UNIT 3: SECTION 2 - ALKANES & HALOGENOALKANES
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What is an alkane?
Hydrocarbon with formula CnH2n Forms maximum bonds with hydrogen
What are the order of fractions?
GAY - Gas POLICEMEN - Petrol NEVER - Naptha KILL - Kerosene GAY - Gas Oil MEN - Mineral Oil BITCH - Bitumen
Conditions for thermal cracking?
High temp (1000°C) High pressure (70atms) Mostly alkenes - polymerisation -> polymers such as plastic: poly(ethene).
Conditions for catalytic cracking AND one advantage?
High temp (450°C) Slight pressure Mostly aromatic HC’s and motor fuels Uses zeolite catalyst Cost-Efficient!
One use of alkanes, and how they are used in this way? One risk to this use?
Alkanes are good fuels. Products are CO2 (POLLUTANT) and H2O and a lot of energy. • Used in power-stations, car engines etc. • If not enough O2, incomplete combustion occurs, leading to more pollutants.
4 Pollutants other than CO2 which are produced during the incomplete combustion of alkanes?
1) CO - Poisonous gas, binds to haemoglobin -> lowers O2 saturation. [REMOVED VIA CAT. CONVERTERS] 2) Carbon - Soot, causes respiratory issues and engine buildup. 3) Smog - Unburnt HC + NOx [NOx made in engine; high pressure high temp] [REMOVED VIA CAT. CONVERTERS] 4) Acid Rain - SO2 from burning of fossil fuels. SO2 dissolves in air -> H2SO4 -> RAINS. • Acid rain destroys vegetation, buildings, aquatic life.
What are free radicals?
Atoms with unpaired electrons, eg. *Cl
How are free radicals formed?
UV light + Halogen molecule -> 2Rads. Photochemical reaction
How are halogenoalkanes formed and what type of reaction is this?
Halogen + Alkane ———> Halogenoalkane + Hydrogen Free-rad substitution
The 3 stages of free radical reactions?
1) Initiation: Cl2 (+UV) ———> 2Cl* 2) Propagation: Chain reaction… Cl* + CH4 —> CH3* + HCl CH3* + Cl2 —> CH3Cl + Cl* And so on.. until all molecules of Cl/CH4 used up. 3) Termination: Free rads used up.. Cl* + CH3* —> CH3Cl OR CH3* + CH3* —> C2H6
What is a CFC? Why are they bad? What is the reaction between CFC’s and O3?
CFC’s are chlorofluorocarbons - All H’s replaced. They were used in refrigerator coolants, aerosol sprays etc. Damaged the Ozone layer. 1) CFCl3 (+UV) —> CFCl2* + Cl* 2) Cl* + O3 —> ClO* + O2 3) ClO* + O3 —> 2O2 + Cl* Overall: 2O3 —> 3O2 (Cl* Catalyst)
What is O3? How is it useful?
Ozone; found in Ozone layer - ACTS AS CHEMICAL SUNSCREEN; PREVENTS SKIN CANCER BY ABSORBING LOTS OF HEAT.
What are CFC alternatives?
HFC’s
Is the bond between a halogen and carbon polar? If so, what causes this polarity?
It is polar because there is a high difference in ELECTRONEGATIVITY.
What is a nucleophile?
Electron donor.
What occurs in a nucleophilic substitution reaction?
Nucleophile replaces Halogen.
Conditions to make Alcohol from Halogenoalkane?
• Weak OH base. • Warm Aqueous soln. • Warm under reflux.
Conditions to make Nitrile from Halogenoalkane?
• CN • Ethanoic soln. • Warm under reflux.
How do you make an Amine from a Halogenoalkane? 2 Step Mechanism; what is the process?
• Ethanoic Soln. • Excess Ammonia (NH3) • Warm under reflux. NH3 attracted to polarity in halogenoalkane bond. Replaces halogen. N has +charge (4 atoms bonded to it) Excess NH3 takes a H from NH3 bonded to HAlkane. H donates electrons to HAlkane and leaves.
What is an elimination reaction? How does it work? What are the conditions?
Elimination reactions take a hydrogen away from the HAlkane; hydrogen leaves its electrons - electrons fed to carbon chain - halogen leaves with its electrons. Results in alkenes being formed. Conditions: • OH • in ethanoic soln. • warmed under reflux
If a mixture of water and ethanol is used in the reaction mixture, what products are made?
Mixture of alkenes and alcohols made.