Grade by Grade
B. Organic Chemistry · Organic Chemistry

Alcohols and Organic Halides

B2.2 B3.1 B3.2 B3.3 B1.1About 75 minutes
Learning goals
  • I can name and draw alcohols, including diols, and classify them as primary, secondary or tertiary.
  • I can explain the boiling points and solubility of alcohols using hydrogen bonding.
  • I can describe how alcohols are made (hydration, fermentation) and how they undergo combustion, substitution and elimination.
  • I can name organic halides, describe their properties and give examples of their uses and risks.

Words to know

alcohol
An organic compound with a hydroxyl group (–OH) bonded to a saturated carbon atom.
hydroxyl group
The –OH functional group. In alcohols it is covalently bonded and does not form OH⁻ ions in water.
primary, secondary, tertiary alcohol
An alcohol whose C–OH carbon is bonded to one, two or three other carbon atoms.
hydrogen bond
A strong dipole-dipole attraction between an H atom bonded to O, N or F and a lone pair on another O, N or F atom.
elimination reaction
A reaction in which atoms are removed from neighbouring carbons, forming a double bond; for example, dehydration of an alcohol to an alkene.
dehydration
An elimination that removes H and OH (as water) from an alcohol.
fermentation
The conversion of sugars to ethanol and carbon dioxide by yeast.
organic halide
A compound in which a halogen atom is bonded to carbon, such as chloromethane.

The –OH family

Hand sanitizer, rubbing alcohol, windshield-washer fluid, antifreeze and the ethanol blended into gasoline are all alcohols: organic compounds with a hydroxyl group, –OH, bonded to a saturated carbon.

The –OH in an alcohol is not a hydroxide ion. Sodium hydroxide, NaOH, is ionic and releases OH⁻ in water, making it basic. In ethanol, the O–H is part of a covalent molecule, so ethanol solutions are neutral.

Naming alcohols

  1. The parent chain is the longest chain that contains the carbon bonded to –OH.
  2. Number so that the –OH carbon gets the lowest number (the –OH has priority over alkyl branches and halogens).
  3. Replace the final -e of the alkane with -ol, and put the locant before -ol: propan-2-ol (older style: 2-propanol).
  4. Two –OH groups: -diol, keeping the -e: ethane-1,2-diol (ethylene glycol). Three: -triol: propane-1,2,3-triol (glycerol).
  5. Ring alcohols: cyclohexanol (the –OH carbon is C1).

Common names you will see on labels: methanol is methyl alcohol or wood alcohol, ethanol is ethyl alcohol, and propan-2-ol is isopropyl alcohol, the main ingredient of rubbing alcohol.

Primary, secondary and tertiary

Look at the carbon that holds the –OH and count how many other carbons it is bonded to:

  • Primary (1°): 1 carbon (or none, in methanol). Example: propan-1-ol.
  • Secondary (2°): 2 carbons. Example: propan-2-ol.
  • Tertiary (3°): 3 carbons. Example: 2-methylpropan-2-ol.

This classification matters because the three kinds react differently when oxidized (Lesson 9).

OHprimary (1°)propan-1-olC–OH carbon bonded to 1 COHsecondary (2°)propan-2-olbonded to 2 COHtertiary (3°)2-methylpropan-2-olbonded to 3 C
Primary, secondary and tertiary alcohols. The highlighted carbon carries the –OH. Count how many carbons are bonded to it.
Worked example

Example 1: Naming and classifying

Name and classify (a) CH₃CH(OH)CH₂CH₃ and (b) CH₃CH₂C(CH₃)(OH)CH₃.

  1. (a) Chain of 4 C; –OH on C2 from the left (C3 from the right): butan-2-ol. The C–OH carbon is bonded to CH₃ and CH₂CH₃, two carbons → secondary.
  2. (b) Write it out: CH₃–CH₂–C(OH)(CH₃)–CH₃. Longest chain: 4 C, with the –OH and a methyl on the same carbon.
  3. Number from the right so the –OH carbon is C2: 2-methylbutan-2-ol. The C–OH carbon is bonded to three carbons → tertiary.

(a) butan-2-ol (2-butanol), secondary. (b) 2-methylbutan-2-ol (2-methyl-2-butanol), tertiary.

Hydrogen bonding: why alcohols are liquids

The O–H bond is very polar: oxygen is much more electronegative than hydrogen, so the H carries a partial positive charge (δ+) and the O a partial negative charge (δ−). The δ+ H of one molecule is strongly attracted to a lone pair on the O of another molecule. This is a hydrogen bond, the strongest kind of intermolecular force (you met it in Grade 11 when you studied water).

Boiling points. Compare molecules with almost the same mass:

  • ethane, C₂H₆ (30 g/mol): −89 °C; methanol, CH₃OH (32 g/mol): 65 °C
  • propane, C₃H₈ (44 g/mol): −42 °C; ethanol, C₂H₅OH (46 g/mol): 78 °C

The alcohols boil more than 100 °C higher because their molecules are held together by hydrogen bonds as well as London forces. More –OH groups mean more hydrogen bonds: ethane-1,2-diol boils at 197 °C.

Solubility. An alcohol molecule has a polar end (–OH), which hydrogen-bonds with water, and a non-polar end (the hydrocarbon chain), which does not. Short alcohols mix with water in any proportion. As the chain grows, the non-polar part takes over and solubility drops.

CH₃CH₂OHCH₃CH₂OHCH₃CH₂OHδ−δ+δ−δ+δ−- - - hydrogen bondO–H of one moleculeattracted to a lonepair on O of another
Hydrogen bonds between ethanol molecules. The partly positive H of one O–H group is attracted to the partly negative O of a neighbour. These attractions must be broken to boil the liquid.
AlcoholFormulaBoiling point (°C)Solubility in water (g/100 mL, about 20 °C)
methanolCH₃OH65mixes completely
ethanolC₂H₅OH78mixes completely
propan-1-olC₃H₇OH97mixes completely
butan-1-olC₄H₉OH118about 7
pentan-1-olC₅H₁₁OH138about 2
hexan-1-olC₆H₁₃OH157about 0.6
As the non-polar chain grows, boiling points rise (more London forces) and solubility in water falls (less of the molecule can hydrogen-bond with water).

Making alcohols

  • Hydration of alkenes (Lesson 6): CH₂=CH₂ + H₂O —(H₂SO₄)→ CH₃CH₂OH. Markovnikov's rule decides where the –OH goes.
  • Fermentation: yeast converts sugars from corn, wheat or fruit into ethanol: C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂. Fuel ethanol blended into Canadian gasoline is made this way, mostly from corn and wheat.

Reactions of alcohols

  1. Combustion: C₂H₅OH + 3 O₂ → 2 CO₂ + 3 H₂O. Ethanol burns with a clean blue flame.
  2. Elimination (dehydration): heating an alcohol with concentrated sulfuric acid removes an H and an OH from neighbouring carbons as water, forming an alkene. This is the reverse of hydration. When there is a choice, the main product is usually the alkene with more alkyl groups on the double bond (Zaitsev's rule): butan-2-ol gives mainly but-2-ene.
  3. Substitution with a hydrogen halide: the –OH is replaced by a halogen: (CH₃)₃C–OH + HCl → (CH₃)₃C–Cl + H₂O.
  4. Oxidation: primary and secondary alcohols can be oxidized; that is the topic of Lesson 9.
CCHHHHOHHethanolconc. H₂SO₄heatCCHHHHethene+H₂OH and OH on neighbouring carbons are removed
Elimination (dehydration) of ethanol. An H from one carbon and the OH from the neighbouring carbon leave together as water, and a double bond forms between the two carbons. One reactant gives two products.
Worked example

Example 2: Predicting elimination products

Predict the organic product(s) of heating (a) propan-2-ol and (b) butan-2-ol with concentrated H₂SO₄.

  1. (a) CH₃–CH(OH)–CH₃: an H can leave from either C1 or C3, but both give the same alkene, CH₂=CH–CH₃.
  2. (b) CH₃–CH(OH)–CH₂–CH₃: removing H from C1 gives but-1-ene; removing H from C3 gives but-2-ene.
  3. Zaitsev's rule: the alkene with more alkyl groups on the double bond (but-2-ene, with a CH₃ on each carbon) is the main product.

(a) propene + H₂O. (b) mainly but-2-ene (as cis and trans isomers), with some but-1-ene, + H₂O.

Worked example

Example 3: A diol

Draw ethane-1,2-diol. Explain why it is used in car antifreeze and why it must be kept away from children and pets.

  1. Structure: HO–CH₂–CH₂–OH (two –OH groups).
  2. Two –OH groups per molecule hydrogen-bond strongly with water, so it mixes completely and lowers the freezing point of the coolant; it also has a high boiling point (197 °C), so it does not boil away.
  3. Safety: it tastes sweet but is toxic: the body converts it into harmful acids that damage the kidneys.

HOCH₂CH₂OH: miscible with water, high boiling point, toxic if swallowed, so store it sealed and clean up spills.

Organic halides

You already name organic halides with prefixes (chloro-, bromo-...). A few facts about them:

  • The C–X bond is polar, so haloalkanes have dipole-dipole forces and boil higher than alkanes of similar size. They do not hydrogen-bond, so they are only slightly soluble in water.
  • Many are denser than water: dichloromethane (1.33 g/mL) sinks below water in a separating funnel.
  • Uses: solvents (dichloromethane), the monomer of PVC (chloroethene), non-stick coatings (PTFE, made from tetrafluoroethene), and some medicines and anaesthetics.
  • Risks: chlorofluorocarbons (CFCs), once used in refrigerators and aerosol cans, break down in the stratosphere and release Cl atoms that destroy ozone. The Montreal Protocol (1987), negotiated in Montréal, phased them out, and the ozone layer is slowly recovering. Some halogenated pesticides, such as DDT, persist in the environment and build up in food chains.

Haloalkanes can also undergo elimination: heating with a strong base such as KOH dissolved in ethanol removes H and X to give an alkene: CH₃CHBrCH₃ + KOH → CH₂=CHCH₃ + KBr + H₂O.

Hands-on activity

Alcohol, water and oil: a mixing and evaporation test

You need: rubbing alcohol (70% or 99% isopropyl alcohol from a pharmacy), water, vegetable oil, 3 clear small glasses or jars, a teaspoon, food colouring, salt, and a grown-up. Grown-up note: rubbing alcohol is flammable and poisonous to drink. Use it in a ventilated room, far from any flame, stove or candle; keep it away from eyes; and put the bottle away right after.

  1. Add a drop of food colouring to 50 mL of water in one jar. Add 50 mL of rubbing alcohol. Swirl. One layer or two?
  2. In a second jar, put 50 mL of water and 50 mL of vegetable oil. Swirl. One layer or two?
  3. Put one drop of water and one drop of rubbing alcohol side by side on a plate or on the back of your hand. Which disappears first? Which feels cooler?
  4. Extension ("salting out"): to the first jar (water + 70% rubbing alcohol), add 3 to 4 heaping teaspoons of salt and stir hard for a minute. Let it stand. What happens?
  5. Explain each result using polarity and hydrogen bonding.

What to look for: alcohol and water mix completely (both hydrogen-bond); oil and water form two layers (oil is non-polar). Rubbing alcohol evaporates faster and feels cooler: propan-2-ol molecules are held by fewer hydrogen bonds per molecule than water, so they escape more easily and take heat from your skin as they evaporate. With enough salt, the mixture separates into two layers: the dissolved ions attract water so strongly that the alcohol is pushed out into its own layer. The top layer is mostly alcohol (it is less dense) and the bottom layer is salty water; notice which layer the food colouring ends up in.

Just for fun

What did one ethanol molecule say to the other at the boiling point?

"I hate to break our bond, but I really need some space."

Your turn

1

Name CH₃CH₂CH₂CH₂OH.

2

Name CH₃CH(OH)CH₃ and classify it.

3

Draw (condensed) 3-methylbutan-2-ol.

4

Why does ethanol (46 g/mol) boil at 78 °C while propane (44 g/mol) boils at −42 °C?

5

Which is more soluble in water: butan-1-ol or hexan-1-ol? Why?

6

What type of reaction turns propan-1-ol into propene? What reagent is used?

7

Name CH₃CHClCH₂CH₃ and say whether it hydrogen-bonds with itself.

More practice: warm-up

1

What is the functional group of an alcohol?

2

What ending do alcohol names use?

3

What is the common name of propane-1,2,3-triol?

4

Is ethanol an acid, a base or neutral in water?

5

What gas is produced with ethanol in fermentation?

More practice: on your own

1

Name (CH₃)₃COH and classify it.

2

Write a balanced equation for the complete combustion of methanol.

3

Predict the main product of dehydrating 2-methylbutan-2-ol.

4

Rank by boiling point: propan-1-ol, ethane-1,2-diol, butane. Explain.

5

Why did the Montreal Protocol target CFCs?

Challenge

1

A student needs pure but-1-ene. Should she dehydrate butan-1-ol or butan-2-ol? Explain.

2

Fermentation of 1.00 kg of glucose (C₆H₁₂O₆, 180.16 g/mol) gives what maximum mass of ethanol (46.07 g/mol)? What volume of CO₂ at SATP (24.8 L/mol)?

3

Explain why methanol, ethanol and propan-1-ol mix with water in all proportions but hexan-1-ol barely dissolves, and predict whether hexan-1-ol dissolves in hexane.

Check your understanding

Quiz

1.

What is the IUPAC name of CH₃CH₂CH(OH)CH₃?

2.

Which alcohol is tertiary?

3.

Why is the boiling point of ethanol much higher than that of propane?

4.

What type of reaction is CH₃CH₂OH → CH₂=CH₂ + H₂O?

5.

Which alcohol is the least soluble in water?

6.

What is the main environmental problem caused by CFCs?

For parents and teachers

Big ideas (B2.2, B3.2, B3.3): alcohols are named with -ol and classified 1°, 2°, 3°. Hydrogen bonding explains their high boiling points and water solubility. Alcohols are made by hydration and fermentation, and they react by combustion, substitution and elimination (dehydration).

How to help: Ask your teen to explain the activity results using "polar", "non-polar" and "hydrogen bond". Ask them to point out the –OH group on labels of hand sanitizer or antifreeze.

Common difficulties:

  • Thinking the –OH of an alcohol makes it a base like NaOH.
  • Classifying alcohols by the number of –OH groups instead of the number of carbons bonded to the C–OH carbon.
  • Forgetting that elimination is the reverse of hydration.

Safety: rubbing alcohol is flammable and toxic if swallowed; methanol and ethylene glycol (antifreeze) are poisonous even in small amounts. Store them sealed, labelled and out of reach of children and pets.