Chemistry Unit 2- Alkenes and Haloalkanes

Chemistry Unit 2- Alkenes and Haloalkanes

1. Alkenes

   • These are hydrocarbons that have one or more carbon-carbon double bond
   • As you can see, the atoms around each carbon form the trigonal planar shape and there is a bond agngle of 120
   • Their double bond means there is restricted rotation around each carbon bond. In alkanes, the atoms have free rotatiomn around each carbon atom because they have a single bond
   • E/Z isomerism is where atoms have the same molecular formula but a different 3-D spacial arrangement around each atom
     • It is also called geometrical isomerism
       • It can only occur if there are 2 different groups attatched to each carbon
         • When trying to work out if a molecule shows E/Z isomerism, you work out where the two heaviest molecules are placed.
           • If they're on the Zame Zide, then they show Z isomerism
           • If they're on opposite sides, i.e. entgegen, then they show E isomerism
           • Example, But-2-ene. The cis but-2-ene shows Z isomerism as the two heaviest molecules are on the same side
             • The trans but-2-ene shows E isomerism as the two heaviest molecules are on different sides
     • They don't show position, chain or functional group isomerism
     • This is caused by the restricted rotation arround each carbon
   • Reactivity/reactions
     • Alkenes are more reactive than alkanes. Why?
       • Alkanes are unsaturated, meaning not all bonds are filled, they can be broken and more can be added to it
         • They have a double bond, meaning there are 4 electrons within that region, which will pull the 2 carbon atoms closer together and make the bond shorter.
           • This then means that there is more negativity within a smaller space.
             • The double bond is said to be a region of high electron density, and this is what makes alkenes more reactive than alkanes.
     • Electrophilic addition:
       • An addition reaction is making 2 smaller molecules into 1 bigger molecule
       • An electrophile is a species that is able to ACCEPT a pair of electrons from an electron rich species
       • An alkene is able to do electrophilic addition with 3 things
         • H-X
           • 1. The electrons in the double bond is given to the hydrogen on the H-BR molecule
             • The hydrogen now has 4 electrons, thats too many, so the Br will take its original electron and also hydrogen's electron to become Br- ion
               • The hydrogen is now bonded to one of the carbons and the other carbon, has had one of its electrons take away from it. It now has a positive charge and is now called a carbocation
                 • So the Br- will now become attracted to the carbocation
                   • A haloalkane will have formed
           • There are two types of products that can be formed: A major yield and a minor yeild
             • You can determine which product will be your major yeild and which will be your minor yeild by looking at the carbocation
               • The carbocation that is attatched to the most carbon atoms is going to be the major yield
                 • Primary
                   • Attatched to only one other carbon atom
                 • Secondary
                   • Attatched to 2 other carbon atoms
                 • Tertiary
                   • Attatched to 3 other carbon atoms
                 • You can't have a quaternary carbocation because the carbon can only bond to 4 things: 3 other carbon, and the positive charge.
                 • Out of all carbocation, a tertiary carbocation is the major product because its the most stable
             • This s only if you have an alkene that is not symmetrical
               • If you have a symmetrical alkene, then you'll get the same product
         • X2
           • When you react an alkene with a halogen, then you get a di-substituted haloalkane
             • You dont get 2 products because the product formed is symmetrical. The X2 doesnt have a permanent dipole however it does have a temporary induced dipole
               • This is because their electronegativties are the same
               • This usually only works with I2 and Br2 because they're big enough to have lasting dipoles
         • H2SO4
           • When you react the alkene with the sulphuric acid, then you'll get an alkyl hydrogen sulphate.
             • The mechanism is the same: 1. The double bond breaks and a pair of electrons are shared with the hydrogen. Then the Hydrogen will give its orginial electron to the O and it will become negatively charged. This will then react, and bond with the carbocation.
             • The alkyl refers to the side group that is attatched to the hydrogen sulphate.
             • Again, you can get a major product and a minor product from this. This will depend on whether the carbocation is primary, seconday or tertiary.
             • Alkyl hydrogen sulphates can be converted into alcohols, by a hydrolysis reaction, i.e. the addition of water
               • Notice how you get sulphuric acid at the end aswell; it acts as a catalyst
                 • This type of reaction is called a hydration reaction
   • Addition polymerisation: This is where alkenes join together in the presence of high pressure and a catalyst
     • The product will be a very long hydrocarbon chain.
       • You can show any polymerisation reaction by opening up the double bond and putting a bracket around it.
         • These polyalkenes are saturated, and are very unreactive
           • Polyethene: Plastic bags. Polypropene: straw, biros, food containers.

2. Haloalkanes

   • Properties:
     • Boiling points
       • A very low, usually gaseous at room temperature. The reason for this is because there are weak intermolecular forces between them i.e. van der Waal's
         • As you go down the homologous series, the boiling points get higher because there are more van der Waal's
           • So more energy is required to break these intermolecular forces.
     • Solubility:
       • Haloalkanes are non polar, and so they aren't soluble in water and therefore are soluble in hydrocarbon because it is also non polar
     • Bond Strength
       • Decreases as you go down the group 7
         • The reason for this is because they get less electronegative as you go down the group
           • This is because shielding increases as well as distance. There is weaker nuclear attraction and so the bond between the group 7 atom and carbon is not as stong and gets easier to break as you go down.
   • Reactivity/Reactions
     • There is a difference in electronegativity between the halogen and the carbon. This will create dipoles
     • They go through Nucleophilic subsitution
       • Substitution is replacing one atom with another
       • A nucleophile is a species that is able to donate a pair of electrons to an electron deficient species
         • Hydroxide/OH- ion
           • Produces an alcohol
           • Also alcoholic Aqeuous
         • Cyanide/CN- ion
           • Produces a nitrile
         • A molecule of ammonia
           • Produces an amine
             • Alcholic solvent
       • Mechanism:
         • The OH- will 'attack' the carbon as it is a delta+ species.
           • The carbon now has too many electrons so the Br atoms takes its original electrons and one of the carbon's.
             • The carbon now has too many electrons so the Br atoms takes its original electrons and one of the carbon's.
               • Equation: C2H5Br + NaOH ------> C2H5OH + NaBr
         • The CN- will 'attack' the carbon as it is a delta+ species
           • The carbon now has too many electrons so the Br atoms takes its original electrons and one of the carbon's.
             • It becomes a Br- ion and reacts with the Na+ or K+ that's just sitting there in the solution
           • We don't use HCN because its a poisonous gas
           • Notice how the lone pair on the CN- is on the carbon
             • Equation: C2H5Br + KCN -----> C2H5CN + KBr
               • C2H5CN: Propanitrile
         • The lone pair on the nitrogen of the ammonia molecule will 'attack' the carbon as it's a delta+ species
           • The carbon now has too many electrons so the Br atoms takes its original electrons and one of the carbon's.
             • It forms a Br- ions
             • There is a positive charge on the carbon because the nitrogen is bonded to 4 things
               • In order to make it stable, we need to 'deprotonate' it. In other words, you take of a proton or H+ ions because it's the same thing
                 • To do this, you need another ammonia molecule which will dative covalently bond with the H+ ions to form ammonium (NH4+)
                   • This will then bond with the Br- ions that was formed before
                     • Equation: C2H5Br + 2NH3 -------> C2H5NH2 + NH4Br
                       • C3H5NH2-- Ethylamine
           • Conditions with ammonia:
             • Sealed container as NH3 is gaseous
               • High pressure: Higher rate of reaction
                 • Alcoholic solution for the haloalkane
         • Conditions for OH- to react:
           • 1. NaOH or the reagent has to be in a polar solvent e.g. water
             • 2. The haloalkane needs to be in a non-polar solvent e.g. alcohol
               • Therefore, an alcoholic aqueous solution is required
                 • The reaction needs to take place in either room temperature or be warm, but NOT heated
     • We know that the reagents are CN-, OH- and ammonia. However, you can't just go to a store and get and OH- or CN- ion of the shelf.
       • It's got to be ionically bonded to something else, e.g. NaOH, NaCN, KOH, KCN etc. in order to stabilise it.
         • We include the reagent in the chemical equation but not the mechanism
         • The Na+, or K+ ion will act as the spectator ion
   • Production
     • 1. Electrophilic addition
       • With H-X
       • With X2
     • Free Radical Substitution
       • This uses an Alkane and a halogen
       • A free radical is a species with an unpaired electron. It is very unstable and will react with almost anything
       • Conditions:
         • A sealed container
           • UV light, this provided the energy to break the halogen bond which is extremely hard to break on its own because they have similar electronegativitys
       • Equation: CH4 + Cl2 ------> CH3Cl + HCl
       • Mechanism:
         • 1. Initiation: This is where the UV light will break the bond between the Cl2. The two electrons that are shared now separates and each Cl gets its own electron and forms a free radical
           • This is called homolytic fission: where a bond breaks and the electrons go seperate ways to form a free radical
           • 2. Propagation I: The chlorine free radical reacts with the methane and forms a methane free radical and a HCl molecule
             • Propagation II: The methane free radical will react with a Cl2 molecule to form chloromethane and a Cl free radical
               • This should in theory start the cycle again because you're back to where you started.
                 • No more UV light is required
                   • 3. Termination: This is where 2 free radicals react with each other and the reaction then stops as you no longer have a free radical
               • You could get other possible propagation steps that lead to different substituted haloalkanes e.g. dichloromethane or even tetrachloromethane
                 • These can be separated using fractional distillation as they will have slightl different boiling points
                 • E.g. you could get a chloromethane react with a cl. free radical. It will form HCl and then will form a chloromethane free radical. This will then react with a chlorine to form dichloromethane
       • Depletion of the Ozone layer
         • This occurs by free radical substitution
         • The ozone layer is made up of O3 and it protects the earth/land from the UV rays from the sun
         • CFC's: These are chlorofluorocarbons. They used to be used as refrigerants or aerosols
           • However they are now banned because they were contributing to the destruction of the ozone layer
             • They are harmless on land, but when they get up in the atmosphere, the bonds start to break from the UV light found in the atmosphere to form free radicals.
               • These free radicals will then react with the ozone layer
                 • Cl. + O3 ------> ClO. + O2
                   • ClO. + O3-----> 2O2 + Cl.
                     • In total : 2O3------> 3O2
                       • Although this isn't harmful, the ozone layer has been destroyed
                     • The cycle will start again
   • Elimination:
     • This is where you turn a haloalkane back into an alkene
       • In order to do this, we need to break 2 bonds.
         • In a haloalkane, the easiest bond to break is the H-X bond because they have a greater difference in electronegativity than a C-H bond does
           • The second bond to break has to be a C-H bond
             • You have to get rid of a H that is ADJACENT to the carbon that you've just pulled off a halogen from in order to form a double bond
               • To do this, we need a base: a hydroxide ion
                 • This will pull off the H+ ions as a base is a proton acceptor
                   • The H+ will obviously take no electrons with it
                     • So the carbon has one extra electron, and it will have a negative charge on it
                       • Both the carbons form another bond (+ and -) and the double bond has formed
           • It will take both it's electrons with it, so there will be a plus charge on the carbon
       • Mechanism:
     • Conditions:
       • The right conditions are required otherwise the OH will act as a nucleophile and not a base
       • 1. Alcoholic solution/solvent only
         • Heat, not warm

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