Chapter 7: The Reactions of Alkynes and Introduction to Multistep Synthesis

Reactions of Alkynes and Introduction to Multistep Synthesis

Overview

This chapter introduces the chemistry of alkynes, including their occurrence, synthetic applications, nomenclature, and the principles guiding their reactions. Alkynes are hydrocarbons containing a carbon-carbon triple bond, and their unique reactivity is central to organic synthesis and multistep reaction strategies.

Naturally Occurring Alkynes

Examples and Biological Roles

• Capillin: A fungicide with an alkyne functional group.

• Ichthyothereol: A convulsant compound found in nature, containing an alkyne moiety.

• Enediyne class: Potent anticancer agents characterized by the presence of two triple bonds and a double bond in a conjugated system.

Application: Alkynes are found in biologically active molecules, including natural toxins and pharmaceuticals.

Synthetic Alkynes

Medicinal Chemistry Applications

• Parsalimide: An analgesic drug containing an alkyne group.

• Supridyl (pargyline): An antihypertensive agent with an alkyne moiety.

Application: Synthetic alkynes are used in drug design for their unique chemical properties.

Nomenclature of Alkynes

General Principles

• Definition: An alkyne is a hydrocarbon with a carbon-carbon triple bond.

• General Formula:

  • Acyclic: $C_nH_{2n-2}$

  • Cyclic: $C_nH_{2n-4}$

• Types:

  • Terminal alkyne: Triple bond at the end of the carbon chain.

  • Internal alkyne: Triple bond within the carbon chain.

• Numbering: The chain is numbered to give the "yne" suffix the lowest possible number.

Example: HC≡CH is named ethyne (acetylene); CH₃C≡CH is 1-butyne (ethylacetylene).

Nomenclature with Substituents

Rules for Numbering and Naming

• Number the chain to give the triple bond the lowest possible number.

• If substituents are present, their positions are indicated by numbers.

• If both directions give the same functional group number, choose the direction that gives the lowest number to the substituent.

Compound	Correct Name	Reason
BrCH₂C≡CCH₂CH₂CH₃	3-bromo-2-chloro-4-octyne	Triple bond at position 4 (2 < 6)
CH₃CH₂C≡CCH₂CH₃	1-bromo-5-methyl-3-hexyne	Substituent at position 1 (1 < 2)

Nomenclature of Multiple Functional Groups

Priority and Numbering

• When both double and triple bonds are present, the chain is numbered to give the lowest number to the double bond.

• Functional group priority: OH > NH₂ > C=C > C≡C

• If a tie occurs, the double bond takes precedence over the triple bond.

Compound	Correct Name	Reason
CH₂=C=CH₂	propadiene (allene)	Two double bonds
CH₃CH=CHCH₂C≡CH	2-hexen-4-yne	Double bond gets lower number

Summary Table: Nomenclature of Alkynes

Type	General Formula	Example	Name
Acyclic terminal	$C_nH_{2n-2}$	HC≡CH	ethyne (acetylene)
Acyclic internal	$C_nH_{2n-2}$	CH₃C≡CCH₃	2-butyne
Cyclic	$C_nH_{2n-4}$	cyclooctyne	cyclooctyne

Key Points

• Alkynes are important in both natural and synthetic organic chemistry.

• Nomenclature follows IUPAC rules, prioritizing the lowest possible numbers for functional groups.

• Functional group priority affects numbering when multiple groups are present.

• Applications include pharmaceuticals, agrochemicals, and natural products.

Additional info: The study notes expand on the brief slide content by providing definitions, examples, and tables for clarity and completeness, suitable for exam preparation.