Chapter 14The Chemistry of Alkynes

Alkynes

• Also known as “Acetylenes”• Naturally occurring alkynes are relatively rare• They do not occur as a petroleum constituents

214.10 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

314.10 Occurrence and Use of Alkynes

Common Nomenclature

• Simple alkynes are named as acetylene derivatives

414.1 Nomenclature of Alkynes

• Certain compounds are derivatives of the propargyl group (HCC-CH2-)

acetylene

IUPAC Nomenclature

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

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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”

614.1 Nomenclature of Alkynes

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

714.1 Nomenclature of Alkynes

Problems

• Name the following compounds:

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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

1014.2 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

1114.2 Structure and Bonding in Alkynes

MO Bonding Picture

1214.2 Structure and Bonding in Alkynes

MO Bonding Picture

1314.2 Structure and Bonding in Alkynes

Heats of Formation

• Alkynes are less stable than isomeric dienes• Internal alkynes are more stable than terminal

alkynes

1414.2 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

1514.3 Physical Properties of Alkynes

IR Spectroscopy of Alkynes

• CC stretch: 2100-2200 cm-1

• Symmetrical alkynes will not show this stretch• C-H stretch: 3300 cm-1

1614.3 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

1714.3 Physical Properties of Alkynes

NMR Spectroscopy of Alkynes

• However, the effect is in the opposite direction

1814.3 Physical Properties of Alkynes

NMR Spectroscopy of Alkynes

• Alkynyl carbons typically appear at 65-80• Propargylic carbons also display smaller

chemical shifts (5-10 ppm lower than alkyl)

1914.3 Physical Properties of Alkynes

Preparation of Alkynes

• Alkynes prepared by elimination of HX from alkyl halides

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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 X2:

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Problems

• Give the alkene products for the following addition rxns:

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Regioselectivity of Second Addition Rxn

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2614.4 Introduction to Addition Reactions of the Triple Bond

Problems

Hydration of Alkynes

• Addition of H2O

• As with alkenes, alkynes can be hydrated by two methods1) Mercury (II) Catalyzed Hydration• The product is a ketone and not an alcohol

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Recall:

Keto-Enol Tautomerism

• Tautomers: constitutional isomers that interconvert rapidly– Equilibrium lies far to the right

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Mechanism of Mercury (II) Catalyzed Hydration

2914.5 Conversion of Alkynes into Aldehydes and Ketones

Mechanism of Hydration

3014.5 Conversion of Alkynes into Aldehydes and Ketones

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Problems1) Give the complete mechanism for the following reaction:

2) What is the product for the following rxn?

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