2.1 Why Physical Properties Matter

Physical properties tell you how a substance will behave without changing its identity. If you know an alcohol’s boiling point, you can decide whether it is safe to heat it on a hot plate. If you know its solubility, you can predict whether it will mix with water, blood, or oil. When you design a medicine, choose a cleaning solvent, or even bake with vanilla extract, these properties guide every practical choice.

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2.2 Intermolecular Forces in Alcohols

Intermolecular forces are the attractions between molecules. They are weaker than the bonds inside a molecule, yet they control boiling, melting, and dissolving. Two main forces act in most alcohols.

2.2.1 Hydrogen Bonding

• The –OH group contains an O–H bond where oxygen is more electronegative than hydrogen.• This difference in electronegativity pulls electron density toward oxygen, leaving hydrogen slightly positive (δ+) and oxygen slightly negative (δ–).• Positive hydrogen on one molecule is attracted to negative oxygen on a neighboring molecule.• These attractions, called hydrogen bonds, are stronger than ordinary dipole-dipole forces and raise boiling point and solubility in water.

2.2.2 London (Dispersion) Forces

• Every molecule has fleeting, uneven electron clouds that create temporary dipoles.• These momentary dipoles attract each other and are called London forces.• They grow stronger as the electron cloud (and therefore the molecular size) increases.• Longer carbon chains have stronger dispersion forces.

2.3 Boiling Point Trends

Boiling occurs when enough energy is supplied to break the attractive forces holding liquid molecules together.

2.3.1 Chain Length

• Within a family of straight-chain (unbranched) alcohols, boiling point rises as the number of carbons increases.– Methanol (CH₃OH) boils at 65 °C.– Ethanol (CH₃CH₂OH) boils at 78 °C.– 1-Butanol (CH₃CH₂CH₂CH₂OH) boils at 118 °C.• Longer chains have more electrons → stronger London forces → more energy needed to separate molecules.

2.3.2 Branching

• A branched alcohol has a lower boiling point than its straight-chain isomer.– 1-Butanol (straight) 118 °C; tert-butyl alcohol (branched) 83 °C.• Branches make molecules more compact. Compact shapes have smaller surface area, leading to weaker dispersion forces.

2.3.3 Comparing with Alkanes and Ethers

• An alcohol and an alkane with the same carbon count differ greatly. Hexane (C₆H₁₄) boils at 69 °C, while 1-hexanol (C₆H₁₃OH) boils at 158 °C.• Ethers, which lack the O–H bond, cannot hydrogen-bond with each other as effectively, so they usually boil lower than their alcohol isomers.

Quick Check

Look at the pair: 2-propanol (isopropyl alcohol) and propyl ether. Which one boils higher, and why?

2.4 Solubility in Water

Water is a polar, hydrogen-bonding solvent. An alcohol will dissolve if it can form strong enough attractions with water molecules.

2.4.1 Role of the –OH Group

• The –OH group can both donate (through H) and accept (through O) hydrogen bonds.• One –OH group makes an alcohol “water-friendly,” but only up to a point.

2.4.2 Carbon Chain Effects

• The hydrocarbon part is non-polar and resists mixing with water.– Methanol, ethanol, and propanol dissolve in any proportion— the polar –OH dominates.– 1-Butanol is only moderately soluble; you can see two layers form if you add enough water.– Pentanol and higher members are almost insoluble; their hydrocarbon “tails” outweigh the single –OH “head.”• Rule of thumb: About one carbon per three hydrogens still dissolves well; longer chains separate.

2.4.3 Temperature and Concentration

• Raising temperature generally increases solubility because molecules move faster and overcome intermolecular forces more easily.• Very high alcohol concentrations (e.g., strong liquor) can pull water away from other dissolved substances, a behavior used in extraction processes.

2.5 Density and Viscosity

Density = mass / volume. Viscosity = resistance to flow.

 

• Most low-molecular-weight alcohols (methanol, ethanol) are slightly lighter than water (density ≈ 0.79 g cm⁻³).• As carbon number increases, density approaches or even exceeds water.• Viscosity grows with chain length because longer molecules tangle and have stronger dispersion forces. Glycerol (three –OH groups, three carbons) is syrupy because abundant hydrogen bonding creates internal “stickiness.”

2.6 Acidity and Basicity of Alcohols

An alcohol can donate the hydrogen of its –OH group, acting as a very weak acid.

 

• Typical pKa values are 15–18, far weaker than carboxylic acids (pKa ≈ 5).• In strong basic conditions (e.g., sodium metal or sodium hydride), an alkoxide ion (RO⁻) forms, which is a strong base and good nucleophile.• Acidity slightly increases when the oxygen is attached to an electron-withdrawing group. For example, 2,2,2-trifluoroethanol (CF₃CH₂OH) is more acidic than ethanol.

 

Knowing these acid-base facts prepares you for Module 3, where alcohols act as reactants in substitution and elimination reactions.

2.7 Examples & Real-World Context

2.7.1 Ethanol vs. Methanol

• Ethanol is the alcohol in beverages and many hand sanitizers. Boils at 78 °C, mixes freely with water, and evaporates fast, making it useful for quick-drying products.• Methanol is common in windshield washer fluid. Boils at 65 °C and is also miscible with water. However, methanol converts in the body to toxic formaldehyde and formic acid, so you must avoid ingestion.• A difference of one carbon creates a huge difference in toxicity.

2.7.2 Propylene Glycol in Cosmetics

• Propylene glycol (PG) has two –OH groups (a diol).• Hydrogen bonding makes PG viscous and able to hold water, working as a skin moisturizer.• Because it mixes with both water and some oils, PG is an effective carrier for fragrances and medicines.

2.8 Safety Tips Linked to Properties

• Low flash point: Alcohol vapors can catch fire easily. Ethanol’s flash point is about 14 °C—common room temperature. Always cap bottles and keep them away from flame.• High volatility: You may not see vapor, but you can smell it. Use good ventilation.• Mixed solubility: Spills can spread quickly on surfaces that have moisture. Wipe them promptly with lots of water.• Toxicity differences: Never taste laboratory alcohols. Methanol and isopropanol are unsafe for drinking.

2.9 How This Module Links to the Rest of the Course

• Module 1 gave you structural language (primary, secondary, tertiary) and naming rules. The same structures drive the trends you have just learned.• The boiling point and solubility ideas in this module explain why some reactions in Module 3 require heat or special solvents.• When you prepare ethers in Module 4, you will manage reaction mixtures whose physical behavior you can now predict.• Knowledge of properties will help you evaluate practical applications in Module 5, from fuel additives to pharmaceutical solvents.

2.10 Key Points Recap

1. Alcohols form hydrogen bonds due to their –OH group.

2. Longer carbon chains raise boiling point but reduce water solubility.

3. Branching lowers boiling point because it reduces surface area.

4. Density and viscosity grow with size and number of –OH groups.

5. Alcohols are very weak acids yet can produce strong basic alkoxides.

6. Physical properties determine laboratory handling and real-world uses.

2.11 Practice Section

Exercise 1: Arrange in Order

Place the following in order of increasing boiling point. Explain your reasoning.a) 2-methyl-2-propanol (tert-butyl alcohol)b) 1-butanolc) Methanol

Exercise 2: Predict Solubility

You mix 3 mL of 1-pentanol with 3 mL of water in a test tube and shake. What do you expect to observe? Why?

Exercise 3: Quick Calculation

At 20 °C ethylene glycol (HOCH₂CH₂OH) has a density of 1.11 g cm⁻³. How much will 25 mL weigh? Show your steps.

Discussion Question

Hand sanitizers often contain 60–70 % ethanol and a small amount of glycerol. Based on this module, explain why glycerol is added even though it is sticky and much less volatile than ethanol.

Mini-Investigation

Look at the safety data sheet (SDS) for isopropyl alcohol in your school’s lab or online. Identify two warnings that can be explained by boiling point or solubility data from this module.

2.12 Answers (Brief Hints)

Exercise 1: c < a < b. Methanol is smallest; tert-butyl alcohol is branched; 1-butanol is longer and unbranched.Exercise 2: Two layers—1-pentanol mostly floats because the hydrocarbon chain is too long for one –OH to keep it in solution.Exercise 3: mass = density × volume = 1.11 g cm⁻³ × 25 cm³ ≈ 28 g.Discussion: Glycerol binds water through multiple hydrogen bonds, slowing evaporation, so skin stays hydrated.Mini-Investigation: Look for flammability warning (flash point) and eye irritation (solubility allows rapid penetration of tissues).