, which also carries a methyl group. This means it is a tertiary alcohol, as the carbon with the -OH group is connected to three other carbons.

Key concepts

Phenol

When students encounter the term 'phenol', it's important to know that it refers to a specific type of chemical compound characterized by a hydroxyl group (-OH) bonded directly to an aromatic hydrocarbon group, typically a benzene ring. A classic example provided in the exercise is 1-chloro-2-hydroxybenzene. This compound is indeed categorized as a phenol because of the presence of the -OH group on the benzene ring.

Phenols are known for their distinct properties, such as acidity stronger than that of alcohols, due to the ability of the aromatic ring to stabilize the negative charge on the oxygen after deprotonation. Understanding the nature of phenols is crucial because they are used in a variety of applications, ranging from disinfectants to the synthesis of more complex organic molecules.

Additionally, phenols can undergo various chemical reactions, such as nitration or halogenation, which further diversifies their applications in different chemical industries.

Primary Alcohol

A primary alcohol is distinguished by the position of the hydroxyl group (-OH) being attached to a carbon atom that is only connected to one other carbon atom. In our exercise, 2,4-dimethyl-1-hexanol serves as an example of a primary alcohol. The -OH group is located on the first carbon (also known as the primary or alpha carbon), and this carbon is bonded only to one other carbon atom, with the rest of its bonds being hydrogen atoms.

In terms of reactivity, primary alcohols can undergo a variety of chemical transformations, including oxidation to aldehydes and then to carboxylic acids. Students should take note that primary alcohols are generally the least reactive amongst alcohols when comparing primary, secondary, and tertiary in terms of resistance to oxidation. This is a key point to understand, particularly when studying reaction mechanisms and synthesis in organic chemistry.

Secondary Alcohol

Secondary alcohols feature an -OH group attached to a carbon that is connected to two other carbon atoms. The exercise introduces 2-methyl-3-pentanol as an example. The carbon carrying the hydroxyl group is the third carbon in the chain, which is attached to two other carbons, making it a secondary carbon.

One interesting aspect of secondary alcohols is their intermediate reactivity. They are more reactive than primary alcohols but less so than tertiary alcohols. When oxidized, secondary alcohols typically form ketones, not continuing further to carboxylic acids as with primary alcohols. This characteristic is essential for understanding the behavior of these molecules during reactions, such as in oxidation or substitution processes, where their reactivity plays a role in determining the outcome.

Tertiary Alcohol

Tertiary alcohols contain a hydroxyl group (-OH) attached to a carbon atom that is in turn bonded to three other carbon atoms. This structure is exemplified by 2-methyl-2-butanol from our textbook problem. Tertiary alcohols are the most sterically hindered and the most resistant to oxidation among the different types of alcohols.

The resistance to oxidation is a notable feature and is due to the absence of a hydrogen atom attached to the carbon bearing the -OH group, which is necessary for the oxidation process. Therefore, tertiary alcohols generally do not readily oxidize to form ketones or carboxylic acids under standard conditions. Moreover, tertiary alcohols can participate in other reaction types, such as eliminations or rearrangements, providing a unique set of reactions and pathways in synthetic chemistry that differ from those available for primary and secondary alcohols.