Naming Alkenes and Unsaturation Test
Alkenes are a family of hydrocarbons containing at least one carbon-carbon double bond. This double bond makes them 'unsaturated' and is the site of chemical reactivity, allowing them to undergo addition reactions which is a key concept for the ESAT.
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
- Alkenes form a homologous series of unsaturated hydrocarbons with the general formula CnH(2n).
- Their functional group is the carbon-carbon double bond, which is the center of their chemical reactivity.C=C
- Naming straight-chain alkenes (C2 to C6) requires a number to indicate the position of the first carbon in the double bond (e.g., but-1-ene, but-2-ene).
- Alkenes undergo addition reactions, where the double bond breaks to form single bonds with new atoms.
- Key addition reactants are hydrogen (H2), halogens (e.g., Br2, Cl2), hydrogen halides (e.g., HBr), and steam (H2O).
- The test for unsaturation involves adding bromine water (orange-brown); an alkene will decolourise it to colourless.
Formulae
Cn H2n To find the molecular formula of a straight-chain or branched alkene that contains exactly ONE carbon-carbon double bond.
Definitions
- Unsaturated Hydrocarbon
- A hydrocarbon molecule that contains one or more carbon-carbon double or triple bonds. It has fewer hydrogen atoms than the maximum possible for its carbon skeleton.
- Homologous Series
- A series of organic compounds with the same functional group and similar chemical properties, in which successive members differ by a CH2 group.
- Addition Reaction
- A chemical reaction in which atoms are added to a molecule containing a double or triple bond, breaking one of the bonds in the process and forming a single, larger molecule.
Worked example
Propene (C3H6) is bubbled through a solution of bromine water (Br2(aq)). What is the balanced chemical equation for the reaction, and what is the molecular formula of the organic product formed?
- 1
Identify the reaction type.
An alkene (propene) reacting with a halogen (bromine) is an addition reaction.
- 2
Recognise that in an addition reaction, the double bond in propene breaks, and a bromine atom adds to each of the two carbons that were part of the double bond.
- 3
The reactants are propene (C3H6) and bromine (Br2).
- 4
The product is a single molecule formed by combining all the atoms of the reactants.
No atoms are lost.
- 5
Determine the product's molecular formula by adding the atoms:
C3 + H6 + Br2 gives C3H6Br2.
- 6
Write the balanced equation.
Since all atoms combine into one product, the molar ratio is 1:1:1.
- 7
Equation:
C3H6 + Br2 → C3H6Br2.
Answer: The balanced equation is C3H6 + Br2 → C3H6Br2. The molecular formula of the product is C3H6Br2.
Common mistakes
- ×Confusing the general formula for alkenes (CnH2n) with that for alkanes (CnH2n+2). Remember the double bond means two fewer hydrogen atoms are present.
- ×Forgetting that the CnH2n formula is only valid for alkenes with a single double bond. Molecules with two double bonds (dienes) will have a different formula (CnH2n-2).
- ×When naming, starting the carbon chain numbering from the wrong end. Always number from the end that gives the double bond the lowest possible number.
- ×Incorrectly identifying the product of an addition reaction. Remember the C=C double bond is the only part that reacts; the rest of the carbon chain remains unchanged.
No-calculator tips
- ✓To quickly check if a given formula like CxHy could be a simple alkene, verify if the number of hydrogens is exactly double the number of carbons (y = 2x).
- ✓In addition reaction questions, simply sum the atoms of the reactants to find the molecular formula of the product. For example, C5H10 reacting with H2 must produce C5H12. No complex drawing is needed if only the formula is asked.