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Chapter 9

Alkyne Notes - Lecture and Textbook Chapter 9

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Cecilia Kutas

CHM247H Jasmyn Lee Chapter 9: Alkynes: An Introduction to Organic Synthesis Alkynes  2 Kinds of alkynes o Internal R-C≡C-R o Terminal R-C ≡C-H 1 CHM247H Jasmyn Lee Alkyne Reactivity  What compounds have similar electronic structure?  Alkenes – What reactions did they undergo? o Halogenation (2 ) o Hydrogenation (H2) o Hydration (H2O) o Halohydrogenation (HX) o Halohydrin Formation (X, OH) o Oxidation 9.1 Naming Alkynes  Use suffix “yne”, start numbering at end nearest C≡C  Compounds with double & triple bonds are “enynes”  Numbering of enyne starts from end nearest multiple bond (either one) Spectroscopy of Alkynes 2 CHM247H Jasmyn Lee 9.2 Preparation of Alkynes: Elimination Reactions of Dihalides Preparation of Alkynes By Elimination  [NaNH 2 sodamide (sodium amide) an extremely strong base] Converting an Alkene to an Alkyne 9.3 Reactions of Alkynes: Addition o2 HX and X 1. Halogenation o Markovnikov o Trans o Different mechanism than alkene addition – if same mechanism occurred, an vinyllic carbocation would form, which is less likely than an alkyllic carbocation 2. Hydrohalogenation o Markovnikov’s rule applies o Gem (geminal) = same C o Can stop at haloalkene o Mechanism analogous to alkene bromination 3 CHM247H Jasmyn Lee 9.4 Hydration of Alkynes 3. Hydration o Similar to alkene reactivity2 H O addition can go i. Oxymercuration – Markovnikov ii. Hydroboration – Anti-Markovnikov i. Oxymercuration (Markovnikov)  In this mechanism we have a) Markovnikov addition of H 2 b) Keto-enol tautomerisation o Enol, (vinylic alcohol) is not isolated but a ketone instead – enol is an intermediate o The individual keto and enol forms are tautomers o Note: this is an equilibrium; tautomers (keto-enol forms) are not related by resonance  Equilibrium favors ketones  Tautomers – constitutional isomers; two isomers that under spontaneous interconversion accompanied by the change in position of a H o Tautomers differ in the position of a double bond and a Hydrogen atom o Tautomers are in equilibrium with each other  Hydration of an internal alkyne results in a mixture of ketones  Analogous to alkene oxymercuration – electrophilic addition of mercury (II) ion to the alkyne gives a vinylic cation, which reacts with2H O and loses a H to yield a mercury containing enol intermediate o Contrast: no treatment with NaBH4is needed to remove mercury; acidic reaction conditions are sufficient to effect replacement of mercury by hydrogen o Tautomerization then gives the ketone 4 CHM247H Jasmyn Lee ii. Hydroboration (Anti-Markovnikov)  Recall: Alkene Hydroboration  A similar reaction occurs with alkynes, but a hindered dialkyl borane is used to prevent addition of two diborane molecules  9-BBN and S2a BH o Are bulky, hindered dialkylboranes that only have 1 hydride to contribute o Internal Alkyne – gives a ketone o Terminal Alkyne – gives an aldehyde o In general – enol is formed first, then rapid tautomerization gives the more stable aldehyde 1. 9-BBN R C C H 3. 2 O2/HO- R C C OH H H enol O R C C H 2 H aldehyde O 1. 9-BBN 2. H O /HO - 2 2 o Internal alkyne gives isomeric products – gives a ketone  Eg/ 2-pentyne with 9-BBN and oxidation gives: 1. 9-BBN - O 2. 2 2 /HO O 9.5 Reduction of Alkynes Hydrogenation  Adds hydrogen across the π bonds  Can reduce alkyne to: 5 CHM247H Jasmyn Lee 3 Types of Reduction (differ by how H is delivered) 1. Deliver H and electron together as H-H, syn addition a. Eg/ H 2Pd-C 2. Add H and e separately, from different sources + - a. 2 Na  2Na + 2e b. 2
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