© Prentice Hall 2001Chapter 51 General Formulas The General Formula for an acyclic alkyne is C n H 2n-2 The Simplest Alkyne is HC  CH, which has the common name Acetylene Used as a fuel used in cutting torches

© Prentice Hall 2001Chapter 52 Alkynes If the triple bond is at the end of the chain, the alkyne is known as a terminal alkyne

© Prentice Hall 2001Chapter 53 Alkynes If the triple bond is not at the end of the chain, the alkyne is known as an internal alkyne

© Prentice Hall 2001Chapter 54 IUPAC Nomenclature of Alkynes 1.Find the longest chain containing the triple bond and change the corresponding “-ane” ending to “-yne” 2.The chain is numbered in direction that gives the triple bond, the lower number 3.If the same number for the triple bond is obtained in numbering from both directions, the number for the substituent nearest the chain end is minimized

© Prentice Hall 2001Chapter 55 Alkyne Structure

© Prentice Hall 2001Chapter 56 Alkyne Structure

© Prentice Hall 2001Chapter 57 Reactivity Considerations In the first step of HCl addition to an alkyne, a  bond breaks because  electrons are attracted to the electrophilic hydrogen atom

© Prentice Hall 2001Chapter 58 Reactivity Considerations In the second step, the positively charged carbocation intermediate reacts rapidly with the negatively charged chloride ion

© Prentice Hall 2001Chapter 59 Reactivity Considerations The product is an alkene which can undergo a second electrophilic addition reaction

© Prentice Hall 2001Chapter 510 Reactivity Considerations

© Prentice Hall 2001Chapter 511 Reactivity Considerations The instability of the transition state for the alkyne reaction can be understood from the Hammond postulate The transition state will resemble the structure of the intermediate, which is a vinyl carbocation The vinyl carbocation has a positive charge on an sp carbon which is more electronegative than an sp 2 carbon

© Prentice Hall 2001Chapter 512 Relative Stabilities of Carbocations

© Prentice Hall 2001Chapter 513 Hydrogen Halide Addition The addition of a hydrogen halide to an alkyne follows Markovnikov’s rule because a secondary vinylic cation is more stable than a primary vinylic cation

© Prentice Hall 2001Chapter 514 Hydrogen Halide Addition When excess hydrogen halide is present, a second equivalent is added Markovnikov’s Rule also is followed for the second addition, forming a geminal dihalide