Reactions of Alcohols – Organic Chemistry III Study Notes

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Reactions of Alcohols

Overview of Alcohol Reactions

Alcohols are versatile organic compounds that undergo a variety of chemical transformations. Their reactivity is largely determined by the nature of the hydroxyl group (-OH) and the structure of the carbon atom to which it is attached. The main types of reactions include dehydration, oxidation, substitution, reduction, esterification, and ether synthesis.

Dehydration to alkenes: Removal of water to form alkenes.
Oxidation to aldehydes or ketones: Increase in oxidation state, forming carbonyl compounds.
Substitution to form alkyl halides: Replacement of the hydroxyl group with a halogen.
Reduction to alkanes: Decrease in oxidation state, forming saturated hydrocarbons.
Esterification: Formation of esters by reaction with acids.
Williamson synthesis of ethers: Formation of ethers via nucleophilic substitution.

Summary Table: Types of Alcohol Reactions

Type of Reaction	Alcohol Reactant	Product
Dehydration	R–OH	Alkene
Oxidation	R–OH	Ketone, Aldehyde, Acid
Substitution	R–OH	R–X (Alkyl halide)
Reduction	R–OH	R–H (Alkane)
Esterification	R–OH	R–O–C(O)R' (Ester)
Williamson Ether Synthesis	R–OH (converted to alkoxide)	R–O–R' (Ether)

Oxidation of Alcohols

General Principles

Oxidation of alcohols is a fundamental transformation in organic chemistry, often accomplished with inorganic oxidizing agents. The process involves the loss of hydrogen or gain of oxygen.

Common oxidizing agents: Potassium dichromate (), chromic acid (), potassium permanganate ().
Oxidation: Loss of , gain of , , or halogen ().
Reduction: Gain of or , loss of , , or .

Oxidation States of Carbons in Alcohols

The oxidation state of the carbon atom bonded to the hydroxyl group determines the product of oxidation.

Primary alcohols (1°): Oxidize to aldehydes, then to carboxylic acids.
Secondary alcohols (2°): Oxidize to ketones.
Tertiary alcohols (3°): Generally resistant to oxidation.

General oxidation sequence:

Alkane → Alcohol → Aldehyde/Ketone → Carboxylic Acid

Oxidation of Primary Alcohols

Primary alcohols are first oxidized to aldehydes. Further oxidation leads to carboxylic acids. To stop at the aldehyde stage, mild oxidizing agents such as pyridinium chlorochromate (PCC) are used.

Reaction: (aldehyde)
Reagent: PCC in

Example:

Heptanol () oxidized to heptanal () using PCC.

Oxidation of Secondary Alcohols

Secondary alcohols are oxidized to ketones using strong oxidizing agents such as sodium dichromate () in sulfuric acid (). The active species is chromic acid ().

Reaction: (ketone)
Color change: Orange (Cr(VI)) to greenish-blue (Cr(III))

Example:

2-Propanol () oxidized to acetone ().

Formation of Chromic Acid

Chromic acid () is generated in situ from sodium dichromate or chromium trioxide in the presence of sulfuric acid and water.

Oxidation Mechanism

The mechanism involves formation of a chromate ester, followed by elimination to yield the oxidized product.

Step 1: Alcohol reacts with chromic acid to form a chromate ester.
Step 2: The chromate ester undergoes elimination, oxidizing the carbinol carbon and reducing chromium.

General mechanism:

(chromate ester)

Oxidation of Tertiary Alcohols

Tertiary alcohols are generally resistant to oxidation because they lack a hydrogen atom on the carbon bearing the hydroxyl group. This property is the basis for the chromic acid test for alcohol classification.

No reaction: Tertiary alcohols do not undergo oxidation under standard conditions.

Summary Table: Alcohol Oxidations

Alcohol Type	Product	Reagent
Primary (1°)	Aldehyde (with PCC) Carboxylic acid (with chromic acid)	PCC, Chromic acid
Secondary (2°)	Ketone	Chromic acid, PCC
Tertiary (3°)	No reaction	—

Key Terms and Concepts

Alcohol: Organic compound containing a hydroxyl group attached to a saturated carbon atom.
Oxidation: Process involving the loss of electrons, often seen as gain of oxygen or loss of hydrogen.
PCC (Pyridinium Chlorochromate): Mild oxidizing agent used to convert primary alcohols to aldehydes and secondary alcohols to ketones.
Chromic Acid (): Strong oxidizing agent used for alcohol oxidation.
Chromate Ester: Intermediate formed during oxidation of alcohols with chromic acid.

Example Equations

Primary alcohol oxidation (PCC):
Secondary alcohol oxidation (Chromic acid):

Additional info: The notes also mention color changes (orange to greenish-blue) as a qualitative test for oxidation, and the resistance of tertiary alcohols to oxidation as a diagnostic tool in laboratory settings.