Transcript Uses of Alkynes
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
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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
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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
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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
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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
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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
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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
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MO Bonding Picture
14.2 Structure and Bonding in Alkynes
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MO Bonding Picture
14.2 Structure and Bonding in Alkynes
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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
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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
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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
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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
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NMR Spectroscopy of Alkynes
• However, the effect is in the opposite direction
14.3 Physical Properties of Alkynes
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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
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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
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Problems
14.4 Introduction to Addition Reactions of the Triple Bond
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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
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Mechanism of Hydration
14.5 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?
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