Question

CHM 260 (Fall 2019) HW #4 (conjugated alkenes/aromatic nomenclature and reactivity) *Due in class Monday, 10/28* 1) Consider the reactions between hexa-1,4-diene and HBr (reaction A) and hexa-2,4-diene and HBr (reaction B) as shown below: (6 pts total) HBr HBO ? ? Reaction A Reaction B - which reaction will involve a carbocation that can be stabilized by resonance? Briefly explain why. - write out the mechanistic steps for reaction of hexa-2,4-diene with HBr (hint: you only need to consider reaction of one of the double bonds with HBr - the other double bond turns out to be equivalent by symmetry) - identify the 1,2 addition product and the 1,4 addition product 2) Consider the reaction between cyclopenta-1,3-diene and HCI, shown below: (4 pts) a HCI - - write out the mechanistic steps for reaction of cyclopenta-1,3-diene with HCl (hint: you only need to consider reaction of one of the double bonds with HCI) - identify the 1,2 addition product and the 1,4 addition product

Answer 1

1) From the reactions given below, Reaction B will involve a carbocation intermediate which can be stabilized by resonance as shown below, Ah-Br M I and II are the resonance structures of the carbocation formed after protonation of alkene The mechanism for 1,2 addition and 1,4-addition product of the reaction is shown below, Mechanism Br w Br protonation Nucleophilic 1.2-addition product Attack Step-11 Step-1 1,4 addition produd protonation Step- Nucleophilic Attack 2) For the given reaction, mechanism of formation of 1,2 and 1,4 addition product is shown below, H ai ci protonation Step- Nucleophilic attack Step-11 1,2 addition product Han protonation Step-1 1,4 addition product Here, 1,2 and 1,4 addition reaction gives the same product.