13.4 - Reactions of Alcohols and Thiols

What is combustion in organic chemistry?

Combustion is a reaction in which an organic compound reacts with oxygen (O₂) to produce carbon dioxide (CO₂), water (H₂O), and energy (heat and light).

What products are formed in the complete combustion of an alcohol?

Complete combustion of an alcohol produces: Carbon dioxide (CO₂) Water (H₂O) Energy

Write the balanced chemical equation for the complete combustion of ethanol (C₂H₅OH).

C2H5OH + 3O2 \rightarrow 2CO2 + 3H_2O

Write the balanced chemical equation for the complete combustion of methanol (CH₃OH).

2CH3OH + 3O2 \rightarrow 2CO2 + 4H2O

Write the balanced chemical equation for the complete combustion of 2-butanol (C₄H₉OH).

2C4H9OH + 11O2 \rightarrow 8CO2 + 10H_2O

What is a dehydration reaction of an alcohol?

A dehydration reaction is an elimination reaction in which an alcohol loses a molecule of water (H₂O) to form an alkene.

What conditions are required for the dehydration of alcohols?

High temperature (~180 °C) and Strong acid catalyst (commonly H₂SO₄).

What atoms are removed during alcohol dehydration, and from where?

–OH is removed from the carbon bearing the hydroxyl group; –H is removed from an adjacent carbon atom. These combine to form H₂O.

What type of organic product forms from the dehydration of an alcohol?

An alkene, formed by the creation of a carbon–carbon double bond between the two carbons that lost –OH and –H.

What is the step-by-step method for writing a dehydration reaction of an alcohol?

1. Identify the carbon bearing the –OH group. 2. Identify adjacent carbon atoms. 3. Remove –OH from the alcohol carbon. 4. Remove –H from an adjacent carbon. 5. Form a C=C double bond between those carbons. 6. Write H₂O as a by-product.

Why can dehydration of a secondary alcohol produce more than one alkene?

Because hydrogen atoms can be removed from more than one adjacent carbon, producing different alkene isomers.

What determines the major product in alcohol dehydration?

The major product forms when hydrogen is removed from the carbon atom with the smaller number of hydrogen atoms, producing the more substituted alkene.

Apply dehydration rules to 2-pentanol. What is the major product?

Remove –OH from carbon 2; Remove –H from carbon 3 (fewer hydrogens than carbon 1); Major product: 2-pentene.

What alkene forms from dehydration of 1-butanol, and why is there only one product?

Product: 1-butene; Reason: Only one adjacent carbon is available for hydrogen removal.

What alkene is produced by dehydration of cyclopentanol?

Cyclopentene.

What alkene is produced by dehydration of 2-methyl-3-hexanol?

2-methyl-2-hexene.

What is oxidation in organic chemistry?

Oxidation is: Loss of hydrogen atoms, and/or Gain of oxygen, resulting in an increase in carbon–oxygen bonds.

What is reduction in organic chemistry?

Reduction is the reverse of oxidation, involving: Gain of hydrogen atoms, and/or Loss of oxygen.

Rank the following from least oxidized to most oxidized: alcohol, aldehyde/ketone, carboxylic acid.

Alcohol < Aldehyde/Ketone < Carboxylic Acid.

What happens when a primary (1°) alcohol is oxidized?

First oxidation → aldehyde; Further oxidation → carboxylic acid; Carbon count remains unchanged.

Describe the oxidation of ethanol step by step.

Ethanol (CH₃CH₂OH) → oxidation → acetaldehyde (CH₃CHO); Acetaldehyde → further oxidation → acetic acid (CH₃COOH).

What happens when a secondary (2°) alcohol is oxidized?

A secondary alcohol oxidizes to a ketone, with no further oxidation possible.

Why do ketones not oxidize further under normal conditions?

Ketones have no hydrogen atoms attached to the carbonyl carbon, preventing further oxidation.

What happens when a tertiary (3°) alcohol is exposed to oxidizing agents?

No reaction occurs because there is no hydrogen atom on the carbon bonded to –OH, and carbon–carbon bonds are too strong to oxidize.

How do thiols undergo oxidation?

Two thiol groups lose hydrogen atoms and form a disulfide bond (–S–S–).

What is the general oxidation reaction of thiols?

2RSH \xrightarrow{ ext{[O]}} RSSR + 2H^+ + 2e^-.

What biological role do disulfide bonds play?

Disulfide bonds cross-link proteins, providing structural strength, such as in hair keratin.

How is hair permanently curled or straightened using redox chemistry?

Reducing agent breaks disulfide bonds; Hair is reshaped; Oxidizing agent reforms disulfide bonds in new positions.

How is ethanol metabolized in the human body?

Ethanol → acetaldehyde (via alcohol dehydrogenase); Acetaldehyde → acetic acid; Acetic acid → CO₂ + H₂O (citric acid cycle).

Why is acetaldehyde dangerous in alcohol metabolism?

Acetaldehyde is toxic, causing impaired coordination, nausea, and cellular damage.

How does Antabuse (disulfiram) treat alcoholism?

It blocks oxidation of acetaldehyde to acetic acid, causing acetaldehyde accumulation and severe unpleasant symptoms.

Why is methanol poisoning especially dangerous?

Methanol oxidizes to formaldehyde and formic acid, which: Destroy retinal tissue; Lower blood pH; Cause blindness, coma, or death.

How is methanol poisoning treated chemically?

Sodium bicarbonate neutralizes formic acid; Ethanol competes for oxidation enzymes, allowing methanol elimination via lungs.

Summarize alcohol oxidation outcomes by classification.

Alcohol Type	Oxidation Product	Further Oxidation
Primary	Aldehyde → Carboxylic acid	Yes
Secondary	Ketone	No
Tertiary	None	Not possible.