Electrophilic addition 2 LO Understand the electrophilic addition mechanism involving alkenes and halogens. Understand the electrophilic addition mechanism.

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Electrophilic addition 2 LO Understand the electrophilic addition mechanism involving alkenes and halogens. Understand the electrophilic addition mechanism involving alkenes and sulfuric acid, H 2 SO 4. Starter questions 1.Outline 2 possible mechanisms for the reaction between pent-1-ene & hydrogen bromide. 2.Why is the H-Br covalent bond polar but the Br-Br covalent bond is non-polar?

Example 1- ethene and bromine, Br 2 Equation for the reaction The e - s in the C=C bond repel the pair of e - s in the Br-Br bond. The electrons in the Br-Br bond are therefore pushed closer to the Br atom furthest from the C=C bond. This creates an induced dipole with one Br atom becoming ∂+ & the other becoming ∂-. The ∂+ Br atom attracts the e - s in the C=C bond. Electrophilic addition involving alkenes and halogens Alkenes can react with halogens in an electrophilic addition mechanism.

Example 2 Outline the mechanism for but-2-ene reacting with chlorine. Name the product formed. Equation for the reaction

Electrophilic addition involving alkenes & sulfuric acid Alkenes can react with aqueous sulfuric acid, H 2 SO 4 in an electrophilic addition mechanism to form an alcohol. There are two steps. Example 1- Step 1- ethene and sulfuric acid, H 2 SO 4 Eqn for step 1:

Step 2- The ethyl hydrogensulfate reacts with water to form an alcohol: The sulfuric acid is reformed, therefore is a catalyst. Overall eqn

Task Step 1) Outline two possible mechanisms for the reaction between propene and aqueous sulfuric acid:

Which product is most likely to form? (Think about stability of carbocations from last lesson) Step 2)- Write eqns for the reaction of the products from step 1 with water to form alcohols.

June ‘12 Exam questions on electrophilic addition June ‘11 Name and outline a mechanism for the reaction below.

June ‘10

(iii) Draw the structure of the compound with Mr = formed when penta-1,4-diene (H 2 C=CHCH 2 CH=CH 2 ) reacts with an excess of bromine. (1 mark) 4(a) Bromine reacts with alkenes, even though bromine is a non-polar molecule. (i) Explain why bromine molecules react with the double bonds in alkenes (2 marks) (ii) Name the type of mechanism involved in this reaction (1 mark) Jan ‘11

(b) Two products are formed when propene reacts with hydrogen bromide. Draw the structure of the intermediate that leads to the formation of the major product in the reaction of propene with hydrogen bromide. Give the name of this type of intermediate. Structure of intermediate Type of intermediate (2 marks)

5(a) Name and outline a mechanism for the reaction of bromine with but-2-ene. Name of mechanism Mechanism (5) (b) But-2-ene can exist as a pair of stereoisomers. (i) State what is meant by the term stereoisomers (2) (ii) Draw the structure of (E)-but-2-ene (1)