Ch 8 Reactions of Alkenes

Question 1

Hoffman’s rule

Answer 1

whenever there is a possibility of forming two alkenes by elimination, formation of the LESS substituted alkene is favoured.

Question 2

Wagner-Meerwein rearrangement

Answer 2

a formed carbocation intermediate may rearrange itself to a more stable carbocation through a 1,2-hydride or a 1,2-alkyl shift, due to the increased stability of a tertiary over a secondary carbocation.

Question 3

mercurinium ion

Answer 3

an organometal- lic cation containing a three-membered ring

Question 4

mercuric acetate

Answer 4

Hg(OCOCH₃)₂

abbreviated Hg(OAc)₂

Question 5

Hg(OCOCH₃)₂

Answer 5

mercuric acetate

Question 6

Hg(OAc)₂

Answer 6

mercuric acetate

Question 7

sodium borohydride

Answer 7

sodium borohydride (NaBH₄) is a reducing agent which replaces the mercuric acetate fragment with a hydrogen atom during demurcuriation.

Question 8

are alkene additions generally endothermic or exothermic?

Answer 8

a majority of these reactions are exothermic, due to the fact that the C-C pi-bond is relatively weak (ca. 63 kcal/mole) relative to the sigma-bonds formed to the atoms or groups of the reagent. Remember, the bond energies of a molecule are the energies required to break (homolytically) all the covalent bonds in the molecule. Consequently, if the bond energies of the product molecules are greater than the bond energies of the reactants, the reaction will be exothermic. Note that by convention exothermic reactions have a negative heat of reaction.

Question 9

addition of strong Brønsted acids to an alkene

Answer 9

strong Brønsted acids such as HCl, HBr, HI & H₂SO₄, rapidly add to the C=C functional group of alkenes to give products in which new covalent bonds are formed to hydrogen and to the conjugate base of the acid

Question 10

addition of weak Brønsted acids to alkenes

Answer 10

weak Brønsted acids such as water (pK_a = 15.7) and acetic acid (pK_a = 4.75) do not normally add to alkenes. However, the addition of a strong acid serves to catalyze the addition of water, and in this way alcohols may be prepared from alkenes. For example, if sulfuric acid is dissolved in water it is completely ionized to the hydronium ion, H3O⁺, and this strongly acidic (pK_a = -1.74) species effects hydration of ethene and other alkenes.

CH₂=CH₂ + H3O⁺ ——> HCH₂–CH₂OH + H⁺

Question 11

what is the importance of choosing an appropriate solvent for addition reactions?

Question 12

which type of solvent more favours addition reactions, polar or non-polar?

Answer 12

Because these additions proceed by way of polar or ionic intermediates, the rate of reaction is greater in polar solvents, such as nitromethane and acetonitrile, than in non-polar solvents, such as cyclohexane and carbon tetrachloride.

Question 13

Hammond Postulate

Answer 13

a transition state will be structurally and energetically similar to the species (reactant, intermediate or product) nearest to it on the reaction path

Question 14

rationalisation of Markovnikov’s rule

Answer 14

when an unsymmetrically substituted double bond is protonated, we expect the more stable carbocation intermediate to be formed faster than the less stable alternative, because the activation energy of the path to the former is the lower of the two possibilities

Question 15

two advantages of the oxymercuration method of adding water to a double bond

Answer 15

two advantages of the oxymercuration method of adding water to a double bond are its high anti-stereoselectivity and the lack of rearrangement in sensitive cases. These characteristics are attributed to a mercurinium ion intermediate, in this case it must be d-orbital electrons that are involved in bonding to carbon