9.8 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) Reactions of Alkynes

9.9 Hydrogenation of Alkynes

RCH 2 CH 2 R' cat catalyst = Pt, Pd, Ni, or Rh alkene is an intermediate RC CR'+ 2H22H22H22H2 Hydrogenation of Alkynes

Heats of Hydrogenation 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. CH 3 CH 2 C CH CH 3 C CCH 3

RCH 2 CH 2 R' Alkenes 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. cat H2H2H2H2 RC CR' cat H2H2H2H2RCH CHR' Partial Hydrogenation

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 CaCO 3, which has been poisoned with lead acetate and quinoline. syn-Hydrogenation occurs; cis alkenes are formed. RCH 2 CH 2 R' cat H2H2H2H2 RC CR' cat H2H2H2H2RCH CHR' Lindlar Palladium

+ H 2 Lindlar Pd CH 3 (CH 2 ) 3 (CH 2 ) 3 CH 3 HH (87%) CH 3 (CH 2 ) 3 C C(CH 2 ) 3 CH 3 C C Example

Alkynes  trans-Alkenes 9.10 Metal-Ammonia Reduction of Alkynes

RCH 2 CH 2 R' Another way to convert alkynes to alkenes is by reduction with sodium (or lithium or potassium) in ammonia. trans-Alkenes are formed. RC CR' RCH CHR' Partial Reduction

CH 3 CH 2 CH 2 CH 3 H H (82%) CH 3 CH 2 C CCH 2 CH 3 C C Na, NH 3 Example

four steps (1)electron transfer (2)proton transfer (3)electron transfer (4)proton transfer Metal (Li, Na, K) is reducing agent; H 2 is not involved Mechanism

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

Step (2) Transfer of a proton from the solvent (liquid ammonia) to the anion radical. H NH 2.. R R' C... – C. R'RC C H..– : Mechanism

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

Step (4) Transfer of a proton from the solvent (liquid ammonia) to the carbanion...H NH 2 R' R C C H.. – R' H H C C R..– : Mechanism

Suggest efficient syntheses of (E)- and (Z)-2- heptene from propyne and any necessary organic or inorganic reagents. Problem 9.12

Problem 9.12 Strategy

1. NaNH 2 2. CH 3 CH 2 CH 2 CH 2 Br Na, NH 3 H 2, Lindlar Pd Problem 9.12 Synthesis

9.11 Addition of Hydrogen Halides to Alkynes

HBr Br (60%) Alkynes are slightly less reactive than alkenes CH 3 (CH 2 ) 3 C CH CH 2 Follows Markovnikov's Rule

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

(76%) CH 3 CH 2 C CCH 2 CH 3 2 HF F F C CHH CH 3 CH 2 CH 2 CH 3 Two Molar Equivalents of Hydrogen Halide

HBr (79%) regioselectivity opposite to Markovnikov's rule CH 3 (CH 2 ) 3 C CH CH 3 (CH 2 ) 3 CH CHBr peroxides Free-radical Addition of HBr

9.12 Hydration of Alkynes

expected reaction: enol observed reaction: RCH 2 CR' O H+H+H+H+RC CR' H2OH2OH2OH2O + H+H+H+H+RCCR' H2OH2OH2OH2O + OHRCHCR' ketone Hydration of Alkynes

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 enol OHRCHCR' RCH 2 CR' O ketone Enols

OHC C H + O H H :.. : Mechanism of conversion of enol to ketone

O H C C H + O H H :..:

O H C C H + O H H :.. : :

O H C C H H O ::H +.. :

O H C CHH O : : H +.. :

O H C C H H O : H +.. :

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

H 2 O, H + (89%) via CH 3 (CH 2 ) 2 C C(CH 2 ) 2 CH 3 Hg 2+ O CH 3 (CH 2 ) 2 CH 2 C(CH 2 ) 2 CH 3 OH CH 3 (CH 2 ) 2 CH C(CH 2 ) 2 CH 3 Example of Alkyne Hydration

H 2 O, H 2 SO 4 HgSO 4 CH 3 (CH 2 ) 5 CCH 3 (91%) via Markovnikov's rule followed in formation of enol CH 3 (CH 2 ) 5 C CH 2 OH CH 3 (CH 2 ) 5 C CH ORegioselectivity

9.13 Addition of Halogens to Alkynes

+ 2 Cl 2 ClCl (63%) C Cl 2 CH CH 3 HC CCH 3 Example

Br 2 CH 3 CH 2 CH 2 CH 3 Br Br (90%) CH 3 CH 2 C CCH 2 CH 3 C C Addition is anti

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

1. O 3 2. H 2 O + CH 3 (CH 2 ) 3 C CHCHCHCH CH 3 (CH 2 ) 3 COH (51%)O HOCOH OExample