Uses of Alkynes
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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 (HCC-CH2-) 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 CC 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 9 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 • CC 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 21 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 X2: 22 23 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 H2O • 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 31 Problems 1) Give the complete mechanism for the following reaction: 2) What is the product for the following rxn? 32
