Lecture_I_Day

Lecture_I_Day - Lecture I, Chapter 13, Chem 308 ORGANIC...

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Lecture I, Chapter 13, Chem 308 1 ORGANIC CHEMISTRY 308 LECTURE I CHAPTER 13 Alkynes: Continuing our survey of functional groups. 1. An alkyne is a hydrocarbon with a triple bond. A triple bond introduces two degrees of unsaturation. Therefore the general formula of a hydrocarbon with one triple bond is C n H 2n-2 . Notice that an alkyne is at a higher oxidation level than an alkene because it has two fewer hydrogens. 2. Nomenclature. Systematic is the same as alkenes but no cis, trans or EZ issues. Key points: main chain: must contain the triple bond numbering priority: number from the end of the chain that gives the triple bond the lowest number. suffix “yne” denotes the carbon-carbon triple bond. Some other points a. acetylene is a widely used common name acceptable in the system. b. other alkynes can be given common names as substituted acetylenes (methylacetylene etc or diisopropyl acetylene as opposed to ?)) c. terminal vs internal. Important because of the H, as we shall see. d. The common name propargyl is used for the CH≡CCH 2 - radical. e. In hydrocarbons that have both a double and a triple bond, the chain is numbered from the end that gives a multiple bond the lowest number.
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Lecture I, Chapter 13, Chem 308 2 If there is a tie then the double bond has priority in numbering and also if it is an issue, in determining the main chain. For example 3. Physical properties as expected for hydrocarbons. But alkynes tend to be relatively high boiling compared to alkenes because of greater polarizability and some subtle entropy effects. Still the lower molecular weight alkynes are gases at room temperature. Also terminal alkynes tend to be higher boiling than internal ones because of the polarity of the C-H bond. 4. Structure. You can predict one σ bond and two π bonds based on what you know about orditals.
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Lecture I, Chapter 13, Chem 308 3 a. With two electron groups (two bonds) the geometry is linear and the bond angles are 180 . b. Since π bonds are relatively weak, a triple bond is not close to 3 times as strong as a single bond and not even 1.5 a double bond. c. C-C triple bond length. Generalization: The more s character in the hybrid orbitals forming the σ bond, the shorter the bond.[why?] So a triple bond is shorter than a double bond. The C-H bond length is relatively short in a terminal alkyne. Same effect in C-C single bonds when at least one of the C’s is sp hybridized, the C-C bond is relatively short. Example the 2,3 C-C single bond in 1,3-butadiyne is very short compared to other C-C single bonds. 5. Terminal Alkynes: Acidity. The acidity of H bonded to carbon increases as the hybridization of the carbon has a greater fraction of s character. Thus sp>sp 2 >sp 3 . The effect is due to the increased electronegativity of the s orbital (closer to the nucleus) than a p orbital.
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This note was uploaded on 04/04/2008 for the course CHEM 308 taught by Professor Boikess during the Spring '08 term at Rutgers.

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Lecture_I_Day - Lecture I, Chapter 13, Chem 308 ORGANIC...

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