Naming and Reactions of Alcohols
Alcohols are a family of organic compounds containing the hydroxyl (-OH) group. For the ESAT, you need to know their general formula, how to name simple straight-chain examples, and how they react with sodium metal.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Alcohols form a homologous series with the general formula C(n)H(2n+1)OH, where the -OH is the functional group.
- Naming straight-chain alcohols (C1 to C6) involves using the alkane stem, adding the suffix '-ol', and using a number to indicate the carbon atom the -OH group is attached to (e.g., propan-1-ol).
- The carbon chain is numbered starting from the end closest to the -OH group, giving it the lowest possible number.
- Alcohols react with sodium metal in a redox reaction, producing a salt called a sodium alkoxide and hydrogen gas.
- The observation for the reaction with sodium is effervescence or fizzing, due to the production of hydrogen gas.
Formulae
C(n)H(2n+1)OH To determine the molecular formula for any member of the alcohol homologous series, given the number of carbon atoms (n).
2ROH + 2Na → 2RONa + H2 To predict the products and write the balanced equation for the reaction of any alcohol (ROH) with sodium metal (Na).
Definitions
- Homologous Series
- A series of compounds with the same functional group and similar chemical properties, in which successive members differ by a CH2 group.
- Functional Group
- An atom or group of atoms within a molecule that is responsible for its characteristic chemical reactions. For alcohols, this is the hydroxyl group (-OH).
- Sodium Alkoxide
- The ionic compound (salt) formed when an alcohol reacts with sodium. It has the general formula RONa, where R is the alkyl group.
Worked example
An alcohol has the molecular formula C5H12O. One of its structural isomers is pentan-3-ol. Write the balanced chemical equation for the reaction of pentan-3-ol with sodium.
- 1
Identify the reactants:
The alcohol is pentan-3-ol (C5H11OH) and the metal is sodium (Na).
- 2
Recall the general reaction for an alcohol with sodium:
Alcohol + Sodium → Sodium Alkoxide + Hydrogen.
- 3
Identify the specific products:
The alkoxide will be sodium pentan-3-oxide (C5H11ONa) and the gas will be hydrogen (H2).
- 4
Write the unbalanced equation:
C5H11OH + Na → C5H11ONa + H2.
- 5
Balance the equation.
Since H2 is produced, we need two H atoms from the -OH groups.
Therefore, we must start with two alcohol molecules and two sodium atoms to balance the equation.
- 6
The final balanced equation is:
2C5H11OH + 2Na → 2C5H11ONa + H2.
Answer: 2C5H11OH + 2Na → 2C5H11ONa + H2
Common mistakes
- ×Mistakes in numbering the carbon chain when naming an alcohol. Always number from the end that gives the -OH group the lowest possible number (e.g., pentan-2-ol, not pentan-4-ol).
- ×Forgetting to balance the equation for the reaction with sodium. The 2:2:2:1 stoichiometric ratio is crucial.
- ×Confusing the product gas with water. The reaction produces hydrogen gas (H2), not H2O.
- ×Mixing up the general formula for alcohols (C(n)H(2n+1)OH) with that for alkanes (C(n)H(2n+2)).
No-calculator tips
- ✓To quickly check an alcohol's molecular formula, remember the total number of hydrogens is always 2n+2, the same as the corresponding alkane. For example, propane (C3H8) and propanol (C3H7OH) both have 8 hydrogens in total.
- ✓When balancing the sodium reaction, start with the product H2. Since you need two H atoms, you must have two ROH molecules. This immediately tells you that you will also need two Na atoms and will produce two RONa molecules.