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Chapter 7: Structure and Synthesis of Alkenes Ú Introduction (7-1) Ú Alkenes are hydrocarbons with C=C in their structure. They are also known as “olefins”. Ú We have already covered the basics of alkenes nomenclature, but here are a few important groups with common names that you can use as well. 313 Orbital Description of the Alkene Double Bond (7-2) Ú We already covered this in the first 2 chapters. The carbon of the alkene group are sp2 hybridized, leaving a p orbital available to form a π bond Ú Cis/trans isomerism conversion is not possible 314 Element of Unsaturation (7-3) Ú Alkenes are said to be “unsaturated” because they react with hydrogen to give the corresponding alkane (a saturated compound). Alkenes are said to have one element of unsaturation. 315 Ú The element of unsaturation of a compound can be calculated using the following formula: Index = # C - # H - # halogen + # N +1 2 2 2 316 Practice Questions Ú Question: What is the unsaturation index for: C15H13ClO 317 Ú How many element of unsaturation are present in podophyllotoxin? 318 Nomenclature of cis-trans Isomers (7-5) Ú We have seen before that cis and trans alkenes were possible. Those were called geometric isomers. Ú However, sometimes this notation cannot produce an unambiguous name. 319 Ú In these cases where the substituents are all different, use the E/Z system. This system can be used all the time, replacing the cis/trans notation. – E: entgagen (opposite) – Z: zusammen (together) This system follows the same principles studied in Chapter 5 for the Cahn-Ingold-Prelog notation of chiral carbons. 320 Ú Procedure: A) assign the substituent a priority for each carbon of the double bond as: #1 and #2. – #1 is the highest priority and is determined by the atomic number (or molecular weight) of the atom directly attached to the C=C carbon B) if a priority cannot be determined using the atom directly attached to the C of C=C, go to the next atom until a priority can be determined. 321 Ú Looking at both carbons, determine if the substituents labeled (1) on each carbon of the double bond are on the same side or opposite sides of the double bond – If both (1) are on the same side: notation is Z – If both (1) are on opposite sides: notation is E 322 Practice Questions Ú Draw and label the E and Z isomers for each of the following: 323 Ú What are the configurations of the double bonds in the following compounds? 324 Ú Draw the structure of (Z)-3-isopropyl-2- heptene. 325 Ú Stability of Alkenes (7-7) The stability of alkenes can be summarized by the Zaitsev’s Rule: – More highly substituted double bonds are usually more stable – In a reaction: chemical reactions producing alkenes will tend to produce the most substituted C=C 326 327 Ú Practice Question Classify the following alkenes in order of increasing stability. A B C D 328 Ú Cycloalkenes stability is related to the ring strain involved: C5 or larger are stable C3 and C4 are less stable Small rings (C4 to C7) are usually “cis” (more stable) 329 Ú For larger ring (C8 and larger) it is possible to have “trans” geometry H H H H cis trans Trans cyclodecene is almost as stable as the cis isomer 330 Physical Properties (7-8) Bp and Mp: increases with molecular weights. But branching tends to decrease it (similar to alkanes). – Relatively non-polar 331 Synthesis of Alkenes by Eliminations of Alkyl Halides (7-9) Dehydrohalogenation Is the elimination of a hydrogen an halogen from an alkyl halide: Mechanism: E2 Base: abstract a proton from the carbon adjacent to C-X (needs strong base, HO-, CH3O-) X-: leaves at the same time 332 Ø Must have an anti-coplanar arrangement in order for the reaction to take place Ø Mechanism: concerted (bonds are formed and broken at the same time) X H C - C B H X 333 Works best with 3o halides and hindered secondary halides. C Br NaOH If other types of halides (unhindered 2o and 1o) are used, it is best to use a very bulky base. This helps in avoiding substitution reactions. 334 Ú Example: any of the bases from previous slide would give this product Br Et3N Difficulty: Ø With bulky bases and sterically hindered alkyl halides, it is not unusual to get the Hoffman product instead of the Zaitsev’s product. Zaitsev’s product: most substituted C=C Hoffman product: least substituted C=C 335 Ú Because: Abstraction of the most hindered proton is not possible 336 Ú Since the reaction is concerted, and an anticoplanar arrangement of the proton and leaving group is necessary, there are stereochemical consequences to the reaction. E 337 Elimination from Substituted Cyclohexanes Ú E2 elimination can take place from either the most stable or least stable chair conformation of the molecule. The requirement to follow is an anticoplanar arrangement of H and X in axial positions. 338 339 Ú Therefore, the most substituted alkene may not be produced in these reactions, and the most stable chair conformation may not be the one leading to the product. this H cannot participate in the elimination since it is equatorial Cl H Cl more stable H Base E2 conditions 340 Practice Question Ú What it the major organic product of the E2 elimination of trans-1-chloro-2-methylcyclohexane. 341 Dehalogenation of vicinal dibromides Usually performed by reduction with iodide ion or Zn/acetic acid. - I Br acetone Br Zn Br + ZnBr2 Br Acetic acid is not involved, it is there simply to clean the surface of the metal. 342 Ú Need anticoplanar arrangement for the 2 bromines…or the molecule must be able to reach this conformation for the reaction to proceed. Br H Ph NaI Ph Ph C C acetone H Br H H Ph Ú What is the product of this reaction? H Ph Br Ph Br NaI H 343 Practice Question Ú What is the product of this reaction? Br NaI Br 344 Alkene Synthesis by Dehydration of Alcohols (7-10) Dehydration simply means: removal of water Mechanism is E1 (elimination 1) and will produce a carbocation… therefore, rearrangement to the most stable carbocation is possible. 345 Mechanism of Dehydration of Alcohols 346 Practice Questions Ú Complete the following reactions carried out under acidic conditions and heat. H2SO4 OH OH H2SO4 OH H3PO4 347 Ú For the last reaction from the last question (giving the blue product), show how the reaction takes place by drawing a detailed mechanism. 348 Study Problems 7-33 Label each structure as Z, E or neither. 349 7-44 Predict the major product of the acidcatalyzed dehydration of the following alcohols. 2-pentanol 1-methylcyclopentanol 2,2-dimethyl-1-propanol 350 7-45 Propose a detailed mechanism for the following reaction. Br CH3OH heat O 351 7-45 Propose a mechanism for the following reaction. 352