Hydrocarbons

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Hydrocarbons
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What do we know?

• Organic chemistry
• The chemistry of carbon
• Covalent bonding - sharing of electrons
• From straight and branched chains (actual bond
  angle is 109o and looks more like a zig-zag but
  for neatness we draw as a straight line

• Hydrocarbons containing only carbon and hydrogen
• Alkanes contains only single bonds (saturated)
• Alkenes contains doubles bonds (unsaturated)
• Alkynes contains triple bonds (unsaturated)

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What do we know

• Functional groups
• Are atoms or groups of atoms that determine the
  behaviour of a compound

• Haloalkanes -Cl, -Br
• Acohols -OH
• Carboxylic acids COOH
• Esters -COO-R
• (R hydrocarbon)

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Alkanes

• Alkanes
• CnH2n 2
• are saturated
• Form a homologous series

• Methane CH4
• Ethane C2H6
• Propane C3H8
• Butane C4H10
• Pentane C5H12
• Hexane C6H14
• Heptane C7H16
• Octane C8H18

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groups

• Alkyl group formed from an alkane
• Methyl CH3-
• Ethyl C2H5-
• Propyl C3H7-
• Butyl C4H9-

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Naming rules

• The alkane is named after the longest parent
  chain
• Branch chains are identified and called yl eg
  methyl, ethyl
• Number from the end of the parent chain that
  gives the branch chain the lowest number
• If there are identical branches on one carbon
  atom use di, tri, and tetra eg
  2,2-dimethylpentane

• If there are chains with methyl, ethyl and
  propyl, the order is ethyl, methyl and propyl
  because comes before m and p in the
  alphabet
• The same rules apply to halo alkanes because
  bromo comes before chloro

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General Formula

• Alkanes CnH2n2
• Alkenes CnH2n
• Alkynes CnH2n-2

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Isomers

• These are compounds which have the same molecular
  formula but different structural formula
• Eg C4H10
• butane or
• 2-methylpropane

• Chain or branched isomers
• C5H12
• Pentane
• 2-methyl butane
• 2,2dimethyl propane

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Isomers

• Positional isomers
• These arise when molecules have a functional
  group in different positions
• Eg C3H7OH
• Propan-1-ol and propan 2-ol

• Functional group isomers
• These arise when molecules belong to different
  homologous series
• Eg C2H6O
• Ethanol and methoxymethane

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Isomers

• Geometric isomers
• In alkenes and alkynes the double bond cannot
  rotate and in ethene all the atoms are in the
  same plane.
• If there are different functional groups off each
  carbon, then two possible geometrical isomers are
  possible

• Eg 1,2-dichloroethene
• There are cis (same side) and trans (opposite
  side) isomers

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Carbon Chemistry

• Carbon is a unique element as it can bond with
  itself to form
• Long chains
• Branched chains
• Rings

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Physical Properties of Alkanes

• Alkanes form a homologous series and thus the
  properties of alkanes change gradually as follows
• C1-4 are gases at room temperature, C5-C15 are
  liquids, C16-C20 melt at 16-37 oC respectively.
  Candle wax is about 50 Carbon atoms and Bitumen
  60 Carbons plus

• Densities are less than that of water (so they
  float)
• They dissolve in each other, but not in water
• Non-conductors of heat and electricity

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Where do alkanes come from

• The main source of alkanes is petroleum
• Petroleum is a naturally occurring mixture of
  hydrocarbons
• The liquid component of petroleum is called
  crude oil
• The lighter gaseous part is called natural gas
• Natural gas consists of the lighter hydrocarbons
  mostly methane but with significant amounts of
  ethane, propane and butane

• Natural gas consists of the lighter hydrocarbons
  mostly methane but with significant amounts of
  ethane, propane and butane
• In New Zealand natural gas is obtained from two
  Taranaki fields called Maui and Kapuni
• Mixtures of alkanes are used as fuels which
  produce heat and energy when burned

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Chemical Properties

• Combustion (reaction with oxygen)
• They react with excess oxygen to form CO2 and H2O
• They react with limited oxygen to form CO and H2O
  (incomplete combustion)
• With very limited oxygen they form C and H2O
  (incomplete combustion)

• Ethane fire in Fort Saskatchewan, Alberta. Note
  the hay rolls lying unharmed in the foreground
  while the fire blazes nearby

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Chemical Properties

• Substitution Reactions
• With UV light alkanes undergo substitution
  reactions or at temperatures in the 250-400oC
  range
• Alkanes can be chlorinated to give a mixture of
  products
• Hydrogen chloride gas HCl is also produced
• One hydrogen is substituted (replaced) by one of
  the chlorine atom
• Note only one of the Cl atoms from each Cl2, not
  both are substituted

• CH4 Cl2 CH3Cl HCl
• chloro methane
• CH4 Cl2 CH2Cl2 HCl
• dichloro methane
• CH4 Cl2 CHCl3 HCl
• trichloromethane

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Substitution reaction of Alkanes

• Bromination this is the substitution reaction
  with Br2 (bromine)
• Reacts with alkanes
• Very slow reaction
• Need a catalyst such as UV light
• Substitution reaction
• H is replaced by Br atom from each Br2 molecule

• The orange molecules slowly converted to
  bromoalkanes and hydrogen bromide (both
  colourless)
• The orange colour slowly disappears

Catalyst something which speeds up a reaction
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Chemical Properties of Alkanes

• Cracking
• When large alkane molecules are heated gently or
  strongly over a catalyst they split into smaller
  alkanes and alkenes
• Ethene is always produced
• Need high temperatures and a catalyst

• C10H22 C8H18 C2H4
• decane octane ethene

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Alkenes

• Characteristics
• Contain double bonds
• Unsaturated hydrocarbons
• Form a homologous series ethene, propene,
  pentene, hexene

• Naming
• If there are more than 3 carbons in the chain the
  position of the double bond must be specified eg
  pent-2-ene
• The parent chain must contain the double bond
• The chain is numbered to give the lowest number
  eg pent-2-ene rather than pent-4-ene
• Branches are named as for alkanes

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Geometric Isomerism

• Unlike alkanes the double bond cannot rotate
• All the atoms therefore remain in the same place
• The molecule is described as planar
• If there are different functional groups off each
  carbon, then possible geometrical isomers are
  possible

Cis 1,2-dichloro ethene (same side)
Trans 1,2-dichloro ethene (opposite sides )
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Alkenes physical properties

• Very similar to alkanes
• As they are a homologous series, thee is a
  gradual change in density
• Melting and boiling points as carbon number
  increases

• Combustion
• Similar as for alkanes, but smokier flame due to
  lower carbon/hydrogen ratio

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Chemical properties - alkenes

• Addition reactions
• Saturation with hydrogen using a nickel of
  platium catalyst to form alkanes
• Addition of hydrogen halides
• eg ethene HBr bromoethene
• Hydration with steam (acid catalyst) to produce
  an alcohol
• Eg ethane steam ethanol
• Halogenation
• eg

• Need to use Markownikoffs rule the hydrogen atom
  of the reagent will add to the carbon atom with
  the most hydrogen atoms attached to it

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