3.1 How This Module Fits Into the Course

In Module 1 you looked at what an alcohol is and how to name one. Module 2 showed why alcohols boil, dissolve, and bond the way they do. Now you turn to what alcohols actually do in a reaction flask or in everyday products. By the end of Module 3 you will be able to look at any simple alcohol and predict the main kinds of changes it can take part in.

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3.2 Why Learn Alcohol Reactions?

• They explain how wine becomes vinegar, how rubbing alcohol becomes acetone in the body, and why some fuels are “dehydrated.”• The same reaction patterns appear again when you study other classes such as aldehydes, ketones, and ethers.• Mastering the reactions builds a toolkit you will reuse in Module 4 (making ethers) and Module 5 (industrial applications).

3.3 Quick Review: Structure Drives Reactivity

1. Functional group: ‑OH (hydroxyl group)

2. Classification:• Primary (1°) – carbon with the ‑OH is attached to one other carbon• Secondary (2°) – attached to two carbons• Tertiary (3°) – attached to three carbons

3. Bond polarity: O–H bond is polar, making the hydrogen slightly positive and the oxygen slightly negative.

4. Lone pairs on oxygen can act as a base or a nucleophile (an electron-pair donor).

Keep these points in mind; they explain every reaction in this module.

3.4 Oxidation of Alcohols

3.4.1 What “Oxidation” Means Here

In organic chemistry, oxidation usually means increasing the number of C–O bonds or decreasing the number of C–H bonds on the carbon that holds the ‑OH group.

3.4.2 General Patterns

• 1° alcohol → aldehyde → carboxylic acid• 2° alcohol → ketone• 3° alcohol → generally no reaction under normal conditions

3.4.3 Common Oxidizing Agents

Abbreviation	Formal Name	Typical Use	Conditions
PCC	Pyridinium chlorochromate	Stops at aldehyde	Room temp, no water
KMnO₄	Potassium permanganate	Goes to acid	Warm, basic or acidic
H₂CrO₄	Chromic acid	Goes to acid (1°) or ketone (2°)	Reflux, water present

3.4.4 Step-by-Step Example

Convert 1-butanol (CH₃CH₂CH₂CH₂OH) to butanoic acid.

1. Choose an oxidizer that reaches the acid stage, for example KMnO₄.

2. Write the structural change: CH₃CH₂CH₂CH₂OH → CH₃CH₂CH₂COOH.

3. Balance oxygen in the full equation (not required for most classroom work, but essential in the lab).

3.4.5 Real-World Connection

The “breathalyzer” uses oxidation of ethanol to acetic acid, producing an electric signal proportional to alcohol concentration.

3.5 Dehydration (Elimination of Water)

3.5.1 What Happens

An alcohol loses H₂O to give an alkene. The reaction usually needs a strong acid (H₂SO₄ or H₃PO₄) and heat.

3.5.2 Two Mechanistic Pathways

• E1 (two-step) – common with secondary and tertiary alcohols• E2 (one-step) – more common with primary alcohols or when concentrated acid is not present

3.5.3 Zaitsev’s Rule

When more than one alkene can form, the product with more substituted double bond (fewer H atoms on the double-bonded carbons) usually dominates.

 

Example: Dehydrating 2-butanol mainly gives 2-butene rather than 1-butene.

3.5.4 Laboratory Note

Collect the gaseous alkene over water, dry it with calcium chloride, and keep ignition sources away—the product is often flammable.

3.5.5 Everyday Example

Production of ethene from ethanol in bio-fuel plants uses high temperature alumina (Al₂O₃) instead of strong liquid acids to avoid corrosion.

3.6 Substitution Reactions

3.6.1 Why Substitution Matters

Replacing the ‑OH group lets you transform an alcohol into many other functional groups. You will revisit these products in later chemistry courses.

3.6.2 Converting Alcohols to Alkyl Halides

1. Direct use of hydrogen halides (HX).Example: tert-butyl alcohol + HCl → tert-butyl chloride + H₂O• Tertiary alcohols react fastest (SN1 mechanism).

2. Using reagents such as SOCl₂ (thionyl chloride) or PBr₃.• Gives good yields with primary and secondary alcohols (SN2 mechanism).

3. Why does ‑OH need help? The hydroxide ion is a poor leaving group. Protonation or converting to an intermediate such as a sulfonate turns it into a better leaving group.

3.6.3 Tosylates and Mesylates

• Treating an alcohol with TsCl (tosyl chloride) or MsCl (mesyl chloride) gives a tosylate or mesylate.• The new group is an excellent leaving group; you can then do SN2 reactions with many nucleophiles (CN⁻, I⁻, SH⁻, etc.).

3.6.4 Example Walk-Through

Step 1: Convert 1-propanol to 1-bromopropane with PBr₃.Step 2: React 1-bromopropane with cyanide ion to obtain butanenitrile.You just built a four-carbon nitrile from a three-carbon alcohol using a two-reaction sequence.

3.7 Esterification

3.7.1 Fischer Esterification

An alcohol reacts with a carboxylic acid in the presence of an acid catalyst (often H₂SO₄) to form an ester and water. The reaction is reversible.

 

General equation: RCOOH + R’OH ⇌ RCOOR’ + H₂O

3.7.2 Driving the Reaction Forward

• Remove water as it forms (Dean-Stark apparatus)• Use an excess of either the alcohol or the acid

3.7.3 Industrial and Daily Uses

• Methyl salicylate (oil of wintergreen) is made from salicylic acid and methanol—used in liniments.• Aspirin (acetylsalicylic acid) is produced by reacting salicylic acid with acetic anhydride (a derivative of acetic acid).• Many fruit flavors are mixtures of simple esters such as ethyl butanoate (pineapple aroma).

3.8 Acid–Base Behavior of Alcohols and Alkoxide Formation

3.8.1 Weak Acids

Alcohols are weak acids (pKa about 16). Strong bases such as sodium hydride (NaH) or sodium metal can remove the acidic hydrogen to form an alkoxide (RO⁻ Na⁺).

 

Example: C₂H₅OH + Na → C₂H₅O⁻ Na⁺ + ½ H₂↑

3.8.2 Why Alkoxides Matter

• They are strong nucleophiles.• In Module 4 you will use an alkoxide in the Williamson ether synthesis to build ethers.

3.9 Comparing Reactivity: Primary vs. Secondary vs. Tertiary Alcohols

Reaction Type	1°	2°	3°
Oxidation	First to aldehyde, then acid	To ketone only	None
Dehydration	Requires high heat, E2	Moderate heat, E1	Easiest, E1
Substitution with HX	Slow, SN2	Moderate, SN1/SN2	Very fast, SN1
Alkoxide formation	Easy with strong base	Easy	Easy

Take-home message: The more substituted the carbon bearing ‑OH, the easier it forms carbocations, making substitution and dehydration faster. Oxidation works best on less substituted alcohols where the carbon still has hydrogens attached.

3.10 Safety and Environmental Notes

1. Concentrated acids used in dehydration cause burns; always add acid to water, never water to acid.

2. Chromium-based oxidizers (PCC, H₂CrO₄) are toxic and must be collected for proper disposal.

3. Many alkyl halides formed in substitution steps are irritants and suspected carcinogens—use a fume hood.

4. Produce and capture hydrogen gas safely when forming alkoxides with sodium metal—risk of fire.

3.11 Worked Examples

Example 1: Predict the Product

Question: What is the main organic product when 2-methyl-2-butanol is heated with concentrated H₂SO₄?Solution steps:

1. Identify tertiary alcohol → likely E1 dehydration.

2. Draw possible alkenes: 2-methyl-2-butene and 2-methyl-1-butene.

3. Apply Zaitsev’s rule → choose 2-methyl-2-butene as major product.

Example 2: Multi-Step Synthesis

Goal: Prepare ethyl acetate (CH₃COOCH₂CH₃) starting from ethanol.

1. Oxidize ethanol to acetic acid using KMnO₄.

2. React acetic acid with excess ethanol under acid catalysis (Fischer esterification).

3. Distill the ester from the mixture.

You have made a common solvent and nail polish remover ingredient using two reactions from this module.

3.12 Practice Exercises

1. Identify whether each reaction below is oxidation, dehydration, substitution, or esterification:a) CH₃CH₂CH₂OH + HBr → CH₃CH₂CH₂Br + H₂Ob) CH₃CH(OH)CH₃ →(acid, Δ) CH₃CH=CH₂ + H₂Oc) CH₃CH₂OH →(PCC) CH₃CHOd) CH₃CH₂OH + CH₃COOH →(H⁺) CH₃COOCH₂CH₃ + H₂O

2. Draw the structure of the product when 1-pentanol is treated with SOCl₂. Explain the mechanism type.

3. A secondary alcohol is oxidized and the product has the formula C₃H₆O. Identify the starting alcohol and the product.

4. Suggest a reagent to carry out the following conversions and justify your choice:a) Cyclohexanol to bromocyclohexaneb) 3-methyl-3-pentanol to 3-methyl-2-pentene

Write answers in your notebook before checking the solutions provided at the end of the module handout.

3.13 Discussion Questions

• Why do tertiary alcohols resist oxidation while being so quick to dehydrate?• In many labs chromium-based oxidizers are being replaced by greener options. Propose at least two alternatives and discuss their pros and cons.• Look at the label of a household product containing “isopropyl alcohol.” Predict which reaction pathway might turn it into acetone if left open to air.

3.14 Quick Reference Tables

Table 1: Choosing an Oxidizing Agent

Desired Product	Preferred Reagent	Reason
Aldehyde from 1° alcohol	PCC	Stops at aldehyde, no water
Carboxylic acid from 1° alcohol	KMnO₄, H₂CrO₄	Stronger, water present
Ketone from 2° alcohol	PCC, H₂CrO₄	Both effective

Table 2: Converting Alcohols to Halides

Alcohol Type	Best Reagents	Notes
1°	PBr₃, SOCl₂	SN2, inversion of configuration
2°	PBr₃, SOCl₂	Possible racemization
3°	HCl, HBr (Lucas test)	SN1, carbocation rearrangement possible

3.15 Mini Case Study: From Grape Juice to Vinegar

1. Sugar in grapes ferments to ethanol (Module 1 recap).

2. Ethanol is oxidized by Acetobacter bacteria to acetic acid (Module 3 oxidation).

3. This two-step biological oxidation is the basis of traditional vinegar making.

4. Control of oxygen and temperature prevents over-oxidation to carbon dioxide and water.

Relating the module’s chemistry to food science deepens your understanding of both.

3.16 Summary of Key Points

• Oxidation increases C–O bonds: 1° → aldehyde → acid; 2° → ketone; 3° unreactive.• Dehydration removes water to give alkenes; Zaitsev’s rule predicts the major product.• Substitution turns ‑OH into better leaving groups, often forming alkyl halides or tosylates.• Esterification combines an alcohol and an acid, producing pleasant-smelling esters found in flavors and medicines.• Alcohols can act as weak acids; forming alkoxides sets the stage for ether synthesis in Module 4.• Reactivity trends correlate with the degree of carbon substitution and the stability of carbocations.• Safety: handle strong acids, oxidizers, and volatile halides with care.

 

Spend time working through the exercises and discussion points to make sure you can apply each reaction type, not just recognize it.