3.4 Alkenes Revision Guide (AQA) PDF

3.4 Alkenes Alkenes contain a carbon- Alkenes are unsaturated hydrocarbons General formula is CnH2n carbon double bond...

3.4 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 The arrangement of bonds around the C C Ethene Propene H H H >C=C< is planar and has the bond angle 120o 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 species are called electrophiles. 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 Naming E-Z stereoisomers C C the double bonds H means one cannot First determine the priority groups on both sides of H C be switched to the the double bond H H other. Priority group: The atom with the bigger E -but-2-ene atomic number is classed as the priority atom 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!) N Goalby chemrevise.org 1 Electrophilic addition reactions of alkenes Definition Electrophile: an electron pair acceptor The double bonds in alkenes are areas with high electron density. This attracts electrophiles and the Addition reaction: a reaction where two alkenes undergo addition reactions. molecules react together to produce one 1. Reaction of bromine with alkenes H H Change in functional group: alkene  dihalogenoalkane H H Reagent: Bromine Conditions: Room temperature (not in UV light) C C + Br2  H C C H Mechanism: Electrophilic addition H H Br Br Type of reagent: Electrophile, Br+ 1,2-dibromoethane As the Br2 molecule H H 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 positive charge on a H Brδ+ H Br induces a dipole. Br2 Br Br carbon atom is called a becomes polar and :Br - carbocation. electrophilic (Brδ+). Brδ- 2. Reaction of hydrogen bromide with alkenes Change in functional group: alkenehalogenoalkane H H H H H H Reagent: HCl or HBr H C C C C H + HBr  H C C C C H Conditions: Room temperature Mechanism: Electrophilic addition H H H H H H Br H Type of reagent: Electrophile, H+ But-2-ene 2-bromobutane HBr is a polar H H H H H This reaction can molecule because H Br is more lead to two H3C C C CH3 + electronegative H3C C C CH3 H3C C C CH3 products when the than H. The H δ + is δ+ alkene is attracted to the H H Br H unsymmetrical - electron-rich pi Br δ :Br - bond. ‘Markownikoff’s Rule’ H :Br - In most cases, bromine will be added to the + H H H H carbon with the fewest hydrogens attached to it. C CH2 Major H C C C C H If the alkene is H3C CH3 product unsymmetrical, δ+ δ- :Br - 90% addition of hydrogen H Br H H Br H bromide can lead to H H two isomeric H2C CH CH2 CH3 C + C CH2 CH3 CH2 CH2 CH2 CH3 Minor products. product H Br 10% H 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 and tertiary H H 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 tertiary > secondary >primary electron releasing and reduce the charge on the ion. (If both carbocations are secondary then both will be equally 2 stable and a 50/50 split will be achieved) N Goalby chemrevise.org  3. Reaction of sulfuric acid with alkenes Stage 1 Stage 2 Change in functional group Change in functional group alkene  alkyl hydrogensulfate alkyl hydrogensulfate  alcohol Reagents: concentrated H2SO4 Reagents: water Conditions: room temperature Conditions: warm mixture Mechanism: Electrophilic addition Type of reaction: hydrolysis Type of reagent: Electrophile, H2SO4 CH3CH2OSO2OH + H2O  CH3CH2OH + H2SO4 CH2=CH2 + H2SO4  CH3CH2OSO2OH Stage 1: electrophilic addition H2SO4 is best H H H H H drawn as H H3C C C H + H3C C C H H-OSO2OH H3C C C H in exams. Its real δ+ H H H OSO 2OH structure is - :OSO2OH H O O OSO 2OH δ- S Stage 2: hydrolysis H O O H H H H3C C C H + H2O H3C C CH3 + H2SO4 Overall role of sulfuric acid is that of a H OH catalyst (as it is OSO 2OH regenerated) With unsymmetrical alkenes a minor and major product can also be formed similar to the addition of HBr. The same explanation applies. Definition: Hydrolysis – a reaction where the molecule is split by the addition of water. Direct industrial 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 sulfuric acid reaction. They are reacted with water in the water is added to a molecule presence of an acid catalyst. Essential conditions CH2=CH2 (g) + H2O (g)  CH3CH2OH (l) High temperature 300 to 600°C High pressure 70 atm Catalyst: 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. Testing for alkenes with bromine water Bromine water decolourises in the presence of a double bond. This can be used as a test for the presence of an double bond in a molecule. It can be used quantitatively to show the presence of multiple double bonds in compounds like polyunsaturated oils. N Goalby chemrevise.org 3 Addition Polymers Poly(alkenes) like alkanes are unreactive due to Addition polymers are formed from alkenes 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 polyethene C C C C C C H H H H CH3 H CH3 H CH3 H n C C C C H CH3 H CH3 n propene poly(propene) Add the n’s if writing an equation showing the Poly(propene) is recycled reaction where ‘n’ monomers become ‘n’ repeating units H H If asked to draw one repeating unit, don’t add the n on to your diagram, C C because n represents a large number H CH3 It is best to first draw out H CH3 You should be able e.g. For but-2-ene the monomer with groups H CH3 to draw the polymer of atoms arranged around C C C C repeating unit for any H3C CH CH CH3 the double bond alkene H3C H CH3 H Poly(chloroethene) is a polymer that is water proof, an electrical insulator and doesn’t react with acids. H H In its pure form it is a rigid plastic due to the strong intermolecular bonding between polymer chains prevents them moving over each C C other. In this un-plasticised form it is used make uPVC window frame coverings and guttering. If a plasticiser is added the intermolecular forces are weakened which H Cl allows the chains to move more easily, resulting in more flexibility in the polymer. In this form PVC is used to make insulation on electrical wires, and waterproof clothing. N Goalby chemrevise.org 4