Transcript File

Unit 2 The World of Carbon
Fuels
-substances which burn releasing energy.
Petrol – from fractional distillation of crude oil and reforming of
naphtha fraction.
Reforming produces - branched chain alkanes,
- cycloalkanes
- aromatic compounds.
- decreases ‘knocking’ from auto-ignition.
Lead compounds improve combustion but no longer used.
Winter petrol blends have more volatile hydrocarbons than
summer blends.
Alternative fuels
Ethanol - from fermentation of sugar cane; renewable.
Methanol - from synthesis gas (from steam reforming of CH4)
Advantages: virtually complete combustion; no aromatic
carcinogens; cheaper than petrol; less explosive than petrol
Disadvantages: absorbs water making in corrosive to engine;
toxic; increases greenhouse gases unless CH4 from biogas
Methane- renewable if formed by anaerobic fermentation of
organic waste e.g. manure; main constituent of biogas.
Hydrogen – can be produced by electrolysis of water using
solar energy; clean burning, only produces water; difficult to
store and distribute.
Hydrocarbons
Compounds of hydrogen and carbon only
Homologous series: families of compounds which
• share general formula
• have similar chemical properties (react the same)
• show regular changes in physical properties with
increasing size (by CH2 each time)
Hydrocarbons
Alkanes
• CnH2n+2
• single C-C bonds, saturated
• slow, substitution reactions
Cycloalkanes
• CnH2n
• single C-C bonds, saturated cyclic compounds
• slow, substitution reactions
Alkenes
• CnH2n
• at least 1 double C=C bond, unsaturated
• fast, addition reactions
Hydrocarbons
Alkynes
• CnH2n-2
• at least 1 triple C=C bonds, unsaturated
• fast, addition reactions
Naming hydrocarbons
Name indicates
• number of carbon atoms in molecule:
meth-1, eth-2, prop-3, but-4, pent-5, hex-6, hept-7, oct-8
(monkeys eat peanut butter penguins hate hairy oranges)
• family e.g. -ane, -ene, -yne
Naming hydrocarbons
Choose longest carbon chain, be sure to include functional
group e.g. double bond
Number carbons, if any branches are present keep numbers
as low as possible (this might mean going from right to left!)
e.g.
Branches name indicates number of carbons
-CH3, methyl; -C2H5, ethyl; -C3H7, propyl etc
Isomers – same molecular formula, different structures (moving
double bond produces isomer)
Addition reactions
Alkenes
addition of
• a halogen (1)
• hydrogen (2)
• a hydrogen halide (3)
• water (4)
Addition reactions
Alkynes
• two-stage process:
• alkyne→ alkene→
alkane
• possibility of isomers
being produced.
• compared to an
alkene, complete
addition to an alkyne
will require twice the
quantity of
• halogen (1)
• hydrogen (2)
• hydrogen halide (3)
Aromatic compounds
Benzene
• very important feedstock
• ‘mother’ of all aromatics
• C6H6
• does not decolourise bromine rapidly, hence saturated
• each carbon forms only 3 bonds, leaving a ‘spare’ electron
• the ‘spare’ electrons can move (delocalised) around the ring
Aromatic compounds
• basically benzene with at least 1 hydrogen substituted with
another atom/group of atoms
• phenyl group, -C6H5 benzene ring minus 1 hydrogen
• used in manufacture of, e.g., plastics (including Bakelite),
explosives, drugs, dyes
Alcohols (Alkanols)
• contain the hydroxyl functional group, -OH
• names end in –ol e.g. methanol CH3OH; ethanol C2H5OH
• need to specify carbon with –OH from propanol onwards
• when naming, longest carbon chain must include –OH
e.g.
• primary alcohols (with exception of methanol) have 1 C
attached to –COH, secondary alcohols have 2C, tertiary have
3C attached to -COH
Reactions of alcohols
• made by catalytic hydration of alkenes or reduction of ketones
and aldehydes
• dehydrated, using aluminium oxide as a catalyst, to alkenes
• undergo combustion producing carbon dioxide and water
• primary and secondary alcohols undergo partial oxidation
with mild oxidising agents:
- acidified potassium dichromate (H+/Cr2O72-, orange to green)
- acidified potassium permanganate (H+/MnO4-, purple to colourless)
- hot copper (II) oxide (black to red)
Partial oxidation of alcohols
• primary alcohols partially oxidised to alkanals (aldehydes) which
can be partially oxidised further to alkanoic acids
• secondary alcohols partially oxidised to alkanones (ketones),
these cannot be further partially oxidised
• tertiary alcohols do no undergo partial oxidation
Alkanals (aldehydes) and
alkanones (ketones)
• made by partial oxidation of primary and secondary alkanols
(alcohols) (see previous slide)
• can be reduced to the alcohol from which they were made
• aldehydes can be further partially oxidised to alkanoic acids
• contain the carbonyl group, C=O
• carbonyl group polar as O greater electronegativity than C
• when naming, carbon chain must include carbonyl group
• number carbons from end closest to carbonyl group e.g.
Experimentally distinguishing
aldehydes from ketones
• ketones are resistant to partial oxidation, because the carbonyl
group is flanked by two C atoms
• alkanals (aldehydes) can be partially oxidised to the
corresponding alkanoic acid