tailieunhanh - Lecture Organic chemistry - Chapter 12: Reactions of alkenes

Lecture Organic chemistry - Chapter 12: Reactions of alkenes. In this chapter, the following content will be discussed: Addition reactions of alkenes, catalytic hydrogenation, electrophilic additions, hydroboration-oxidation,.and other contents. | Chapter 12: Reactions Of Alkenes Tear resistant Fundamental building blocks for organic transformations and polymer materials synthesis Polyethylene Addition Reactions Of Alkenes π Bond is unsaturated: Reacts by addition. C C + A B C C A B exothermic (usually) We can calculate ∆Hº of addition reaction from bond strength data, using DHºπ-bond = 65 kcal mol-1. Note, however: ∆Sº is negative (~ -30 .) Catalyst 1. Catalytic Hydrogenation C C + H H C C H H Typical catalysts are heterogeneous: insoluble solids Why catalysts? Enable a lower Ea mechanism. Alkenes + H2 no reaction without catalyst Pd/C, PtO2 ( Pt) H2 E Catalyzed pathway Uncatalyzed pathway Stereospecific H-H addition from the same side (syn) of double bond. + H2 cat. H H CH3 CH2CH3 cis trans cat. D2 + SS D D H H R R Stereochemistry + Mechanism Reversibility causes cis to trans isomerization Hydrogenation “Hardens” Oils, But Generates Trans Fatty Acids The fatty acid molecules in butter and hard (stick) margarines are highly | Chapter 12: Reactions Of Alkenes Tear resistant Fundamental building blocks for organic transformations and polymer materials synthesis Polyethylene Addition Reactions Of Alkenes π Bond is unsaturated: Reacts by addition. C C + A B C C A B exothermic (usually) We can calculate ∆Hº of addition reaction from bond strength data, using DHºπ-bond = 65 kcal mol-1. Note, however: ∆Sº is negative (~ -30 .) Catalyst 1. Catalytic Hydrogenation C C + H H C C H H Typical catalysts are heterogeneous: insoluble solids Why catalysts? Enable a lower Ea mechanism. Alkenes + H2 no reaction without catalyst Pd/C, PtO2 ( Pt) H2 E Catalyzed pathway Uncatalyzed pathway Stereospecific H-H addition from the same side (syn) of double bond. + H2 cat. H H CH3 CH2CH3 cis trans cat. D2 + SS D D H H R R Stereochemistry + Mechanism Reversibility causes cis to trans isomerization Hydrogenation “Hardens” Oils, But Generates Trans Fatty Acids The fatty acid molecules in butter and hard (stick) margarines are highly saturated, whereas those in vegetable oils have a high proportion of cis-alkene functions (more than 90%). Partial hydrogenation of these oils yields soft (tub) margarine. Conditions of hydrogenation also isomerize cis double bonds to trans. Trans fatty acids are not metabolized in same way as cis counterparts: accumulate in cell membranes Ù increased risk of breast cancer and heart disease. FDA: phase out by 2018. Oleic acid is a cis unsaturated fatty acid making up 55–80% of olive oil Elaidic acid is the principal trans unsaturated fatty acid often found in partially hydrogenated vegetable oils. Stearic acid is a saturated fatty acid found in animal fats and is the intended product in full hydrogenation. 8/11/2015 © Univesity of California 7 On 7 November 2013, the FDA issued a preliminary determination that trans fats are not "generally recognized as safe", which was widely seen as a precursor to a reclassification of trans fats as a "food additive," meaning they could not be used

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