Alkanols (also called alcohols) are organic compounds with a hydroxyl group (-OH) attached to a carbon atom. They form a homologous series with the general formula CnH2n+1OH, where n represents the number of carbon atoms.
Quick Summary
- Alkanols contain the hydroxyl functional group (-OH)
- General formula: CnH2n+1OH
- Named by replacing the -e in alkanes with -ol (methane → methanol)
- Used as solvents, fuels, antiseptics, and in making perfumes
- Solubility in water decreases as carbon chain length increases
Understanding Alkanols
Alkanols are important organic compounds you encounter daily. The methylated spirit used in laboratories is mainly ethanol. The hand sanitizer NAFDAC approved during COVID-19 contains alcohol. Even palm wine contains ethanol produced by yeast.
The Hydroxyl Group
The hydroxyl group (-OH) makes alkanols different from alkanes. This group gives alkanols their special properties. When you add -OH to an alkane, you get an alkanol. The -OH group is polar, which means it attracts water molecules.
General Formula and Structure
All alkanols follow the pattern CnH2n+1OH. Let’s see how this works:
- If n = 1: C1H3OH (methanol)
- If n = 2: C2H5OH (ethanol)
- If n = 3: C3H7OH (propanol)
- If n = 4: C4H9OH (butanol)
Notice that each member differs from the next by CH2. This pattern defines a homologous series.
Naming Alkanols
The IUPAC system names alkanols by replacing the final -e in the alkane name with -ol. For example:
- Methane becomes methanol
- Ethane becomes ethanol
- Propane becomes propanol
- Butane becomes butanol
For longer chains, you must show where the -OH group is attached. We use numbers for this. Butan-1-ol means the -OH is on the first carbon. Butan-2-ol means it’s on the second carbon.
Classification of Alkanols
We classify alkanols based on which carbon atom holds the -OH group:
Primary alkanols: The -OH group attaches to a carbon atom that connects to only one other carbon atom. Examples include ethanol and propan-1-ol. These are most common.
Secondary alkanols: The -OH group attaches to a carbon atom that connects to two other carbon atoms. Propan-2-ol is an example. Hospitals use this as rubbing alcohol.
Tertiary alkanols: The -OH group attaches to a carbon atom that connects to three other carbon atoms. 2-methylpropan-2-ol is an example.
Physical Properties
The first three alkanols (methanol, ethanol, propanol) are liquids at room temperature in Lagos or Kano. They have these properties:
Smell: Lower alkanols have a sharp, distinctive smell. You can recognize methylated spirit by its smell.
Solubility: Methanol and ethanol mix completely with water. This happens because the -OH group forms hydrogen bonds with water molecules. As the carbon chain gets longer, alkanols become less soluble in water. Octanol barely dissolves in water.
Boiling Points: Alkanols have higher boiling points than alkanes with the same number of carbon atoms. Ethanol boils at 78°C while ethane boils at -89°C. The hydrogen bonds between alkanol molecules need more energy to break.
Density: All alkanols are less dense than water. They float on water.
Comparison Table: Alkanols vs Alkanes
| Property | Alkanols | Alkanes |
|---|---|---|
| Functional Group | -OH (hydroxyl) | None |
| General Formula | CnH2n+1OH | CnH2n+2 |
| Solubility in Water | Lower members soluble | Insoluble |
| Boiling Point (C2) | Ethanol: 78°C | Ethane: -89°C |
| Reactivity | More reactive | Less reactive |
| Common Use | Solvents, antiseptics | Fuels, lubricants |
Chemical Reactions of Alkanols
1. Combustion
Alkanols burn in oxygen to produce carbon dioxide and water. Ethanol burns with a blue flame:
C2H5OH + 3O2 → 2CO2 + 3H2O
This reaction releases energy. Spirit burners use this principle.
2. Oxidation
When you heat primary alkanols with oxidizing agents like acidified potassium dichromate, they form aldehydes first, then carboxylic acids:
Ethanol → Ethanal → Ethanoic acid
Secondary alkanols form ketones when oxidized. Tertiary alkanols resist oxidation under normal conditions.
3. Esterification
Alkanols react with carboxylic acids to form esters. These smell fruity and are used in perfumes:
Ethanol + Ethanoic acid → Ethyl ethanoate + Water
4. Dehydration
Concentrated sulfuric acid removes water from alkanols at high temperatures to form alkenes:
C2H5OH → C2H4 + H2O
Uses of Alkanols
Methanol: Used as industrial solvent and in making formaldehyde. NDLEA warns against drinking it because it causes blindness and death.
Ethanol: Used in alcoholic drinks (beer, palm wine), as fuel additive, in methylated spirit, and as antiseptic. Hospitals use it to sterilize equipment.
Propanol: Used as industrial solvent and in making cosmetics.
Higher Alkanols: Used in detergents and plasticizers.
Common Exam Mistakes
WAEC examiners report these frequent errors:
- Confusing general formulas: Students write CnH2nOH instead of CnH2n+1OH. Remember the +1 before OH.
- Wrong naming: Writing “ethyl alcohol” instead of ethanol in IUPAC nomenclature questions.
- Incomplete combustion equations: Forgetting to balance equations or omitting water as a product.
- Classification errors: Calling all alkanols “primary” without checking the carbon bonding pattern.
- Oxidation products: Not stating that primary alkanols give aldehydes then acids, while secondary give only ketones.
- Solubility trends: Saying all alkanols dissolve in water. Only lower members (C1-C3) are very soluble.
Practice Questions
Multiple Choice Questions
1. What is the general formula for alkanols?
(a) CnH2n
(b) CnH2n+2
(c) CnH2n+1OH ✓
(d) CnH2n-2
2. Which alkanol is commonly used in methylated spirit?
(a) Methanol
(b) Ethanol ✓
(c) Propanol
(d) Butanol
3. What type of alcohol is propan-2-ol?
(a) Primary alcohol
(b) Secondary alcohol ✓
(c) Tertiary alcohol
(d) Quaternary alcohol
4. Which product forms when ethanol undergoes complete combustion?
(a) Carbon monoxide and water
(b) Carbon dioxide and hydrogen
(c) Carbon dioxide and water ✓
(d) Carbon and water vapor
Essay Questions
1. (a) Define alkanols and state their general formula. (2 marks)
(b) Write the structural formulas for the first three members of the alkanol homologous series. (3 marks)
(c) Explain why ethanol has a higher boiling point than ethane. (3 marks)
Examiner Tip: For part (c), mention hydrogen bonding between ethanol molecules and compare with weaker van der Waals forces in ethane.
2. (a) Distinguish between primary, secondary, and tertiary alkanols. Give one example of each. (6 marks)
(b) State two physical properties of alkanols. (2 marks)
Examiner Tip: “Distinguish” means explain the differences clearly. Show the structural differences in your examples.
3. (a) Write a balanced equation for the complete combustion of propanol. (3 marks)
(b) Describe what happens when ethanol is heated with concentrated sulfuric acid. (3 marks)
(c) State two uses of ethanol. (2 marks)
Examiner Tip: In part (b), name the process (dehydration) and write the equation showing ethene formation.
Memory Aids
To remember the general formula:
“Can Harry 2n+1 Operate Huge machines?” = CnH2n+1OH
For naming:
“Take E, add OL” = Take the -e from alkane, add -ol
Oxidation products:
“1° makes 2 (aldehyde, acid), 2° makes 1 (ketone), 3° makes none”
Solubility trend:
“Short chains swim, long chains sink” = Lower alkanols dissolve in water, higher ones don’t
Related Topics
To understand alkanols better, explore these related chemistry topics:
- Alkanes (the parent compounds without -OH group)
- Alkanoic Acids (formed when primary alkanols are oxidized)
- Esters (formed when alkanols react with acids)
- Organic Chemistry Introduction (classification and nomenclature)
- Homologous Series (pattern shared by alkanol family)