Chapter 14 The Chemistry of Alkynes Alkynes Also

Chapter 14 The Chemistry of Alkynes

Alkynes • Also known as “Acetylenes” • Naturally occurring alkynes are relatively rare • They do not occur as a petroleum constituents Carotatoxin 14. 10 Occurrence and Use of Alkynes 2

Uses of Alkynes • Acetylene is a chemical feedstock for many important compounds • Occur in antiviral and antifungal drugs – Efavirenz 14. 10 Occurrence and Use of Alkynes 3

Common Nomenclature • Simple alkynes are named as acetylene derivatives acetylene • Certain compounds are derivatives of the propargyl group (HC C-CH 2 -) 14. 1 Nomenclature of Alkynes 4

IUPAC Nomenclature • Follow the rules for naming alkenes except, change “-ane” to “-yne” 5

IUPAC Nomenclature • Principal groups that have priority over the triple bond receive preference in numbering and suffix – Recall: Carboxylic acid > anhydride > ester > acid halide amide > nitrile > aldehyde > ketone > alcohol > thiol > amine – Substituent groups containing a triple bond = alkynyl groups • Named by replacing final “-e” with “-yl” 14. 1 Nomenclature of Alkynes 6

Substitutive Nomenclature • If both C=C and C C are present, the bond with the lowest number gets precedence • However, if the rule is ambiguous, the double bond gets precedence 14. 1 Nomenclature of Alkynes 7

Problems • Name the following compounds: 8

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Structure and Bonding • Alkynes display linear geometry • cis-trans isomerism cannot occur in alkynes • Cycloalkynes smaller than cyclooctyne cannot be isolated under ordinary conditions 14. 2 Structure and Bonding in Alkynes 10

MO Bonding Picture • Alkynes utilize sp hybrid orbitals • Electrons in an sp hybrid orbital are held closer to the nucleus on average 14. 2 Structure and Bonding in Alkynes 11

MO Bonding Picture 14. 2 Structure and Bonding in Alkynes 12

MO Bonding Picture 14. 2 Structure and Bonding in Alkynes 13

Heats of Formation • Alkynes are less stable than isomeric dienes • Internal alkynes are more stable than terminal alkynes 14. 2 Structure and Bonding in Alkynes 14

Boiling Points and Solubilities • Alkyne boiling points are not much different from those of analogous alkenes and alkanes • Similarly, alkynes have low densities and are insoluble in water 14. 3 Physical Properties of Alkynes 15

IR Spectroscopy of Alkynes • C C stretch: 2100 -2200 cm-1 • Symmetrical alkynes will not show this stretch • C-H stretch: 3300 cm-1 14. 3 Physical Properties of Alkynes 16

NMR Spectroscopy of Alkynes • The reason for the unusual acetylenic proton chemical shift is similar to that described for vinylic protons 14. 3 Physical Properties of Alkynes 17

NMR Spectroscopy of Alkynes • However, the effect is in the opposite direction 14. 3 Physical Properties of Alkynes 18

NMR Spectroscopy of Alkynes • Alkynyl carbons typically appear at d 65 -80 • Propargylic carbons also display smaller chemical shifts (5 -10 ppm lower than alkyl) 14. 3 Physical Properties of Alkynes 19

Preparation of Alkynes • Alkynes prepared by elimination of HX from alkyl halides 20

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Addition Rxns of Alkynes • Similar to addition reactions for alkenes – Alkyne addition rxns are often slower than correspondig alkene additions – Regioselectivity follows a similar course • Addition of HX and X 2: 22

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Problems • Give the alkene products for the following addition rxns: 24

Regioselectivity of Second Addition Rxn 25

Problems 14. 4 Introduction to Addition Reactions of the Triple Bond 26

Hydration of Alkynes • Addition of H 2 O • As with alkenes, alkynes can be hydrated by two methods 1) Mercury (II) Catalyzed Hydration • The product is a ketone and not an alcohol Recall: 27

Keto-Enol Tautomerism • Tautomers: constitutional isomers that interconvert rapidly – Equilibrium lies far to the right 28

Mechanism of Mercury (II) Catalyzed Hydration 14. 5 Conversion of Alkynes into Aldehydes and Ketones 29

Mechanism of Hydration 14. 5 Conversion of Alkynes into Aldehydes and Ketones 30

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Problems 1) Give the complete mechanism for the following reaction: 2) What is the product for the following rxn? 32