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 Occurrence and Use of Alkynes Carotatoxin

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

Common Nomenclature Simple alkynes are named as acetylene derivatives Nomenclature of Alkynes Certain compounds are derivatives of the propargyl group (HC  C-CH 2 -) acetylene

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” Nomenclature of Alkynes

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 Nomenclature of Alkynes

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 Structure and Bonding in Alkynes

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

MO Bonding Picture Structure and Bonding in Alkynes

MO Bonding Picture Structure and Bonding in Alkynes

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

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 Physical Properties of Alkynes

IR Spectroscopy of Alkynes C  C stretch: cm -1 Symmetrical alkynes will not show this stretch  C-H stretch: 3300 cm Physical Properties of Alkynes

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

NMR Spectroscopy of Alkynes However, the effect is in the opposite direction Physical Properties of Alkynes

NMR Spectroscopy of Alkynes Alkynyl carbons typically appear at  Propargylic carbons also display smaller chemical shifts (5-10 ppm lower than alkyl) Physical Properties of Alkynes

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

Introduction to Addition Reactions of the Triple Bond Problems

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 27 Recall:

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

Mechanism of Mercury (II) Catalyzed Hydration Conversion of Alkynes into Aldehydes and Ketones

Mechanism of Hydration Conversion of Alkynes into Aldehydes and Ketones

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