© Prentice Hall 2001 Chapter 5 1

General Formulas

The General Formula for an acyclic alkyne is CnH2n-2

The Simplest Alkyne is HCCH, which has the common name Acetylene

Used as a fuel used in cutting torches

© Prentice Hall 2001 Chapter 5 2

Alkynes If the triple bond is at the end of the

chain, the alkyne is known as a terminal alkyne

© Prentice Hall 2001 Chapter 5 3

Alkynes If the triple bond is not at the end of the

chain, the alkyne is known as an internal alkyne

© Prentice Hall 2001 Chapter 5 4

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 2001 Chapter 5 5

Alkyne Structure

© Prentice Hall 2001 Chapter 5 6

Alkyne Structure

© Prentice Hall 2001 Chapter 5 7

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 2001 Chapter 5 8

Reactivity Considerations In the second step, the positively

charged carbocation intermediate reacts rapidly with the negatively charged chloride ion

© Prentice Hall 2001 Chapter 5 9

Reactivity Considerations The product is an alkene which can

undergo a second electrophilic addition reaction

© Prentice Hall 2001 Chapter 5 10

Reactivity Considerations

© Prentice Hall 2001 Chapter 5 11

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 sp2 carbon

© Prentice Hall 2001 Chapter 5 12

Relative Stabilities of Carbocations

© Prentice Hall 2001 Chapter 5 13

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

CH3CH2C CH

H

CH3CH2C CH2

Br

CH3CH2C CH2

Br

© Prentice Hall 2001 Chapter 5 14

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

CH3CH2C CH2

Br

HBr

CH3CH2CCH3

Br

Br