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UH CHEM 3331 - Alkynes-intro,properties,synthesis
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CHEM 3331 1nd Edition Lecture 20Outline of Last Lecture I. Addition of HalogensII. HydrogenationIII. EpoxidationIV. DihydroxylationV. Oxidative CleavageVI. PolymerizationOutline of Current Lecture I. AlkynesII. NomenclatureIII. UsesIV. Physical propertiesV. Structure and acidityVI. Preparation Current LectureI. AlkynesAlkynes are hydrocarbons with a triple bond between two carbons. The simplest alkyne is ethyne, also known as acetylene. A molecule with a carbon triple bonded to a nitrogen is not analkyne. Alkanes are CnH2n+2, alkenes are CnH2n, and alkynes are CnH2n-2.II. NomenclatureEthyne: HC≡CHPropyne: H3CC≡CH1-butyne: HC≡CCH2CH3These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.2-butyne: H3C-C≡C-CH3Double bond gets priority before triple bonds. H2C=CCH2C≡CH is 1-penten-4yne.Cyclooctyne is the most stable cycloalkyne. (CH3)2-CH-C≡C-CH3 is 4-methyl-1-pentyne.H-C≡C-CH2 this is a terminal alkyne since the triple bond is at the terminal end. The hydrogen connected to the triple bonded carbon is an acetylenic hydrogen. H3C-C≡C-CH2 this is an internal alkyne since the triple bond is between two internal carbons.III. UsesSmallest member is the most useful. Acetylene (ethyne) is used in welding. It is explosive so it is dissolved in acid to be stored for transportation. Propyne is less explosive.Ethyne is produced from coal not petroleum.3C +CaOCaC2+ COCaC2 +H2O Ca(OH)2 + HC≡CHCaO is prepared from limestone. CaCO3(-CO2) CaOIV. Physical PropertiesEthyne and propyne are gases. Butyne is a liquid, lighter than water, and non polar. They have a slight sweet smell. They also burn with a lot of soot.V. Structure and acidityThe two adjacent carbons contain a sigma bond and two pi bonds. The triple bond shortens the length between the two carbons. The sigma bond is 88kcal/mol. The first pi bond is 62kcal/mol. The second pi bond is 48kcal/mol.These two carbons are sp hybridized, which has 50% s character. S contains most of its density at the nucleus. Sp holds electrons tighter than sp2 and sp3. H has an easier time leaving. Alkynes are acidic. H directly connected to triple bonded carbons can leave. Alkynes have a pKa of 25. Alkenes have a pKa of 44. Alkanes have a pKa of about 50. Alcohols have a pKa of 16-18.The stronger acid has to give up the H+. For an alkyne to be an acid it must be paired with a molecule that has a larger pKa value.H2C-C≡C:- is an acetylide, which are strong nucleophiles and strong bases. They aren’t sterically bulky since bulk is removed by at least three carbons. It can be used in Sn2 and E2. They are used as reagents in reactions. Carbon is a nucleophilic atom. You can add as many carbons as you want to acetylide.H3C-C≡C:- + CH3-I  CH3-C≡C-CH3 +I-VI.


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UH CHEM 3331 - Alkynes-intro,properties,synthesis

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