4.2Core Organic Chemistry
Alkanes — OCR A Level Chemistry A H432 Revision Notes & Topical Questions
5 min read 9 parts Lesson 6 of 29
Lesson summary
Complete PDF with figures and original page layoutPage 1 OCR A Level Chemistry A Your notes Alkanes Contents Introduction to Alkanes Reactions of Alkanes Free Radical Substitution of Alkanes Page 2 Introduction to Alkanes …
Part 1 of 7Bonding in alkanes
Alkanes
Alkanes a homologous series of saturated hydrocarbons joined by sigma (σ) bonds, with general formula CnH2n+2
Polarity
- Polarity carbon and hydrogen have very similar electronegativities, so the C–H σ bonds are non-polar
- Alkanes have no δ+ or δ− charges.
- They do not react with polar reagents because they have no electron-deficient areas to attract nucleophiles and no electron-rich areas to attract electrophiles.
Shape
- Shape each carbon has four valence electrons and forms four σ bonds with no lone pairs
- The arrangement is tetrahedral with bond angles of 109.5°.
- Equal repulsion acts between all four bonding pairs.
Intermolecular forces
- Intermolecular forces alkanes are non-polar and have only weak induced dipole–dipole forces
- (London dispersion)
- OCR allows both permanent and induced dipole–dipole forces to be called van der Waals forces.
Part 2 of 7Homologous series data
| Alkane | Carbons | Formula | Boiling point / °C | State at room temperature | Melting point / °C |
|---|---|---|---|---|---|
| Methane | 1 | CH4 | −162 | Gas | −183 |
| Ethane | 2 | C2H6 | −89 | Gas | −172 |
| Propane | 3 | C3H8 | −42 | Gas | −188 |
| Butane | 4 | C4H10 | 0 | Gas | −138 |
| Pentane | 5 | C5H12 | 36 | Liquid | −130 |
Part 3 of 7Boiling points of alkanes
Chain length
- Chain length boiling point increases with molecular size
- London forces are very small for methane and increase as the molecules get larger.
Branching
- Branching branched alkanes have lower boiling points than unbranched alkanes with the same number of carbons
- The more branched the chain, the lower the boiling point tends to be.
- Induced dipole–dipole forces act only over very short distances.
- Short bulky molecules pack less closely than long thin ones and have a smaller surface area.
Part 4 of 7Low reactivity of alkanes
Two reasons
- Two reasons high bond enthalpies and very low polarity of the σ bonds
Bond enthalpy
- C–C is 346 kJ mol−1 and C–H is 411 kJ mol−1
- C–H is stronger because hydrogen has only one shell, so the nuclei are closer.
- The attraction of the nuclei to the shared electron pair is greater.
Polarity
- Polarity the electronegativities of C and H are almost equal, so the bonds have very low polarity
Part 5 of 7Complete and incomplete combustion
Complete combustion
- Complete combustion in excess oxygen all carbon and hydrogen are oxidised to carbon dioxide and water
- Alkanes are burnt on a large scale as fuels.
Incomplete combustion
- Incomplete combustion in limited oxygen not all carbon is fully oxidised
- Some carbon forms carbon monoxide.
- With even less oxygen, carbon forms as soot.
- It often occurs in car engines because of limited oxygen.
Car exhaust pollutants
- Car exhaust pollutants carbon monoxide (CO), oxides of nitrogen (NO and NO2) and volatile organic compounds (VOCs)
- They have serious environmental and health effects.
Carbon monoxide
- Carbon monoxide a toxic, odourless gas causing dizziness, loss of consciousness and death
- It binds strongly to haemoglobin, which then cannot bind and carry oxygen to organs or carry carbon dioxide away.
Part 6 of 7Free radical substitution: steps
1Reaction
- Reaction a hydrogen atom of an alkane is substituted by a halogen
- (chlorine or bromine)
- Alkanes are unreactive, so ultraviolet light is needed (a high temperature of about 400 °C is also mentioned).
- The bromine colour disappears only when an alkane is mixed with it in sunlight, showing UV is essential.
2Initiation
- UV energy breaks the Cl–Cl or Br–Br bond by homolytic fission
- This forms two radicals.
3Propagation
- Propagation a chain reaction in which the reaction grows
- A halogen radical breaks a C–H bond homolytically to give an alkyl radical and a hydrogen halide.
- The alkyl radical attacks another halogen molecule, forming the halogenoalkane and regenerating the halogen radical.
4Termination
- Termination two radicals react to form a single unreactive molecule
- Several products are possible.
Part 7 of 7Limitations of free radical substitution
1Mixture of products
- Mixture of products the reaction is unsuitable for making a specific halogenoalkane
- With enough halogen all hydrogens are replaced, e.g. ethane becomes C2Cl6 or C2Br6.
2Impurities from termination
- Impurities from termination any radical can combine with another
- Two •CH3 radicals form ethane, CH3CH3, an impurity when making a chloroalkane.
3Further substitution
- Further substitution excess chlorine with methane gives CH2Cl2, CHCl3 and CCl4
- CH3Cl + •Cl → HCl + •CH2Cl, then •CH2Cl + Cl2 → CH2Cl2 + •Cl.
4Substitution at a middle carbon
- Substitution at a middle carbon in propane the radical can form on the middle carbon
- CH3CH2CH3 + •Br → CH3•CHCH3 + HBr.
- CH3•CHCH3 + Br2 → CH3CH(Br)CH3 + Br•.
- The radical dot must be shown on the correct carbon.
Key terms
| Term | Meaning |
|---|---|
| Alkane | A saturated hydrocarbon with general formula CnH2n+2 |
| Sigma bond | A covalent bond formed by head-on overlap of orbitals |
| Free radical substitution | A reaction in which a hydrogen atom is replaced by a halogen through radical intermediates |
| Initiation | The first step, forming radicals by homolytic fission |
| Propagation | Chain steps in which radicals are used and regenerated |
| Termination | Two radicals combine to form a stable molecule |
Avoid these mistakes
1
Saying alkanes are unreactive because C–H bonds are polar — they are non-polar and strong
2
Saying branched alkanes have higher boiling points — they pack less closely and have lower ones
3
Forgetting UV light as the condition for free radical substitution
4
Writing a termination step that forms radicals — termination removes two radicals to give a molecule
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