Alkynes: Naming, Reactions, and Synthesis - Prof. Kevin Shaughnessy, Study notes of Chemistry

Download Alkynes: Naming, Reactions, and Synthesis - Prof. Kevin Shaughnessy and more Study notes Chemistry in PDF only on Docsity! Chapter 8 Outline  8.1: Naming Alkynes o Alkynes are named using the –yne ending. The rules are similar to those for alkenes. o The parent hydrocarbon is the one containing both carbons of the alkyne or alkynes. Numbering starts on the side closest to the alkyne. o Molecules with both alkenes and alkynes are known as enynes • The en ending comes before yne. For example, 1-hexen-4-yne or 3-hexen-1-yne • Numbering starts on the side closest to the first multiple bond whether double or triple • If the alkene and alkyne would have the same starting number, the alkene should have the lower number  8.3: Reactions of Alkynes: Addition of HX and X2 o Alkynes react similarly to alkenes. The alkyne π-electrons act as nucleophiles. The difference is that there are two pairs of π-electrons. o HBr and other HX reagents add to alkynes in a similar manner to alkenes • Using 1 equivalent of HBr, a bromoalkane is formed. • For a terminal alkyne, the H is added to the less substituted carbon and the Br to the more substituted one. Internal alkynes give a mixture of products unless they are symmetrical. • The addition is usually trans. • If an excess of HBr is used, two equivalents of HBr are added to give a dihalo compound. Both halogens are added to the same carbon (more substituted). o Bromine and chlorine add to give dihalo or tetrahalo products. • One equivalent of Br2 or Cl2 will give a dibromo- or dichloroalkene. The halogens are added trans. • An excess of the halogen will result in addition of two equivalents to give a tetrabromo- or tetrachloro alkane.  8.4: Hydration of Alkynes o Acid and water alone does not result in hydration of alkynes. Using a catalytic mercury salt (HgSO4) along with water and H2SO4 does result in hydration of the alkyne. • As with alkenes, the oxygen is added to the more substituted carbon • Unlike alkenes, an alcohol product is not formed. The initially formed hydroxyalkene (enol) rearranges to a more stable ketone form.  Ketones and enols are examples of tautomers—molecules that rapidly interconvert by movement of one or more atoms (usually H). 2 o Hydroboration will place the oxygen on the less-substituted carbon of the alkyne (if terminal) • Again the product is a carbonyl compound. Starting from a terminal alkyne, the product is an aldehyde.  8.5: Reduction of Alkynes o The two π-bonds of an alkyne can be reduced. • Reducing ethyne to ethene releases 176 kJ/mol • Reducing ethene to ethane releases 137 kJ/mol • Since reducing the triple bond is more exothermic, this reaction is more favorable thermodynamically than reduction of an alkene. Therefore, it is possible to stop at the alkene stage o Hydrogenation of an alkyne with H2 and Pd/C will give complete reduction to the alkane. o Using Lindlar's catalyst, which is a palladium catalyst deactivated with lead and quinine, reduction can be stopped at the alkene product. The H2 is added in cis fashion to give the Z-alkene. o The E-alkene product can be obtained by reduction using Li in liquid ammonia.  8.6: Oxidative Cleavage of Alkynes o Ozone and KMnO4 cleave alkynes to give carboxylic acid products. o If the alkyne is terminal, the terminal carbon is lost as CO2  8.7: Alkyne Acidity o C-H bonds are not very acidic, but the acidity depends upon the hybridization • The pKa of an alkyne C-H is 25, an alkene C-H is 44, and an alkane C-H is 60 • Only the alkyne can be deprotonated under normal conditions • Using a strong base (NaNH2, sodium amide), an alkyne can be deprotonated to give an acetylide anion and ammonia o Why are alkynes more acidic than alkene or alkane C-H bonds? • The sp-hybridized carbon of an alkyne has more s character (50% s) than an sp2 or sp3-hybridized carbon • The increased s-character means that the sp orbital holds the electrons closer to the nucleus. This effectively makes the carbon more electronegative and more stable as an anion. Thus, the C-H bond is more acidic.  8.8: Alkylation of Acetylide Anions o Acetylide anions react with methyl or primary alkyl halides to form alkylated products. • The acetylide anion acts as a nucleophile and attacks the C-X bond displacing the halide.