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5. Alkenes Iedxcel

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5. Alkenes Iedxcel 5 Alkenes Alkenes contain a carbon- Alkenes are unsaturated hydrocarbons General formula is CnH2n carbon double bond somewhere in their structure H H H H C C C H C C Ethene Propene H H H H H H H H H Numbers need to be H added to the name when H C C C C H But-2-ene C C C C H But-1-ene positional isomers can occur H H H H H H H H π bonds are exposed and have high C=C double covalent bond electron density. consists of one sigma (σ) bond and one pi (π) bond. They are therefore vulnerable to attack by species which ‘like’ electrons: these C-C sigma bond C-C pi bond species are called electrophiles. Formation of σ bond C C One sp2 orbital from each carbon overlap to form a single C-C bond called a sigma σ bond sigma σ bond Rotation can occur around a sigma bond Formation of π bond p orbitals The π bond is formed by sideways overlap of two p orbitals on each carbon atom forming a π-bond above and below the plane of molecule. C-C sigma bond The π bond is weaker than C-C pi bond the σ bond. The pi bond leads to resultant high electron density above There is Restricted rotation about a pi bond and below the line between the two nuclei N Goalby chemrevise.org 1 Stereoisomerism Stereoisomers have the same structural formulae Alkenes can exhibit a type of isomerism but have a different spatial arrangement of atoms. called E-Z stereoisomerism E-Z isomers exist due to restricted E-Z stereoisomers arise when: rotation about the C=C bond (a) There is restricted rotation around the C=C double bond. (b) There are two different groups/atoms attached both ends of Single carbon-carbon covalent the double bond bonds can easily rotate H H H H two different groups H C C H But-1-ene H C C H attached either end of C C the restricted double H H C C H bond- leads to EZ H two identical groups attached to H H H H isomers one end of the restricted double Z- but-2-ene bond – no E-Z isomers But-1-ene is a structural isomer of But-2- H H These are two ene but does not show E-Z isomerism C H isomers as the lack H of rotation around Skeletal formulae can also represent E-Z isomerism C C the double bonds H means one cannot Z-but-2-ene H C be switched to the H H other E-but-2-ene E -but-2-ene : The atom with the bigger Naming E-Z stereoisomers Priority Group atomic number is classed as the priority atom First determine the priority groups on both sides of the double bond Priority Priority Cl H group Cl Cl group side 1 side 2 C C C C Z-1,2-dichloroethene H H H Cl E-1,2-dichloroethene If the priority atom is on the same side of the If the priority atom is on the opposite side of double bond it is labelled Z from the german the double bond it is labelled E from the zusammen (The Zame Zide!) german entgegen (The Epposite side!) cis-trans isomerism is a special case of EIZ isomerism in which two of the substituent groups are the same. H H H H C H H C C H H C C H C C H C H H H H H H Z- but-2-ene E- but-2-ene Can also be called Can also be called Cis- but-2-ene trans- but-2-ene N Goalby chemrevise.org 2 Addition Reactions of Alkenes Addition reaction: a reaction where two molecules react together to produce one 1. Reaction of Alkenes with Hydrogen Change in functional group: alkene alkane H H H H Reagent: hydrogen H C C H Conditions: Nickel Catalyst C C + H2 Type of reaction: Addition/Reduction H H H H ethene ethane Electrophilic Addition: Reactions of Alkenes A π bond is weaker than a σ bond so less energy is needed to break π bond Definition Electrophile: an The π bonds in alkenes are areas with high electron density. This is more electron pair acceptor accessible to electrophilic attack by electrophiles. Alkenes undergo addition reactions. 2. Reaction of Alkenes with bromine/chlorine H H Change in functional group: H H alkene dihalogenoalkane H C C H C C + Br Reagent: Bromine (dissolved in organic solvent) 2 Conditions: Room temperature (not in UV light) H H Br Br Mechanism: Electrophilic Addition 1,2-dibromoethane Type of reagent: Electrophile, Br+ Type of Bond Fission: Heterolytic As the Br molecule H H 2 H H H H approaches the alkene, the pi bond electrons + The INTERMEDIATE C C H C C H H C C H repel the electron pair in formed, which has a the Br-Br bond. This δ+ positive charge on a H H Br carbon atom is called a INDUCES a DIPOLE. Br2 Br Br Br becomes polar and :Br - CARBOCATION δ+ ELECTROPHILIC (Br ). Brδ- 3. Reaction of Hydrogen Bromide with alkenes Change in functional group: H H H H H H alkenehalogenoalkane H C C C C H + HBr H C C C C H Reagent: HCl or HBr Conditions: Room temperature H H H H H H Br H Mechanism: Electrophilic Addition But-2-ene 2-bromobutane Type of reagent: Electrophile, H + Type of Bond Fission: Heterolytic H H H HBr is a polar molecule H H H because Br is more H3C C C CH3 + electronegative than H. H3C C C CH3 H3C C C CH3 The H δ + is attracted to δ+ H H the electron-rich pi bond. Br H - Brδ :Br - N Goalby chemrevise.org 3 If the alkene is unsymmetrical, addition of hydrogen bromide can lead to two isomeric products. But-1-ene will form a mixture of 1-bromobutane and 2-bromobutane on reaction with hydrogen bromide - H :Br + H H H H C CH2 Major H C C C C H H3C CH3 product δ+ δ- 90% H Br H H Br H :Br - H2C CH CH2 CH3 H H + CH2 CH2 CH2 CH3 Minor C C CH2 CH3 product Br H H 10% WHY? In electrophilic addition to alkenes, the major product is This carbocation intermediate H H H formed via the more stable carbocation intermediate. is more stable because the methyl groups on either side H C C C H In exam answers + of the positive carbon are •Draw out both carbocations and identify as primary, secondary electron releasing and reduce H H and tertiary the charge on the ion which •State which is the more stable carbocation e.g. secondary more stabilises it. stable than primary •State that the more stable carbocation is stabilised because the The order of stability for carbocations is methyl groups on either (or one) side of the positive carbon are electron releasing and reduce the charge on the ion. tertiary > secondary >primary •(If both carbocations are secondary then both will be equally stable and a 50/50 split will be achieved) 4. Reaction of Potassium Manganate(VII) with Alkenes Change in functional group: alkene diol H H H H H Reagent: KMnO4 in an acidified solution KMnO C C C H 4 H C C C H Conditions: Room temperature H H Type of reaction: Oxidation H OH OH H - propene Propane-1,2-diol Observation: purple colour of MnO4 ion will decolourise to colourless This reaction with its colour change can be used as a test for the alkene functional group. It would not change colour with alkanes 5. Reaction of Bromine Water with Alkenes Reagent: Bromine dissolved in water H H H H Conditions: Room temperature + BrOH Type of reaction: Addition C C H C C H Observation: Orange colour of bromine water H H Br OH will decolourise to colourless This reaction with its colour change is used as a test for the alkene functional group. N Goalby chemrevise.org 4 hydration of alkenes to form alcohols This reaction can be called Industrially alkenes are converted to alcohols in one step rather than the hydration: a reaction where two in the above sulphuric acid reaction. They are reacted with water in water is added to a molecule the presence of an acid catalyst. Essential Conditions = High temperature 300 to 600°C CH2 CH2 (g) + H2O (g) CH3CH2OH (l) High pressure 70 atm Catalyst of concentrated H3PO4 The high pressures needed mean this cannot be done in the laboratory. It is preferred industrially, however, as there are no waste products and so has a high atom economy. It would also mean separation of products is easier (and cheaper) to carry out. See equilibrium chapter for more on the industrial conditions for this reaction. Addition Polymers Addition polymers are formed from alkenes Poly(alkenes) like alkanes are unreactive due to the strong C-C and C-H bonds. This is called addition polymerisation be able to recognise the repeating unit in a poly(alkene) n H H H H H H Monomer Polymer Ethene Poly(ethene) C C C C C C H H H H CH3 H CH3 H CH3 H C C n C C H CH3 H CH3 n Add the n’s if writing an equation showing the propene reaction where ‘n’ monomers become ‘n’ repeating poly(propene) units Poly(propene) is recycled Poly(ethene): is used to make plastics bags, Poly(propene) is a stiffer polymer, used in buckets, bottles.
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