Prof. Jayaraman Sivaguru. Prof. Jayaraman Sivaguru

Transcription

1 hapter 9 Alkynes 9-1 Naturally Occurring Alkynes Some alkynes occur naturally. For example, O 3 ( 2 ) 10 ( 2 ) 4 O istrionicotoxin: defensive toxin in the poison dart frogs of entral and South America Tariric acid: occurs in seed of a Guatemalan plant. 9-2

2 Naming Alkynes Include the triple bond in the parent chain; replace ane in the parent alkane name with yne Number the parent chain such that the first multiple bond has the lowest number possible In the event of a tie between a double and a triple bond, the double bond wins The group is called an ethynyl substituent 9-3 Industrial preparation of acetylene is by dehydrogenation of ethylene Acetylene Acetylene and ethyne are both acceptable IUPA names for

3 ycloalkynes yclononyne is the smallest cycloalkyne stable enough to be stored at room temperature for a reasonable length of time. yclooctyne polymerizes on standing. 9-5 Physical Properties of Alkynes Physical properties of alkynes are similar to those of alkanes and alkenes of comparable molecular weight Low boiling points Low density Low solubility in water (lacking many other polar functional groups) 9-6

4 Geometry and Bond Lengths The bond is shorter and stronger than comparable = bonds and R bonds are also shorter Like alkenes, alkynes have significant electron density between their carbons. Nucleophilicity: Like alkenes, alkynes are nucleophilic and undergo electrophilic addition Alkynes are linear and the triply bound carbons have sp hybridization 9-7 Mix together (hybridize) the 2s orbital and one of the three 2p orbitals. 2p 2s Each carbon is connected to a hydrogen by a σ bond. The two carbons are connected to each other by a σ bond and two π bonds. sp ybridization in Acetylene 2p 2sp One of the two π bonds in acetylene is shown here. The second π bond is at right angles to the first. This is the second of the two π bonds in acetylene. 9-8

5 omparing Alkynes to Other ydrocarbons Pr a J. f o r a y an am u r u g a v i S Note the remarkable acidity of acetylene!, North Dakota State University M341: Organic hemistry (chapter 9) 9-9 Alkynes are Relatively Acidic Relative to alkanes and alkenes, terminal alkynes are remarkably acidic a r a y an am u r u g a v Si Deprotonation of R leaves a lone pair in a hybrid orbital with high s character. The high s character means that the lone pair is close to the nucleus and thus stabilized Pr.J f o, North Dakota State University M341: Organic hemistry (chapter 9) 9-10

6 arbon: ybridization and Electronegativity pk a = : sp 3 pk a = 45 : + + sp 2 pk a = : sp Electrons in an orbital with more s character are closer to the nucleus and more strongly held Bases to Deprotonate R Alkynes are acidic relative to alkanes and alkenes, but still require fairly strong bases to be significantly deprotonated Water is not basic enough and neither is hydroxide! 9-12

7 Bases to Deprotonate R Alkynes are acidic relative to alkanes and alkenes, but still require fairly strong bases to be significantly deprotonated Amide ion (N 2 ) is usually the base of choice arbanions of alkanes and alkenes also work well 9-13 Alkylation of Terminal Alkynes An alkylation reaction replaces an or other functional group with an alkyl group Terminal alkynes can be alkylated via a nucleophilic acetylide anion, the conjugate base of the alkyne The strongly nucleophilic acetylide reacts with an alkyl halide via S N 2! 9-14

8 Alkylation of Terminal Alkynes Acetylene can be subjected to two distinct alkylations or a terminal alkyne can be alkylated once Note: the scope of the electrophile in S N 2 still applies! It must be a methyl or primary alkyl (psuedo)halide! The Brønsted basic acetylide anion promotes elimination in secondary and tertiary alkyl halides 9-15 Example: Alkylation of Acetylene NaN 2 Na N Br (70-77%) 9-16

9 ( 3 ) 2 2 ( 3 ) 2 2 ( 3 ) 2 2 NaN 2, N 3 3 Br Na (81%) NaN 2, N Br 1. NaN 2, N Br (81%) 9-17 Forming Alkynes via Elimination Alkylation is a carbon-carbon bond forming reaction, but we can also form alkynes using functional group transformations Dehydrogenation is one example Elimination of a dihalide (two eliminations to make two π bonds) is another Two different types of halides may be used: geminal or vicinal These methods are typically applied only to make terminal alkynes; double eliminations of internal dihalides have regioselectivity issues. 9-18

10 Double Elimination of Geminal Dihalides In a geminal dihalide, the halogen substituents have a 1,1- relationship. Both halogens are bound to the terminal carbon to prevent regioselectivity issues Three equivalents of base are necessary because of rapid and irreversible deprotonation of the terminal alkyne product Water returns a proton to the acetylide in workup O Geminal dihalide Alkyne ( 3 ) 3 2 l 2 ( 3 ) 3 ( 3 ) 3 ( 3 ) 3 l Na NaN 2, N 3 NaN 2, N 3 NaN 2, N 3 (slow) (slow) (fast) 9-20

11 Double Elimination of Vicinal Dihalides In a vicinal dihalide, the halogen substituents have a 1,2-relationship. One halogen is bound to the end carbon; the other is bound to the adjacent carbon Three equivalents of base are again necessary Nice route from alkenes to alkynes if dehydrogenation is not possible (or too expensive ) 9-21 Vicinal dihalide Alkyne 3 ( 2 ) 7 2 Br Br 3 ( 2 ) NaN 2, N O (54%) 9-22

12 Reactions of Alkynes Reactions of alkynes fall into two broad categories Reactions of terminal alkynes via acetylide anions Alkylation (discussed during synthesis of alkynes) Other S N 2-like processes (discussed during synthesis of alkynes) Elimination (discussed during synthesis of alkynes) Nucleophilic addition to polarized π bonds (later) Additions to one or both of the π bonds ydrogenation Electrophilic addition of X Electrophilic halogenation Ozonolysis Nucleophilic addition (later) 9-23 ydrogenation of Alkynes eats of ydrogenation kj/mol 275 kj/mol Alkyl groups stabilize triple bonds in the same way that they stabilize double bonds. Internal triple bonds are more stable than terminal ones. Alkene is an intermediate. 9-24

13 ydrogenation of Alkynes ydrogenation of an alkyne in the presence of Pt or another metal proceeds all the way to the alkane ydrogens add in a syn manner, just like reactions of alkenes (the second hydrogenation really just is hydrogenation of an alkene!) an we stop hydrogenation at the alkene stage? 9-25 Lindlar ydrogenation Using a poisoned or slowed catalyst mixture called the Lindlar catalyst, selective hydrogenation to the cis alkene is possible Lindlar hydrogenation is very attractive because it avoids the regioselectivity and stereoselectivity issues of elimination-based methods (e.g., dehydration). 9-26

14 ydrohalogenation of Alkynes X adds to alkynes to form alkenyl halides - Addition follows Markovnikov s rule Kinetic studies point to a concerted Ad E 3 addition of and X! rate = k[rr][x] Double Dehydrohalogenation The reaction can be stopped at the alkenyl halide stage, or a second equivalent of X can be used to give a geminal dihalide an this dihalide be re-eliminated to form the starting alkyne? No! 9-28

15 Metal-Ammonia Reduction of Alkynes Alkynes trans-alkenes Partial Reduction R R' R R' Another way to convert alkynes to alkenes is by reduction with sodium (or lithium or potassium) in ammonia. trans-alkenes are formed. R 2 2 R' Example 2 3 (82%) Na, N 3 Mechanism Metal (Li, Na, K) is reducing agent; 2 is not involved Four steps (1) electron transfer (2) proton transfer (3) electron transfer (4) proton transfer 9-30

16 Step (1): Transfer of an electron from the metal to the alkyne to give an anion radical. R Mechanism M + R' + M.. R R' Step (2): Transfer of a proton from the solvent (liquid ammonia) to the anion radical. R.. R R' R' + : N 2 N Step (3): Transfer of an electron from the metalvto the alkenyl radical to give a carbanion. R. R' + Mechanism M. R' Step (4): Transfer of a proton from the solvent (liquid ammonia) to the carbani R N 2 R' R' R R M + : N

17 Suggest efficient syntheses of (E)- and (Z)-2- heptene from propyne and any necessary organic or inorganic reagents Synthesis 2, Lindlar Pd 1. NaN Br Na, N

18 Addition of ydrogen alides to Alkynes Br 3 ( 2 ) 3 3 ( 2 ) 3 2 Alkynes are slightly less reactive than alkenes. Follows Markovnikov's Rule R Br : Br : Br (60%) Termolecular Rate-determining Step Observed rate law: rate = k[alkyne][x] Two Molar Equivalents of ydrogen alide F Free-radical Addition of Br (more on this in h 10) 3 ( 2 ) 3 Br peroxides regioselectivity opposite to Markovnikov's rule F F (76%) 3 ( 2 ) 3 Br (79%) 9-36

19 ydration of Alkynes By analogy to alkenes, alkynes can also undergo hydration, addition of and O across the π bond The product is a special kind of alkene containing an attached O group, an enol keto form The enol undergoes a spontaneous isomerization reaction called tautomerization As in hydration of alkenes, an acid catalyst is essential enol O Enols R R' R 2 R' Enols are tautomers of ketones, and exist in equilibrium with them. Keto-enol equilibration is rapid in acidic media. Ketones are more stable than enols and predominate at equilibrium. O ketone 9-38

20 Mechanism of onversion of Enol to Ketone + : O : O 9-39 Mechanism of onversion of Enol to Ketone : O : + : O 9-40

21 Mechanism of onversion of Enol to Ketone + : O : O : 9-41 Mechanism of onversion of Enol to Ketone : O + O : 9-42

22 arbocation is stabilized by electron delocalization (resonance). + Key arbocation Intermediate : O + O 9-43 Mechanism of Tautomerization Tautomerization amounts to migration of an from O to (or vice versa). owever, tautomerization is not intramolecular! Acid donates a proton to first Then conjugate base removes a proton from O Tautomerization can also be catalyzed by base, in which case the steps are reversed. 9-44

23 Examples of ydration Mercury(II) is commonly used as a Lewis acid catalyst along with acid The reaction follows Markovnikov s rule Be wary of hydration of asymmetric internal alkynes regioselectivity issues! 9-45 alogenations of Alkynes In the presence of 1 equivalent of X 2, the dihaloalkene can be isolated. The halogen substituents are trans, implying anti addition (halenium ion?!) Use of 2 equivalents leads to a tetrahaloalkane Only bromination and chlorination are synthetically useful Addition is anti 9-46

24 Ozonolysis of Alkynes In the presence of ozone, alkynes split like alkenes to form carboxylic acids an be a useful reaction for characterizing alkynes 9-47 Acetylene: A Useful Two- Fragment bonds can be formed on either end of acetylene via alkylation. It can be used to introduce a two-carbon fragment into a chain Subsequent partial hydrogenation gives us access to all the reactions of alkenes! 9-48

25 Retrosynthesis - Example: 1,2-epoxybutane Design a synthesis of 1,2-epoxybutane starting from ethyl bromide and acetylene as the only carbon sources 9-49