Chapter9_Slides(3)

Chapter9_Slides(3) - Organic Chemistry CHE 275 Chapter 9...

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Unformatted text preview: Organic Chemistry CHE 275 Chapter 9 Alkynes Naturally Occurring Alkynes Some alkynes occur naturally. For example, O CH3(CH2)10C C(CH2)4COH Tariric acid: occurs in seed of a Tariric Guatemalan plant Guatemalan Nomenclature Acetylene and ethyne are both acceptable IUPAC names for HC CH HC 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 Structure linear geometry for acetylene 120 pm H C C H 106 pm CH3 106 pm 121 pm C C 146 pm H 106 pm 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 C σ Bonds in Acetylene Each carbon is Each connected to a hydrogen by a σ bond. The two carbons are connected to each other by a σ bond and two π bonds. π Bonds in Acetylene One of the two One π bonds in acetylene is acetylene shown here. The second π The bond is at right angles to the first. π Bonds in Acetylene This is the second This of the two π bonds in acetylene. acetylene. Electrostatic Potential in Acetylene The region of highest negative charge lies above and below the molecular plane in ethylene. ethylene The region of highest The negative charge encircles the molecule around its molecule center in acetylene. center acetylene Acidity of Acetylene and Terminal Alkynes H C C Acidity of Hydrocarbons In general, hydrocarbons are exceedingly weak acids exceedingly Compound pKa HF 3.2 H2O 15.7 NH3 36 H2C CH2 CH4 45 60 Acetylene Acetylene is a weak acid, but not nearly Acetylene as weak as alkanes or alkenes. Compound HF HC CH pKa 3.2 H2O 15.7 NH3 36 H2C CH2 CH4 45 60 Carbon: Hybridization and Electronegativity C pKa = 60 H H C C pKa = 45 H+ + H+ + –C C pKa = 26 C C H H+ + C : sp3 : sp2 C – – C: sp Electrons in an orbital with more s character are closer to the nucleus and more strongly held. Sodium Acetylide Objective: Objective: Prepare a solution containing sodium acetylide NaC CH Will treatment of acetylene with NaOH be effective? NaOH + HC CH NaC CH + H2O Sodium Acetylide No. Hydroxide is not a strong enough base to deprotonate acetylene. deprotonate .. – HO: + .. H C CH weaker acid pKa = 26 .. HO .. – H + :C stronger acid pKa = 15.7 In acid-base reactions, the equilibrium lies to the side of the weaker acid. CH Sodium Acetylide Solution: Use a stronger base. Sodium amide Solution: is a stronger base than sodium hydroxide. NaNH2 + HC CH NaC CH + NH3 .. – H2N : + H C CH stronger acid pKa = 26 .. H2N – H + :C weaker acid pKa = 36 Ammonia is a weaker acid than acetylene. The position of equilibrium lies to the right. CH 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 Alkylation of Acetylene and Terminal Alkynes H—C C—H C—H R—C R—C C—H C—H R—C R—C C—R C—R 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 sodium salt of a terminal (monosubstituted) alkyne. Alkylation of Acetylene HC NaNH2 NaNH CH NH3 HC CNa CH3CH2CH2CH2Br HC C CH2CH2CH2CH3 (70-77%) Alkylation of a Terminal Alkyne (CH3)2CHCH2C CH NaNH2, NH3 NaNH (CH3)2CHCH2C CNa CH3Br (CH3)2CHCH2C (81%) C—CH3 Example: Dialkylation of Acetylene H—C H—C C—H C—H 1. NaNH2, NH3 1. 2. CH3CH2Br CH3CH2—C C—H 1. NaNH2, NH3 2. CH3Br CH3CH2—C C—CH3 (81%) Limitation SN2 reactions are effective only with primary alkyl halides For Secondary and tertiary alkyl halides, elimination reactions are more effective. Acetylide Ion as a Base E2 predominates over SN2 when alkyl E2 when halide is secondary or tertiary halide H—C H—C – C: H C C X E2 E2 H—C H—C C —H + C C + : X– Preparation of Alkynes by "Double Dehydrohalogenation" H X H H C C C C H X X X Geminal dihalide Geminal Vicinal dihalide The most frequent applications are in preparation The of terminal alkynes. of Geminal dihalide → Alkyne (CH3)3CCH2—CHCl2 (CH 1. 3NaNH2, NH3 2. H2O (CH3)3CC CH (56-60%) Geminal dihalide → Alkyne (CH3)3CCH2—CHCl2 (CH3)3CCH (CH3)3CC (slow) CHCl CH H2O (CH3)3CC NaNH2, NH3 NaNH CNa NaNH2, NH3 (slow) NaNH2, NH3 (fast) Vicinal dihalide → Alkyne CH3(CH2)7CH—CH2Br CH Br 1. 3NaNH2, NH3 2. H2O CH3(CH2)7C (54%) CH 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) Hydrogenation of Alkynes RC CR' + 2H2 cat RCH2CH2R' RCH catalyst = Pt, Pd, Ni, or Rh alkene is an intermediate Heats of Hydrogenation CH3CH2C CH 292 kJ/mol CH3C CCH3 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. 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. Example C(CH2)3CH3 + H2 CH3(CH2)3C Lindlar Pd CH3(CH2)3 (CH2)3CH3 C C H H (87%) Useful In Industry • Large scale synthesis of vitamin A performed by Hoffman-LaRoche Partial Reduction RC CR' RCH CHR' RCH2CH2R' RCH Another way to convert alkynes to alkenes is by reduction with sodium (or lithium or potassium) in ammonia. trans-Alkenes are formed. Example CH3CH2C CH CCH2CH3 Na, NH3 CH3CH2 H C C CH2CH3 H (82%) Suggest efficient syntheses of (E)- and (Z)-2heptene from propyne and any necessary organic or inorganic reagents. Strategy Better drawing, why? Synthesis 1. NaNH2 1. 2. CH3CH2CH2CH2Br H2, Lindlar Pd Na, NH3 Retrosynthetic Analysis • Write out the reactions in reverse to first Molecule to be made + Br Strategies for Synthesis • Then write out reactions in the forward direction • Be sure to look out for incompatabilities Problem Br 2-Bromopentane • Devise a synthesis of 2-bromopentene from acetylene Retrosynthetic Analysis Say, you are only allow to use simple alkyl halides and acetylene, and any reagents. Br Br + Na Solution Problem OH 1-Hexanol • Can we synthesize 1-hexanol from acetylene? Retrosynthetic Analysis OH Br + Na Solution Addition of HX CH3(CH2)3C CH HBr HBr CH3(CH2)3C CH2 Br (60%) Follows Markovnikov's rule Alkynes are slightly less reactive than alkenes Intermolecular RateDetermining Step H RC H .. Br : .. CH CH .. Br : .. Observed rate law: rate = k[alkyne][HX]2 Observed Two Molar Equivalents of Hydrogen Halide CH3CH2C CH CCH2CH3 2 HF H CH3CH2 F C C H F (76%) CH2CH3 Hydration of Alkynes expected reaction: RC CR' + H2O H+ RCH CR' OH enol observed reaction: RC CR' + H2O H+ RCH2CR' O ketone Enols 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 Mechanism of conversion of enol to ketone .. :O H + :O H C H C H Mechanism of conversion of enol to ketone .. :O H : O: H H C C + H Mechanism of conversion of enol to ketone H .. :O H C C + H : O: H Mechanism of conversion of enol to ketone H .. :O H C C + H O: H Key Carbocation Intermediate Carbocation is stabilized by electron delocalization (resonance) electron .. :O H C C + .. +O H H C C H Example of Alkyne Hydration CH3(CH2)2C C(CH2)2CH3 H2O, H+ via Hg2+ CH3(CH2)2CH O CH3(CH2)2CH2C(CH2)2CH3 (89%) OH C(CH2)2CH3 Regioselectivity 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 CH2 Hydroboration/Oxidation of Alkynes • BH3 (borane) adds to alkynes to give a vinylic borane • Oxidation with H2O2 produces an enol that converts to the ketone or aldehyde • Process converts alkyne to ketone or aldehyde with orientation opposite to mercuric ion catalyzed hydration Addition of Halogens to Alkynes Cl HC C CCH3 + 2 Cll2 Cl2CH C CH3 Cl (63%) Addition is anti CH3CH2 CH3CH2C CCH2CH3 Br2 Br Br C C CH2CH3 Br (90%) Ozonolysis of Alkynes gives two carboxylic acids by cleavage of triple bond, and results in the highest oxidation of each product. Example CH3(CH2)3C CH 1. 1. 2. O O CH3(CH2)3COH (51%) O3 H2O + HOCOH ...
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