Transcript SYNTHESIS - Teachnet UK

SYNTHESIS
PART ONE
HYDROCARBONS
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.
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.
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
•
•
•
•
•
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.
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
Saturated & Unsaturated
• The alkanes have only single C-C
bonds. We say they are
saturated.
• The alkenes have a double C=C
bond. We say they are
unsaturated.
• 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
• We can test to see if a
hydrocarbon is an alkene.
• Alkenes turn bromine water from
orange/brown to colourless.
H H
I I
C=C + Br2
I I
H H
• Alkanes will not do this
H H
I I
Br-C-C-Br
I I
H H
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.
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
Paraffin
soaked into
mineral wool
heat
Click here to
complete the
cracking exercise
Ethene gas
collecting
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
•
C=C + H-O-H
H-C-C-O-H
•
I I
I I
•
H H
H H
•
ethene + steam
ethanol
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
How are they made?
• Alkenes have a double C=C 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
There are two main types of polymer.
They are thermosetting and thermoplastic polymers.
Thermoplastic
Thermosetting
• 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
• 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
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.
• 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.
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