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AS Level Chemistry 9701
14. Hydrocarbons
Written by: Pranav I
Formatted by: Pranav I
Index
14.1 The homologous group of alkanes
- General formula: CnH2n+2
- All the C-C bonds are single covalent bonds
- All of the carbon atoms display sp3 hybridization
- Tetrahedral arrangement of atoms around each C → bond angle: 109.5°
- Hydrocarbons: compounds containing hydrogen and carbon only
- Saturated hydrocarbons → resulting in the maximum number of hydrogen atoms in the molecule
14.2 Reactions of alkanes
- Generally unreactive compounds
- Small difference in electronegativity between C and H
- The strength of C-H bonds is high
- Non-polar molecules → NOT attacked by nucleophiles and electrophiles
Combustion of alkanes
- Uses of alkanes as fuel:
- To generate electricity in power stations
- To heat our homes and cook our food
- To provide energy in industrial processes
- To provide fuel for transport (list vehicles)
- Complete combustion of alkanes
- Occurs in plenty of oxygen
- All the carbon is fully oxidized to form CO2 (not CO)
- All the hydrogen is oxidized to form H2O
\[
\text{C}_3\text{H}_8 + 5\text{O}_2 \rightarrow 3\text{CO}_2 + 4\text{H}_2\text{O}
\]
- Incomplete combustion of alkanes
- Some of the carbon is only partially oxidized to form CO
- CO is a toxic gas that bonds with haemoglobin the the blood → can no longer transport oxygen around the body
- Oxides of nitrogen contribute to acid rain and photochemical smog
- Unburnt hydrocarbons contribute to photochemical smog (and some are carcinogens)
\[
\text{C}_3\text{H}_8 + 3\text{O}_2 \rightarrow 3\text{CO} + 4\text{H}_2\text{O}
\]
Reducing traffic emissions
- Reactions in catalytic converters fitted in exhaust systems (once warmed up):
- Oxidation of CO to form CO2
- Reduction of nitrogen oxides to form harmless N2
- Oxidation of unburnt hydrocarbons to form CO2 and H2O
- Cannot reduce CO2 amounts (GHG) → a pollutant
- Precious metals coated on a honeycomb structure to provide a large surface area (mechanism)
\[
2\text{CO} + 2\text{NO} \rightarrow 2\text{CO}_2 + \text{N}_2 \quad \text{and} \quad 2\text{CO} + \text{O}_2 \rightarrow 2\text{CO}_2
\]
Free-radical substitution of alkanes by Cl2 (or Br2)
- Reagent: Cl2 or Br2
- Conditions: UV light
- Initiation step
- Breaking of the Cl-Cl bond
- Each Cl atom takes one electron from the bond pair → homolytic fission
- Produces two Cl free radicals (Cl⋅)
\[
\text{Cl}_2 \rightarrow{\text{UV light}} \text{Cl}^\cdot + \text{Cl}^\cdot
\]
- Propagation steps
- A possible step
\[
\text{C}_2\text{H}_6 + \text{Cl}^\cdot \rightarrow \text{C}_2\text{H}_5^\cdot + \text{HCl}
\]
-
-
- A chlorine free radical attacks the ethane molecule
- A C-H bond breaks homolytically
- An ethyl free radical and HCl are produced
- Another possible step
-
\[
\text{C}_2\text{H}_5^\cdot + \text{Cl}_2 \longrightarrow \text{C}_2\text{H}_5\text{Cl} + \text{Cl}^\cdot
\]
-
-
- The ethyl free radical attacks a chlorine molecule
- Forms chloroethane
- Regenerates a chlorine free radical
- The first step can be repeated, then the second and so on → chain reaction
- A mixture of products is formed as per the varying availability of Cl2
- Mechanism reactions for the formation of other compounds:
-
\[
\text{C}_2\text{H}_5\text{Cl} + \text{Cl}^\cdot \longrightarrow \text{C}_2\text{H}_4\text{Cl}^\cdot + \text{HCl}
\quad \text{and} \quad
\text{C}_2\text{H}_4\text{Cl}^\cdot + \text{Cl}_2 \longrightarrow \text{C}_2\text{H}_5\text{Cl}_2 + \text{Cl}^\cdot
\]
- Termination steps
- Two free radicals react to form one molecule
- The chain reaction stops here since there are no free radicals to carry on the reaction
- The more halogen has in the mixture, the greater the number of halogen atoms in the haloalkanes
14.3 The alkenes
- General formula: CnH2n
- Unsaturated hydrocarbons → contain C=C double bonds
- Alkenes are chemical feedstock for industries
| # | Reagent | Acts on | Reaction type | Conditions | Example |
|---|---|---|---|---|---|
| 1 | Al2O3 (catalyst) | Alkanes | Cracking | Heat | \( \text{C}_6\text{H}_{14} \rightarrow \text{C}_4\text{H}_8 + \text{C}_2\text{H}_6 \) |
| 2 | NaOH | Haloalkanes | Elimination | Heat & ethanol | \( \text{C}_2\text{H}_5\text{Cl} + \text{NaOH} \rightarrow \text{C}_2\text{H}_4 + \text{NaCl} + \text{H}_2\text{O} \) |
| 3 | H2SO4 | Alcohols | Elimination (dehydration) | Concentrated & heat | \( \text{C}_2\text{H}_5\text{OH} \rightarrow \text{C}_2\text{H}_4 + \text{H}_2\text{O} \) |
Addition reactions of the alkenes
Addition of hydrogen, H2(g)
- Reagent: H2(g)
- Conditions: heat & Pt/Ni catalyst
\[
\text{C}_2\text{H}_4 + \text{H}_2 \rightarrow{\text{Ni, heat}} \text{C}_2\text{H}_6
\]
Addition of hydrogen halides, HX(aq)
- Reagent: HX(aq)
- Hydrogen halides → HF, HCl, HBr and HI
- Conditions: room temperature
- Alkene bubbled through a concentrated solution of the hydrogen halide
\[
\text{C}_2\text{H}_4 + \text{HCl} \rightarrow \text{C}_2\text{H}_5\text{Cl}
\]
✅ Definition
An intermediate is a species which is formed at a particular step of the reaction
- Stable enough to reach with another substance
- Not stable enough to be a product
- When two or more products can be formed, the more stable carbonation intermediate is more readily formed
- The major product is the haloalkane which has the X atom bonded to the C=C carbon atom with the highest number of alkyl groups attached to it
🚨 Order of carbocation intermediate preference: tertiary > secondary > primary
Addition of steam, H2O(g)
- Reagent: H2O(g)
- Conditions: heat, pressure & conc. H3PO4 catalyst
\[
\text{C}_2\text{H}_4 + \text{H}_2\text{O} \rightarrow{\text{H}_2\text{SO}_4} \text{C}_2\text{H}_5\text{OH}
\]
Addition of steam, X2(aq)
- Reagent: X2(aq)
- Conditions: room temperature
- The alkene is bubbled through a solution of the halogen
- The color of the halogen molecules in the solution is removed
- Bromine water test → to test for the presence of C=C
- Unsaturated compounds decolorize bromine water
\[
\text{C}_2\text{H}_4 + \text{Br}_2 \rightarrow \text{C}_2\text{H}_4\text{Br}_2
\]
The mechanism of electrophilic addition to alkenes
- There is a high electron density around the C=C bond → open to attack by electrophiles
- The π bond breaks since it is further away from the nucleus (greater shielding)
- Hydrogen halides
- Hydrogen halides, like HBr, are polar molecules due to the difference in electronegativity between the H atom and the halogen (Br) atom → H has a δ+ charge and the halogen atom has a δ– charge
- The H atom acts as an electrophile
- Halogen molecules
- As the alkene and halogen approach each other, the area of high electron density around the C=C bond repels the pair of electrons in the X-X away from the nearer X
- Nearer X → slightly positive; Further X → slightly negative
14.4 Oxidation of the alkenes
- Reagent: KMnO4 → potassium manganate(VII)
- The products formed depend on the conditions of the reaction
Cold dilute acidified manganate solution, KMnO4
- Observation: pale purple to colorless
- General reaction: Alkene + water + oxygen → alkanediol
\[
\text{C}_2\text{H}_4 + \text{H}_2\text{O} + \text{O}_2 \rightarrow{ \text{cold dilute KMnO}_4} \text{HO}-\text{CH}_2-\text{CH}_2-\text{OH}
\]
Hot concentrated acidified manganate solution, KMnO4
- C=C double bond is broken completely
- Carbon dioxide, aldehydes, ketones, or carboxylic acids are produced depending on the position of C=C
- If a carbon atom is bonded to two hydrogen atoms, we get oxidation to CO2
- If a carbon atom is bonded to one hydrogen atom and one alkyl group, we get oxidation to a carboxylic acid group (-COOH)
- If a carbon atom is bonded to two alkyl groups, we get oxidation to a ketone group (>C=O)
