Chapter 9 Alkynes Dr. Wolf's CHM 201 & 202 1

Sources of Alkynes Dr. Wolf's CHM 201 & 202 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 Dr. Wolf's CHM 201 & 202 2

Nomenclature Dr. Wolf's CHM 201 & 202 3

Acetylene and ethyne are both acceptable IUPAC names for Nomenclature HC CH Acetylene and ethyne are both acceptable IUPAC names for 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 (CH3)3CC CCH3 4,4-Dimethyl-2-pentyne Dr. Wolf's CHM 201 & 202 4

Physical Properties of Alkynes The physical properties of alkynes are similar to those of alkanes and alkenes. Dr. Wolf's CHM 201 & 202 5

Structure and Bonding in Alkynes: sp Hybridization Dr. Wolf's CHM 201 & 202 5

linear geometry for acetylene Structure linear geometry for acetylene C H 120 pm 106 pm 121 pm CH3 C C H 146 pm 106 pm Dr. Wolf's CHM 201 & 202 6

Cyclooctyne polymerizes on standing. Cyclononyne is the smallest cycloalkyne stable enough to be stored at room temperature for a reasonable length of time. Cyclooctyne polymerizes on standing. Dr. Wolf's CHM 201 & 202 14

Bonding in acetylene is based on sp-hybridization for each carbon Mix together (hybridize) the 2s orbital and one of the three 2p orbitals 2p 2p 2sp 2s Dr. Wolf's CHM 201 & 202 8

Bonding in acetylene is based on sp-hybridization for each carbon Mix together (hybridize) the 2s orbital and one of the three 2p orbitals 2p Each carbon has two half-filled sp orbitals available to form s bonds. 2sp Dr. Wolf's CHM 201 & 202 8

s Bonds in Acetylene Each carbon is connected to a hydrogen by a s bond. The two carbons are connected to each other by a s bond and two p bonds. Dr. Wolf's CHM 201 & 202 9

p Bonds in Acetylene One of the two p bonds in acetylene is shown here. The second p bond is at right angles to the first. Dr. Wolf's CHM 201 & 202 9

p Bonds in Acetylene This is the second of the two p bonds in acetylene. Dr. Wolf's CHM 201 & 202 9

The region of highest negative charge lies above and below the molecular plane in ethylene. The region of highest negative charge encircles the molecule around its center in acetylene. Dr. Wolf's CHM 201 & 202 18

Table 9.1 Comparison 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 Dr. Wolf's CHM 201 & 202 18

Acidity of Acetylene and Terminal Alkynes H C Dr. Wolf's CHM 201 & 202 17

In general, hydrocarbons are exceedingly weak acids Compound pKa HF 3.2 H2O 16 NH3 36 45 CH4 60 H2C CH2 Dr. Wolf's CHM 201 & 202 20

Acetylene is a weak acid, but not nearly as weak as alkanes or alkenes. Compound pKa HF 3.2 H2O 16 NH3 36 45 CH4 60 26 HC CH H2C CH2 Dr. Wolf's CHM 201 & 202 20

Electronegativity of carbon increases with its s character 10-60 C H H+ + : sp3 C H C : sp2 10-45 H+ + C C 10-26 C H C : sp H+ + Electrons in an orbital with more s character are closer to the nucleus and more strongly held. Dr. Wolf's CHM 201 & 202 21

Objective: Prepare a solution containing sodium acetylide Will treatment of acetylene with NaOH be effective? NaC CH H2O NaOH + HC CH NaC Dr. Wolf's CHM 201 & 202 22

No. Hydroxide is not a strong enough base to deprotonate acetylene. H2O NaOH + HC CH NaC HO .. : HO H .. – – + + H C CH : C CH weaker acid pKa = 26 stronger acid pKa = 16 In acid-base reactions, the equilibrium lies to the side of the weaker acid. Dr. Wolf's CHM 201 & 202 23

Solution: Use a stronger base Solution: Use a stronger base. Sodium amide is a stronger base than sodium hydroxide. NH3 NaNH2 + HC CH NaC .. .. – – : + H2N + H2N H C CH 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. Dr. Wolf's CHM 201 & 202 23

Preparation of Alkynes by Alkylation of Acetylene and Terminal Alkynes Dr. Wolf's CHM 201 & 202 26

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 Dr. Wolf's CHM 201 & 202 25

Alkylation of acetylene and terminal alkynes H—C C—H R—C C—H R—C C—R Dr. Wolf's CHM 201 & 202 27

Alkylation of acetylene and terminal alkynes – : H—C SN2 + R X X– : C—R H—C + 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. Dr. Wolf's CHM 201 & 202 28

Example: Alkylation of acetylene NaNH2 HC CH HC CNa NH3 CH3CH2CH2CH2Br HC C CH2CH2CH2CH3 (70-77%) Dr. Wolf's CHM 201 & 202 29

Example: Alkylation of a terminal alkyne CH (CH3)2CHCH2C NaNH2, NH3 CNa (CH3)2CHCH2C CH3Br (81%) C—CH3 (CH3)2CHCH2C Dr. Wolf's CHM 201 & 202 30

Example: Dialkylation of acetylene H—C C—H 1. NaNH2, NH3 2. CH3CH2Br C—H CH3CH2—C 1. NaNH2, NH3 2. CH3Br C—CH3 CH3CH2—C (81%) Dr. Wolf's CHM 201 & 202 31

Effective only with primary alkyl halides Limitation Effective only with primary alkyl halides Secondary and tertiary alkyl halides undergo elimination Dr. Wolf's CHM 201 & 202 32

E2 predominates over SN2 when alkyl halide is secondary or tertiary – : H—C H C C— X E2 + C H—C —H X– : Dr. Wolf's CHM 201 & 202 33

Preparation of Alkynes by Elimination Reactions Dr. Wolf's CHM 201 & 202 34

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. Dr. Wolf's CHM 201 & 202 35

Geminal dihalide ® Alkyne (CH3)3CCH2—CHCl2 1. 3NaNH2, NH3 2. H2O (56-60%) (CH3)3CC CH Dr. Wolf's CHM 201 & 202 36

Geminal dihalide ® Alkyne (CH3)3CCH2—CHCl2 NaNH2, NH3 (slow) (CH3)3CCH CHCl (slow) NaNH2, NH3 (CH3)3CC CH H2O NaNH2, NH3 (fast) (CH3)3CC CNa Dr. Wolf's CHM 201 & 202 37

Vicinal dihalide ® Alkyne CH3(CH2)7CH—CH2Br Br 1. 3NaNH2, NH3 2. H2O (54%) CH3(CH2)7C CH Dr. Wolf's CHM 201 & 202 38

Reactions of Alkynes Dr. Wolf's CHM 201 & 202 1

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) Addition of Halogens (Section 9.13) Ozonolysis (Section 9.14) Dr. Wolf's CHM 201 & 202 2

Hydrogenation of Alkynes Dr. Wolf's CHM 201 & 202 3

alkene is an intermediate Hydrogenation of Alkynes cat RC CR' + 2H2 RCH2CH2R' catalyst = Pt, Pd, Ni, or Rh alkene is an intermediate Dr. Wolf's CHM 201 & 202 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. Dr. Wolf's CHM 201 & 202 6

Partial Hydrogenation RC CR' cat H2 RCH CHR' cat H2 RCH2CH2R' Alkynes could be used to prepare alkenes if a catalyst were available that is active enough to catalyze the hydrogenation of alkynes, but not active enough for the hydrogenation of alkenes. Dr. Wolf's CHM 201 & 202 8

syn-Hydrogenation occurs; cis alkenes are formed. Lindlar Palladium RC CR' cat H2 RCH CHR' cat H2 RCH2CH2R' There is a catalyst that will catalyze the hydrogenation of alkynes to alkenes, but not that of alkenes to alkanes. It is called the Lindlar catalyst and consists of palladium supported on CaCO3, which has been poisoned with lead acetate and quinoline. syn-Hydrogenation occurs; cis alkenes are formed. Dr. Wolf's CHM 201 & 202 9

Example CH3(CH2)3C C(CH2)3CH3 + H2 Lindlar Pd CH3(CH2)3 (CH2)3CH3 C H (87%) Dr. Wolf's CHM 201 & 202 10

Metal-Ammonia Reduction of Alkynes Alkynes ® trans-Alkenes Dr. Wolf's CHM 201 & 202 11

trans-Alkenes are formed. 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. trans-Alkenes are formed. Dr. Wolf's CHM 201 & 202 8

Example CH3CH2C CCH2CH3 Na, NH3 CH3CH2 H C CH2CH3 H (82%) 10 Dr. Wolf's CHM 201 & 202 10

Metal (Li, Na, K) is reducing agent; H2 is not involved Mechanism Metal (Li, Na, K) is reducing agent; H2 is not involved four steps (1) electron transfer (2) proton transfer (3) electron transfer (4) proton transfer Dr. Wolf's CHM 201 & 202 13

Mechanism Step (1): Transfer of an electron from the metal to the alkyne to give an anion radical. M . + R R' C .. – M+ Dr. Wolf's CHM 201 & 202 14

Mechanism Step (2) Transfer of a proton from the solvent (liquid ammonia) to the anion radical. . R' R C H H NH2 .. R R' C . – NH2 .. – : Dr. Wolf's CHM 201 & 202 15

Mechanism Step (3): Transfer of an electron from the metal to the alkenyl radical to give a carbanion. M+ R' R C H .. – R . . C C R' + M H Dr. Wolf's CHM 201 & 202 16

Mechanism Step (4) Transfer of a proton from the solvent (liquid ammonia) to the carbanion . H NH2 .. R' R C – H C R NH2 .. – : R' Dr. Wolf's CHM 201 & 202 17

Suggest efficient syntheses of (E)- and (Z)-2- heptene from propyne and any necessary organic or inorganic reagents. Dr. Wolf's CHM 201 & 202 18

1. NaNH2 2. CH3CH2CH2CH2Br Na, NH3 H2, Lindlar Pd 19 Dr. Wolf's CHM 201 & 202 19

Addition of Hydrogen Halides to Alkynes Dr. Wolf's CHM 201 & 202 21

Follows Markovnikov's Rule HBr CH3(CH2)3C CH CH3(CH2)3C CH2 Br (60%) Alkynes are slightly less reactive than alkenes Dr. Wolf's CHM 201 & 202 22

Termolecular transition state .. Br H : RC CH .. Br H : Observed rate law: rate = k[alkyne][HX]2 Dr. Wolf's CHM 201 & 202 23

Reaction with two moles of a hydrogen halide yields a geminal dihalide CH3CH2C CCH2CH3 2 HF F C H CH3CH2 CH2CH3 (76%) Dr. Wolf's CHM 201 & 202 24

Free-radical addition of HBr occurs when peroxides are present CH3(CH2)3C CH CH3(CH2)3CH CHBr peroxides (79%) regioselectivity opposite to Markovnikov's rule Dr. Wolf's CHM 201 & 202 25

Hydration of Alkynes Dr. Wolf's CHM 201 & 202 26

expected reaction: Hydration of Alkynes H+ RC CR' H2O + OH RCH CR' observed reaction: H+ + RC CR' H2O RCH2CR' O Dr. Wolf's CHM 201 & 202 27

enols are regioisomers of ketones, and exist in equilibrium with them OH RCH CR' RCH2CR' O enol ketone enols are regioisomers 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 Dr. Wolf's CHM 201 & 202 28

Mechanism of conversion of enol to ketone .. : O H C H + : O H H Dr. Wolf's CHM 201 & 202 30

Mechanism of conversion of enol to ketone .. : O H C H + : O H H Dr. Wolf's CHM 201 & 202 30

Mechanism of conversion of enol to ketone .. : O H H H C C + : O : H Dr. Wolf's CHM 201 & 202 30

Mechanism of conversion of enol to ketone .. O : : O H H H C C + Dr. Wolf's CHM 201 & 202 30

Mechanism of conversion of enol to ketone .. O : : O H H H C C + Dr. Wolf's CHM 201 & 202 30

Mechanism of conversion of enol to ketone .. : H O + : O H H C C Dr. Wolf's CHM 201 & 202 30

Key carbocation intermediate is stabilized by electron delocalization (resonance) H C + .. : O C H .. + Dr. Wolf's CHM 201 & 202 30

Example CH3(CH2)2C C(CH2)2CH3 via Hg2+ H2O, H+ OH CH3(CH2)2CH CH3(CH2)2CH2C(CH2)2CH3 (89%) Dr. Wolf's CHM 201 & 202 32

Markovnikov's rule followed in formation of enol H2O, H2SO4 CH3(CH2)5C CH CH3(CH2)5CCH3 HgSO4 (91%) via CH3(CH2)5C CH2 OH Dr. Wolf's CHM 201 & 202 32

Addition of Halogens to Alkynes Dr. Wolf's CHM 201 & 202 35

Example Cl (63%) C Cl2CH CH3 HC CCH3 + 2 Cl2 36 Dr. Wolf's CHM 201 & 202 36

Addition is anti CH3CH2 Br Br2 C CH3CH2C CCH2CH3 Br CH2CH3 (90%) 37 Dr. Wolf's CHM 201 & 202 37

gives two carboxylic acids by cleavage of triple bond Ozonolysis of Alkynes gives two carboxylic acids by cleavage of triple bond Dr. Wolf's CHM 201 & 202 38

Example CH3(CH2)3C CH 1. O3 2. H2O CH3(CH2)3COH (51%) O HOCOH O + 39 Dr. Wolf's CHM 201 & 202 39

End of Chapter 9 Dr. Wolf's CHM 201 & 202