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Alcohols and Phenols

Table of Contents

Overview and Definitions

Alcohols and phenols are organic compounds that both contain the functional group $–\mathrm{OH}$ (the hydroxyl group), but they differ in what that hydroxyl group is attached to.

Example: ethanol, $\mathrm{CH_3CH_2OH}$

Example: phenol, $\mathrm{C_6H_5OH}$

This structural difference leads to substantial differences in physical properties, acid–base behavior, and reactivity.

Classification and Nomenclature of Alcohols and Phenols

Classification of Alcohols

Alcohols are commonly classified based on the type of carbon that bears the $–\mathrm{OH}$ group:

Example: $\mathrm{CH_3CH_2OH}$ (ethanol)

Example: $\mathrm{(CH_3)_2CHOH}$ (2-propanol, isopropanol)

Example: $\mathrm{(CH_3)_3COH}$ (tert-butanol)

This classification is particularly important for understanding their typical reaction pathways (e.g. in oxidation and substitution).

Nomenclature of Alcohols

IUPAC naming principles (building on general organic nomenclature):

  1. Choose the longest carbon chain containing the carbon bearing the $–\mathrm{OH}$ group as the parent.
  2. Replace the alkane ending -ane with -ol.
  3. Number the chain so that the carbon bearing $–\mathrm{OH}$ gets the lowest possible number.
  4. Indicate the position of the $–\mathrm{OH}$ group with a number.
  5. Name and number substituents as usual.

Examples:

Short, commonly used trivial names also exist:

Nomenclature of Phenols

For simple phenols where only one $–\mathrm{OH}$ group is attached to benzene, the parent compound is simply called phenol.

Substituents on the ring are named using the benzene ring of phenol as the parent:

Examples:

The prefixes ortho- (o-), meta- (m-), para- (p-) are traditional for disubstituted benzenes:

Structure and Physical Properties

Bonding and Geometry

In phenols:

Hydrogen Bonding and Boiling Points

Because of the polar O–H bond and lone pairs on oxygen, alcohols and phenols can engage in intermolecular hydrogen bonding:

Example (approximate boiling points at 1 bar):

Phenols also show relatively high boiling points for their molar mass, due to hydrogen bonding and the rigid aromatic ring.

Solubility in Water

Hydrogen bonding with water leads to:

Phenol is only moderately soluble in water, because the aromatic ring is hydrophobic while the $–\mathrm{OH}$ group is hydrophilic. Substituents that are electron-withdrawing or that contain additional polar groups can increase water solubility.

Preparation of Alcohols

Only the most typical laboratory and industrial routes are sketched here, without detailed mechanisms.

Hydration of Alkenes

Alkenes can be converted to alcohols by addition of water across the C=C double bond.

General pattern:

$$
\mathrm{R_2C{=}CHR' + H_2O \longrightarrow R_2CHOH{-}CHR'}
$$

This process usually requires an acid catalyst (e.g. $\mathrm{H_2SO_4}$) and follows regiochemistry rules (which carbon gets the $–\mathrm{OH}$ group) depending on the specific method.

Example:

$$
\mathrm{CH_2{=}CH_2 + H_2O \xrightarrow[\text{cat.}]{H^+} CH_3CH_2OH}
$$

This is an important industrial route for ethanol.

Reduction of Carbonyl Compounds

Carbonyl compounds can be reduced to the corresponding alcohols:

General pattern:

$$
\mathrm{RCHO + [H] \longrightarrow RCH_2OH}
$$

$$
\mathrm{R_2C{=}O + [H] \longrightarrow R_2CHOH}
$$

Reducing agents often used in the laboratory include $\mathrm{NaBH_4}$ and $\mathrm{LiAlH_4}$, each with specific application ranges.

Fermentation (Ethanol)

Ethanol can be produced biologically by fermentation of sugars using microorganisms such as yeast:

$$
\mathrm{C_6H_{12}O_6 \longrightarrow 2 \, C_2H_5OH + 2 \, CO_2}
$$

This process operates under mild conditions and is central to alcoholic beverages and many bioethanol processes.

Preparation of Phenols

Some common routes to phenols include:

From Aryl Halides (Industrial Processes)

Substitution of an aryl halide (e.g. chlorobenzene) with hydroxide can give phenol under harsh conditions (high temperature and pressure) or in multistep processes used industrially.

Very simplified overall idea:

$$
\mathrm{C_6H_5Cl + NaOH \longrightarrow C_6H_5OH + NaCl}
$$

Actual industrial processes may involve different intermediates (e.g. via sulfonation and subsequent hydrolysis, or via cumene oxidation).

From Diazonium Salts

Anilines (aromatic amines) can be converted to diazonium salts, which in turn can be hydrolyzed to phenols:

Overview:

$$
\mathrm{Ar{-}N_2^+ \, X^- + H_2O \longrightarrow Ar{-}OH + N_2 + HX}
$$

This route is important in the synthesis of substituted phenols in the laboratory.

Acid–Base Properties

Alcohols as Very Weak Acids

The O–H bond in alcohols can donate a proton, but alcohols are only very weak Brønsted acids:

$$
\mathrm{R{-}OH + H_2O \rightleftharpoons R{-}O^- + H_3O^+}
$$

The alkoxide ion $\mathrm{R{-}O^-}$ is formed only to a small extent in neutral water. Strong bases (e.g. reactive metals such as Na, or metal hydrides) can deprotonate alcohols:

Example:

$$
\mathrm{2 \, R{-}OH + 2 \, Na \longrightarrow 2 \, R{-}O^-Na^+ + H_2}
$$

The resulting alkoxides are stronger bases and useful nucleophiles in organic synthesis.

Alcohols as Bases

Because oxygen has lone pairs, alcohols can accept protons (act as Brønsted bases) or coordinate to Lewis acids (e.g. $\mathrm{BF_3}$, $\mathrm{AlCl_3}$, metal cations).

Protonation in strongly acidic media:

$$
\mathrm{R{-}OH + H^+ \longrightarrow R{-}OH_2^+}
$$

This protonated species often appears as an intermediate in substitution or elimination reactions.

Phenols as Weak Acids

Phenols are significantly more acidic than typical aliphatic alcohols. Phenol dissociates in water:

$$
\mathrm{C_6H_5OH + H_2O \rightleftharpoons C_6H_5O^- + H_3O^+}
$$

The phenoxide ion $\mathrm{C_6H_5O^-}$ is resonance-stabilized: the negative charge is delocalized over the aromatic ring, which stabilizes the conjugate base and makes deprotonation easier.

Consequences:

$$
\mathrm{C_6H_5OH + NaOH \longrightarrow C_6H_5O^-Na^+ + H_2O}
$$

Substituents on the aromatic ring can strongly influence the acidity of phenols:

Typical Reactions of Alcohols

Here, only reactions where the $–\mathrm{OH}$ group is directly involved are considered.

Oxidation of Alcohols

Oxidation states of carbon change when alcohols are oxidized. In organic chemistry, “oxidation” often means an increase in the number of C–O bonds or a decrease in C–H bonds.

Oxidation of Primary Alcohols

Primary alcohols can be oxidized in steps:

  1. To aldehydes:

$$
\mathrm{RCH_2OH \xrightarrow[\text{mild}]{[O]} RCHO}
$$

  1. Further to carboxylic acids (if stronger or prolonged conditions are used):

$$
\mathrm{RCH_2OH \xrightarrow[\text{strong}]{[O]} RCOOH}
$$

Oxidation of Secondary Alcohols

Secondary alcohols are oxidized to ketones:

$$
\mathrm{R_2CHOH \xrightarrow{[O]} R_2C{=}O}
$$

Further oxidation can sometimes lead to cleavage of carbon–carbon bonds under very strong conditions but, in many controlled reactions, ketones are the main products.

Tertiary Alcohols

Tertiary alcohols cannot be easily oxidized to carbonyl compounds without breaking C–C bonds, because the carbon bearing the $–\mathrm{OH}$ group has no hydrogen directly attached to it.

Dehydration of Alcohols (Elimination)

Alcohols can undergo elimination of water to form alkenes. This is typically promoted by strong acids (e.g. concentrated $\mathrm{H_2SO_4}$) and heat.

Example:

$$
\mathrm{CH_3CH_2OH \xrightarrow[\Delta]{H_2SO_4} CH_2{=}CH_2 + H_2O}
$$

The general mechanism (for many cases) involves:

  1. Protonation of the hydroxyl group (making it a better leaving group).
  2. Loss of water to form a carbocation (for many secondary and tertiary alcohols).
  3. Loss of a proton to form the double bond.

For some primary alcohols, alternative pathways (e.g. via concerted mechanisms) can operate.

Nucleophilic Substitution (Conversion to Alkyl Halides)

Alcohols can react with hydrogen halides (e.g. HCl, HBr) or with reagents like $\mathrm{PCl_3}$, $\mathrm{SOCl_2}$ to form alkyl halides:

Example:

$$
\mathrm{R{-}OH + HBr \longrightarrow R{-}Br + H_2O}
$$

Here:

Secondary and tertiary alcohols often react more readily via carbocation intermediates; primary alcohols usually react more slowly and can involve other pathways.

Esterification (Formation of Esters)

Alcohols react with carboxylic acids (or derivatives) to form esters, in the presence of an acid catalyst:

General equation (Fischer esterification):

$$
\mathrm{RCOOH + R'OH \xrightleftharpoons[H_2O]{H^+} RCOOR' + H_2O}
$$

Example:

$$
\mathrm{CH_3COOH + HOCH_2CH_3 \rightleftharpoons CH_3COOCH_2CH_3 + H_2O}
$$

Esters often have characteristic, pleasant odors and are widely used as fragrances and flavor compounds.

Typical Reactions of Phenols

Phenols share some reactions with alcohols but are also strongly influenced by the aromatic ring and higher acidity.

Formation of Phenoxide and Nucleophilic Reactions

Because phenols are more acidic, phenoxide ions form readily with bases:

$$
\mathrm{Ar{-}OH + OH^- \rightleftharpoons Ar{-}O^- + H_2O}
$$

Phenoxide ions are often more nucleophilic than the neutral phenol and can undergo reactions such as:

$$
\mathrm{Ar{-}O^- + RCOCl \longrightarrow Ar{-}OCO{-}R + Cl^-}
$$

$$
\mathrm{Ar{-}O^- + R{-}X \longrightarrow Ar{-}OR + X^-}
$$

Electrophilic Aromatic Substitution (Ring Reactions)

The $–\mathrm{OH}$ group on a benzene ring is a strongly activating, ortho/para-directing substituent in electrophilic aromatic substitution (EAS) reactions.

This means:

Examples:

Nitration of Phenol

With diluted nitric acid, phenol can be nitrated to give mainly 2-nitrophenol and 4-nitrophenol:

$$
\mathrm{C_6H_5OH + HNO_3 \longrightarrow NO_2C_6H_4OH + H_2O}
$$

With more vigorous conditions, more than one nitro group can be introduced (e.g. picric acid, 2,4,6-trinitrophenol).

Bromination of Phenol

Phenol reacts readily with bromine water, often without a catalyst, to give polybrominated products such as 2,4,6-tribromophenol:

$$
\mathrm{C_6H_5OH + 3 \, Br_2 \longrightarrow 2,4,6{-}Br_3C_6H_2OH + 3 \, HBr}
$$

The ease and pattern of substitution reflect the activating effect of the $–\mathrm{OH}$ group.

Oxidation of Phenols

Phenols can be oxidized relatively easily compared with simple alcohols, leading to products such as quinones or complex polymeric oxidation products, depending on conditions and substitution pattern.

Simplified example (phenol to a quinone-like structure):

$$
\mathrm{Ar{-}OH \xrightarrow{[O]} \text{quinone-like product}}
$$

Specific oxidizing agents and mechanisms depend on the structure of the phenol and are widely used in dye chemistry and in biological systems.

Ethers from Alcohols and Phenols (Brief Overview)

Although ethers are treated in a separate chapter, it is useful to note that both alcohols and phenols can serve as nucleophiles in forming ethers:

$$
\mathrm{R{-}O^- + R'X \longrightarrow R{-}O{-}R' + X^-}
$$

Phenoxide ions are particularly good nucleophiles in such reactions.

Selected Uses and Importance

Alcohols

Phenols

Summary of Key Differences Between Alcohols and Phenols

These structural and reactivity differences underpin the widespread use and varied chemistry of alcohols and phenols in both laboratory and industrial contexts, as well as their roles in biological and environmental systems.

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