© Prentice Hall 2001Chapter 51 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 52 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

© Prentice Hall 2001Chapter 53 Hydrogen Halide Addition in the presence of Peroxide Hydrogen peroxide has same effect on hydrogen bromide addition to an alkyne as to an alkene

© Prentice Hall 2001Chapter 54 Halogen Addition to Alkynes Halogens add to alkynes as well as to alkenes Excess halogen leads to the addition of a second equivalent

© Prentice Hall 2001Chapter 55 Addition of Water to Alkynes Water adds to alkynes in the presence of acid to yield an enol

© Prentice Hall 2001Chapter 56 Addition of Water to Alkynes However the initially formed enol reacts further to produce a ketone Such isomers, differing only in the placement of a hydrogen atom, are called tautomers

© Prentice Hall 2001Chapter 57 Mercuric-Ion-Catalyzed Addition of Water to Alkynes

© Prentice Hall 2001Chapter 58 Hydroboration–Oxidation Hydroboration of an internal alkyne leads to a ketone

© Prentice Hall 2001Chapter 59 Addition of Water to an Alkyne

© Prentice Hall 2001Chapter 510 Addition of Hydrogen to an Alkyne

© Prentice Hall 2001Chapter 511 Addition of Hydrogen to an Alkyne

© Prentice Hall 2001Chapter 512 Acidity of a Hydrogen Bonded to an sp Carbon The acidities of hydrocarbons can be compared with the acidities of compounds having hydrogen attached to other second-row elements

© Prentice Hall 2001Chapter 513 Acidity of a Hydrogen Bonded to an sp Carbon The conjugate bases have the following relative base strength because the stronger the acid, the weaker the conjugate base

© Prentice Hall 2001Chapter 514 Acidity of a Hydrogen Bonded to an sp Carbon Relative electronegativities of carbon sp > sp 2 > sp 3 The amide ion is strong enough to remove the proton from an sp carbon

© Prentice Hall 2001Chapter 515 Acidity of a Hydrogen Bonded to an sp Carbon Hydroxide ion is too weak to remove a proton from an sp carbon

© Prentice Hall 2001Chapter 516 Syntheses Using Acetylide Ions Alkylation reactions work best with primary alkyl halides and methyl halides

© Prentice Hall 2001Chapter 517 Starting with 1-butyne, how would you prepare the following ketone? At this point the only way we know to form a ketone is to add water to an alkyne in the presence of a catalyst Introduction to Multistep Synthesis

© Prentice Hall 2001Chapter 518 If the alkyne used has identical substituents on both sp carbons, only one ketone will result 3-Hexyne can be obtained from the starting material by removing the hydrogen bonded to the sp carbon and reacting with an alkyl halide Introduction to Multistep Synthesis

© Prentice Hall 2001Chapter 519 Introduction to Multistep Synthesis Overall we have a two-step synthesis

© Prentice Hall 2001Chapter 520 In designing a synthesis it is best to start from the product and work backwards as we did in this example Introduction to Multistep Synthesis

© Prentice Hall 2001Chapter 521 The thought process is known as retrosynthetic analysis and is indicated by using open arrows Introduction to Multistep Synthesis