The Structure of Alkynes

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1 Synthesis of Acetylene Alkynes eating coke with lime in an electric furnace to forms calcium carbide. Then drip water on the calcium carbide. 3 ao a 2 coke lime O * a O a(o) 2 *This reaction was used to produce light for miners lamps and for the stage. The Structure of Alkynes Arrange ethane, ethene, and ethyne in order of increasing - bond length. A) ethane < ethene < ethyne B) ethene < ethane < ethyne ) ethyne < ethene < ethane D) ethane < ethyne < ethene A triple bond is composed of a σ bond and two π bonds Acidity of ydrocarbons Acidity of Acetylene and Terminal Alkynes In general, hydrocarbons are very weak acids ompound 2 pk a F O 16 N

2 Acetylene is a weak acid, but not nearly as weak as alkanes or alkenes. ompound pk a F O N Acetylene Which one of the following is the strongest acid? A) water B) ammonia ) 1-butene D) 1-butyne arbon: ybridization and Electronegativity : sp : sp : sp Electrons in an orbital with more s character are closer to the nucleus and more strongly held. Which one of the following statements best explains the greater acidity of terminal alkynes (R ) compared with monosubstituted alkenes (R= 2 )? A) The sp-hybridized carbons of the alkyne are less electronegative than the sp 2 carbons of the alkene. B) The two π bonds of the alkyne are better able to stabilize the negative charge of the anion by resonance. ) The sp-hybridized carbons of the alkyne are more electronegative than the sp 2 carbons of the alkene. D) The question is incorrect - alkenes are more acidic than alkynes. The stronger the acid, the weaker its conjugate base top 252 2

3 Solution: Use a stronger base. Sodium amide is a stronger base than sodium hydroxide. NaN N 2 Na N N : : 2 N 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. Which of the following bases is strong enough to completely deprotonate propyne? A) N 3 B) 3 O ) NaN 2 D) NaO Synthesis Using Acetylide Ions: Formation of Bond Preparation of Various Alkynes by alkylation reactions with Acetylide or Terminal Alkynes Alkylation of Acetylene and Terminal Alkynes Alkylation of Acetylene and Terminal Alkynes : R X S N 2 R : X R R 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( monosubstituted) alkyne. 3

4 Example: Alkylation of Acetylene NaN 2 N 3 Na (70-77%) Br Which alkyl halide will react faster with the acetylide ion ( Na) in an S N 2 reaction? A) bromopropane B) 2-bromopropane ) tert-butyl iodide D) 1-bromo-2-methylbutane Example: Alkylation of a Terminal Alkyne Example: Dialkylation of Acetylene ( 3 ) 2 2 ( 3 ) 2 2 NaN 2, N 3 3 Br Na NaN 2, N Br 1. NaN 2, N Br ( 3 ) 2 2 (81%) (81%) 3 Limitation Acetylide Ion as a Base E2 predominates over S N 2 when alkyl halide is secondary or tertiary. Effective only with primary alkyl halides Secondary and tertiary alkyl halides undergo elimination : X E2 : X 4

5 onsider the reaction of each of the following with cyclohexyl bromide. For which one is the ratio of substitution to elimination highest? A) NaO 2 3, ethanol, 60 B) NaS 2 3, ethanol-water, 25 ) NaN 2, N 3, -33 D) Na, N 3, -33 Preparation of Alkynes by Elimination Reactions Preparation of Alkynes by "Double Dehydrohalogenation" Geminal dihalide Alkyne X ( 3 ) 3 2 ll 2 X X X 1. 3NaN 2, N O Geminal dihalide Vicinal dihalide The most frequent applications are in preparation of terminal alkynes. ( 3 ) 3 (56-60%) Geminal dihalide Alkyne ( 3 ) 3 2 ll 2 ( 3 ) 3 ( 3 ) 3 l l NaN 2, N 3 NaN 2, N 3 (slow) (slow) In addition to NaN 2, what other base can be used to convert 1,1-dichlorobutane into 1-butyne? A) NaO 3 B) NaO 2 O ( 3 ) 3 Na NaN 2, N 3 (fast) ) NaO 2 3 D) KO( 3 ) 3 5

6 Vicinal dihalide Alkyne 3 ( 2 ) 7 2 Br Br 3 ( 2 ) 7 (54%) 1. 3NaN 2, N O Which of the following compounds yield 1- heptyne on being treated with three moles of sodium amide (in liquid ammonia as the solvent) followed by adding water to the reaction mixture? A) 1,1,2,2-tetrachloroheptane B) 1-bromo-2-chloroheptane ) 1,1,2-trichloropentane D) all of the above Reactions of Alkynes Reactions of Alkynes Acidity ydrogenation Metal-Ammonia Reduction Addition of ydrogen alides ydration Addition of alogens Ozonolysis Atomic Force Microscopy of Acetylene Lawrence Berkeley Laboratory (LBL) ydrogenation of Alkynes 6

7 Imaging: acetylene on Pd(111) at 28 K Molecular Image Tip cruising altitude ~700 pm _z = 20 pm Why don t we see the Pd atoms? Because the tip needs to be very close to image the Pd atoms and would knock the molecule away Excitation of frustrated rotational modes in acetylene molecules on Pd(111) at T = 30 K Tip e - TIP p z O π orbital alculated image (Philippe Sautet) The STM image is a map of the pi-orbital of distorted acetylene Surface atomic profile Tip cruising altitude ~500 pm _z = 2 pm If the tip was made as big as an airplane, it would be flying at 1 cm from the surface and waving up an down by 1 micrometer ((( ) ( ))) M. Salmeron (LBL) 1 cm (± 1 _m) M. Salmeron (LBL) ydrogenation of Alkynes Partial ydrogenation R R' 2 2 cat R 2 2 R' R R' 2 cat R R' 2 cat R 2 2 R' catalyst = Pt, Pd, Ni, or Rh alkene is an intermediate 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. Lindlar Palladium Example R R' 2 cat R R' 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 ao 3, which has been poisoned with lead acetate and quinoline. syn-ydrogenation occurs; cis alkenes are formed. R 2 2 R' 3 ( 2 ) 3 3 ( 2 ) 3 ( 2 ) 3 3 (87%) Lindlar Pd 2 ( 2 ) 3 3 7

8 Partial Reduction Metal-Ammonia Reduction of Alkynes R R' R R' R 2 2 R' Alkynes trans-alkenes Another way to convert alkynes to alkenes is by reduction with sodium (or lithium or potassium) in ammonia. trans-alkenes are formed. Example Na, N (82%) 2 3 8

9 ow would you accomplish the following conversion? A) NaN 2 B) 2, Lindlar Pd ) Na, N 3 D) either B or Metal (Li, Na, K) is reducing agent; 2 is not involved; proton comes from N 3 four steps Mechanism (1) electron transfer (2) proton transfer (3) electron transfer (4) proton transfer Answer Select the most effective way to synthesize cis- 2-pentene from 1-propyne. A) 1) NaN 2) Br 3) 2, Pd B) 1) NaN 2) 2 3 Br 3) 2, Lindlar Pd ) 1) NaN 2) I 3) 2, Lindlar Pd D) 1) NaN 2) Br 3) Na,N 3 Select the most effective way to synthesize cis- 2-pentene from 1-propyne. A) 1) NaN 2) Br 3) 2, Pd B) 1) NaN 2) 2 3 Br 3) 2, Lindlar Pd ) 1) NaN 2) I 3) 2, Lindlar Pd D) 1) NaN 2) Br 3) Na,N 3 Which reagent would accomplish the transformation of 3-hexyne into trans-3- hexene? A) /Ni 2 B) 2, Lindlar Pd ) Na, N 3 D) NaN, N 2 3 9

10 Answer Problem Which reagent would accomplish the transformation of 3-hexyne into trans-3- hexene? A) /Ni 2 B) 2, Lindlar Pd ) Na, N 3 D) NaN, N 2 3 Suggest an efficient syntheses of (E)-( and (Z)-2-( heptene from propyne and any necessary organic or inorganic reagents. Problem Strategy Problem Strategy Problem Synthesis 2, Lindlar Pd 1. NaN Br Na, N 3 Which would be the best sequence of reactions to use in order to prepare cis-3-nonene from 1-butyne? A) 1. NaN 2 in N 3 ; 2. 1-bromopentane; 3. 2, Lindlar Pd B) 1. NaN 2 in N 3 ; 2. 1-bromopentane; 3. Na, N 3 ) 1. 2, Lindlar Pd; 2. NaN 2 in N 3 ; bromopentane D) 1. Na, N 3 ; 2. NaN 2 in N 3 ; bromopentane 10

11 Follows Markovnikov's Rule Addition of ydrogen alides to Alkynes 3 ( 2 ) 3 Br 3 ( 2 ) 3 Br 2 (60%) Alkynes are slightly less reactive than alkenes Two Molar Equivalents of ydrogen alide Free-radical Addition of Br F 3 ( 2 ) 3 Br peroxides 3 ( 2 ) 3 Br F (79%) regioselectivity opposite to Markovnikov's rule F (76%) ydration of Alkynes expected reaction: R R' 2 O R R' observed reaction: R R' 2 O enol O R 2 R' O ketone 11

12 Enols R R' R 2 R' enol O enols are regioisomers of ketones,, and exist in equilibrium with them keto-enol enol equilibration is rapid in acidic media ketones are more stable than enols and predominate at equilibrium O ketone Mechanism of conversion of enol to ketone Mechanism of conversion of enol to ketone.. : O.. : O : O : O Mechanism of conversion of enol to ketone Mechanism of conversion of enol to ketone : O:.. : O.. : O : O: 12

13 Key arbocation Intermediate Mechanism of conversion of enol to ketone arbocation is stabilized by electron delocalization (resonance)..... : O O.. : O : O: Useful for symmetrical starting alkynes to produce a single product. Unsymmetrical starting alkynes that are not terminal produce a mixture of ketones nonnon-regioselectively. Markovnikov's rule followed in formation of enol, Useful with terminal alkynes. O 3 ( 2 ) 5 Regioselectivity 2 O, 2 SO 4 gso 4 via O 3 ( 2 ) ( 2 ) 5 3 (91%) Aldehyde vs. Ketone What is the product of the acid catalyzed hydration of 1-hexyne? A) B) ) D) 13

14 Example Addition of alogens to Alkynes 3 2 l 2 l 2 l 3 l (63%) Addition is anti Br 2 3 Br (90%) Br 2 3 Ozonolysis of Alkynes gives two carboxylic acids by cleavage of triple bond Example 3 ( 2 ) 3 1. O O What product is formed when 2-butyne is subjected to ozonolysis? A) B) O 3 ( 2 ) 3 O O OO O ) D) (51%) 14

15 an you identify and name the function? Alkynes Synthesis & Functions 15

16 Example What is the structure of ompound Y in the following synthetic sequence? A) pentane B) cis-2-pentene ) trans-2-pentene D) 2-pentyne 16

Chapter 22 Carbonyl Alpha-Substitution Reactions

John E. McMurry www.cengage.com/chemistry/mcmurry Chapter 22 Carbonyl Alpha-Substitution Reactions The α Position The carbon next to the carbonyl group is designated as being in the α position Electrophilic