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

Introduction Alkynes contain a triple bond. General formula is CnH2n–2. Two elements of unsaturation for each triple bond. Some reactions resemble the reactions of alkenes, like addition and oxidation. Some reactions are specific to alkynes.

Nomenclature: IUPAC Find the longest chain containing the triple bond. Change -ane ending to -yne. Number the chain, starting at the end closest to the triple bond. Give branches or other substituents a number to locate their position.

Examples of Nomenclature All other functional groups, except ethers and halides, have a higher priority than alkynes.

Physical Properties Nonpolar, insoluble in water. Soluble in most organic solvents. Boiling points are similar to alkane of same size. Less dense than water. Up to four carbons, gas at room temperature.

Ethyne Commonly called acetylene. It is used in welding torches. The oxyacetylene flame reaches temperatures as high as 2800 °C. Thermodynamically unstable. When it decomposes to its elements, it releases 234 kJ (56 kcal) of energy per mole.

Synthesis of Acetylene from Coal CaC2 + 2 H2O H—C≡C—H + Ca(OH)2 Heat coke with lime in an electric furnace to form calcium carbide. Then drip water on the calcium carbide.

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

Structure and Bonding in Alkynes: sp Hybridization 5

Molecular Structure of Acetylene Triple-bonded carbons have sp hybrid orbitals. A sigma bond is formed between the carbons by overlap of the sp orbitals. Sigma bonds to the hydrogens are formed by using the second sp orbital. Since the sp orbitals are linear, acetylene will be a linear molecule.

Overlap of the p Orbitals of Acetylene Each carbon in acetylene has two unhybridized p orbitals with one nonbonded electron. It is the overlap of the parallel p orbitals that forms the triple bond (two pi orbitals).

Bond Lengths Triple bonds are shorter than double or single bonds because of the two pi overlapping orbitals.

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

Acidity of Acetylene and Terminal Alkynes H C Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 17

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

Acidity of Alkynes Terminal alkynes are more acidic than other hydrocarbons due to the higher s character of the sp hybridized carbon. Terminal alkynes can be deprotonated quantitatively with strong bases such as sodium amide (–NH2). Hydroxide (HO–) and alkoxide (RO–) bases are not strong enough to deprotonate the alkyne quantitatively.

Sodium Acetylide Objective: Prepare a solution containing sodium acetylide Will treatment of acetylene with NaOH be effective? NaC CH H2O NaOH + HC CH NaC 22

No. Hydroxide is not a strong enough base to deprotonate acetylene. Sodium Acetylide No. 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

Preparation of Alkynes by Alkylation of Acetylene and Terminal Alkynes 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 25

Alkylation of Acetylene and Terminal Alkynes H—C C—H R—C C—H R—C C—R 27

Acetylide Ions in SN2 Reactions One of the best methods for synthesizing substituted alkynes is a nucleophilic attack by the acetylide ion on an unhindered alkyl halide. SN2 reaction with 1 alkyl halides lengthens the alkyne chain. Unhindered alkyl halides work better in an SN2 reaction: CH3X > 1°.

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

Effective only with primary alkyl halides Limitation Effective only with 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

Preparation of Alkynes by Elimination Reactions 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. 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) (CH3)3CCH CHCl NaNH2, NH3 (CH3)3CC CH (slow) H2O NaNH2, NH3 (CH3)3CC CNa (fast) 37

Vicinal dihalide  Alkyne Br CH3(CH2)7CH—CH2Br 1. 3NaNH2, NH3 2. H2O (54%) CH3(CH2)7C CH KOBut in DMSO can be used in place of amide. 38

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

Metal-Ammonia Reduction Addition of Hydrogen Halides Hydration Reactions of Alkynes Acidity Hydrogenation Metal-Ammonia Reduction Addition of Hydrogen Halides Hydration Addition of Halogens Ozonolysis 2

Hydrogenation of Alkynes cat RC CR' + 2H2 RCH2CH2R' catalyst = Pt, Pd, Ni, or Rh 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 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. 8

syn-Hydrogenation occurs; cis alkenes are formed. Lindlar Catalyst 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. 9

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

Metal-Ammonia Reduction of Alkynes Alkynes  trans-Alkenes 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. 8

Example CH3CH2C CCH2CH3 Na, NH3 CH3CH2 CH2CH3 H (82%) C 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 13

Mechanism Step (1): Transfer of an electron from the metal to the alkyne to give an anion radical. M . + R R' C .. – M+ 14

Mechanism 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 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 16

Mechanism 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

Suggest efficient syntheses of (E)- and (Z)-2- heptene from propyne and any necessary organic or inorganic reagents. 18

Strategy 19

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

Addition of Hydrogen Halides to Alkynes 21

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

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

Figure 9.4

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

Anti-Markovnikov Addition of Hydrogen Bromide to Alkynes By using peroxides, hydrogen bromide can be added to a terminal alkyne anti-Markovnikov. The bromide will attach to the least substituted carbon, giving a mixture of cis and trans isomers.

Hydration of Alkynes 26

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

Keto–Enol Tautomerism Enols are not stable, and they isomerize to the corresponding aldehyde or ketone in a process known as keto–enol tautomerism. Keto-enol equilibration is rapid in acidic media

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

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

Mechanism of Conversion of Enol to Ketone : + .. 30

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

Addition of Halogens to Alkynes 26

Example Cl (63%) C Cl2CH CH3 HC CCH3 + 2Cl2 36

Addition is anti CH3CH2 Br Br2 C CH3CH2C CCH2CH3 Br CH2CH3 (90%) When alkyne and the halogen are in equal ratio, a dihaloalkene can be isolated. 37

gives two carboxylic acids by cleavage of triple bond 9.14 Ozonolysis of Alkynes gives two carboxylic acids by cleavage of triple bond 38

Example CH3(CH2)3C CH 1. O3 2. H2O + CH3(CH2)3COH (51%) O HOCOH 39