Synthesis

• Eight homologous series studied at AS

  • Alkanes
  • Alkenes
  • Alcohols
  • Carboxylic acids
  • Aldehydes
  • Ketones
  • Halogenoalkanes
  • Esters
• Alkanes


  Don't have a functional group, made up of carbon-carbon single bonds and hydrogen (has the general formula CnH2n+2)
• Alkenes


  The functional group is the double carbon carbon bond
• Alcohols


  The functional group is the -OH (hydroxyl) group bound to a saturated carbon atom (the general formula for an alcohol is CnH2n+1 OH)
• Carboxylic acids


  The functional group is the (carboxyl) group (the general formula for a carboxylic acid is CnH2n+1 COOH)
• Aldehydes


  The functional group is
• Ketones


  The functional group is part of the carbon chain, rather than at the end like in aldehydes
• Halogenoalkanes


  The functional group is a halogen replacing one of the hydrogens
• Esters


  Their functional group is
• Naming convention


  The area to the right of the functional group makes up the first part of the name, and the area to the left of the functional group makes up the second part of the name
• Radical substitution mechanism of alkanes

  1. Initiation

  2. Propagation

  3. Termination
• The reactions of halogens with alkanes are substitution reactions
• Addition reactions are when two reactants make one product, whereas substitution is where two reactants make two products (one of which is a haloalkane)
• In these reactions one of the hydrogens in the alkane is swapped with one of the halogen atoms, and the swapped hydrogen atom then bonds with the other halogen atom to form a compound
• Radical substitution reactions require UV in order to occur (a form of radiation/energy)
• Initiation


  The halogen molecule undergoes homolytic fission (the diatomic molecule's bond is split evenly down the middle, giving each halide ion one electron from the bond)
• Free radicals


  The stable molecule's reaction halide ions produced are very reactive (they are drawn with a dot to show the extra unpaired electron)
• Propagation


  The second part of the mechanism, consisting of a pair of steps 'P1' and 'P2'
• Propagation


  1. P1: A stable alkane molecule is attacked by a radical, who removes one of its hydrogens, causing the remaining stable alkane molecule to become a radical

  2. P2: The alkane radical reacts with a stable diatomic halogen molecule to form a halogen-alkane compound and another halogen radical
• Termination


  The final stage of the mechanism where any two free radicals combine to form a stable molecule
• If there was excessive amounts of the halogen present, this would lead to excessive numbers of halide free radicals in the vessel where this takes place, then halide free radicals could continue stripping hydrogens from alkane compounds until there was no hydrogen left to take (i.e: CCl4)
• To prevent this occurring, you can make sure that the alkane is in excess instead
• Cracking


  • Reactant: an alkane (usually long chain)
  • Products: a shorter alkane and an alkene
  • Conditions: Low pressure, high temperature (450 degrees c) and a zeolite catalyst
• Hydrogenation of alkenes


  • When alkenes react with hydrogen, the pi bond in the double bond is broken, making two single bonds which allow the hydrogen atoms to become part of the compound
  • Therefore the alkene becomes an alkane
  • For this reaction to occur, a nickel catalyst and temperature of 150 degrees is needed
  • The number of double bonds and moles of hydrogen must be the same in order for all double bonds to be broken
• In naturally occurring molecules, the hydrogen atoms are on the same side of the double bond
• Polyunsaturated


  Species with more than one double bond
• The more saturated, the higher the melting point, so hydrogen may be added to polyunsaturated fats to 'harden' them
• In this process, a cis fat can stay cis or become trans
• Trans fatty acids are linked to heart disease as they raise low-density-lipoprotein levels
• Alkene hydration reaction


  • Occurs at over 100 degrees, and Phosphoric acid (H3PO4) or sulphuric acid (H2SO4) can be used to catalyse it
  • Produces an alcohol
  • In the reaction water vapour is split into a hydrogen and a hydroxide before bonding into the compound
  • Symmetrical alkenes produce only one product, but non-symmetrical can produce two
• Reaction of alkenes with halogens (halogenation)


  • These reactions happen at room temperature
  • When bromine water reacts with alkenes, it goes from orange to decolourized
  • Produces haloalkanes
• Reactions of alkenes with hydrogen halides

  • There are no specific conditions for this reaction
  • Produces haloalkanes
• Electrophilic addition mechanism


  • Electrophile- electron pair acceptor
  • The reaction is called an addition reaction because the halide is added across the double bond, and because the two reactants make one product
  • This is the reaction of any alkene with molecules such as halogens and hydrogen halides
  • Halogen atoms are non-polar, as both atoms in the diatomic molecule have the same electronegativities, meaning that the electron pair is exactly in the middle
  • The double bond in alkenes contain twice the number of electrons as single bonds
  • Due to this increased electron density, the electrons in the double bond will repel the electrons in the single bond of the bromine, pushing the majority of the density to one side and inducing creating a dipole (the closest atom becomes electron deficient, the other becoming electron-rich)
  • The side of the diatomic molecule with the delta positive charge will be the one closest to the double bond, and as opposites attract, the electrons will leave the weaker pi bond (destroying it) to join with the delta positive side of the molecule
  • The new electrons at the previously delta positive side repel the shared pair of electrons and the delta negative side, breaking the bond and leading to two halide ions
  • This is heterolytic fission, as one side gets both the electrons from the originally shared pair
  • The previously delta positive halide then joins onto the compound, and because one of the carbons is now positively charged (as it has in effect lost one of its electrons that was originally in the pi bond to form the bond holding the first halide in place), and because the compound contains carbon, the overall compound is called a Carbocation(carbon containing cation)
  • The halide ion that was originally the delta negative part of the molecule has become a bromide ion as due to the heterolytic fission it has gained one electron that originally belonged to the delta positive side
  • This negative halide ion is now attracted to the positive carbocation, and bonds to become part of the new halogenoalkane molecule
  • Electrophilic addition does not happen with alkanes as they do not have a double bond, so there is not a great enough electron density to convert the halogen molecule into a dipole
  • If the reaction occurs with a polar molecule, the dipole is pre-existing, not created, and the more electronegative element will be delta negative
• Addition polymers from alkenes

  • Addition polymerisation- formation of a very long molecular chain (addition polymer) by repeated addition reactions of many unsaturated alkene molecules (monomers)
  • When alkenes are exposed to a high temperature and pressure with a catalyst present, it breaks their double bond
  • If there are lots of alkene monomers, when the double bonds are broken, they join together in a long chain to form a polymer(typically 2000-20,000 monomers long)
  • The repeat unit is the section of polymer we draw when asked what the polymer version of a certain monomer would be like
  • It is the shortest repeating unit of the monomer
  • The name of the polymer formed from monomers is the name of the monomer with 'poly' in front
  • Always make your monomer look like an ethene molecule (so put the double bond in the middle, rest condensed to fit on the four branches available)
  • When asked to draw a repeat unit, show that many monomers joined together without the brackets or n
  • To identify the monomer used to make a polymer, simply find the shortest repeating unit and revert it back to normal alkene state (putting the double bond back, having no bonds extending outwards that aren't filled ..etc)
• Oxidation reactions of alcohols

  • The symbol for an oxidising agent is [O]
  • When oxidising alcohols, we tend to use potassium or sodium dichromate as an oxidising agent
  • For the reaction to occur, there must be heat present, and the dichromate ions must be in acidified conditions
  • When primary alcohols are oxidised, an aldehyde(identifiable by its CHO) and water are produced
  • Aldehydes can be oxidised further under the same conditions to produce only a carboxylic acid
  • Secondary alcohols when oxidised produce a ketone and water
• Dehydration of alcohols


  • Dehydration- the removal of H2O
  • Alcohols are dehydrated by reacting them with concentrated phosphoric acid (dehydrating agent)
  • The products are an alkene and water
  • When water is produced from this reaction, it is formed from the functional group of the alcohol and the second hydrogen adjacent to the carbon attached to the functional group
  • Because the reaction consists of one reactant becoming two products (one of which is much smaller), it is called an elimination reaction
  • This is because the smaller molecule (water in this case) has been eliminated from the starting molecule
  • Whenever an alkene is produced from a reaction,you should check whether E/Z isomerism is possible
• Halide substitution


  The halide ion used takes the place of the OH functional group of the alcohol. The general equation for this is: ROH + HX -> RX + H2O. The reagents would be an alcohol and a hydrogen halide in the presence of a sulfuric acid catalyst. HCl would be used as the reagent to make a chloroalkane, whereas for bromine we'd create the hydrogen halide necessary by reacting sodium bromide with sulfuric acid in situ. For iodine we'd use sodium iodide, with phosphoric acid instead of sulfuric acid. This is because sulfuric acid oxidises iodide ions to iodine, so the yield of desired iodoalkane would be very low.
• Hydrolysis of Halogenoalkanes


  • Hydrolysis- reaction with water/aqueous hydroxide ions that breaks down a chemical compound
  • A source of heat is required, therefore it is sometimes asked that this reaction is carried out under reflux as this is a source of heat
  • This is a substitution reaction, and the products are an alcohol and a halide (and an H+ ion if the reaction was done with water)
  • Hydrolysis equation for when aqueous hydroxide ions are used:

  Hydrolysis equation for when water is used:

  The most common source of aqueous hydroxide ions for this experiment is aqueous sodium hydroxide
• Nucleophilic substitution mechanism (occurs in hydrolysis)

  • Halide atoms such as chlorine tend to be more electronegative than hydrogen or carbon
  • This means that the bond between the halide and the carbon in the halogenoalkane is polar, with the shared pair of electrons closer to the halide atom than the carbon atom
  • This means that there is a dipole, and so the halide atom is delta negative, and the carbon atom is delta positive
  • The hydroxide ion involved in this hydrolysis reaction has a lone pair of electrons which causes it to be attracted to the electron deficient carbon (they are nucleophiles) to join with it
  • As the hydroxide ion moves closer due to the attraction, its lone pair repel the shared electrons between the carbon and halogen atoms
  • The shared electrons are thus repelled completely onto the halogen, breaking the bond via heterolytic fission
  • Nucleophile- electron pair donor
• Esterification


  Esters are produced when carboxylic acids are heated with alcohols in the presence of an acid catalyst. The catalyst is usually concentrated sulphuric acid