Organic 1 Lecture 5 PDF

2-Rearrangement of Carbocations Rearrangement of the initially formed 2º-carbocation to a 3º- carbocation by a 1,2-shift of a methyl group (a shift of a neighboring alkyl group or hydrogen) 1 ‫ م‬03:32 29/10/2024 3-Addition of Lewis Acids (Electrophili...

2-Rearrangement of Carbocations Rearrangement of the initially formed 2º-carbocation to a 3º- carbocation by a 1,2-shift of a methyl group (a shift of a neighboring alkyl group or hydrogen) 1 ‫ م‬03:32 29/10/2024 3-Addition of Lewis Acids (Electrophilic Reagents) Lewis acids like the halogens, boron hydrides and certain transition metal ions are able to bond to the alkene pi- electrons, and the resulting complexes rearrange or are attacked by nucleophiles to give addition products 2 ‫ م‬03:32 29/10/2024  4- Oxidation of alkenes Alkenes undergo a number of reactions in which the carbon-carbon double bond is oxidized. Several oxidizing agents, e.g. KMnO4, K2Cr2O7, O3, and peroxy acids may be used. (a) Oxidation by potassium permanganate to glycol OH, H2O CH2 CH2 + KMnO4 CH2 CH2 cold OH OH Ethene 1,2-ethanediol (ethylene glycol) 3 ‫ م‬03:32 29/10/2024  (b) Oxidation by ozone to ozonoid(ozonolysis of alkenes). H3C CH3 H3C CH3 (1) O3, CH2Cl2, - 78oC C C C O + O C H3C H (2) Zn/ HOAc H3C H 4 ‫ م‬03:32 29/10/2024  Alkynes ❑The general molecular formula for an acyclic (noncyclic) alkyne is CnHn ❑Each carbon is sp hybridized, so each has two sp orbitals and two p orbitals ❖Synthesis of Alkynes by Double Elimination (E2 mechanis) ✓from dihaloalkanes through dehydrohalogenation using alkoxide bases ✓if we apply this concept using 2 halides on vicinal or geminal carbons, the E2 reaction will take place twice resulting in the formation of 2  bonds and thus an Alkyne 5 ‫ م‬03:32 29/10/2024  Reactions of Alkynes An alkyne is an electron rich molecule. In other words, it is a nucleophile and consequently it will react with electrophiles The mechanism of the additions is presumably the same as with the alkenes, the initial step involving reaction between the nucleophilic unsaturated carbon atom and the electrophilic reagent atom 6 ‫ م‬03:32 29/10/2024 1.Addition by Electrophilic Reagents o The reactions are even more exothermic than the additions to alkenes o It takes place and generally display Markovnikov Rule regioselectivity and anti-stereoselectivity. o The products of these additions are themselves substituted alkenes and can therefore undergo further addition 7 ‫ م‬03:32 29/10/2024  2-Hydration of Alkynes and Tautomerism ❑As with alkenes, the addition of water to alkynes requires a strong acid, usually sulfuric acid, and is facilitated by mercuric sulfate ❑However, unlike the additions to double bonds which give alcohol products, addition of water to alkynes gives ketone products ❑Due to enol-keto tautomerization, The initial product from the addition of water to an alkyne is an enol (a compound having a hydroxyl substituent attached to a double-bond), and this immediately rearranges to the more stable keto tautomer 8 ‫ م‬03:32 29/10/2024  3-Oxidations Reactions of alkynes with oxidizing agents such as potassium permanganate and ozone usually result in cleavage of the triple-bond to give carboxylic acid products RC≡CR' + [O] ——> RCO2H + R'CO2H 9 ‫ م‬03:32 29/10/2024