Chapter6 - Organic Chemistry CHE 275 Chapter 6 Reactions of...

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Unformatted text preview: Organic Chemistry CHE 275 Chapter 6 Reactions of Alkenes: Addition Reactions •The characteristic reaction of alkenes is addition to the double bond. + A—B C C A C C B Addition Reactions of Alkenes + H—H σ π σ σ •exothermic Δ H ° = –136 kJ/mol •catalyzed by finely divided Pt, Pd, Rh, Ni C C H C C H H H H H H H H H Hydrogenation of Ethylene H 2 , Pt (73%) Example CH 2 H 3 C H 3 C CH 3 H H 3 C H 3 C • What three alkenes yield 2-methylbutane on catalytic hydrogenation? H 2 , Pt Problem H H C C A B X Y H H Mechanism of Catalytic Hydrogenation H C C A B X Y H H H Mechanism of Catalytic Hydrogenation H H H H C C A B X Y Mechanism of Catalytic Hydrogenation H H H H C C A B X Y Mechanism of Catalytic Hydrogenation H H H C C A B X Y H Mechanism of Catalytic Hydrogenation H H H C C A B X Y H Mechanism of Catalytic Hydrogenation • can be used to measure relative stability of isomeric alkenes • correlation with structure is same as when heats of combustion are measured Heats of Hydrogenation CH 3 CH 2 CH 2 CH 3 126 119 115 Heats of Hydrogenation of Isomers •Ethylene 136 •Monosubstituted 125-126 • cis-Disubstituted 117-119 • trans-Disubstituted 114-115 •Terminally disubstituted 116-117 •Trisubstituted 112 •Tetrasubstituted 110 Heats of Hydrogenation (kJ/mol) Match each alkene of Problem 6.1 with its correct heat of hydrogenation. 126 kJ/mol 118 kJ/mol 112 kJ/mol highest heat of hydrogenation; least stable isomer lowest heat of hydrogenation; most stable isomer Problem Stereochemical Aspects of Alkene Hydrogenation: (1) syn addition of both H atoms to double bond (2) hydrogenation is stereoselective, corresponding to addition to less crowded face of double bond syn addition anti addition syn-Additon Vs. anti-Addition CO 2 CH 3 CO 2 CH 3 (100%) H 2 , Pt Example of Syn Addition CO 2 CH 3 CO 2 CH 3 H H (2) hydrogenation is stereoselective, corresponding to addition to less crowded face of double bond A reaction in which a single starting material can give two or more stereoisomeric products but yields one of them in greater amounts than the other (or even to the exclusion of the other) is said to be stereoselective . Stereochemical Aspects of Alkene Hydrogenation H 3 C CH 3 H 3 C H H 2 , cat Both products correspond to syn addition of H 2 . CH 3 H 3 C H 3 C H H H CH 3 H 3 C H 3 C H H H Example H 3 C CH 3 H 3 C H H 2 , cat Only this product is formed Addition of H 2 prefers less hindered approach CH 3 H 3 C H 3 C H H H Example CH 3 H 3 C H 3 C H H H H 3 C CH 3 H 3 C H H 2 , cat Top face of double bond blocked by this methyl group Example H 2 , cat H 2 adds to bottom face of double bond. CH 3 H 3 C H 3 C H H H H 3 C CH 3 H 3 C H Example + E — Y δ + δ – C C E Y C C General Equation for Electrophilic Addition + H — X δ + δ – C C H X C C When E + is a Hydrogen Halide CH 3 CH 2 CH 2 CH 3 H H CH 3 CH 2 CH 2 CHCH 2 CH 3 Br (76%) CHCl 3 , -30°C C C H Br Example • Electrophilic addition of hydrogen halides to alkenes proceeds by rate-determining formation of a carbocation intermediate....
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Chapter6 - Organic Chemistry CHE 275 Chapter 6 Reactions of...

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