Alkenes: Structure and Reactivity

Size: px

Start display at page:

Download "Alkenes: Structure and Reactivity"

Ilene Francis
6 years ago
Views:

1 6 Alkenes: Structure and Reactivity آلکن ها: ساختار و واکنش پذیری Dr Morteza Mehrdad University of Guilan, Department of Chemistry, Rasht, Iran Based on McMurry s Organic Chemistry, 7 th edition

naturally occurring materials Flavors, fragrances,

2 2 Alkene - Hydrocarbon With Carbon-Carbon Double Bond Also called an olefin but alkene is better Includes many naturally occurring materials Flavors, fragrances, vitamins طعم دهنده ها خوشبوکننده ها ویتامین ها سقز از درخت ب ن ه

3 3 Why this Chapter? C=C double bonds are present in most organic and biological molecules To examine consequences of alkene stereoisomerism To focus on general alkene reaction: electrophilic addition

4 4 6.1 Industrial Preparation and Use of Alkenes Ethylene and propylene are the most important organic chemicals produced Made by thermal cracking of light alkanes (petroleum)

5 5 6.2 Calculating Degree of Unsaturation محاسبه درجه سیرنشدگی Relates molecular formula to possible structures Degree of unsaturation سیرنشدگی :درجه number of multiple bonds or rings Formula for a saturated acyclic ) خطی )سیرشده compound is C n H 2n+2 Each ring or multiple bond replaces 2 H's سیرشده سیرنشده

6 Example: C 6 H 10 MW = 6 x 12 + 10 x 1= 82 Saturated is C 6 H 14 Therefore 4 H's are not present This has two

6 6 Example: C 6 H 10 MW = 6 x x 1= 82 Saturated is C 6 H 14 Therefore 4 H's are not present This has two degrees of unsaturation Two double bonds? 2 = or two rings? درجه سیرنشدگی or ring and double bond? or triple bond?

7 7 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 Organoxygen compounds (C,H,O) - if connected by single bonds These don't affect the total count of H's

8 8 Organonitrogen compounds Nitrogen has three bonds So if it connects where H was, it adds a connection point Subtract one H for equivalent degree of unsaturation in hydrocarbon

9 Summary - Degree of Unsaturation Method 1 Add the number of halogens to the number of hydrogens. Ignore the number of oxygens. Subtract the number of nitrogens from the number of hydrogens. Method 2 Method 3 H sat 2C 2 X N (ignore O,S) Degree of Unsaturation H sat - H 2 act = درجه سیرنشدگی )2- ظرفیت( تعداد اتم

10 :مثال درجه سیرنشدگی = C 20 H 32 ClN = 2 )2- ظرفیت( تعداد اتم + 2 2 + 20(4-2) + 32(1-2) +1(1-2)+1(3-2) 2 Diazepam, marketed as an antianxiety medication under the name Valium, has three rings,

10 10 :مثال درجه سیرنشدگی = C 20 H 32 ClN = 2 )2- ظرفیت( تعداد اتم (4-2) + 32(1-2) +1(1-2)+1(3-2) 2 Diazepam, marketed as an antianxiety medication under the name Valium, has three rings, eight double bonds, and the formula C 16 H? ClN 2 O. How many hydrogens does diazepam have? (Calculate the answer; don t count hydrogens in the structure.) = 5 No.H + No.X = = 33 - No.N = 33-1 = 32 C n H 2n+2 = C 20 H = / 2 = 5 = 2+16(4-2) +X(1-2)+1(1-2)+2(3-2)+1(2-2) = 11 2 = -X+35 = 11 2 X= 13 C 16 H 13 ClN 2 O

11 6.3 Naming of Alkenes Name the parent hydrocarbon change ending to ene Number carbons in chain so that double bond carbons have lowest possible numbers Alkenes higher priority than alkanes: even shorter chain Multiple C=C are named as diene triene tetraene etc Rings have cyclo prefix double bond always C#1, C#2 11

12 12 Many Alkenes Are Known by Common Names

6.4 Cis-Trans Isomerism in Alkenes Carbon atoms in a double bond are sp 2 -hybridized Three equivalent orbitals at 120º separation in plane Fourth orbital is atomic p orbital Combination of electrons

13 6.4 Cis-Trans Isomerism in Alkenes Carbon atoms in a double bond are sp 2 -hybridized Three equivalent orbitals at 120º separation in plane Fourth orbital is atomic p orbital Combination of electrons in two sp 2 orbitals of two atoms forms bond between them Additive interaction of p orbitals creates a bonding orbital Occupied orbital prevents rotation about -bond Rotation prevented by bond - high barrier, about 268 kj/mole in ethylene 13

14 14 Rotation of Bond Is Prohibitive This prevents rotation about a carbon-carbon double bond (unlike a carbon-carbon single bond). Creates possible alternative structures

15 15 The presence of a carbon-carbon double bond can create two possible structures cis isomer - two similar groups on same side of the double bond trans isomer - similar groups on opposite sides Each carbon must have two different groups for these isomers to occur

16 16 Cis, Trans Isomers Require That End Groups Must Differ in Pairs 180 rotation superposes Bottom pair cannot be superposed without breaking C=C

17 Sequence Rules: The E,Z Designation قواعد ترتیب تقدم : سیستم E, Z نامگذاری Neither compound is clearly cis or trans Substituents on C1 are different than those on C2 We need to define similarity in a precise way to distinguish the two stereoisomers Cis, trans nomenclature only works for disubstituted double bonds E/Z Nomenclature for 3 or 4 substituents

18 E,Z Stereochemical Nomenclature Priority rules )قواعد ترتیب تقدم( of Cahn, Ingold, and Prelog Compare where higher priority groups are with respect to bond and designate as prefix E -entgegen, opposite sides Z - zusammen, together on the same side Hi(C1)-Low(C1)-Hi(C2)-Lo(C2) 18

20 Ranking Priorities: Cahn-Ingold-Prelog Rules RULE 1 Must rank atoms that are connected at comparison point Higher atomic number gets higher priority Br > Cl > S > P > O > N > C > H

21 21 Extended Comparison RULE 2 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

22 22 Dealing With Multiple Bonds: RULE 3 Substituent is drawn with connections shown and no double or triple bonds

24 Stability of Alkenes پایداری آلکن ها Cis alkenes are less stable than trans alkenes Compare heat given off on hydrogenation: H o Less stable isomer is higher in energy And gives off more heat tetrasubstituted > trisubstituted > disubstituted > monosusbtituted hyperconjugation ) شدن موقت )مزدوج stabilizes ازدحام فضایی 2/8 ناپایدارتر از kj/mol

25 25 Comparing Stabilities of Alkenes Evaluate heat given off when C=C is converted to C-C More stable alkene gives off less heat ( G o cis > G o trans) 2/8 kj/mol

26 trans-butene generates 4 kj less heat than cis-butene CH 3 CH 2 CH 2 CH 3 but the numbers aren t exactly the same 4 kj/mol (energy difference) 2.8 kj/mole (quilibrium data) for two reasons: 1. There is probably some experimental error, since heats of hydrogenation are difficult to measure accurately. 2. Heats of reaction and equilibrium constants don t measure exactly the same thing. Heats of reaction measure H, equilibrium constants measure G a (slight difference between the two) 26

stability order: This order of stability is due to:

27 Alkenes become more stable with increasing substitution: As a general rule, alkenes follow the stability order: This order of stability is due to: )مزدوج شدن موقت( Hyperconjugation )قدرت پیوند( bond strength

positive interaction برهم کنش پایدارکننده ای بین اوربیتال

28 28 )مزدوج شدن موقت( Hyperconjugation Electrons in neighboring filled orbital stabilize vacant antibonding orbital net positive interaction برهم کنش پایدارکننده ای بین اوربیتال ضدپیوندی پرنشده C=C شده پیوند C-H برروی استخالف های مجاور و اوربیتال پر

29 Alkyl groups are better than H 29

30 30 Hyperconjugation involves a sigma orbital component, usually a C-C or C-H bond. Resonance involves pi orbitals Alternative Explanation: )قدرت پیوند( bond strength sp 3 sp 2 bond > sp 3 sp 3 bond

31 6.7 Electrophilic Addition of Alkenes General reaction mechanism: electrophilic addition Attack of electrophile (such as HBr) on bond of alkene Produces carbocation and bromide ion Carbocation is an electrophile, reacting with nucleophilic bromide ion 31

32 32 Electrophilic Addition Energy Path Two step process First transition state is high energy point

33 Electrophilic Addition for preparations The reaction is successful with HCl and with HI as well as HBr HI is generated from KI and phosphoric acid 33

34 6.8 Orientation of Electrophilic Addition: قاعده مارکونیکوف Markovnikov s Rule In an unsymmetrical alkene, HX reagents can add in two different ways, but one way may be preferred over the other If one orientation predominates, the reaction is regiospecific )جهت ویژه( Markovnikov observed in the 19 th century that in the addition of HX to alkene, the H attaches to the carbon with the most H s and X attaches to the other end (to the one with the most alkyl substituents) This is Markovnikov s rule. 34

35 Example of Markovnikov s Rule Addition of HCl to 2-methylpropene Regiospecific one product forms where two are possible جهت ویژه واکنشی که افزایش فقط در یکی از دوجهت ممکن صورت می گیرد If both ends have similar substitution, then not regiospecific 35

36 Markovnikov s Rule )به بیان دیگر( (restated) More highly substituted carbocation forms as intermediate rather than less highly substituted one Tertiary cations and associated transition states are more stable than primary cations 36

37 6.9 Carbocation Structure and Stability Carbocations are planar and the tricoordinate carbon is surrounded by only 6 electrons in sp 2 orbitals The fourth orbital on carbon is a vacant

37 Carbocation Structure and Stability Carbocations are planar and the tricoordinate carbon is surrounded by only 6 electrons in sp 2 orbitals The fourth orbital on carbon is a vacant p-orbital The stability of the carbocation (measured by energy needed to form it from R-X) is increased by the presence of alkyl substituents (Hyperconjugation stabilizes C+) اوربیتال p خالی از الکترون

The stability of carbocation (R + ) can be determined by measuring energy needed to form it from R-X: تعیین پایداری کربوکاتیون می تواند توسط اندازه گیری انرژی الزم

38 The stability of carbocation (R + ) can be determined by measuring energy needed to form it from R-X: تعیین پایداری کربوکاتیون می تواند توسط اندازه گیری انرژی الزم برای تشکیل آن از R-X بدست آید R-X R + + :X - 3º alkyl halides dissociate to give R + more easily than 2º and 1º تفکیک هالید نوع سوم بسیار سریع تر نوع اول و دوم است

آبی( کاسته می شود Hyperconjugation )مزدوج شدن موقت( هرچه تعداد گروههای آلکیل واقع بر روی

39 Inductive stabilization of پایداری القایی cation species 39 هرچه تعداد گروههای آلکیل بیشتری به کربن مثبت متصل باشد چگالی الکترونی بیشتر و میزان کم الکترونی کربن مثبت ( رنگ آبی( کاسته می شود Hyperconjugation )مزدوج شدن موقت( هرچه تعداد گروههای آلکیل واقع بر روی کربوکاتیون بیشتر باشد امکان بیشتری برای مزدوج شدن موقت و ایجاد کربوکاتیونی پایدارتر وجود خواهد داشت.

40 The Hammond Postulate پذیره هاموند If carbocation intermediate is more stable than another, why is the reaction through the more stable one faster? چرا پایداری یک کربوکاتیون بر سرعت تشکیل آن اثر گذاشته و ساختار محصول نهایی را تعیین می کند The relative stability of the intermediate is related to an equilibrium constant ( Gº) پایداری نسبی حدواسط کربوکاتیون به وسیله ( Gº) تعیین می شود The relative stability of the transition state (which describes the size of the rate constant) is the activation energy ( G ) اما سرعت واکنش به وسیله انرژی فعال سازی ) ( G بدست می آید The transition state is transient and cannot be examined حاالت گذار حاالت گذرایی هستند و به دلیل عمر بسیار کوتاه خود قابل بررسی نیستند

41 Transition State Structures A transition state is the highest energy species in a reaction step یک حالت گذار عبارت از پرانرژی ترین جزء در یک مرحله واکنش است By definition, its structure is not

41 41 Transition State Structures A transition state is the highest energy species in a reaction step یک حالت گذار عبارت از پرانرژی ترین جزء در یک مرحله واکنش است By definition, its structure is not stable enough to exist for one vibration برطبق تعریف ساختار آنها به اندازه کافی پایدار نیست که قابل مشاهده باشند But the structure controls the rate of reaction اما ساختار آنها سرعت واکنش را کنترل می نمایند So we need to be able to guess about its properties in an informed way We classify them in general ways and look for trends in reactivity the conclusions are in the Hammond Postulate The structure of the transition state resembles the structure of the nearest stable species. T.S. for Endergonic steps resemble products. T.S. for Exergonic steps resemble reactants. ساختار یک حالت گذار به ساختار نزدیک ترین گونه شیمیایی پایدار شباهت دارد T.S. مراحل انرژی خواه از لحاظ ساختاری به فرآورده ها و T.S. مراحل انرژی زا از لحاظ ساختاری به واکنش دهندگان شباهت دارد

quential states on a reaction path that are close in energy are likely to be close in structure - G. S.

42 Examination of the Hammond Postulate A transition state should be similar to an intermediate that is close in energy Sequential states on a reaction path that are close in energy are likely to be close in structure - G. S. Hammond واکنش آهسته تر منجر به تولید حدواسط پایدارتری می شود در مرحله انرژی زا ترازهای انرژی حالت گذار و واکنش دهنده مشابه هستند واکنش سریع تر حدواسط پایدارتری تولید می کند در مرحله انرژی خواه ترازهای انرژی حالت گذار و فرآورده مشابه هستند 42

state است تشکیل یک کربوکاتیون با پروتون دار شدن یک آلکن مرحله ای انرژی خواه است.

43 43 Competing Reactions and the Hammond Postulate Normal Expectation: Faster reaction gives stable intermediate حدواسط شبیه به حالت گذار Intermediate resembles transition state است تشکیل یک کربوکاتیون با پروتون دار شدن یک آلکن مرحله ای انرژی خواه است. بنابراین حالت گذار پروتون دار شدن آلکن از لحاظ ساختاری باید به حدواسط کربوکاتیونی شباهت داشته باشد

44 Mechanism of Electrophilic Addition: Rearrangements of Carbocations Carbocations undergo structural rearrangements following set patterns 1,2-H and 1,2-alkyl shifts occur Goes to give more stable carbocation Can go through less stable ions as intermediates

45 Alkyl Shifts Can Also Occur 45

Also called an olefin but alkene is better General formula C n H 2n (if one alkene present) Can act as weak nucleophiles

Alkenes - Structure, Stability, Nomenclature Also called an olefin but alkene is better General formula C n H 2n (if one alkene present) unsaturated - contain fewer than maximum H's possible per C Can