Organic Chemistry Sh.Javanshir Faculty of Chemistry Iran University of Science & Technology

Chapter 4. Alkenes: Structure and Reactivity Based on McMurry’s 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 درجه غير اشباعی 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

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21 Alkene Group Names

22 فصل چهارم : آلکن ها X Y Z 120 o sp 2 2p P-P  bond Sp 2 -Sp 2 Sigma bond

23 فصل چهارم : آلکن ها وجود پیوند دوگانه باعث محدودیت چرخش حول محور پیوند می شود. تبدیل نشدن ایزومر ها و قابلیت جداسازی آنها دیاستریومر ها یا ایزومر های هندسی trans -2-Butene cis -2-Butene == 180° rotation results in identical (superimposable) structures

24 مثالهای قطبیت  = 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

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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

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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

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34 Some examples

35 Practice problem:

36 Solution

37 Problem:

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39 فصل چهارم : آلکن ها 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