Alkynes Reactions PDF

Alkynes Lecture 2 Organic chemistry Halogen Stabilization of Carbonations Resonance stabilizes a molecule by delocalizing charge and electron density. Halogens stabilize an adjacent positive charge by resonance. Carbocation A is stabilized by resonance. Halogenation of Alkynes Halogens X2 (X = Cl or...

Alkynes Lecture 2 Organic chemistry Halogen Stabilization of Carbonations Resonance stabilizes a molecule by delocalizing charge and electron density. Halogens stabilize an adjacent positive charge by resonance. Carbocation A is stabilized by resonance. Halogenation of Alkynes Halogens X2 (X = Cl or Br) add to alkynes just as they do to alkenes. Addition of one mole of X2 forms a trans dihalide, which can then react with a second mole of X2 to yield a tetrahalide. Halogenation of Alkynes Halogenation of Alkynes In the presence of strong acid or Hg2+ catalyst, the elements of H2O add to the triple bond to form an enol initially. The enol is unstable and rearranges to a ketone. Hydration of Internal vs. Terminal Alkynes Internal alkynes undergo hydration with concentrated acid to form ketones. Terminal alkynes require the presence of an additional Hg2+ catalyst (usually HgSO4) to yield methyl ketones by Markovnikov addition of water. Hydration of Internal vs. Terminal Alkynes Keto-Enol Tautomerization Tautomers are constitutional isomers that differ in the location of a double bond and a hydrogen atom. A and B are tautomers: A is the enol form and B is the keto form of the tautomer. Keto-Enol Tautomerization An enol tautomer has an O−H group bonded to a C=C. A keto tautomer has a C=O and an additional C−H bond. Equilibrium favors the keto form largely because the C=O is much stronger than a C=C. Tautomerization, the process of converting one tautomer into another, is catalyzed by both acid and base. Hydration Mechanism Hydroboration-Oxidation of Alkynes Hydroboration−oxidation is a two-step reaction sequence that also converts an alkyne to a carbonyl compound. Hydroboration-Oxidation of Alkynes Addition of borane forms an organoborane. Oxidation with basic H2O2 forms an enol. Tautomerization of the enol forms a carbonyl compound. The overall result is addition of H2O to a triple bond. Hydroboration-Oxidation of Alkynes Hydroboration−Oxidation of Internal vs. Terminal Alkynes. Hydroboration−oxidation of an internal alkyne forms a ketone, just as the acid-catalyzed hydration did. However, hydroboration−oxidation of a terminal alkyne forms an aldehyde. BH2 adds to the less substituted, terminal carbon resulting in antiMarkovnikov addition of water. Hydroboration-Oxidation of Alkynes Reduction of Alkynes Alkynes are reduced to alkanes by addition of H2 over a metal catalyst. The reaction occurs in two steps through an alkene intermediate. Complete reduction to the alkane occurs when palladium on carbon (Pd/C) is used as catalyst Reduction of Alkynes Hydrogenation can be stopped at the alkene stage if the less active Lindlar catalyst is used. The Lindlar catalyst is chemically deactivated palladium metal on calcium carbonate. The hydrogenation occurs with syn stereochemistry, giving a cis alkene product. Note: The Lindlar Catalyst will not reduce double bonds. Oxidation Reaction of Alkynes with KMnO4 Alkynes are oxidized by the same reagents that oxidize alkenes. Disubstituted alkynes react with potassium permanganate to yield vicinal diketones (1,2‐diketones) in mild conditions. While, warm and basic KMnO4 cleaves the triple bond. Oxidation Reaction of Alkynes with KMnO4 Ozonolysis Ozonolysis of alkynes produces carboxylic acids. Used to find location of triple bond in an unknown compound. Reactions of Acetylide Ions Acetylide anions are strong nucleophiles and react with unhindered alkyl halides(1o alkyl halides) to yield products of nucleophilic substitution. The mechanism of substitution is SN2, and thus the reaction is fastest with CH3X and 1o alkyl halides. Reactions of Acetylide Ions Elimination vs. Substitution with Acetylide Ions Steric hindrance around the leaving group causes 2° and 3° alkyl halides to preferentially undergo elimination by an E2 mechanism, as shown with 2-bromo-2-methylpropane. Thus, nucleophilic substitution with acetylide anions forms new carbon-carbon bonds in high yield only with unhindered CH3X and 1° alkyl halides. Reactions of Acetylide Ions Reactions of Acetylide Ions Synthesis using Alkynes You can now begin to consider (for example) how to prepare a fivecarbon product from three smaller precursor molecules using the reactions you have learned. To plan a synthesis of more than one step, we use the process of retrosynthetic analysis—that is, working backwards from a desired product to determine the starting materials from which it is made. Retrosynthetic Analysis Retrosynthetic analysis is the method of working backwards from a target compound to starting materials. To write a synthesis working backwards, an open arrow (⇒) is used to indicate that the product is drawn on the left and the starting material on the right. Retrosynthetic Analysis In designing a synthesis, reactions are often divided into two categories: 1. Those that form new carbon-carbon bonds. 2. Those that convert one functional group into another—that is, functional group interconversions. Retrosynthetic Analysis Example of a Retrosynthetic Synthesis Devise a synthesis of the following compound from starting materials having two carbons or fewer. Retrosynthetic Analysis Thinking backwards... 1. Form the carbonyl group by hydration of a triple bond. 2. Form a new C-C bond using an acetylide anion and a 1° alkyl halide (two 2-carbon structures are converted to a 4-carbon product). 3. Prepare the acetylide anion from acetylene by treatment with base. Retrosynthetic Analysis Steps to develop a Retrosynthetic Analysis Title