Compounds of Carbon

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Compounds of Carbon

• Chapter 8

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Why is carbon important?

• Carbon makes up over 90 of all chemical
  compounds
• They form the basis of living systems
• Carbohydrates all have carbon
• Proteins contain carbon
• Fats contain carbon

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How does carbon form so many compounds?

• Carbon has 4 valence electrons, all available for
  bonding with other atoms
• Carbon can form strong covalent bonds with other
  carbon atoms
• Bonds between carbons can be single or multiple

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Hydrocarbons

• Hydrocarbons are made up of different compounds
  of hydrogen and carbon.
• There are many different hydrocarbons
• They make up the majority of the petroleum and
  natural gas industry
• Hydrocarbons can be classified into several
  families or homologous groups.
• The simplest hydrocarbon is methane and it
  belongs to the alkane series.

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Homologous groups

• A series of compounds with similar properties in
  which each member differs form the previous one
  by CH2- is called a homologous group
• Members of the same homologous groups tend to
  have very similar chemical properties. So
  organising carbon compounds into homologous
  series simplifies the study of hydrocarbons.

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Alkanes

• Alkanes consist of carbon and hydrogen only.
• They contain only single bonds
• Look at the table, each alkane differs by CH2-
• The alkanes have a general formula of CnH2n2
• If a compound has 16 carbons, then 2 x 16 2
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• So it would have the fomula C16H34

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Representing alkane molecules

• When drawing hydrocarbons we use structural
  formulas
• These are very similar to valence structures
  except they dont show the unbonded pairs.
• In structural formulas we focus on the location
  of the atoms relative to one another in the
  molecule as well as the number and location of
  chemical bonds.

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• The above diagram shows the first three alkanes.
• You will notice
• Each carbon atom forms a single covalent bond to
  four other atoms
• Each hydrogen atom forms a single covalent bond
  to one carbon atom
• The four atoms bonded to each carbon atom are
  arranged in a tetrahedral shape.

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Your Turn

• How would we draw the structural formula for C4H10

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• There are two possible ways.
• The first has the four carbon atoms in a
  continuous chain. The overall molecule is said to
  be linear.
• These are called straight-chain molecules
• The second one is not linear, this is called a
  branched chain molecule

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Isomer

• Molecules which have the same chemical formula
  but can form different arrangements of their
  atoms are called isomers.
• Same number of atoms just arranged differently.
• Structural isomers have similar chemical
  properties but can differ in some physical
  properties such as melting and boiling
  temperatures

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Alkanes

• The alkane series contains only single bonds.
• The alkanes are known to be saturated
  hydrocarbons as the carbons are saturated with
  hydrogens
• Meaning each carbon is completely bonded to
  either hydrogen or carbon, there are no unbonded
  carbons.

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Alkenes

• Alkenes contains one double bond between two
  carbons.
• Like alkanes, alkenes also differ by one CH2-
  group.
• The alkenes also form a homologous series.
• The alkenes generally have the formula
• CnH2n.

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Representing Alkene Molecules

• Like ethene, propene C3H6 also has one
  carbon-carbon double bond.

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Butene

• Butene (C4H8), like butane has more than one
  isomer.
• The alkenes are classified as unsaturated
  hydrocarbons. The double bond means that alkenes
  contain less hydrogen than the maximum amount
  possible.

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Semistructural formulas

• When we want to summarise the structural formula
  without indicating the 3D arrangement we use
  semistructural formulas.
• The semi structural
• Formula for propene is
• C3H6

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Your Turn

• Page 140
• Questions 3 - 6

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Naming Carbon Compounds

• How do we name carbon compounds?
• How do we distinguish between structural isomers?
• There are a set of rules put in place by which
  chemists can derive a systematic name for a given
  compound.

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Straight-chain hydrocarbons

• The first part of the name refers to the number
  of hydrocarbons.
• The second part refers to type of bonds
• ane if all carbon-carbon bonds are single
• ene if one C-C bond is a double
• yne if one C-C bond is a triple
• Pentane, pentene and pentyne all have 5 carbons
  bonded in a linear or straight chain.

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Unsaturated compounds

• Unsaturated hydrocarbons contain at least one
  multiple bond.
• Butene has three isomers, two of which are
  straight chained, as the carbon chain becomes
  longer the number of isomers increases.
• To name straight-chain alkenes, first number the
  carbon atoms in the chain, starting with the end
  that will give the first carbon atom involved in
  the double bond the smallest number possible.

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• The number starts at the end closest to the
  double bond.
• The isomer is named according to the first carbon
  atom involved in the double bond.
• The first isomer is but-1-ene.
• The other isomer is but-2-ene

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Branched Hydrocarbons

• An alkyl group usually forms a branch in a
  branched chain hydrocarbon.
• An alkyl group is an alkane molecule less on
  hydrogen atom
• It is named after the alkane from which it is
  derived.
• -CH3 is a methyl group.
• -C2H5 (-CH2CH3) is an ethyl group

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Branched Hydrocarbons

• Systematic naming requires us to
• Identify the longest continuous chain of carbon
  atoms in a molecule.
• Identify the side group that forms the branch in
  the chain
• Number the carbon atoms from one of the ends of
  the longest carbon chain so that the side group
  is attached to the carbon atom with the smallest
  possible number.

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C4H10

• The longest chain of carbons has 3 carbons and
  all the bonds are single.
• Therefore the molecule is derived from propane.
• Identify the side group
• The side group is a methyl group.
• Number the carbons. The methyl group is on the
  second carbon.
• The compound is therefore 2-methylpropane.

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Worked Example 8.4, page 143
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Some are done for us

• Take a look at page 144 of your text.
• Which are the alkanes?
• Which are the alkenes?

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Functional Groups

• -CH3 methyl
• -OH alkanol
• -Cl (or F, or B or I) chloro (or fluoro etc)
• -NH2 - amino

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Your Turn

• Page 147
• Questions 7 and 8

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Chemical Properties of Alkanes

• The most significant reaction of alkanes is
  combustion. Alkanes burn in oxygen, releasing
  large quantities of energy.
• If the oxygen supply is sufficient the products
  released are carbon dioxide and water.
• This energy released is what we use as a source
  of heat, to produce electricity for domestic and
  industrial use.

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Equations for Combustion of Reactions

• This figure shows the rearrangement of atoms that
  occur when the hydrocarbon methane burns in
  oxygen.
• Have the atoms of each element been conserved?
• The equation is
• CH4 2O2 ? CO2 2H2O

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Chemical Properties of Alkenes

• Due to the double bond in alkenes they react much
  more readily and with more chemicals than the
  alkanes.
• Alkenes, in particular, ethene and propene, are
  not used for fuels but rather as starting
  materials to manufacture a huge range of
  compounds such as alcohols, antifreeze and
  plastics.
• Apart from combustion, the reactions of alkenes
  usually involve the addition of a small molecule
  to produce a single product.

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Addition reactions of ethene

• Reaction with Bromine solution
• Ethene reacts with bromine solution as shown.
• In addition reactions, bonding new atoms to the
  two carbons on either side of the double bond,
  converts the CC double bond to a C-C single bond.

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Reaction with Steam

• Large amounts of ethanol are now made by the
  addition of steam and ethene using a phosphoric
  acid catalyst.
• This ethanol is used as a reagent for industrial
  purposes and as a solvent in cosmetics and
  pharmaceuticals.
• This is not the ethanol that people drink.

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Formation of polyethene

• An addition reaction of ethene is involved in
  making polyethene.
• As seen previously the CC bond is converted to a
  C-C bond and a saturated product is formed.
• In this case there is no other reactant to add to
  the ethene molecules. Polyethene is formed when
  ethene molecules themselves join together to form
  a long chain.

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• Polyethene is usually written as (-CH2-CH2-)n
• Where n is a large number
• A molecule made by linking a large number of
  small molecules such as ethene is called a
  polymer (meaning many units). Each small molecule
  is called a monomer (one unit).
• This type of reaction is addition polymerisation

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Your Turn

• Page 151
• Questions 9-10

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Polymers

• Polymers are long chained molecules
• Each one can contain tens of thousands of atoms.
• Cotton, wool and silk are some naturally
  occurring polymers.
• Synthetically made polymers are generally
  superior to natural polymers as they have been
  designed for specific properties.

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Synthetic polymers

• Include
• Cling wrap
• Drugs
• Clothing
• Domestic appliances
• Cars
• Sporting equipment
• Check out table 8.8 on page 153

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Not Plastic

• We frequently use the term plastic when referring
  to polymers.
• The term plastic refers to the property of a
  material not the material itself.
• A substance is plastic if it can be moulded
  into different shapes easily.
• Many polymers are indeed plastic, some however
  are not.
• The materials used to make powerpoints are
  brittle and cannot be reshaped.

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What are polymers?

• Polymers are large covalently bonded molecules.
• They contain tens of thousands of atoms
• They are formed by joining together smaller
  molecular units called monomers.
• The size of the polymer varies, a polymer can
  consist of varying sizes of molecules formed from
  different numbers of monomers.

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Polymers

• There are two types of polymerisation processes.
• Addition polymerisation
• Condensation polymerisation
• The polymers formed by addition polymerisation
  often have the monomer included in the name of
  the polymer.
• Polyethene is formed by the monomer ethene.
• Condensation polymers are named after the
  chemical bond they form.
• Polyesters contain monomers joined by an ester
  functional group

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Addition Polymers

• Most polymers are built around atoms of carbon
  like their monomers.
• Covalent bonds form between the monomers to
  produce a polymer molecule.

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Addition Polymers

• Suitable monomers for addition polymerisation are
  unsaturated molecules.
• What are unsaturated molecules?
• The double bond between the two carbon atoms
  react and new covalent bonds are formed between
  carbon atoms on nearby molecules creating long
  chains.

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Polyethene

• Read page 155 156.
• Why is High-density polyethene stronger and more
  rigid than low-density polyethene?

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Structure, properties and applications

• Two very important properties of polymers are
  tensile strength and softening temperature.
• Tensile strength is a measure of the materials
  resistance to breaking under tension. It
  determines the structural uses of the polymer.
• The softening point affects the way the polymer
  can be moulded.
• Both tensile strength and softening point are
  determined by the strength of the forces between
  polymer chains.

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Thermoplastics

• Thermoplastics are polymers that can be moulded
  and shaped.
• The tensile strength and softening point are
  affected by
• Degree of branching
• Nature of atoms of groups of atoms attached to
  the carbon chain
• How the atoms or groups of atoms are arranged
  along the chain.

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Cross-linking

• Another factor that affects the properties of a
  polymer is cross-linking. A cross-link is a
  covalent bond between polymer chains.
• The more cross-links the stronger and rigid the
  polymer.
• The strong covalent bonds in 3D bind all the
  atoms together to form one large lattice.

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Thermosets

• Thermosets are polymers with extensive
  cross-linking.
• They do not soften on heating as thermoplastics
  do.
• When heat is applied the covalent bonds break and
  the thermosetting polymer will decompose rather
  than soften.

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Degree of branching

• Low denisty polyethene contains a higher degree
  of branching which lowers the density, hardness
  and melting point of a polymer.
• Low denisty polyethene is more flexible and is
  used in cling wraps and squeeze bottles.
• High density polyethene is harder and less
  flexible, used in pipes and toys

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Nature of side groups

• Look at table 8.9 on page 156.
• Which are the bulky side groups which would lower
  density by getting in the way?
• These bulky side groups prevent the chains from
  stacking close together and forming strong rigid
  structures.

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Your Turn

• Question 15
• Page 160

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Cross-linking

• Cross linking involves breaking the double bond
  in a polymer and using this double bond to bond
  to carbons in the next chain.

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Extensive cross-linking

• Where there is extensive cross-linking the
  structure will be rigid and cannot be re-shaped.
• Thermosetting polymers have extensive
  cross-linking.
• They char and burn when heated.
• They contain covalent bonds between the chains as
  well as within the chains

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Occasional cross-linking

• Elastomers are polymers that can be stretched or
  pulled out of shape and then will regain their
  original shape.
• They contain covalent bonds within the chains.
• They only contain a few covalent bonds between
  chains.
• They only have occasional cross-linking.

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Guess What

• We have finished unit 1
• WOOHOO.
• be prepared for a topic test