© 2011 Pearson Education, Inc. 1 Organic Chemistry 6 th Edition Paula Yurkanis Bruice Chapter 6 The Reactions of Alkynes An Introduction to Multistep Synthesis

© 2011 Pearson Education, Inc. 2 Acetylenes and Birth Control Note the “yn,” as in alkyne, in the generic name

© 2011 Pearson Education, Inc. 3 An alkyne is a hydrocarbon that contains a carbon–carbon triple bond General formula: C n H 2n–2 (acyclic); C n H 2n–4 (cyclic)

© 2011 Pearson Education, Inc. 4 Nomenclature In common nomenclature, alkynes are named as substituted acetylenes:

© 2011 Pearson Education, Inc. 5 In IUPAC nomenclature: The yne suffix is assigned the lowest number. Then substituents are assigned the lowest possible set of numbers.

© 2011 Pearson Education, Inc. 6 Naming a compound that has more than one functional group… A compound containing two double bonds or two triple bonds:

© 2011 Pearson Education, Inc. 7

8 Alkene and Alkyne Mixed Functional Groups Longest chain with both functional groups Find the longest chain bearing both functional groups. Assign ene and yne suffixes the lowest set of numbers.

© 2011 Pearson Education, Inc. 9 If there is a tie between a double bond and a triple bond, the ene suffix is assigned the lower number: A chain is numbered to give the lowest possible number to the functional group with the higher priority:

© 2011 Pearson Education, Inc. 10 Complex Mixed Functional Group Examples Do not forget to designate alkene stereochemistry if necessary. Longest chain most functional groups Vinyl or ethenyl Ethynyl Longest chain most functional groups

© 2011 Pearson Education, Inc. 11 The Structure of Alkynes A triple bond is composed of a  bond and two  bonds

© 2011 Pearson Education, Inc. 12 Electrophilic Addition of Hydrogen Halides to Alkynes One Equivalent of HCl: Excess HCl: Z-Stereoisomer Gem regioisomer

© 2011 Pearson Education, Inc. 13 The initial addition of the proton can occur with equal ease to either of the sp carbons and the geminal regioisomer always results:

© 2011 Pearson Education, Inc. 14 Stereoselectivity of Alkyne Addition Reactions Trans addition of hydrohalides HCl and HBr occur because of a  -complex intermediate: Adding excess halide anion increases the yield of trans addition product.

© 2011 Pearson Education, Inc. 15 Regioselectivity of Alkyne Addition Reactions Use Markovnikov’s rule for HCl and HBr additions to terminal alkynes:

© 2011 Pearson Education, Inc. 16 The second hydrohalide addition affords a geminal dihalide:

© 2011 Pearson Education, Inc. 17 The transition state for the alkyne addition is less stable than for an alkene addition:

© 2011 Pearson Education, Inc. 18 Relative Stabilities of Carbocations A vinylic cation (sp) is formed in the first step of the alkyne addition reaction whereas an alkyl cation (sp 2 ) is formed in the first step of the alkene addition reaction High s character, cation interacts with positive nucleus. Less s character, more stable cation

© 2011 Pearson Education, Inc. 19 The halogens Cl 2 and Br 2 also add to alkynes:

© 2011 Pearson Education, Inc. 20 Addition of Water to an Alkyne One method of synthesizing ketones.

© 2011 Pearson Education, Inc. 21 Hg 2+ is added to increase the rate of water addition to terminal alkynes:

© 2011 Pearson Education, Inc. 22 Mechanism involves a prototropic shift: Acid CatalyzedBase Catalyzed

© 2011 Pearson Education, Inc. 23 How to Draw Tautomers: Recognize protons  to a carbonyl and then protonate oxygen and deprotonate carbon to afford enol species: No protons  to a carbonyl, no enol tautomers: Two sets of  protons Two sets of tautomers Proton from solvent

© 2011 Pearson Education, Inc. 24 Hydroboration–Oxidation of Internal Alkynes

© 2011 Pearson Education, Inc. 25 Hydroboration–Oxidation of Terminal Alkynes Bulky borane required to distinguish terminal end from substituted end:

© 2011 Pearson Education, Inc. 26 Formation of Ketone versus Aldehyde

© 2011 Pearson Education, Inc. 27 Addition of Hydrogen Formation of Cis Alkene Catalytic reduction of an alkyne affords an alkane without buildup of the alkene intermediate: Use a “poisoned” catalyst developed by Lindlar to obtain the alkene:

© 2011 Pearson Education, Inc. 28 Lindlar Catalyst The components of Lindlar catalyst: Palladium Metal Calcium Carbonate Lead Oxide Origin of the cis stereochemistry: The poison The substrate The catalyst Delivery of hydrogen atoms to one side by the solid- phase catalyst

© 2011 Pearson Education, Inc. 29 Lindlar’s Synthesis of Vitamin A

© 2011 Pearson Education, Inc. 30 Conversion of Internal Alkynes to Trans Alkenes

© 2011 Pearson Education, Inc. 31 Reason for trans stereochemistry: electron repulsion and steric hindrance. Stable Unstable Repulsion Steric Hindrance

© 2011 Pearson Education, Inc. 32 Acidity of a Hydrogen Bonded to an sp Hybridized Carbon High s character, sp orbital penetrates nucleus. Lower s character, sp 3 orbital penetrates nucleus to a lesser degree

© 2011 Pearson Education, Inc. 33 Electronegativities versus Acid Strengths relative base strength weakest base strongest base

© 2011 Pearson Education, Inc. 34 The stronger the acid, the weaker its conjugate base: An amide anion cannot remove a hydrogen bonded to an sp 2 or sp 3 carbon.

© 2011 Pearson Education, Inc. 35 Synthesis Using Acetylide Ions: Formation of C–C Bond We can convert terminal alkynes to longer internal alkynes by addition of alkyl halides Known as an S N 2 reaction

© 2011 Pearson Education, Inc. 36 Designing a Synthesis Example 1

© 2011 Pearson Education, Inc. 37 Example 2

© 2011 Pearson Education, Inc. 38 Example 3

© 2011 Pearson Education, Inc. 39 Example 4

© 2011 Pearson Education, Inc. 40 Example 5

© 2011 Pearson Education, Inc. 41 Example 6

© 2011 Pearson Education, Inc. 42 Example 7