Chapter 9 Alkynes Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1

9.1 Sources of Alkynes 1

Industrial preparation of acetylene is by dehydrogenation of ethylene. + CH3CH3 H2C CH2 1150°C H2 + H2C CH2 HC CH Cost of energy makes acetylene a more expensive industrial chemical than ethylene. 2

Naturally Occurring Alkynes Some alkynes occur naturally. For example, C(CH2)4COH CH3(CH2)10C O Tariric acid: occurs in seed of a Guatemalan plant. Histrionicotoxin: defensive toxin in poison dart frogs of Central and South America 2

9.2 Nomenclature 3

Acetylene and ethyne are both acceptable IUPAC names for HC CH Nomenclature Acetylene and ethyne are both acceptable IUPAC names for HC CH Higher alkynes are named in much the same way as alkenes except using an -yne suffix instead of -ene. HC CCH3 Propyne HC CCH2CH3 1-Butyne or But-1-yne (CH3)3CC CCH3 4,4-Dimethyl-2-pentyne or 4,4-Dimethyl-pent-2-yne 4

9.3 Physical Properties of Alkynes The physical properties of alkynes are similar to those of alkanes and alkenes. 5

9.4 Structure and Bonding in Alkynes: sp Hybridization 5

Linear geometry for acetylene Structure Linear geometry for acetylene C H 120 pm 106 pm CH3 121 pm 146 pm 6

Cyclooctyne polymerizes on standing. Cycloalkynes Cyclononyne is the smallest cycloalkyne stable enough to be stored at room temperature for a reasonable length of time. Cyclooctyne polymerizes on standing. C 14

sp Hybridization in Acetylene Mix together (hybridize) the 2s orbital and one of the three 2p orbitals. Atomic orbitals Hybridized 2p 2p 2sp Each carbon has two half-filled sp orbitals available to form  bonds. 2s 8

 Bonds in Acetylene Each carbon is connected to a hydrogen by a  bond. The two carbons are connected to each other by a  bond and two  bonds. Figure 9.2 (a) 9

 Bonds in Acetylene One of the two  bonds in acetylene is shown here. The second  bond is at right angles to the first. Figure 9.2 (b) 9

This is the second of the two  bonds in acetylene. Figure 9.2 (c) 9

Figure 9.3 Electrostatic Potential in Acetylene The region of highest negative charge encircles the molecule around its center in acetylene. The region of highest negative charge lies above and below the molecular plane in ethylene. 18

Table 9.1 Structural Features of Ethane, Ethylene, and Acetylene Ethane Ethylene Acetylene C—C distance 153 pm 134 pm 120 pm C—H distance 111 pm 110 pm 106 pm H—C—C angles 111.0° 121.4° 180° C—C BDE 368 kJ/mol 611 kJ/mol 820 kJ/mol C—H BDE 410 kJ/mol 452 kJ/mol 536 kJ/mol hybridization of C sp3 sp2 sp % s character 25% 33% 50% pKa 62 45 26 18

9.5 Acidity of Acetylene and Terminal Alkynes: H C 5

Acidity of Hydrocarbons In general, hydrocarbons are exceedingly weak acids, but alkynes are not nearly as weak as alkanes or alkenes. Compound pKa 26 45 CH4 60 HC CH H2C CH2 20

Carbon: Hybridization and Electronegativity pKa = 62 sp3 : – H+ + sp2 H C : – pKa = 45 H+ + sp C H : – pKa = 26 Electrons in an orbital with more s character are closer to the nucleus and more strongly held. 21

Sodium Acetylide Objective: Prepare a solution containing sodium acetylide Will treatment of acetylene with NaOH be effective? NaC CH H2O NaOH + HC CH NaC NO 22

Hydroxide is not a strong enough base to deprotonate acetylene. Sodium Acetylide Hydroxide is not a strong enough base to deprotonate acetylene. HO .. : H C CH – + weaker acid pKa = 26 stronger acid pKa = 15.7 In acid-base reactions, the equilibrium lies to the side of the weaker acid. 23

Sodium Acetylide Solution: Use a stronger base. Sodium amide is a stronger base than sodium hydroxide. NH3 NaNH2 + HC CH NaC – H2N .. : H C CH + stronger acid pKa = 26 weaker acid pKa = 36 Ammonia is a weaker acid than acetylene. The position of equilibrium lies to the right. 23

9.6 Preparation of Alkynes by Alkylation of Acetylene and Terminal Alkynes 5

Preparation of Alkynes There are two main methods for the preparation of alkynes: Carbon-carbon bond formation alkylation of acetylene and terminal alkynes Functional-group transformations elimination 25

Alkylation of Acetylene and Terminal Alkynes H—C C—H Remove 1st H with NaNH2, then alkylate with RX. R—C C—H R—C C—R Remove 2nd H with NaNH2, then alkylate with RX. 27

Alkylation of Acetylene and Terminal Alkynes SN2 C – : H—C + R X C—R H—C + X– : The alkylating agent is an alkyl halide, and the reaction is nucleophilic substitution. The nucleophile is sodium acetylide or the sodium salt of a terminal (monosubstituted) alkyne (these are strong bases). Effective only with methyl and 1o alkyl halides, 2o and 3o alkyl halides undergo elimination. 28

Example: Alkylation of Acetylene NaNH2 HC CH HC CNa NH3 CH3CH2CH2CH2Br (70-77%) HC C CH2CH2CH2CH3 29

Example: Alkylation of a Terminal Alkyne CH (CH3)2CHCH2C NaNH2, NH3 CNa (CH3)2CHCH2C CH3Br (81%) C—CH3 (CH3)2CHCH2C 30

Example: Dialkylation of Acetylene H—C C—H 1. NaNH2, NH3 2. CH3CH2Br C—H CH3CH2—C (81%) 1. NaNH2, NH3 2. CH3Br C—CH3 CH3CH2—C 31

Limitation on Alkylation of Acetylides Alkylation of an acetylide is effective only with methyl or primary alkyl halides Secondary and tertiary alkyl halides undergo elimination. 32

E2 predominates over SN2 when alkyl halide is secondary or tertiary. Acetylide Ion as a Base E2 predominates over SN2 when alkyl halide is secondary or tertiary. H C X C – : H—C E2 + C H—C —H X– : 33

9.7 Preparation of Alkynes by Elimination Reactions Elimination of vicinal and geminal dihalides yield alkynes. 5

Preparation of Alkynes by Double Dehydrohalogenation X C H X C H Geminal dihalide Vicinal dihalide The most frequent applications are in preparation of terminal alkynes. 35

Geminal dihalide  Alkyne (CH3)3CCH2—CHCl2 1. 3NaNH2, NH3 2. H2O (56-60%) (CH3)3CC CH 36

Geminal dihalide  Alkyne (CH3)3CCH2—CHCl2 NaNH2, NH3 (slow) (Dehydrohalogenation) (CH3)3CCH CHCl NaNH2, NH3 (CH3)3CC CH (slow) (Dehydrohalogenation) H2O NaNH2, NH3 (CH3)3CC CNa (fast) (Loss of a proton) (Regain proton) 37

Vicinal dihalide  Alkyne Br CH3(CH2)7CH—CH2Br 1. 3NaNH2, NH3 2. H2O (54%) CH3(CH2)7C CH 38

9.8 Reactions of Alkynes 5

Hydrogenation (Section 9.9) Metal-Ammonia Reduction (Section 9.10) Reactions of Alkynes Acidity (Section 9.5) Hydrogenation (Section 9.9) Metal-Ammonia Reduction (Section 9.10) Addition of Hydrogen Halides (Section 9.11) Hydration (Section 9.12) Hydroboration-oxidation (not in text) Addition of Halogens (Section 9.13) Ozonolysis (Section 9.14) 2

9.9 Hydrogenation of Alkynes 5

Hydrogenation of Alkynes cat RC CR' + 2H2 RCH2CH2R' catalyst = Pt, Pd, Ni, or Rh Twice the H2 is needed here compared to reaction with an alkene since there are two pi bonds in the alkyne. An alkene is an intermediate. 4

Heats of Hydrogenation CH3CH2C CH CH3C CCH3 292 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. 6

Partial Hydrogenation RC CR' cat H2 RCH CHR' cat H2 RCH2CH2R' Alkynes can be used to prepare alkenes if a catalyst is used that is active enough to catalyze the hydrogenation of alkynes, but not active enough for the hydrogenation of alkenes. 8

syn-Hydrogenation occurs; cis alkenes are formed. Lindlar Catalyst RC CR' cat H2 RCH CHR' cat H2 RCH2CH2R' A catalyst that will catalyze the hydrogenation of alkynes to alkenes, but not of alkenes to alkanes is called the Lindlar catalyst and consists of palladium supported on CaCO3, which has been poisoned with lead acetate and quinoline. (Poisoning reduces activity of the catalyst.) syn-Hydrogenation occurs; cis alkenes are formed. 9

Example + H2 CH3(CH2)3C C(CH2)3CH3 Lindlar Pd CH3(CH2)3 (CH2)3CH3 C H (87%) C 10

9.10 Metal-Ammonia Reduction of Alkynes Alkynes  trans-Alkenes 5

Partial Reduction RC CR' RCH CHR' RCH2CH2R' Another way to convert alkynes to alkenes is by reduction with sodium (or lithium or potassium) in ammonia. This reaction goes by a multistep mechanism. In this reaction, trans-alkenes are formed. 8

Example CH3CH2C CCH2CH3 Na, NH3 CH3CH2 CH2CH3 H (82%) C 10

Mechanism The metal (Li, Na, K) is the reducing agent; H2 is not involved in this reaction. Four steps: (1) electron transfer (2) proton transfer (3) electron transfer (4) proton transfer 13

Mechanism M . + R R' C .. – M+ . R' R C H – . R C C R' .. .. NH2 .. – Step (1): Transfer of an electron from a metal atom to the alkyne to give an anion radical. M . + R R' C .. – M+ Step (2): Transfer of a proton from the solvent (liquid ammonia) to the anion radical. . R' R C H – . R C C R' .. .. NH2 .. – : H NH2 15

Mechanism M+ R' R C H .. – R . . C C R' + M H Step (3): Transfer of an electron from a 2nd metal atom to the alkenyl radical to give a carbanion. M+ R' R C H .. – R . . C C R' + M H 16

Mechanism .. H NH2 R' H C R NH2 .. – : R' R C H .. – Step (4): Transfer of a proton from the solvent (liquid ammonia) to the carbanion. .. H NH2 R' H C R NH2 .. – : R' R C H .. – 17

Propose efficient syntheses of (E)- and (Z)-2- heptene from propyne and other necessary reagents. Strategy 19

Synthesis 1. NaNH2 2. CH3CH2CH2CH2Br H2, Lindlar Pd Na, NH3 19

9.11 Addition of Hydrogen Halides to Alkynes 5

Follows Markovnikov's Rule HBr CH3(CH2)3C CH CH3(CH2)3C CH2 Br (60%) Alkynes are slightly less reactive than alkenes. Markovnikov’s rule is followed. 22

Termolecular Rate-determining Step .. Br H : RC CH .. Br H : Observed rate law: rate = k[alkyne][HX]2 23

Two Molar Equivalents of Hydrogen Halide CH3CH2C CCH2CH3 2 HF (76%) F C H CH3CH2 CH2CH3 24

Free-radical Addition of HBr CH3(CH2)3C CH (79%) CH3(CH2)3CH CHBr peroxides Regioselectivity opposite to Markovnikov's rule. As with alkenes, the anti-Markovnikov product is formed in presence of peroxide. 25

9.12 Hydration of Alkynes 5

Hydration of Alkynes expected reaction: H+ RC CR' H2O + OH RCH CR' enol observed reaction: RCH2CR' O H+ RC CR' H2O + ketone 27

Enols are tautomers of ketones, and exist in equilibrium with them. OH RCH CR' RCH2CR' O enol ketone 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. 28

Mechanism of Conversion of Enol to Ketone : H + O C .. .. : O H H H C C + : O : H 30

Mechanism of Conversion of Enol to Ketone .. : H O + : O H H C C O H C : + .. 30

Key Carbocation Intermediate Carbocation is stabilized by electron delocalization (resonance). .. O C H .. + : O H H C C + 30

Example of Alkyne Hydration CH3(CH2)2C C(CH2)2CH3 via OH CH3(CH2)2CH C(CH2)2CH3 Hg2+ H2O, H+ (89%) O CH3(CH2)2CH2C(CH2)2CH3 32

Markovnikov's rule followed in formation of enol Regioselectivity Markovnikov's rule followed in formation of enol O H2O, H2SO4 CH3(CH2)5C CH CH3(CH2)5CCH3 HgSO4 (91%) via CH3(CH2)5C CH2 OH 32

9.13 Addition of Halogens to Alkynes 5

Both pi bonds react with excess X2. Example Cl (63%) C Cl2CH CH3 HC CCH3 + 2Cl2 Both pi bonds react with excess X2. 36

Addition is anti as with an alkene. CH3CH2 Br Br2 C CH3CH2C CCH2CH3 Br CH2CH3 (90%) One addition 1mol of X2. Addition is anti as with an alkene. 37

9.14 Ozonolysis of Alkynes Gives two carboxylic acids by cleavage of triple bond 5

Reduction is not needed in step 2. 1. O3 Example CH3(CH2)3C CH Reduction is not needed in step 2. 1. O3 2. H2O + CH3(CH2)3COH (51%) O HOCOH 39

Hydroboration-Oxidation of Terminal Alkynes Gives the anti-Markovnikov enol 5

Disecondaryisoamyl borane, Sia2BH This is similar to hydroboration-oxidation of alkenes. Disiaborane (a bulky borane) is used to prevent subsequent reaction with the alkene. Addition of the borane is syn as before. Sia2BH RC CH RCH CH-BSia2 THF H2O2 RCH CH-BSia2 RCH CH-OH HOˉ an enol which rearranges to an aldehyde. 8

End of Chapter 9 Alkynes 5