Alcohols

1. Introduction

Alcohols are organic compounds in which one or more hydroxyl groups ($\ce{-OH}$) are directly attached to a saturated ($sp^3$) carbon atom of an alkyl group. Their general formula is $\ce{R-OH}$, where $\ce{R}$ is an alkyl group. The simplest members are methanol $\ce{CH3OH}$ and ethanol $\ce{CH3CH2OH}$.

The $\ce{-OH}$ group is the functional group that decides most of their chemistry. Because oxygen is far more electronegative than carbon and hydrogen, both the $\ce{C-O}$ and $\ce{O-H}$ bonds are polar, which explains hydrogen bonding, solubility in water and the reactions of alcohols.

Classification by number of $\ce{-OH}$ groups:

• Monohydric: one $\ce{-OH}$, e.g. ethanol $\ce{CH3CH2OH}$.
• Dihydric (glycols): two $\ce{-OH}$, e.g. ethane-1,2-diol $\ce{HOCH2-CH2OH}$.
• Trihydric: three $\ce{-OH}$, e.g. propane-1,2,3-triol (glycerol) $\ce{HOCH2-CHOH-CH2OH}$.

This chapter deals mainly with monohydric alcohols, general formula $\ce{C_nH_{2n+1}OH}$ or $\ce{C_nH_{2n+2}O}$.

2. Nomenclature, Isomerism and Classification of Monohydric Alcohols

2.1 Classification (1 degree, 2 degree, 3 degree)

Monohydric alcohols are classified by the nature of the carbon bearing the $\ce{-OH}$ group:

• Primary (1 degree): the $\ce{-OH}$ carbon is joined to one (or zero) other carbon, e.g. ethanol $\ce{CH3-CH2-OH}$.
• Secondary (2 degree): the $\ce{-OH}$ carbon is joined to two other carbons, e.g. propan-2-ol $\ce{CH3-CHOH-CH3}$.
• Tertiary (3 degree): the $\ce{-OH}$ carbon is joined to three other carbons, e.g. 2-methylpropan-2-ol $\ce{(CH3)3C-OH}$.

This classification matters because rate of many reactions (oxidation, dehydration, reaction with Lucas reagent) follows the order 3 degree > 2 degree > 1 degree or the reverse, and is the basis of tests that distinguish the three classes.

2.2 Nomenclature

IUPAC rules: select the longest carbon chain containing the $\ce{-OH}$ carbon, replace the terminal -e of the parent alkane by -ol, and number the chain to give the $\ce{-OH}$ carbon the lowest locant.

2.3 Isomerism

Monohydric alcohols show:

• Chain isomerism: same $\ce{-OH}$ position, different carbon skeleton (butan-1-ol vs 2-methylpropan-1-ol).
• Position isomerism: same skeleton, different position of $\ce{-OH}$ (propan-1-ol vs propan-2-ol).
• Functional isomerism: alcohols are functional isomers of ethers, e.g. $\ce{C2H6O}$ is either ethanol $\ce{CH3CH2OH}$ or dimethyl ether $\ce{CH3-O-CH3}$.
• Metamerism appears in ethers, not alcohols.