Chapter 9 Alcohol Reactions

1. Chapter 9 Alcohol Reactions • Reactions of Alcohols with Acids and Bases • Preparations of Alkoxides • To deprotonate an alcohol, Base more basic than RO- • Alkali Metal Reductions give Alkoxides • 2 H2O + Na (Li, K, Cs) 2 Na+OH- + H2 (Violent!) • 2 ROH + Na 2 Na+OR- + H2 • Reactivity: MeOH > primary > secondary > tertiary Electron donating R groups destabilize RO- anion

2. 3) Uses of Alkoxides • Strong base for E2 reactions • Ether synthesis by SN2 reactions (stay tuned!) • Protonation of Alcohols form a better Leaving Group • X- + ROH RX + OH- (OH- is a poor leaving group) • Strong acid converts OH- leaving group to H2O (good leaving group) • Synthesis of Haloalkanes from Alcohols • Only I- and Br- are nucleophilic enough to work • Works best for primary alcohols

3. 4) Reaction of Secondary and Tertiary ROH with H+ a) Secondary and Tertiary alcohols easily lose water to from carbocations • If the acid contains a good nucleophile, you get SN1 substitution • If there is not nucleophile or have high Temperature, you get E1 • Dehydration = loss of an H2O molecule Use non-nucleophilic acid = H2SO4, H3PO4 • As usual, Tertiary ROH only does SN1, E1 while Secondary ROH can do SN2, SN1, or E1 in a strong acid Catalytic

4. Carbocation Rearrangement • Hydride Shift • Sometimes we get mixtures of products from 2o and 3o ROH reactions • How does the rearrangement occur? • Carbocation intermediate rearranges • Hydrogen and an electron moves = hydride shift • Mechanism • H:- and + trade places • Very fast (faster than SN1/E1) if new C+ more stable

5. Orbital Picture of the Hydride Shift Mechanism • Primary ROH/RX won’t form carbocations, so don’t do Hydride Shift • Secondary and Tertiary ROH give mixture of products with nucleophile

6. Mixture of E1 Products are also observed at high Temp., no nucleophile B. Alkyl Shifts • If the carbocation doesn’t have a H- positioned to shift, an alkyl group can move = Alkyl Shift

7. Hydride and Alkyl Shifts occur at about the same rate • Very fast if rearranged carbocation is more stable • Formation of tertiary carbocation is faster than secondary • Concerted Hydride and Alkyl Shifts in Primary Alcohols • Primary alcohol will not form a carbocation, but sometimes rearranged products are observed anyway • Only observed with much heat and much time • Concerted mechanism explains how • Ester Formation from Alcohols • Esters are derivatives of organic and inorganic acids

8. Synthesis of Organic Esters Subject of chapters 19 and 20 • Synthesis of Haloalkanes from Alcohols involves Inorganic Esters • R—OH + HBr R+ multiple products • Use Inorganic Reagent to make the water leaving group • 1o or 2o ROH + PBr3 1o or 2o RBr + H3PO3 • Mechanism • Reaction also works to make iodoalkanes with PI3 • Chlorination of an alcohol requires thionyl chloride = SOCl2

9. Mechanism • Amine works as a base to remove H+ from reaction • Alkyl Sulfonates in Substitution Reactions • Preparation of Alkyl Sulfonates (good leaving groups) • Substitution reactions take advantage of the good sulfonate leaving group