Organic Chemistry Sh.Javanshir Faculty of Chemistry Iran University of Science & Technology Chapter 4. Alkenes: Structure and Reactivity Based on McMurrys Organic Chemistry, 6 th edition 3 Alkene - Hydrocarbon With Carbon- Carbon Double Bond Includes many naturally occurring materials Flavors, fragrances, vitamins Important industrial products These are feedstocks for industrial processes 4 Lycopene molecular structure Ethene is the hormone that causes tomatoes to ripen. Lycopene is a bright red carotene and carotenoid pigment and phytochemical found in tomatoes and other red fruits & vegetables, such as red carrots, watermelons and papayas (but not strawberries or cherries).carotenecarotenoid phytochemicaltomatoescarrotswatermelonspapayas 5 Alkenes 6 Degree of Unsaturation Relates molecular formula to possible structures Degree of unsaturation: number of multiple bonds or rings Formula for saturated a acyclic compound is C n H 2n+2 Each ring or multiple bond replaces 2 H's 7 Degree of Unsaturation 8 Example: C 6 H 10 Saturated is C 6 H 14 Therefore 4 H's are missing This has two egrees of unsaturation Two double bonds? or triple bond? or two rings or ring and double bond 9 Degree of Unsaturation With Other Elements Organohalogens (X: F, Cl, Br, I), Halogen replaces hydrogen C 4 H 6 Br 2 and C 4 H 8 have one degree of unsaturation Oxygen atoms: these don't affect the total count of H's 10 Degree of Unsaturation With Other Elements 11 Compounds with the same degree of unsaturation can have many things in common and still be very different Degree of Unsaturation and Variation 12 If C-N Bonds Are Present Nitrogen has three bonds So if it connects where H was, it adds a connection point, and there an extra H Subtract one H for equivalent degree of unsaturation in hydrocarbon 13 Count pairs of H's below C n H 2n+2 Add number of halogens to number of H's (X equivalent to H) Don't count oxygens (oxygen links H) Subtract NH's DU=X-(Y/2-Z/2)+1 X=nbr of C, y=nbr of H, Z=nbr of N Summary - Degree of Unsaturation 14 Naming of Alkenes Find longest continuous carbon chain for root Number carbons in chain so that double bond carbons have lowest possible numbers Rings have cyclo prefix 15 Alkene Nomenclature: longest chain must contain the C=C 16 Alkene Nomenclature 17 Cycloalkene nomenclature 18 Problem: nomenclature these alkenes 19 Problem: nomenclature these alkenes 20 21 Alkene Group Names 22 : X Y Z 120 o sp 2 2p P-P bond Sp 2 -Sp 2 Sigma bond 23 : . trans -2-Butene cis -2-Butene == 180 rotation results in identical (superimposable) structures 24 = 0.33 D = 0 25 Rotation of Bond Is Energetically Costly This prevents rotation about a carbon-carbon double bond (unlike a carbon-carbon single bond). Creates possible geometric isomers (cis/trans) 26 E, Z Stereochemical Nomenclature Priority rules of Cahn, Ingold, and Prelog Compare where higher priority group is with respect to bond and designate as prefix E -entgegen, opposite sides Z - zusammen, together on the same side 27 28 Ranking Priorities: Cahn-Ingold-Prelog Rules Must rank atoms that are connected at comparison point Higher atomic number gets higher priority Br > Cl > O > N > C > H In this case,The higher priority groups are opposite: (E )-1-bromo-1-chloro-propene 29 2-chloro-2-butenes 30 If atomic numbers are the same, compare at next connection point at same distance Compare until something has higher atomic number Do not combine always compare Extended Comparison 31 32 Dealing With Multiple Bonds Substituent is drawn with connections shown and no double or triple bonds Added atoms are valued with no ligands themselves 33 34 Some examples 35 Practice problem: 36 Solution 37 Problem: 38 39 : trans -2-Butene cis -2-Butene H+H+ 76% 24% E Butane Cis Trans G c G t Ht=27.6 kcal/mol Hc=28.6 kcal/mol 40 Reaction of alkenes: Electrophilic Addition of HX to Alkenes General reaction mechanism: electrophilic addition Attack of electrophile (such as HBr) on bond of alkene produces carbocation and bromide ion Carbocation is itself an electrophile, reacting with nucleophilic bromide ion 41 Writing Organic Reactions