Alkenes. Structure, Nomenclature, and an introduc1on to Reac1vity Thermodynamics and Kine1cs

Transcription

1 Alkenes Structure, Nomenclature, and an introduc1on to Reac1vity Thermodynamics and Kine1cs 1

2 Alkene - Hydrocarbon With Carbon- Carbon Double Bond Also called an olefin but alkene is be>er Hydrocarbon that contains a C- C double bond Includes many naturally occurring materials Flavors, fragrances, vitamins 2

3 Industrial Prepara1on and Use of Alkenes Ethylene and propylene are important organic chemicals produced 3

4 Thermal Cracking Thermal cracking may take place w/o catalyst at very high temperatures 4

5 Calcula1ng Degree of Unsatura1on Relates molecular formula to possible structures Degree of unsatura0on: number of mul1ple bonds or rings Formula for a saturated acyclic compound is C n H 2n+2 Alkene has fewer hydrogens than an alkane with the same number of carbons C n H 2n because of double bond Each ring or mul1ple bond replaces 2 H's 5

6 Vinylic and Allylic Carbons vinylic carbon: the sp 2 carbon of an alkene allylic carbon: a carbon adjacent to a vinylic carbon 6

7 Name the parent hydrocarbon Naming Alkenes Number the carbons in chain so that double bond carbons have lowest possible numbers 7

8 Naming Alkenes Write the full name- Number subs1tuents according to: 1) Posi1on in chain, 2) Alphabe1cally Rings have cyclo prefix 8

9 Cis- Trans Isomerism in Alkenes Carbon atoms in a double bond are sp 2 - hybridized Three equivalent orbitals at 120º separa1on in plane Fourth orbital is atomic p orbital Combina1on of electrons in two sp 2 orbitals of two atoms forms σ bond between them Addi1ve interac1on of p orbitals creates a π bonding orbital Subtrac1ve interac1on creates a π an1- bonding orbital Occupied π orbital prevents rota1on about σ- bond Rota1on prevented by π bond - high barrier, about 268 kj/ mole in ethylene 9

10 Cis- Trans Isomerism in Alkenes Rota1on of π bond is prohibi1ve This prevents rota1on about a carbon- carbon double bond (unlike a carbon- carbon single bond). Creates possible alterna1ve structures 10

11 Cis- Trans Isomerism in Alkenes 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 11

12 Cis- Trans Isomerism in Alkenes Cis- Trans Isomeriza1on requires that end groups differ in pairs Bo>om pair cannot be superposed without breaking C=C 12

13 Alkene Stereochemistry and the E,Z Designa1on Cis- Trans naming system discussed thus far only works with disubs+tuted alkenes Tri, Tetra- subs1tuted double bonds, and conjugated systems require more general method, referred to as the E,Z system. 13

14 Alkene Stereochemistry and the E,Z Designa1on (Con1nued): E,Z Stereochemical Nomenclature Priority rules 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 14

15 RULE 1 Alkene Stereochemistry and the E,Z Designa1on Cahn- Ingold- Prelog Rules Must rank atoms that are connected at comparison point Higher atomic number gets higher priority Br > Cl > S > P > O > N > C > H 15

16 Alkene Stereochemistry and the E,Z Designa1on Cahn- Ingold- Prelog Rules RULE 2 If atomic numbers are the same, compare at next connec1on point at same distance Compare un1l something has higher atomic number Do not combine always compare 16

17 Alkene Stereochemistry and the E,Z Designa1on Cahn- Ingold- Prelog Rules RULE 3 Mul1ple- bonded atoms are equivalent to the same number of single- bonded atoms Subs1tuent is drawn with connec1ons shown and no double or triple bonds Added atoms are valued with 0 ligands themselves 17

18 Isomers for Compounds with Two Double Bonds 18

19 Stability of Alkenes Cis alkenes are less stable than trans alkenes Compare heat given off on hydrogena1on: ΔH o Less stable isomer is higher in energy And gives off more heat tetrasubs1tuted > trisubs1tuted > disubs1tuted > monosusb1tuted hyperconjuga:on stabilizes 19

20 Rela1ve Stabili1es of Alkenes Evaluate heat given off when C=C is converted to C- C More stable alkene gives off less heat trans- Butene generates 5 kj less heat than cis- butene 20

21 Stability of Alkenes- Hyperconjuga1on Electrons in neighboring filled σ orbital stabilize vacant an1bonding π orbital net posi1ve interac1on Alkyl groups are be>er than H 21

22 Each Family Can Be Put in One of Four Groups The families in a group react in similar ways. 22

23 Electrophiles An electrophile has a posi1ve charge, a par1al posi1ve charge, or an incomplete octet. 23

24 Nucleophiles A nucleophile has a nega1ve charge or a lone pair. 24

25 Curved Arrows first step of the reac1on Curved arrows are used to show the mechanism of a reac1on. The mechanism of a reac1on is the step- by- step descrip1on of the process by which reactants are converted into products. 25

26 Curved Arrows second step of the reac1on The curved arrow shows where the electrons start from and where they end up. 26

27 An Electrophilic Addi1on Reac1on Transi1on states have par1ally formed bonds. 27

28 Reac1on Coordinate Diagram for Each Step of the Addi1on of HBr to 2- Butene 28

29 Reac1on Coordinate Diagram for the Addi1on of HBr to 2- Butene The rate- limi1ng step of the reac1on is the step that has its transi1on state at the highest point of the reac1on coordinate diagram. 29

30 Electrophilic Addi1on of Alkenes The reac1on is successful with HCl and with HI as well as HBr HI is generated from KI and phosphoric acid 30

31 Orienta1on of Electrophilic Addi1ons: 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 orienta1on predominates, the reac1on is regioselec0ve Markovnikov observed in the 19 th century that in the addi1on of HX to alkene, the H a>aches to the carbon with more H s and X a>aches to the other end (to the one with more alkyl subs1tuents) This is Markovnikov s rule 31

32 Example of Markovnikov s Rule Addi1on of HCl to 2- methylpropene Regiospecific one product forms where two are possible If both ends have similar subs1tu1on, then not regiospecific 32

33 Markovnikov s Rule More highly subs1tuted carboca1on forms as intermediate rather than less highly subs1tuted one Ter1ary ca1ons and associated transi1on states are more stable than primary ca1ons 33

34 Markovnikov s Rule 34

35 Carboca1on Structure and Stability Carboca1ons 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 carboca1on (measured by energy needed to form it from R- X) is increased by the presence of alkyl subs1tuents 35

36 Carboca1on Stability A induc1ve stabilized ca1on species 36

37 Carboca1on Structure and Stability A plot of ΔH dissocia1on shows that more highly subs1tued alkyl halides dissociate more easily than less highly subs1tuted ones 37

38 The Hammond Postulate If a carboca1on intermediate is more stable than another, why is the reac1on through the more stable one faster? the rela1ve stability of the intermediate is related to an equilibrium constant (ΔGº) the rela1ve stability of the transi1on state (which describes the size of the rate constant) is the ac1va1on energy (ΔG ) the transi1on state is transient and cannot be examined What does the Hammond Postulate state? the structure of a transi1on state resembles the structure of the nearest stable species. Transi1on states for endergonic steps structurally resemble products, and transi1on states for exergonic steps structurally resemble reactants 38

39 The Hammond Postulate (Con1nued): Transi1on State Structures A transi0on state is the highest energy species in a reac1on step By defini1on, its structure is not stable enough to exist for one vibra1on But the structure controls the rate of reac1on So we need to be able to guess about its proper1es in an informed way We classify them in general ways and look for trends in reac1vity the conclusions are in the Hammond Postulate 39

40 Examina1on of the Hammond A transi1on state should be similar to an intermediate that is close in energy Sequen1al states on a reac1on path that are close in energy are likely to be close in structure - G. S. Hammond Postulate 40

41 Compe1ng Reac1ons and the Hammond Postulate Normal Expecta1on: Faster reac1on gives more stable intermediate Intermediate resembles transi1on state 41

42 Evidence for the Mechanism of Electrophilic Addi1on: Carboca1on Rearrangments Carboca1ons undergo structural rearrangements following set pa>erns 1,2- H and 1,2- alkyl shits occur Goes to give moststable carboca1on Can go through less stable ions as intermediates 42

43 Hydride shits in biological molecules 43