Alkenes

• • Alkenes are a homologous series of unsaturated hydrocarbons that share the general formula CnH2n
  • Referred to as unsaturated as they contain double bonds known as pi bonds, as well as sigma bonds
• Sigma bonds --> orbitals directly between bonding atoms overlap
• Pi bonds --> sideways overlap of adjacent p-orbitals above and below the bonding carbon atoms
• • Each carbon atom contributes one electron to the electron pair in the pi bond
  • The pi electron density is concentrated above and below the line joining the nuclei of the bonding atoms
  • The pi bond locks the two carbon atoms in position & prevents them from rotating around the double bond
• • The shape around each of the carbon atoms in the double bond is trigonal planar because:
  • there are three regions of electron density around each of the carbon atoms
  • the three regions repel each other as far apart as possible, so the bond angle around each carbon atom is 120 degrees
  • all of the atoms are in the same plane
• Stereoisomerism --> occurs in compounds with the same structural formula but a different arrangement in space
• • The pi bonds in alkenes restricts the rotation of the molecule potentially causing E/Z isomerism to occur
  • A form of stereoisomerism
  • Sigma bonds can rotate so steroisomerism can't occur in alkanes
• E/Z isomerism can only occur if:

  • There is a carbon double bond
  • Each carbon is bonded to a different atom / group
• • E (entgegen) isomers - atom / groups on the opposite sides
  • Z ( zusammen) - atom / groups on the same side
• CIP ( Cahn, Ingold & Prelog ):

  • CIP rules are used to assign stereoisomers as either E or Z
  • This uses the atomic number of the atom directly bonded to the carbon double bond
  • Highest atomic numbers are diagonally opposite = E - isomers
  • Highest atomic numbers are not diagonally oppposite = Z - isomers
  • If atomic numbers are equal, adjacent atoms are taken into account: longer alkyl chains take priority
• Cis - Trans isomers:

  • Cis - Trans isomers are a type of E / Z where two of the substituent groups on the carbon double bond are hydrogen
  • Trans = diagonally opposite
  • Cis = not diagonally opposite
• Using CIP rules:

  • In this system the atoms attached to each carbon atom in a double bond are given a priority based upon their atomic number
  • If the group of higher priority are on the same side of the double bond, the compound is the z isomer
  • If the groups of higher priority are diagonally placed across the double bond, the compound is the E isomer
  • The higher the atomic number, the higher the priority
• Reactions of alkenes:

  • Alkenes are much more reactive than alkanes because of the presence of the pi bond
  • Being on the outside of the double bond, the pi electrons are more exposed than the electrons in the sigma bond
  • A pi bond readily breaks & alkenes undergo addition reactions relatively easily
  • The pi bonds is weaker than the sigma bonds and is therefore broken more readily
• Addition reactions of alkenes:

  • hydrogen in the presence of a nickel catalyst
  • halogens
  • hydrogen halides
  • steam in the presence of an acid catalyst
• Hydrogenation of alkenes:

  • Alkene mixed with hydrogen & passed over a nickel catalyst at 423K
• Halogenation of alkenes:

  • Alkenes undergo a rapid addition reaction with the halogens chlorine or bromine at room temperature
  • Bromine water turns colourless in the presence of alkenes
• Addition reactions of alkenes with hydrogen halides:

  • Alkenes react with gaseous hydrogen halides at room temperature to form haloalkanes
  • If the alkene is a gas, like ethene, than the reaction takes place when the two gases are mixed
  • If the alkene is a liquid, then the hydrogen halide is bubbled through it
  • Alkenes also react with concentrated hydrochloric or concentrated hydrobromic acid, which are solutions of the hydrogen halides in water
• Hydration reactions of alkenes:

  • Alcohols are formed when alkenes react with steam, H2O (g) in the presence of a phosphoric acid catalyst, H3PO4
• Electrophile --> an electron pair acceptor. These are often positive ions or have a delta positive dipole
• • Heterolytic fission is the splitting of a covalent bond where one atom takes both electrons, forming a pair of ions
• • Curly arrows show the movement of electrons:
  • Half head = one electron moving
  • Full head = two electrons moving
• Electrophilic addition

  Alkenes react via the mechanism of electrophilic addition<br>Step 1: The pi bond gives alkenes a region of high electron density above & below the molecule. This can induce a dipole into non-polar molecules.<br>Step 2: The pi bond electron pair are attracted to the atom with a positive dipole to form a covalent bond, and the small molecule is broken by heterolytic fission. The result is the formation of a positive carbocation & a negatively charged ion<br>Step 3: A pair of electrons from the negative ion are donated to the positive carbon in the carbocation forming a covalent bond
• • The double bond in an alkene represents a region of high electron density because of the presence of the pi electrons
  • The high electron density of the pi electrons attracts electrophiles
  • An electrophile is an atom or group of atoms that is attracted to an electron-rich centre & accepts an electron pair. An electrophile is usually a positive ion or a molecule containing an atom with a partial delta positive charge
• Carbocation --> A molecule in which a carbon atom has a positive charge & three bonds
• Carbocation stability:

  • Linked to the electron - donating ability of the alkyl groups
  • Each alkyl groups donates & pushes electrons towards the positive charge of the carbonation
  • The positive charge is spread over the alkyl groups
  • The more alkyl groups attached to the positively charged carbon atom, the more the more the charge is spread out, making the ion more stable
  • Therefore tertiary carbocations are more stable than secondary carbocations, which are more stable than primary carbocations
• Markownikoff's rule:

  • When an asymmetrical alkene reacts with a hydrogen halide or water molecule there are 2 possible isomers formed. But one isomer is more commonly produced than the other
  • Markownikoff's rule states that the hydrogen being added bonds to the carbon with the most hydrogen's present
• Carbonium ion stability:

  • The stability of the carbonium ions formed is determined by the inductive effect of adjacent alkyl groups.
  • The more alkyl groups the greater the stability
  • Primary carbonium ions --> induction from one alkyl group only
  • Secondary carbnium ions --> induction from 2 alkyl groups gives better stabilisation than primary
  • Tertiary carbonium ions --> Induction from three alkyl groups gives even more stabilisation
• Addition polymerisation:

  • Polymers are extremely large molecules formed from many thousands of repeat units of smaller molecules known as monomers
  • Unsaturated alkene molecules undergo addition polymerisation to produce long saturated chains containing no double bonds
  • Industrial polymerisation is carried out at high temperatures & high pressure using catalyst
  • Addition polymers have high molecular masses
  • Synthetic polymers are usually named after the monomer that reacts to form their giant molecules prefixed by poly
• • A repeating unit is the specific arrangement of atoms in the polymer molecule that repeats over and over again
  • The repeat unit is always written in square brackets
  • After the bracket you place a letter n to show that there is a large number of repeats
• Poly(ethene) --> made by heating a large number of ethene monomers at high pressure. One of the most commonly used polymers
• Poly(chloroethene) --> also known as poly(vinyl chloride) or PVC, can be prepared to make a polymer that is flexible or rigid
• Disposing of waste polymers:

  • Polymers are readily available, cheap to purchase, and more convenient than the alternatives
  • The lack of reactivity that makes polymers suitable for storing food & chemicals safely also presents chemists with challenge in their disposal
  • Many alkene based polymers are non-biodegradable. The growing amount of polymer waste has serious environmental effects. E.g: killing marine life
• Recycling:

  • Recycling polymers reduces their environmental impact by conserving finite fossil fuels as well as decreasing the amount of waste going to landfill
  • Discarded polymers have to be sorted by type
  • Once sorted, the polymers are chopped into flakes, washed, dried, and melted
  • The recycled polymer is cut into pellets and used by manufactures to make new products
• PVC recycling:

  • The disposal & recycling of PVC is hazardous due to the high chlorine content & the range of additives present in the polymer
  • Dumping PVC in landfill is not sustainable and, when burnt PVC releases hydrogen chloride, a corrosive gas and other pollutants like toxic dioxins
• Using waste polymers as fuel:

  • Waste polymers can be incinerated to produce heat, generating steam to drive a turbine producing electricity
• Feedstock recycling:

  • Describes the chemical & thermal processes that can reclaim monomers, gases or oil from waste polymers
  • The products from feedstock recycling resemble those produced from crude oil in refineries
  • These materials can be used as raw materials for the production of new polymers
  • Able to handle unsorted and unwashed polymers
• Biodegradable & photodegradable polymers:

  • Bioplastics produced from plant starch, cellulose, plant oils, and proteins offer a renewable & sustainable alternative to oil-based products
  • Conserves valuable oil reserves
• Biodegradable polymers:

  • Biodegradable polymers are broken down by microorganisms into water, carbon dioxide, and biological compounds
  • Usually made from starch or cellulose, or contains additives that alter the structure of traditional polymers so that microorganisms can break them down.
  • Compostable polymer degrade & leave no visible or toxic residue. Compostable polymers based on poly(lactic acid) are becoming more common as an alternative to alkene based polymers
• Photodegradable polymers:

  • These polymers contain bonds that are weakened by absorbing light to start the degradation
  • Alternatively, light absorbing additives are used