SYNTHESIS

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SYNTHESIS

• PART ONE
• HYDROCARBONS

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What is organic chemistry?

• In chemistry chemicals which contain carbon are
  classed as organic.
• Carbon is a non metal and can form four bonds
  with other non metals.
• This means it can form many, many different
  substances in many different shapes.
• Some organic substances are simple like methane
  and others are very complex like sugars ,
  proteins and DNA.

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Hydrocarbons

• Organic compounds that contain only hydrogen and
  carbon are called hydrocarbons.
• There are two main groups you have to know about
  alkanes and alkenes.

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Alkanes

• The simplest alkane is called methane.
• It consists of one carbon atom joined to four
  hydrogen atoms
• We write its formula as CH4

Carbon atom
Hydrogen atom
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• The next alkanes in the series are
• Ethane C2H6
• Propane C3H8
• Butane C4H10
• As we add another carbon atom in the chain we
  need more hydrogen atoms.
• Can you see how these two are related?
• If we double the number of carbon atoms and add
  two we get the number of hydrogen atoms.
• CLICK HERE TO COMPLETE THE ALKANES WORKSHEET.

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The alkenes

• In these hydrocarbons one of the carbons has a
  double bond.
• This means that there are two bonds holding two
  of the carbons together instead of only one

Carbon atom
Double carbon carbon bond
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Saturated Unsaturated

• The alkanes have only single C-C bonds. We say
  they are saturated.
• The alkenes have a double CC bond. We say they
  are unsaturated.

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• The simplest alkene is called ethene
• It has the formula C2H4
• There are always twice as many hydrogen atoms as
  carbon atoms in the alkenes.
• The other two alkanes you need to know are
  propene and butene

CLICK HERE TO COMPLETE ALKENE WORKSHEET
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• We can test to see if a hydrocarbon is an alkene.
• Alkenes turn bromine water from orange/brown to
  colourless.
• H H H H
• I I I I
• CC Br2 Br-C-C-Br
• I I I I
• H H H H
• Alkanes will not do this

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CRACKING

• When crude oil is separated by fractional
  distillation there are always a higher of the
  longer chained fractions.
• The shorter chained hydrocarbons are more useful
  so the long chained molecules are cracked to give
  more shorter chain hydrocarbons.

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Cracking in the School Laboratory

• Paraffin is soaked into ceramic wool and placed
  at the end of a boiling tube (see diagram)
• The aluminium oxide is heated strongly first.
  This is the catalyst which speeds the reaction
  up.
• The paraffin is heated to vaporise it and then
  the ethene gas is collected in the test tubes.

Catalyst
Ethene gas collecting
Paraffin soaked into mineral wool
heat
Click here to complete the cracking exercise
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What can we do with the ethene?

• The ethene can be used as a fuel but because it
  contains a double bond but it is much more useful
  as a raw material in the chemical industry.
• If it is reacted with water(steam) ethanol
  (alcohol) can be made.
• H H H H
• I I
  I I
• CC H-O-H H-C-C-O-H
• I I
  I I
• H H H
  H
• ethene steam ethanol

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Polymers

• Ethene and other alkenes are most useful for
  making polymers.
• Plastics are polymers.
• Given the right conditions small alkenes can join
  together to form very large molecules.
• The small molecules are called monomers and the
  large molecules are called polymers .
• The process is called polymerisation

Click here to complete the crossword
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How are they made?

• Alkenes have a double CC bond
• The double bond breaks and the smaller alkenes
  join together.

ETHENE ETHENE ETHENE
POLYETHENE
Alkenes join to form polymer
Double bond breaks
Click here to complete the polymer worksheet
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There are two main types of polymer.They are
thermosetting and thermoplastic polymers.

• Thermoplastic
• They can stretch or bend
• They soften melt when heated
• Then can be remoulded into new shapes
• They have no cross links between the polymer
  molecules

• Thermosetting
• Are rigid, they break when bent
• They char or decompose when heated
• They cannot be moulded into new shapes
• They have cross links between the polymer
  molecules

Click here to complete the matching exercise
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Properties of polymers

• Polyethene polypropene are made from
  unsaturated monomers.
• These monomers ADD on to each other so we call
  them addition polymers.
• Other polymers are formed by condensation. This
  means another molecule water is made as well as
  the polymer.
• Some polymers have branched chains.
• This affects the strength of the polymer.

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• HDPE (high density polyethene) has few branches
  and is not as strong as LDPE (low density
  polyethene) which has many branches.
• Some times plasticisers have to be added to alter
  the properties.
• These allow the polymer chains to slide over each
  other which makes the polymer softer more
  flexible.
• uPVC is the hard polymer used to make window
  frames. If plasticiser is added to this the
  polymer it produces a soft , flexible substance
  that can be used for floor covering.

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Getting rid of polymers

• Some polymers are hard to dispose of.
• Thermoplastic polymers can be recycled
• Many polymers are not biodegradeable (will not
  rot) and are not affected by acids and other
  chemicals.
• There are however some new polymers around that
  have additives which allow them to decompose.

Click to complete the cloze exercise