Notes about different reactions required for term 2

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Browning, Chin

Chapter 6 Alkenes: their stability and reaction mechanism Stability of Alkenes Generally, cis is less stable than trans, due to steric strain between the substituted chains (R group). Less stable compounds have less G and H values. 0 The more substituted the compound is, the more stable the compound, and the greater their H and G values. Stability in alkenes are attributed by two factors: 1. Hyperconjugation The interaction of a paired pi orbital and a neighbouring filled C H sigma bond on a substituent. 2. Bond Strength A sp sp carbon bond is stronger than a sp sp carbon bond Electrophile Addition Reactions, in this case with binary acids (HBr, HCl) 1. The electron in H of the binary acid is transferred to the electronegative ion, usually Br or Cl. 2. H of the binary acid is now electrophile, and Br or Cl is now nucleophile 3. H attacks a electron pair in the C = C bond, making itself attached to one of the less substituted C in the C = C bond (Markovnikovs rule) 4. The other C in the C = C bond is now a carbocation, having only 6 electrons in its valence 5. Up until this point, the intermediate formed will be very unstable and so it drives further step, addition of the anion. 6. A Br or Cl anion now attacks this carboncation, forming a bond with the carboncation. ** Therefore, the anions would usually making bond with more substituted carbon, whereas the + H gets the less substituted carbon. Stability of Carbocations (which is formed in step 4 of the above reaction mechanism) 1. Similar to the stability of alkenes, the more substituted it is, the more stable it is. Because with more R group around that carbocation, more inductive effect takes place. The positive charge in the carbocation can be shared between those relatively electronegative R groups, making the structure more stable, than a less substituted carbocation in which the negative charge is not shared as much. 2. Hyperconjugation also contributes to its stability for the same reason as discussed in alkenes. Hammond postulate w.r.t the formation of carbocations The structure of a transition state resembles the structure of the nearest stable speices(product, being the formed carbocation or reactant, being the alkene). Transition states for endergonic steps structurally resemble products and transition states for exergonic steps structurally resemblereactants. In other words, the engery level of the transition state is closer to that of the reactant in the exergonic reaction, and is closer to that of the product in the endergoinc reaction. Evidence of Electrophilic Additions In an alkene with a parent chain more than two carbons, carbocation structure could be rearranged.In essence, a most stable carbocation structure that a compound can produce is always formed during an electrophilic addition reaction, even if the formation requires moving the existing R group (methyl shift) or H atoms (hydride shift) even changing the ring size (alkyl shift) attached on the compound around.
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