3 Addition reactions / PAG 7

Addition reactions of alkenes Addition reaction: a reaction where two molecules react together to produce one Change in functional group: alkene  alkane Reagent: hydrogen Conditions: Nickel Catalyst Type of reaction: Addition/Reduction 1. Reaction of Alkenes with Hydrogen C C H H H H + H2  ethane C C H H H H H H ethene 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 alkenes undergo addition reactions 2. Reaction of alkenes with bromine/chlorine Change in functional group: alkene  dihalogenoalkane Reagent: Bromine Conditions: Room temperature (not in UV light) Mechanism: Electrophilic Addition Type of reagent: Electrophile, Br+ Type of Bond Fission: Heterolytic C C H H Br Br H H C C H H H H + Br2  1,2-dibromoethane As the Br2 molecule approaches the alkene, the pi bond electrons repel the electron pair in the Br-Br bond. This INDUCES a DIPOLE. Br2 becomes polar and ELECTROPHILIC (Brδ+ ). The INTERMEDIATE formed, which has a positive charge on a carbon atom is called a CARBOCATION C C H H H H C C H H Br Br H H C + C Br H H H H Br Br δ + δ – :Br – N Goalby chemrevise.org 4 3. Reaction of Hydrogen Bromide with Alkenes Change in functional group: alkenehalogenoalkane Reagent: HCl or HBr Conditions: Room temperature Mechanism: Electrophilic Addition Type of reagent: Electrophile, H+ C + HBr  H H C C C H H H H H H C C H C H Br C H H H H H H H But-2-ene 2-bromobutane HBr is a polar molecule because Br is more electronegative than H. The H δ + is attracted to the electron-rich pi bond. This reaction can lead to two products when the alkene is unsymmetrical Major product 90% Minor product 10% If the alkene is unsymmetrical, addition of hydrogen bromide can lead to two isomeric products. ‘Markownikoff’s Rule’ In most cases, bromine will be added to the carbon with the fewest hydrogens attached to it C C H H H C H H H H + This carbocation intermediate is more stable because the methyl groups on either side of the positive carbon are electron releasing and reduce the charge on the ion which stabilises it. WHY? The order of stability for carbocations is tertiary > secondary >primary In electrophilic addition to alkenes, the major product is formed via the more stable carbocation intermediate. In exam answers •Draw out both carbocations and identify as primary, secondary and tertiary •State which is the more stable carbocation e.g. secondary more stable than primary •State that the more stable carbocation is stabilised because the methyl groups on either (or one) side of the positive carbon are electron releasing and reduce the charge on the ion. •(If both carbocations are secondary then both will be equally stable and a 50/50 split will be achieved) C + C H CH3 H H H3C C C H CH3 H H H3C Br C C CH3 H H H3C δ +δ – :Br – H Br H2C CH CH2 CH3 H Br H3C C + CH2 CH3 H C + C CH2 CH3 H H H H :Br – :Br δ – + δ – CH2 CH2 CH2 CH3 Br C C H C H Br C H H H H H H H 4. Reaction of alkenes with steam to form alcohols Industrially alkenes are converted to alcohols in one step. They are reacted with steam in the presence of an acid catalyst. CH2=CH2 (g) + H2O (g)  CH3CH2OH (l) This reaction can be called hydration: a reaction where water is added to a molecule Reagent : steam Essential Conditions High temperature 300 to 600°C 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. The alkenes are relatively reactive because of the relatively low bond enthalpy of the π-bond.