Chapter 9 Alkynes Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
9.1 Sources of Alkynes
Acetylene Industrial preparation of acetylene is by dehydrogenation of ethylene. CH 3 CH 3 800 C H 2 C CH 2 + H 2 1150 C H 2 C CH 2 HC CH + H 2 Cost of energy makes acetylene a more expensive industrial chemical than ethylene.
Naturally Occurring Alkynes Some alkynes occur naturally. For example, CH 3 (CH 2 ) 10 C O C(CH 2 ) 4 COH Tariric acid: occurs in seed of a Guatemalan plant.
Naturally Occurring Alkynes Some alkynes occur naturally. For example, Histrionicotoxin: defensive toxin in the poison dart frogs of Central and South America
9.2 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 CCH 3 Propyne HC CCH 2 CH 3 1-Butyne or But-1-yne (CH 3 ) 3 CC CCH 3 4,4-Dimethyl-2-pentyne or 4,4-Dimethyl-pent-2-yne
9.3 Physical Properties of Alkynes The physical properties of alkynes are similar to those of alkanes and alkenes.
9.4 Structure and Bonding in Alkynes: sp Hybridization
Structure Linear geometry for acetylene 120 pm H C C H 106 pm 106 pm 121 pm CH 3 C C H 146 pm 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
sp Hybridization in Acetylene Mix together (hybridize) the 2s orbital and one of the three 2p orbitals. 2p 2p 2sp 2s
sp Hybridization in Acetylene 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 σ bonds. 2sp
σ Bonds in Acetylene 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. Figure 9.2 (a)
π Bonds in Acetylene One of the two π bonds in acetylene is shown here. The second π bond is at right angles to the first. Figure 9.2 (b)
π Bonds in Acetylene This is the second of the two π bonds in acetylene. Figure 9.2 (c)
Figure 9.3 Electrostatic Potential in Acetylene 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.
Table 9.1 Structural Features 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 sp 3 sp 2 sp % s character 25% 33% 50% pk a 62 45 26
9.5 Acidity of Acetylene and Terminal Alkynes H C C Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Acidity of Hydrocarbons In general, hydrocarbons are exceedingly weak acids, but alkynes are not nearly as weak as alkanes or alkenes. Compound pk a HC CH 26 H 2 C CH 2 45 CH 4 60
Carbon: Hybridization and Electronegativity pk a = 62 C H H + + C : sp 3 H pk a = 45 : C H + + sp C C C 2 pk a = 26 C C H H + + C C : sp Electrons in an orbital with more s character are closer to the nucleus and more strongly held.
Sodium Acetylide Objective: Prepare a solution containing sodium acetylide NaC CH Will treatment of acetylene with NaOH be effective? NaOH + HC CH NaC CH + H 2 O
Sodium Acetylide No. Hydroxide is not a strong enough base to deprotonate acetylene... HO.. :.. + H C CH HO.. H + : C CH weaker acid pk a = 26 stronger acid pk a = 15.7 In acid-base reactions, the equilibrium lies to the side of the weaker acid.
Solution: Use a stronger base. Sodium amide is a stronger base than sodium hydroxide. NaNH 2 + HC CH NaC CH + NH 3.... H 2 N : + H C CH H H + : C 2 N CH stronger acid pk a = 26 Sodium Acetylide weaker acid pk a = 36 Ammonia is a weaker acid than acetylene. The position of equilibrium lies to the right.
9.6 Preparation of Alkynes by Alkylation of Acetylene and Terminal 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
Alkylation of Acetylene and Terminal Alkynes H C C H R C C H R C C R
Alkylation of Acetylene and Terminal Alkynes H C C: + R X S N 2 H C C R + : X 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.
Example: Alkylation of Acetylene HC CH NaNH 2 NH 3 HC CNa CH 3 CH 2 CH 2 CH 2 Br HC C CH 2 CH 2 CH 2 CH 3 (70-77%)
Example: Alkylation of a Terminal Alkyne (CH 3 ) 2 CHCH 2 C CH NaNH 2, NH 3 (CH 3 ) 2 CHCH 2 C CNa CH 3 Br (CH 3 ) 2 CHCH 2 C (81%) C CH 3
Example: Dialkylation of Acetylene H C C H 1. NaNH 2, NH 3 2. CH 3 CH 2 Br CH 3 CH 2 C C H 1. NaNH 2, NH 3 2. CH 3 Br CH 3 CH 2 C C CH 3 (81%)
Limitation Effective only with primary alkyl halides Secondary and tertiary alkyl halides undergo elimination
Acetylide Ion as a Base E2 predominates over S N 2 when alkyl halide is secondary or tertiary. H C C: H C E2 C X H C C H + C C + : X
9.7 Preparation of Alkynes by Elimination Reactions
Preparation of Alkynes by Double Dehydrohalogenation H X H H C C C C H X X X Geminal dihalide Vicinal dihalide The most frequent applications are in preparation of terminal alkynes.
Geminal dihalide Alkyne (CH 3 ) 3 CCH 2 CHCl 2 1. 3NaNH 2, NH 3 2. H 2 O (CH 3 ) 3 CC CH (56-60%)
Geminal dihalide Alkyne (CH 3 ) 3 CCH 2 CHCl 2 (CH 3 ) 3 CCH CHCl (CH 3 ) 3 CC CH NaNH 2, NH 3 NaNH 2, NH 3 (slow) (slow) H 2 O (CH 3 ) 3 CC CNa NaNH 2, NH 3 (fast)
Vicinal dihalide Alkyne CH 3 (CH 2 ) 7 CH CH 2 Br Br 1. 3NaNH 2, NH 3 2. H 2 O CH 3 (CH 2 ) 7 C CH (54%)
9.8 Reactions of Alkynes Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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)
9.9 Hydrogenation of Alkynes
Hydrogenation of Alkynes RC CR' + 2H 2 cat RCH 2 CH 2 R' catalyst = Pt, Pd, Ni, or Rh Alkene is an intermediate.
Heats of Hydrogenation CH 3 CH 2 C CH CH 3 C CCH 3 292 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.
Partial Hydrogenation RC CR' H 2 RCH CHR' cat H 2 cat RCH 2 CH 2 R' 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.
Lindlar Catalyst RC CR' H 2 RCH CHR' cat H 2 cat 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. RCH 2 CH 2 R' syn-hydrogenation occurs; cis alkenes are formed.
Example CH 3 (CH 2 ) 3 C C(CH 2 ) 3 CH 3 + H 2 Lindlar Pd CH 3 (CH 2 ) 3 (CH 2 ) 3 CH 3 C C H (87%) H
9.10 Metal-Ammonia Reduction of Alkynes Alkynes trans-alkenes
Partial Reduction RC CR' RCH CHR' 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.
Example CH 3 CH 2 C CCH 2 CH 3 Na, NH 3 CH 3 CH 2 H C C H (82%) CH 2 CH 3
Mechanism Metal (Li, Na, K) is reducing agent; H 2 is not involved Four steps (1) electron transfer (2) proton transfer (3) electron transfer (4) proton transfer
Mechanism Step (1): Transfer of an electron from the metal to the alkyne to give an anion radical. R M + C C R' + M. R Ċ.. C R'
Mechanism Step (2): Transfer of a proton from the solvent (liquid ammonia) to the anion radical. R. C.. C R' R C. C R'.. H NH 2 H.. : NH 2
Mechanism Step (3): Transfer of an electron from the metal to the alkenyl radical to give a carbanion. R C. C R' + M. R C M +.. C H H R'
Mechanism Step (4): Transfer of a proton from the solvent (liquid ammonia) to the carbanion. R C.. C.. H NH 2.. R H : NH 2 C C H R' H R'
Suggest efficient syntheses of (E)- and (Z)-2- heptene from propyne and any necessary organic or inorganic reagents.
Strategy
Synthesis 1. NaNH 2 2. CH 3 CH 2 CH 2 CH 2 Br H 2, Lindlar Pd Na, NH 3
9.11 Addition of Hydrogen Halides to Alkynes
Follows Markovnikov's Rule CH 3 (CH 2 ) 3 C CH HBr CH 3 (CH 2 ) 3 C CH 2 Br (60%) Alkynes are slightly less reactive than alkenes.
Termolecular Rate-determining Step.. H Br :.. RC CH.. H Br :.. Observed rate law: rate = k[alkyne][hx] 2
Two Molar Equivalents of Hydrogen Halide CH 3 CH 2 C CCH 2 CH 3 2 HF H F CH 3 CH 2 C C CH 2 CH 3 H F (76%)
Free-radical Addition of HBr CH 3 (CH 2 ) 3 C CH HBr peroxides CH 3 (CH 2 ) 3 CH CHBr (79%) regioselectivity opposite to Markovnikov's rule
9.12 Hydration of Alkynes
Hydration of Alkynes expected reaction: RC CR' + H 2 O H + RCH CR' observed reaction: RC CR' + H 2 O H + OH enol RCH 2 CR' O ketone
Enols RCH CR' RCH 2 CR' enol OH O ketone Enols are tautomers 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 H + : O H H C.. : O C H
Mechanism of Conversion of Enol to Ketone.. : O H H : O: H C C + H
Mechanism of Conversion of Enol to Ketone H H C.. : O C + H : O: H
Mechanism of Conversion of Enol to Ketone H H C.. : O C H + O: H
Key Carbocation Intermediate Carbocation is stabilized by electron delocalization (resonance)... : O H +.. O H H C C + H C C
Example of Alkyne Hydration CH 3 (CH 2 ) 2 C C(CH 2 ) 2 CH 3 H 2 O, H + Hg 2+ via OH O CH 3 (CH 2 ) 2 CH C(CH 2 ) 2 CH 3 CH 3 (CH 2 ) 2 CH 2 C(CH 2 ) 2 CH 3 (89%)
Regioselectivity Markovnikov's rule followed in formation of enol O CH 3 (CH 2 ) 5 C CH H 2 O, H 2 SO 4 CH 3 (CH 2 ) 5 CCH 3 HgSO 4 (91%) via OH CH 3 (CH 2 ) 5 C CH 2
9.13 Addition of Halogens to Alkynes
Example HC CCH 3 + 2Cl 2 Cl Cl 2 CH C CH 3 Cl (63%)
Addition is anti CH 3 CH 2 Br 2 CH 3 CH 2 C CCH 2 CH 3 C C Br Br CH 2 CH 3 (90%)
9.14 Ozonolysis of Alkynes gives two carboxylic acids by cleavage of triple bond
Example CH 3 (CH 2 ) 3 C CH 1. O 3 2. H 2 O O CH 3 (CH 2 ) 3 COH (51%) + O HOCOH