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Organic basics
Alkanes (properties and reactions)
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Olivia Johansson
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Boiling
point of alkanes
Depends on
size
and
shape
Smallest
alkanes
Gases
at room temperature and pressure, have very
low
boiling points
Larger alkanes
Liquids, have
higher
boiling points
Alkane
bonding
1.
Covalent
bonds inside molecules
2. Induced
dipole-dipole
interactions (
London
forces) between molecules
Longer
carbon chain
Stronger induced
dipole-dipole
interactions
Longer
molecules
More
energy
required to overcome
induced dipole-dipole
interactions, boiling point rises
Branched
-chain alkane
Lower
boiling point than
straight-chain
isomer
Alkanes burn completely in
oxygen
Close
packing of branched-chain alkanes
Not possible,
reduced
surface contact between molecules,
reduced
induced dipole-dipole interactions
Combustion of alkanes
Alkanes +
oxygen
→ carbon dioxide +
water
Smaller
alkanes
Turn into
gases
more easily,
burn
more easily
Larger alkanes
Release
more energy
per mole when
burned
Alkanes make excellent
fuels
because they release so much
energy
when burned
Burning alkanes in limited
oxygen
produces carbon
monoxide
and water
Carbon monoxide
is poisonous, binds to
haemoglobin
better than oxygen
Lack of
oxygen
from carbon
monoxide
can lead to oxygen deprivation and be fatal
Complete combustion of ethane
C2H6 + 3.5O2 →
2CO2
+
3H2O
Homolytic
fission
Bond
breaks
evenly, each bonding atom receives one electron, forms two
radicals
Heterolytic
fission
Bond
breaks unevenly, one atom receives both electrons, forms a
cation
and an anion
Radicals are very
reactive
due to
unpaired
electron
Photochemical
reaction of alkanes with
halogens
Hydrogen
atom substituted by
halogen
(chlorine or bromine)
Mechanism
of halogen-alkane photochemical reaction
1. Initiation (free radicals produced)
2.
Propagation
(free radicals used up and created in chain reaction)
3.
Termination
(free radicals joined to form stable molecules)
Excess methane
Reduces chance of
by-products
forming
Free
radical substitution
can occur at any point along carbon chain, producing mixture of
isomers