Chapter 7 The Carbon is sp hybridized:

Size: px

Start display at page:

Download "Chapter 7 The Carbon is sp hybridized:"

Lesley Perkins
7 years ago
Views:

1 Alkynes Chapter 7 1 The Carbon is sp hybridized: sp sp 2

2 There are 2 left over p orbitals for each C: 3 Nomenclature IUPAC: use the infix -yn- to show the presence of a carbon-carbon triple bond Methyl-1-bu tyn e ,6-D imethyl-3-hep tyn e 1,6-ep tadiyne Common names: prefix the substituents on the triple bond to the word acetylene. IUPAC name: Common name: 2-Butyne D imethylacetylene 1-Buten-3-yne Vinylacetylene 4

3 Physical Properties Similar to alkanes and alkenes of comparable molecular weight and carbon skeleton. Name Ethyne Propyne 1-Butyne 2-Butyne 1-Pentyne Formula C C C 3 C C C 3 C 2 C C C 3 C CC 3 C 3 ( C 2 ) 2 C C Melting Point ( C) Boiling Point ( C) Density at 20 C (g/ml) (a gas) (a gas) (a gas) exyne C 3 ( C 2 ) 3 C C Octyne C 3 ( C 2 ) 5 C C Decyne C 3 ( C 2 ) 7 C C Acidity The pk a of acetylene and terminal alkynes is approximately 25, which makes them stronger acids than ammonia but weaker acids than alcohols (Section 4.1). Terminal alkynes react with sodium amide to form alkyne anions. -C C- N 2 -C C: - N 3 pk a 25 pk a 38 (S tronger acid) (Weaker acid) 6

4 Acidity Terminal alkynes can also be converted to alkyne anions by reaction with sodium hydride or lithium diisopropylamide (LDA). Na [( C 3 ) 2 C] 2 N Li Sodium hydride Lith ium diisoprop ylamide (LDA) Because water is a stronger acid than terminal alkynes, hydroxide ion is not a strong enough base to convert a terminal alkyne to an alkyne anion. C C O - C C- 2 O pk a 25 pk a 15.7 (Weaker acid) (Stronger acid) K eq = Alkylation of Alkyne Anions Alkyne anions are both strong bases and good nucleophiles. They participate in nucleophilic substitution reactions with alkyl halides to form new C-C bonds to alkyl groups; they undergo alkylation. Because alkyne anions are also strong bases, alkylation is practical only with methyl and 1 halides With 2 & 3 halides, elimination is the major reaction. 8

5 Alkylation of Alkyne Anions Alkylation of alkyne anions is the most convenient method for the synthesis of terminal alkynes. C C - Na Sodium acetylid e 1-omob utane 1-exyne Na - Alkylation can be repeated and a terminal alkyne can be converted to an internal alkyne. C 3 C 2 C C - Na Sodium butynide C 3 C 2 - omoethan e C 3 C 2 C CC 2 C 3 3-exyne Na - 9 Alkyne Anions as Strong Bases With 2 & 3 halides, elimination is the major reaction: C C - Na Sodium acetylide Because alkyne anions are also strong bases, alkylation is practical only with methyl and 1 halides, however- elimination C C Na - (Ch 9) omocyclohexane Acetylene Cycloh exene 10

6 Preparation from Alkenes Treatment of a vicinal dibromoalkane with two moles of base, most commonly sodium amide, results in two successive dehydrohalogenation reactions (removal of and X from adjacent carbons) and formation of an alkyne. C 3 C=CC 3 2-Buten e 2 C 2 Cl 2 C 3 C-CC 3 N 3 ( l) 2NaN 2-33 o C Sodiu m amide C 3 C CC 3 2Na 2N 3 2-Butyne 11 Preparation from Alkenes For a terminal alkene to a terminal alkyne, 3 moles of base are required: 2 3 NaN 2 C 3 ( C 2 ) 3 C= C 2 C 3 ( C 2 ) 3 C-C exene 1,2-Dibromohexane C 3 ( C 2 ) 3 C C - Na Sodium salt of 1-hexyne 2 O C 3 ( C 2 ) 3 C C 1-exyne 12

7 Addition of X 2 Alkynes add one mole of bromine to give a dibromoalkene. Addition shows anti stereoselectivity. 3 C C C 3 C CC 3 3 COO, Li 2 C C anti addition C 3 2-Bu tyne (E)-2,3-D ibromo-2-b utene 13 Addition of X 2 The intermediate in bromination of an alkyne is a bridged bromonium ion. 3 C C C C 3 C C C C 3 C C 3 3 C C 3 3 C C C C 3 14

8 Addition of X Alkynes undergo regioselective addition of either 1 or 2 moles of X, depending on the ratios in which the alkyne and halogen acid are mixed. C 3 C C Propyne C 3 C= C 2 C 3 CC 3 2-omopropene 2,2-Dibromopropane Follows Markovnikov s rule, but why? 15 Addition of X The intermediate in addition of X is a 2 vinylic carbocation in preference to a 1 vinylic carbocation. C 3 C C - C 3 C=C 2 A 2 vinylic carbocation Reaction of the vinylic cation (an electrophile) with halide ion (a nucleophile) gives the product. C 3 C=C 2 C 3 C=C 2 2-omopropen e 16

9 In the addition of the second mole of X, Step 1 is reaction of the electron pair of the remaining pi bond with to form a carbocation. Of the two possible carbocations, the favored one is the resonance-stabilized 2 carbocation. C 3 C C 2 slower faster Addition of X C 3 C C 2 1 Carb ocation C 3 C C 2 C 3 C C 2 Resonance-stabilized 2 carbocation C 3 CC 3 17 Reduction Treatment of an alkyne with hydrogen in the presence of a transition metal catalyst, most commonly Pd, Pt, or Ni, converts the alkyne to an alkane. C 3 C CC 3 2-Butyne 2 2 Pd, Pt, or Ni 3 atm C 3 C 2 C 2 C 3 Butane No selectivity-complete reduction!! 18

10 Reduction With the Lindlar catalyst, reduction stops at addition of one mole of 2. This reduction shows syn stereoselectivity. Lin dlar 3 C C 3 catalyst C 3 C CC 3 2 C C 2-Butyne Pt on CaCO 3 cis-2-buten e 19 Dissolving Metal Reduction Reduction of an alkyne with Na or Li in liquid ammonia converts an alkyne to an alkene with anti stereoselectivity. R R R 2 Na N 3 ( l) R 2 NaN 2 4-Octyne 2 Na N 3 ( l) trans- 4-Octene 20

11 ydroboration Addition of borane to an internal alkyne gives a trialkenylborane. Addition is syn stereoselective. 3 B 3 TF 3-exyne B R R A trialkenylborane (R = cis- 3-hexenyl group) 21 ydroboration Treating an alkenylborane with 2 O 2 in aqueous NaO gives an enol. Enol: A compound containing an O group on one carbon of a carbon-carbon double bond. 2-Butyne 1. B O 2, NaO O O 2-Buten-2-ol (an enol) 2-Butanone (a ketone) K e q = 6.7 x 10 6 (for keto-enol tautomerism) Combination of alkene and alcohol 22

12 ydroboration An enol is in equilibrium with a keto form by migration of a hydrogen from oxygen to carbon and migratrion of the double bond from C=C to C=O. Keto forms generally predominate at equilibrium. Keto and enol forms are tautomers and their interconversion is called tautomerism. O O K e q = 6.7 x Buten-2-ol (an enol) 2-Butanone (a ketone) (for keto-enol tautomerism) 23 ydroboration To prevent dihydroboration with terminal alkynes, it is necessary to use a sterically hindered dialkylborane, such as (sia) 2 B. B- Di-sec-isoamylborane [(sia) 2 B] Treatment of a terminal alkyne with (sia) 2 B results in stereoselective and regioselective hydroboration. 1-Octyne ( sia) 2 B B( sia) 2 An alkenylborane 24

13 B- Di-sec-isoamylborane [(sia) 2 B] 25 R R C B C R NaO 2 O 2 R C O C 26

14 ydroboration/oxidation of an internal alkyne gives a ketone: 3-exyne ydroboration 1. B O 2, NaO 3-exanone ydroboration/oxidation of a terminal alkyne gives an aldehyde: 1. (sia) 2 B O 1-Octyne 2. 2 O 2, NaO O O An enol Octanal 27 Addition of 2 O: hydration In the presence of sulfuric acid and g(ii) salts, alkynes undergo addition of water. C 3 C C Propyne O O 2 SO 4 2 O C gso 3 C= C 2 C 3 CC Propen-2-ol Propanone (an enol) (Acetone) (Omit mechanism) 28

15 We use a method called a retrosynthesis and an open arrow to symbolize a step in a retrosynthesis. target molecule Organic Synthesis starting materials Retrosynthesis: A process of reasoning backwards from a target molecule to a set of suitable starting materials. 29 Organic Synthesis Target molecule: cis-3-hexene. disconnect here cis-3-exene 3-exyne - :C C: - Acetylide dianion 2 C3 C2 omoethane 30

16 Organic Synthesis Starting materials are acetylene and bromoethane. C C Acetylene 1. NaN 2 3. NaN 2 2. C 3 C 2 1-Butyne 4. C 3 C 2 3-exyne 5. 2 Lin dlar catalyst cis-3-exene 31 Organic Synthesis Target molecule: 2-heptanone An acid-catalyzed h yd ration of this alkyne gives a mixture of 2-heptanone and 3-heptanone O 2-eptyne 2-ep tanone An acid-catalyzed hydration of this alkyne gives 2-heptan on e 1-eptyne C C - Acetylide anion 1-omopentane 32

17 Organic Synthesis Starting materials are acetylene and 1-bromopentane. C C 1. NaN ep tyne 3. 2 O 2 SO 4, gso 4 O 2-eptanone 33

Electrophilic Addition Reactions

Electrophilic Addition Reactions Electrophilic addition reactions are an important class of reactions that allow the interconversion of C=C and C C into a range of important functional groups. Conceptually,