8Chapter 09

8Chapter 09 - Chapter 9 Alkynes Dr. Wolf's CHM 201 &...

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Unformatted text preview: Chapter 9 Alkynes Dr. Wolf's CHM 201 & 202 9-1 9.1 Sources of Alkynes Dr. Wolf's CHM 201 & 202 9-2 Acetylene Acetylene Industrial preparation of acetylene is by dehydrogenation of ethylene CH3CH3 H2C CH2 800°C 1150°C H2C CH2 + H2 HC CH + H2 cost of energy makes acetylene a more expensive industrial chemical than ethylene Dr. Wolf's CHM 201 & 202 9-3 9.2 Nomenclature Dr. Wolf's CHM 201 & 202 9-4 Nomenclature 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 (CH3)3CC HC CCH2CH3 1-Butyne CCH3 4,4-Dimethyl-2-pentyne Dr. Wolf's CHM 201 & 202 9-5 9.3 Physical Properties of Alkynes The physical properties of alkynes are The similar to those of alkanes and alkenes. similar Dr. Wolf's CHM 201 & 202 9-6 9.4 Structure and Bonding in Alkynes: sp Hybridization Dr. Wolf's CHM 201 & 202 9-7 Structure Structure linear geometry for linear acetylene acetylene H 120 pm C C 106 pm CH3 Dr. Wolf's CHM 201 & 202 106 pm 121 pm C C 146 pm H H 106 pm 9-8 Cyclononyne is the Cyclononyne smallest cycloalkyne stable enough to be stored at room temperature stored for a reasonable length for of time. Cyclooctyne polymerizes Cyclooctyne on standing. Dr. Wolf's CHM 201 & 202 9-9 Bonding in acetylene is based on spp-hybridization Bonding in acetylene is based onss -hybridization p Bonding sp Bonding for each carbon for each carbon Mix together (hybridize) the 2s orbital Mix and one of the three 2p orbitals one 2p 2p 2sp 2s Dr. Wolf's CHM 201 & 202 9-10 Bonding in acetylene is based on spp-hybridization Bonding in acetylene is based onss -hybridization p Bonding sp Bonding for each carbon for each carbon Mix together (hybridize) the 2s orbital Mix and one of the three 2p orbitals one 2p Each carbon has two Each half-filled sp orbitals sp available to form σ bonds. available Dr. Wolf's CHM 201 & 202 2sp 9-11 σ Bonds in Acetylene σBonds in Acetylene Bonds Bonds 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. Dr. Wolf's CHM 201 & 202 9-12 π Bonds in Acetylene πBonds in Acetylene Bonds Bonds One of the two π bonds in acetylene is shown here. The second π bond is at right angles to the first. Dr. Wolf's CHM 201 & 202 9-13 π Bonds in Acetylene πBonds in Acetylene Bonds Bonds This is the second of the two π bonds in acetylene. Dr. Wolf's CHM 201 & 202 9-14 The region of highest negative charge lies above and below the molecular plane in ethylene. Dr. Wolf's CHM 201 & 202 The region of highest negative charge encircles the molecule around its center in acetylene. 9-15 Table 9.1 Comparison of ethane, ethylene, and acetylene Ethane Ethane Ethylene Acetylene Acetylene C—C distance 153 pm 134 pm 120 pm C—H distance 111 pm 110 pm 106 pm 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 sp3 sp2 sp 25% 33% 50% 62 45 26 H—C—C angles hybridization of C % s character pKa Dr. Wolf's CHM 201 & 202 9-16 9.5 Acidity of Acetylene and Terminal Alkynes H Dr. Wolf's CHM 201 & 202 C C 9-17 In general, hydrocarbons are In general, hydrocarbons are In In exceedingly weak acids exceedingly weak acids exceedingly exceedingly Compound pKa HF 3.2 H2O 16 NH3 36 H2C CH2 CH4 Dr. Wolf's CHM 201 & 202 45 60 9-18 Acetylene is a weak acid, but not nearly Acetylene is a weak acid, but not nearly as weak as alkanes or alkenes. as weak as alkanes or alkenes. Compound HF CH 3.2 H2O 16 NH3 HC pKa 36 H2C CH2 CH4 Dr. Wolf's CHM 201 & 202 26 45 60 9-19 Electronegativity of carbon Electronegativity of carbon Electronegativity Electronegativity increases with its ssccharacter increases with its haracter C 10-60 H H C C 10 H+ + C : sp33 sp : sp22 sp -45 H+ + C C 10-26 C C H H+ + C C: sp sp Electrons in an orbital with more s character are closer to the nucleus and more strongly held. Dr. Wolf's CHM 201 & 202 9-20 Objective: Objective: Prepare a solution containing sodium acetylide Prepare a solution containing sodium acetylide NaC CH Will treatment of acetylene with NaOH be effective? Will treatment of acetylene with NaOH be effective? NaOH + HC Dr. Wolf's CHM 201 & 202 CH NaC CH + H2O 9-21 No. Hydroxide is not a strong enough No. Hydroxide is not a strong enough No. No. base to deprotonate acetylene. base to deprotonate acetylene. base base NaOH + HC .. – HO: + .. H CH C CH weaker acid pKa = 26 NaC CH + H2O .. – HO H + : C .. stronger acid pKa = 16 In acid-base reactions, the equilibrium lies to the side of the weaker acid. Dr. Wolf's CHM 201 & 202 CH 9-22 Solution: Use a stronger base. Sodium amide Solution: Use a stronger base. Sodium amide is a stronger base than sodium hydroxide. is a stronger base than sodium hydroxide. NaNH2 + HC .. – H2N : + H CH C CH stronger acid pKa = 26 NaC CH + NH3 .. H2N – H + :C 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 CH 9-23 9.6 Preparation of Alkynes Preparation by by Alkylation of Acetylene and Terminal Alkynes Dr. Wolf's CHM 201 & 202 9-24 Preparation of Alkynes 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 9-25 Alkylation of acetylene and terminal alkynes Alkylation of acetylene and terminal alkynes H—C H—C R—C —C C—H C—H R—C —C Dr. Wolf's CHM 201 & 202 C—H C—H C—R C— 9-26 Alkylation of acetylene and terminal alkynes Alkylation of acetylene and terminal alkynes H—C H—C – C: + R X SN2 H—C H—C C—R + : X– C—R • The alkylating agent is an alkyl halide, and the reaction is nucleophilic substitution. • The nucleophile is sodium acetylide or the The sodium salt of a terminal (monosubstituted) alkyne. alkyne. Dr. Wolf's CHM 201 & 202 9-27 Example: Alkylation of acetylene Example: Alkylation of acetylene Example: HC CH NaNH2 NaNH HC NH3 CNa CH3CH2CH2CH2Br HC C CH2CH2CH2CH3 (70-77%) Dr. Wolf's CHM 201 & 202 9-28 Example: Alkylation of a terminal alkyne Example: Alkylation of a terminal alkyne Example: (CH3)2CHCH2C CH NaNH2, NH3 (CH3)2CHCH2C CNa CH3Br (CH3)2CHCH2C Dr. Wolf's CHM 201 & 202 (81%) C—CH3 9-29 Example: Dialkylation of acetylene Example: Dialkylation of acetylene Example: Dialkylation Example: Dialkylation H—C H—C C—H C—H 1. NaNH2, NH3 2. CH3CH2Br 2. CH CH3CH2—C C—H 1. NaNH2, NH3 2. CH3Br 2. CH CH3CH2—C C—CH3 (81%) Dr. Wolf's CHM 201 & 202 9-30 Limitation Limitation Effective only with primary alkyl halides Secondary and tertiary alkyl halides undergo elimination Dr. Wolf's CHM 201 & 202 9-31 E2 predominates over SN2 when alkyl E2 predominates over SN2when alkyl when when halide is secondary or tertiary halide is secondary or tertiary halide halide H—C H—C – C: H C C— X C— E2 H—C H—C Dr. Wolf's CHM 201 & 202 C —H + C C + : X– 9-32 9.7 Preparation of Alkynes by Elimination Reactions Dr. Wolf's CHM 201 & 202 9-33 Preparation of Alkynes by Preparation of Alkynes by Preparation Preparation "Double" Dehydrohalogenation ""Double" Dehydrohalogenation Double" "Double" H X H H C C C C H X X X Geminal dihalide Vicinal dihalide The most frequent applications are in preparation The of terminal alkynes. of Dr. Wolf's CHM 201 & 202 9-34 Geminal dihalide → Alkyne Geminal dihalide → Alkyne Geminal Geminal (CH3)3CCH2—CHCl2 1. 3NaNH2, NH3 2. H2O (CH3)3CC CH (56-60%) Dr. Wolf's CHM 201 & 202 9-35 Geminal dihalide → Alkyne Geminal dihalide → Alkyne Geminal Geminal (CH3)3CCH2—CHCl2 (CH3)3CCH (CH3)3CC Dr. Wolf's CHM 201 & 202 (slow) CHCl CH H2O (CH3)3CC NaNH2, NH3 CNa NaNH2, NH3 (slow) NaNH2, NH3 (fast) 9-36 Vicinal dihalide → Alkyne Vicinal dihalide → Alkyne Vicinal Vicinal CH3(CH2)7CH—CH2Br Br 1. 3NaNH2, NH3 2. H2O CH3(CH2)7C CH (54%) Dr. Wolf's CHM 201 & 202 9-37 9.8 Reactions of Alkynes Dr. Wolf's CHM 201 & 202 9-38 Reactions of Alkynes Reactions of Alkynes Acidity (Section 9.5) Acidity (Section 9.5) Hydrogenation (Section 9.9) Hydrogenation (Section 9.9) Metal-Ammonia Reduction (Section 9.10) Metal-Ammonia Reduction (Section 9.10) Addition of Hydrogen Halides (Section 9.11) Addition of Hydrogen Halides (Section 9.11) Hydration (Section 9.12) Hydration (Section 9.12) Addition of Halogens (Section 9.13) Addition of Halogens (Section 9.13) Ozonolysis (Section 9.14) Ozonolysis (Section 9.14) Dr. Wolf's CHM 201 & 202 9-39 9.9 Hydrogenation of Alkynes Dr. Wolf's CHM 201 & 202 9-40 Hydrogenation of Alkynes Hydrogenation of Alkynes RC CR' + 2H2 cat RCH2CH2R' catalyst = Pt, Pd, Ni, or Rh alkene is an intermediate Dr. Wolf's CHM 201 & 202 9-41 Heats of hydrogenation Heats of hydrogenation CH3CH2C CH 292 kJ/mol CH3C CCH3 275 kJ/mol Alkyl groups stabilize triple bonds in the Alkyl same way that they stabilize double same bonds. Internal triple bonds are more bonds. stable than terminal ones. stable Dr. Wolf's CHM 201 & 202 9-42 Partial Hydrogenation Partial Hydrogenation RC CR' H2 cat RCH CHR' H2 cat cat 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 Dr. Wolf's CHM 201 & 202 9-43 Lindlar Palladium Lindlar Palladium RC CR' H2 cat RCH CHR' H2 cat 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 9-44 Example Example CH3(CH2)3C C(CH2)3CH3 + H2 Lindlar Pd CH3(CH2)3 (CH2)3CH3 C C H H (87%) Dr. Wolf's CHM 201 & 202 9-45 9.10 Metal-Ammonia Reduction of Metal-Ammonia Alkynes Alkynes Alkynes → trans-Alkenes Alkynes trans Dr. Wolf's CHM 201 & 202 9-46 Partial Reduction 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 9-47 Example Example CH3CH2C CCH2CH3 Na, NH3 CH3CH2 H C C CH2CH3 H (82%) Dr. Wolf's CHM 201 & 202 9-48 Mechanism Mechanism Metal (Li, Na, K) is reducing agent; Metal H2 is not involved four steps (1) electron transfer (2) proton transfer (3) (4) Dr. Wolf's CHM 201 & 202 electron transfer proton transfer 9-49 Mechanism Mechanism Step (1): Transfer of an electron from the metal to the alkyne to give an anion radical. R C C Dr. Wolf's CHM 201 & 202 R' + M . R M+ – .. C . C R' 9-50 Mechanism Mechanism Step (2) Transfer of a proton from the solvent Step (liquid ammonia) to the anion radical. (liquid R . C – C .. H Dr. Wolf's CHM 201 & 202 R R' .. NH2 C H . C R' .. : NH2 – 9-51 Mechanism Mechanism Step (3): Transfer of an electron from the metal to the alkenyl radical to give a carbanion. R C . C H Dr. Wolf's CHM 201 & 202 R R' + M . C H M+ – .. C R' 9-52 Mechanism Mechanism Step (4) Transfer of a proton from the solvent (liquid ammonia) to the carbanion . H R C .. C– H Dr. Wolf's CHM 201 & 202 R' .. NH2 H R C H .. : NH2 – C R' 9-53 Suggest efficient syntheses of (E)- and (Z)-2heptene from propyne and any necessary organic or inorganic reagents. Dr. Wolf's CHM 201 & 202 9-54 1. NaNH2 1. 2. CH3CH2CH2CH2Br H2, Lindlar Pd Dr. Wolf's CHM 201 & 202 Na, NH3 9-55 9.11 Addition of Hydrogen Halides Addition to Alkynes to Dr. Wolf's CHM 201 & 202 9-56 Follows Markovnikov's Rule Follows Markovnikov's Rule CH3(CH2)3C CH HBr CH3(CH2)3C CH2 Br (60%) Alkynes are slightly less reactive than alkenes Dr. Wolf's CHM 201 & 202 9-57 Termolecular transition state Termolecular transition state H RC H .. .. Br : .. CH .. Br : .. Observed rate law: rate = k[alkyne][HX]2 Dr. Wolf's CHM 201 & 202 9-58 Reaction with two moles of a hydrogen Reaction with two moles of a hydrogen halide yields a geminal dihalide halide yields a geminal dihalide CH3CH2C CCH2CH3 2 HF H C C H CH3CH2 F F (76%) Dr. Wolf's CHM 201 & 202 CH2CH3 9-59 Free-radical addition of HBr occurs when Free-radical addition of HBr occurs when peroxides are present peroxides are present CH3(CH2)3C CH HBr CH3(CH2)3CH peroxides CHBr (79%) regioselectivity opposite to Markovnikov's rule Dr. Wolf's CHM 201 & 202 9-60 9.12 Hydration of Alkynes Dr. Wolf's CHM 201 & 202 9-61 Hydration of Alkynes Hydration of Alkynes expected reaction: RC CR' + H2O H+ RCH CR' OH observed reaction: RC CR' + H2O H+ RCH2CR' O Dr. Wolf's CHM 201 & 202 9-62 RCH CR' OH enol RCH2CR' O 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 9-63 Mechanism of conversion of enol to ketone Mechanism of conversion of enol to ketone .. :O H + :O C H C H H Dr. Wolf's CHM 201 & 202 9-64 Mechanism of conversion of enol to ketone Mechanism of conversion of enol to ketone .. :O H + :O C H C H H Dr. Wolf's CHM 201 & 202 9-65 Mechanism of conversion of enol to ketone Mechanism of conversion of enol to ketone .. :O H H C H C + : O: H Dr. Wolf's CHM 201 & 202 9-66 Mechanism of conversion of enol to ketone Mechanism of conversion of enol to ketone H .. :O H Dr. Wolf's CHM 201 & 202 C C + H : O: H 9-67 Mechanism of conversion of enol to ketone Mechanism of conversion of enol to ketone H .. :O H Dr. Wolf's CHM 201 & 202 C C + H : O: H 9-68 Mechanism of conversion of enol to ketone Mechanism of conversion of enol to ketone H .. :O H Dr. Wolf's CHM 201 & 202 C C H +O: H 9-69 Key carbocation intermediate is stabilized by Key carbocation intermediate is stabilized by Key Key electron delocalization (resonance) electron delocalization (resonance) electron electron .. :O H C Dr. Wolf's CHM 201 & 202 C + .. +O H H C H C 9-70 Example Example CH3(CH2)2C C(CH2)2CH3 H2O, H+ via OH Hg 2+ CH3(CH2)2CH C(CH2)2CH3 O CH3(CH2)2CH2C(CH2)2CH3 (89%) Dr. Wolf's CHM 201 & 202 9-71 Markovnikov's rule followed in formation of enol Markovnikov's rule followed in formation of enol CH3(CH2)5C CH H2O, H2SO4 HgSO4 O CH3(CH2)5CCH3 (91%) via OH CH3(CH2)5C Dr. Wolf's CHM 201 & 202 CH2 9-72 9.13 Addition of Halogens to Addition Alkynes Alkynes Dr. Wolf's CHM 201 & 202 9-73 Example Example Cl HC C CCH3 + 2 Cll2 Cl2CH C CH3 Cl (63%) Dr. Wolf's CHM 201 & 202 9-74 Addition is anti Addition is anti CH3CH2 CH3CH2C CCH2CH3 Br2 Br C C CH2CH3 Br (90%) Dr. Wolf's CHM 201 & 202 9-75 9.14 Ozonolysis of Alkynes gives two carboxylic acids by cleavage gives of triple bond of Dr. Wolf's CHM 201 & 202 9-76 Example Example CH3(CH2)3C CH 1. 2. O O CH3(CH2)3COH O3 H2 O + HOCOH (51%) Dr. Wolf's CHM 201 & 202 9-77 End of Chapter 9 Dr. Wolf's CHM 201 & 202 9-78 ...
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This document was uploaded on 01/03/2012.

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