Organic Chemistry 4 th Edition Paula Yurkanis Bruice Chapter 6 Reactions of Alkynes Introduction to Multistep Synthesis Irene Lee Case Western Reserve University Cleveland, OH ©2004, Prentice Hall

Alkynes are hydrocarbons that contain a carbon–carbon triple bond General formula: C n H 2n–2 (acyclic); C n H 2n–4 (cyclic)

Nomenclature In common nomenclature, alkynes are named as substituted acetylenes

3-bromo-2-chloro-4-octyne not 6-bromo-7-chloro-4-octyne because 2 < 6 A substituent receives the lowest number if there is no functional group suffix, or if the same number for the functional group suffix is obtained in both directions 1-bromo-5-methyl-3-hexyne not 6-bromo-2-methyl-3-hexyne because 1 < 2

The Structure of Alkynes A triple bond is composed of a  bond and two  bonds

Commercial Use of Ethyne

Acidity of a Hydrogen Bonded to an sp Hybridized Carbon pK a = 25 pK a = 44 pK a = 50

Electronegativities versus Acid Strengths relative base strength weakest base strongest base

The stronger the acid, the weaker its conjugate base top 252

Synthesis Using Acetylide Ions: Formation of C–C Bond

Addition Reactions of Alkynes

Sequential Addition of HCl

An alkyne is less reactive than an alkene

Relative Stabilities of Carbocations

Addition of Hydrogen Halides In the presence of 1 mole equivalent of HBr The secondary vinylic cation is more stable

In the presence of excess HBr, a carbocation is formed by adding the electrophile to the sp 2 carbon bonded to the greater number of hydrogen

A secondary vinylic cation is about as stable as a primary cation, therefore a pi-complex may be the actual reaction intermediate Many (but not all) alkyne addition reactions are stereoselective

Addition of Halogens to Alkynes

Addition of Water keto tautomer enol tautomer tautomerization

Examples of Water Addition

Hg 2+ is added to increase the rate of water addition to terminal alkynes

Hydroboration–Oxidation of Internal Alkynes

Hydroboration–Oxidation of Terminal Alkynes

Formation of Ketone versus Aldehyde

Addition of Hydrogen Formation of Cis Alkene

Conversion of Internal Alkynes to Trans Alkenes

Reason for trans addition:

Designing a Synthesis ? Example 1

Example 2

Example 3