CHEM 261 Mar 9, same

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1 93 EM 261 Mar 9, 2017 Review: ydrogenation The addition of 2 to an alkene Example: cyclohexene 2 Pd same or stereochemistry: S R Aside: = deuterium = 2 (1 p + and 1 e - ) Product is a meso compound (has stereogenic centers but is achiral due to plane of symmetry within molecule Example: androstane (steroid, male hormone) 2 Pt 2 comes from the bottom only because the methyl group adjacent to the alkene shields the top face methyl group sterically hinders the approach Example: fats (esters of glycerol) glycerol or glycerin:

2 94 triglyceride: ester molecule composed of glycerol and three fatty acids Aside: the colour of margarine is artificial, a yellow dye Pearl (maragon) is added, hence the name margarine Additionally, diacetyl (on the right) is added and this gives it the familiar buttery flavour Terminology Unsaturated fat = double bond in the fatty acid, induces kinks in the carbon chain Saturated fat = single bonds in the carbon chain (not including glycerol moiety) Trans-unsaturated fat: R R Trans-Unsaturated Fat Trans-polyunsaturated fat = multiple double bonds, all with trans configuration is-polyunsaturated fat = multiple double bonds, all with cis configuration

3 95 alogenation The addition of a halide (X 2 ) to an alkene Reaction is fast and does not require heat or light as with alkane Addition occurs in anti or trans fashion eterolytic chemistry (movement of 2 electrons) General reaction X X 2 X X= l or Anti or trans addition Examples: Alkene + 2 Anti or trans addition Free rotation about - bond yclopentene 2

4 96 Mechanisms for the above reactions: Alkene sp 3 sp 2 Inversion of configuration Anti-addition sp 3 sp 3 omonium ion S N 2-like attack

5 97 yclopentene 1-methylcyclohex-1-ene (achiral) Trans-1,2-dibromocyclopentane l l 2 l l l l l racemic mixture Additional examples for your reference: 1. 2

6 98 2. I l Iodine monochloride Il -l l I I

7 99 Reaction of Alkenes with alogen and Water/Alcohol General reaction X 2 R- R X X = l,, F R = alkyl/ Markovnikov s Rule - positive species adds to the least substituted end of = - negative species adds to the more substituted end of = (stabilized positive charge) Example 1 l 2 2 l a chlorohydrin Mechanism l 2 l l l Aside: alohydrins are useful compounds for further reactions. The chlorohydrin below can be converted into an epoxide by an intramolecular (within a molecule) S N 2 reaction. The epoxide may then be converted into a 1,2-diol (intermolecular S N 2). epoxide = oxirane = cyclic 3-membered ether l Base e.g. Na S N 2 - intramolecular intramolecular rxns are FAST S N 2 - intermolecular ethylene oxide - reactive 2 ethylene glycol

8 100 Example 2! +! l I +! - l! - racemate I l I Example 3 an ether 3 l 2 3 l Example 4 (Markovnikov rule) 3 l S N 2 - intermolecular l 2 ether 3 3 l 3 l l l l

9 101 Example pent-4-en-1-ol 5 2 l 4 + xygen is a better nucleophile than bromide and intramolecular (within a molecule) cyclization is FAST. Product is a racemate an this reaction occur with chlorine? Yes. Iodine? Yes. Example I 2 * I 6 racemate 5-membered ring formation is favored à FAST 6-membered ring formation à K, but much slower Terminology: Intramolecular within the same molecule Intermolecular amongst molecules

10 102 ydration and Ether Formation General Reactions: Addition of or R + No Acid No reaction + eg), l 2 S 4, I Alcohol Non-stereospecific Syn and Anti + R R = alkyl R Ether Non-stereospecific Syn and Anti Example 1 (follows Markvnikov s Rule) 3-2 S 4 3 Elimination isopropyl alcohol Mechanism