This is Part 3 of the long series of Berry Berry Easy essential and concise notes for SPM Chemistry (Form 5) topic on Carbon Compounds. In the previous notes (Part 2), the focus is placed on the alkane group (CnH2n+2) and also the IUPAC naming comvention. The Berry Berry Teacher hopes that all of you are well versed in the IUPAC naming convention. Also for those who have understood the properties of the alkane group, it is time to move on to the alkene group. Try to find an analogy between the two groups.
Further back, Part 1 was on the types of compounds, hydrocarbons, combustions and homologous series and sources of hydrocarbon. Be sure to read all the notes, and take note that this is a long chapter, so there will be more notes on this topic. So for now, try to understand part 3.
SPM Chemistry Form 5 – Terminology and Concepts: Carbon Compounds (Part 3)
Family of Hydrocarbon – Alkene
1. General formula: CnH2n
Where n = 2, 3, 4 … (n = number of carbon)
2. Alkenes are unsaturated hydrocarbons which contain one or more carbon-carbon (C = C) double bonds in molecules.
3. The functional group in alkenes is carbon-carbon double (C = C) bond.
|Name of alkene||Molecular formula of alkene|
- Molecular formula is a chemical formula that shows the actual number of atoms of each type of elements present in a molecule of the compound.
Example: molecular formula of butene is C4H2x4 = C4H8
4. Physical properties of alkenes
|Name||Molecularformula||RMM||Density(g cm-3)||Physical state at 25°C|
- Solubility in water – all members in alkenes are insoluble in water but soluble in many organic solvent (benzene and ether).
- Density of alkene – the density of water is higher than density of alkene.
When going down the series, relative molecular mass of alkenes is higher due to the higher force of attraction between molecules and alkene molecules are packed closer together.
- Electrical conductivity – all members in alkenes do not conduct electricity.
Alkenes are covalent compounds and do not contain freely moving ions.
- Boiling and melting points – all alkenes in general have low boiling points and melting points. Alkenes are held together by weak attractive forces between molecules (intermolecular forces) van der Waals’ force. When going down the series, more energy is required to overcome the attraction. Hence, the boiling and melting points increases.
5. Chemical properties of alkenes
- Reactivity of alkenes
Alkenes are more reactive (unsaturated hydrocarbon).
Alkenes have carbon-carbon (C = C) double bonds which is more reactive than carbon-carbon (C-C) single bonds. All the reaction occur at the double bonds.
- Combustion of alkenes
Complete combustion of hydrocarbons (alkenes)
CxHy + (x + y/4) O2 –> xCO2 + y/2 H2O
C2H4 + 3O2 –> 2CO2 + 2H2O
(Alkenes burn with sootier flames than alkanes. It is because the percentage of carbon in alkene molecules is higher than alkane molecules and alkenes burn plenty of oxygen to produce carbon dioxide and water)
Incomplete combustion occurs when insufficient supply of oxygen
C2H4 + O2 –> 2C + 2H2O
C2H4 + 2O2 –> 2CO + 2H2O
(The flame in the incomplete combustion of alkenes is more smoky than alkanes)
- Polymerisation reaction of alkenes
Polymers are substances that many monomers are bonded together in a repeating sequence.
Polymerisation is small alkene molecules (monomers) are joined together to form a long chain (polymer).
nCH2 = CH2 –> -(- CH2 – CH2 -)-n
ethene (monomer)(unsaturated compound) –> polyethene polymer (saturated compound)
It must be carry out in high temperature and pressure.
- Addition of hydrogen (Hydrogenation)
Addition reaction is atoms (or a group of atoms) are added to each carbon atom of a carbon-carbon multiple bond to a single bond.
C2H4 + H2 –> C2H6 (catalyst: nickel and condition: 200°C)
Example: margarine (produce from hydrogenation of vegetable oils).
- Addition of halogen (Halogenation)
Halogenation is the addition of halogens to alkenes (no catalyst of ultraviolet light is needed).
Alkene + Halogen –> Dihaloalkane
C2H4 + Br2 –> C2H4Br2
In this reaction the brown colour of bromine decolourised (immediately) to produce a colourless organic liquid.
Bromination is also used to identify an unsaturated (presence of a carbon-carbon double bond) organic compound in a chemical test.
- Addition of hydrogen halides
Hydrogen halides (HX) are hydrogen chlorine, hydrogen bromide, hydrogen iodide and etc. This reaction takes place rapidly in room temperature and without catalyst.
CnH2n + HX –> CnH2n+1X
C2H4 + HBr –> C2H5Br (Bromoethane)
(There are two products for additional of hydrogen halide to propene. The products are 1-bromopropane and 2-bromopropane).
- Addition of water (Hydration)
Alkenes do not react with water under ordinary condition. It can react with a mixture of alkene and steam pass over a catalyst (Phosphoric acid, H3PO4). The product is an alcohol.
CnH2n + H2O –> CnH2n+1OH
C2H4 + H2O –> C2H5OH
- Additional of acidified potassium manganate(VII), KMnO4
CnH2n + [O] + H2O –> CnH2n(OH)2
C2H4 + [O] + H2O –> C2H5(OH)2
The purple colour of KMnO4 solution decolourised immediately to produce colourless organic liquid. Also used to identify the presence of a carbon-carbon double bond in a chemical test.