Outline for CH 231 with Professor Shaughnessy at UA-Elem Organic Chem II
Outline for CH 231 with Professor Shaughnessy at UA-Elem Organic Chem II
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This 3 page Class Notes was uploaded by an elite notetaker on Friday February 6, 2015. The Class Notes belongs to a course at University of Alabama - Tuscaloosa taught by a professor in Fall. Since its upload, it has received 11 views.
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Date Created: 02/06/15
Chapter 8 Outline I 81 Naming Alkynes o Alkynes are named using the iyne 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 lhexen4yne or 3hexenlyne 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 I 83 Reactions of Alkynes Addition of HX and X2 0 Alkynes react similarly to alkenes The alkyne n electrons act as nucleophiles The difference is that there are two pairs of nelectrons 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 0 Bromine and chlorine add to give dihalo or tetrahalo products One equivalent of Brz or C12 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 I 84 Hydration of Alkynes 0 Acid and water alone does not result in hydration of alkynes Using a catalytic mercury salt HgSO4 along with water and H2804 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 I Ketones and enols are examples of tautomersimolecules that rapidly interconvert by movement of one or more atoms usually H o Hydroboration will place the oxygen on the lesssubstituted carbon of the alkyne if terminal 39 Again the product is a carbonyl compound Starting from a terminal alkyne the product is an aldehyde 85 Reduction of Alkynes o The two n bonds of an alkyne can be reduced 39 Reducing ethyne to ethene releases 176 kJmol 39 Reducing ethene to ethane releases 137 kJmol 39 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 0 Hydrogenation of an alkyne with H and Pd C will give complete reduction to the alkane 0 Using Lindlar s catalyst which is a palladium catalyst deactivated with lead and quinine reduction can be stopped at the alkene product The H is added in cis fashion to give the Zalkene o The Ealkene product can be obtained by reduction using Li in liquid ammonia 86 OXidative Cleavage of Alkynes 0 Ozone and KMnO4 cleave alkynes to give carboxylic acid products 0 If the alkyne is terminal the terminal carbon is lost as C02 87 Alkyne Acidity o CH bonds are not very acidic but the acidity depends upon the hybridization 39 The pKa of an alkyne CH is 25 an alkene CH is 44 and an alkane C H is 60 39 Only the alkyne can be deprotonated under normal conditions 39 Using a strong base NaNHz sodium amide an alkyne can be deprotonated to give an acetylide anion and ammonia 0 Why are alkynes more acidic than alkene or alkane CH bonds 39 The sphybridized carbon of an alkyne has more s character 50 s than an sp2 or sp3hybridized carbon 39 The increased scharacter 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 CH bond is more acidic 88 Alkylation of Acetylide Anions o Acetylide anions react with methyl or primary alkyl halides to form alkylated products 39 The acetylide anion acts as a nucleophile and attacks the CX bond displacing the halide I A new CC bond is formed Organic synthesis requires ways to connect carbons together to make small molecules into bigger ones 0 The alkyne starting material can be any alkyne that has at least one hydrogen on an alkyne carbon terminal alkyne o The alkyl halide must either be a methyl or primary alkyl halide I Secondary and tertiary alkyl halides undergo a different undesired reactioni elimination of HX I 89 Introduction to Organic Synthesis 0 Most organic chemistry research and applications involve the synthesis of compleX molecules starting from simple starting materials Organic synthesis is the art of designing a series of chemical reactions to convert a starting compound into a new molecule 0 A key step in most syntheses are those that create new CC bonds So far we know one way to do this alkylation of an acetylide anion o Other key steps in organic syntheses are functional group transformations For example converting an alkyne into an alcohol by reducing the alkyne to an alkene and then hydrating the alkene 0 Synthesis strategy There are several useful strategies when trying to design a synthetic route I First look at the starting compound you are given and try to find those carbons in the product Usually the carbon framework of the starting material will be retained in the product Often the functional group in the starting material will be related to functional groups in the product as well I Look at what must be done to convert the starting material into the product I What carbons need to be added and where I What changes to the functional groups must occur I Sometimes it is useful to work backwards This is known as retrosynthetic analysis basically synthesizing backwards I Look at the product and think about reactions that you know that could make the product from a precursor For example if your product is a ketone you know that hydration of an alkyne would give a ketone I Then draw the starting material you would need to give the desired product I Now repeat the process on the new intermediate compound you39ve identified Keep doing this until you find a compound that can be made in one step from the given starting material 0 We will focus more on synthesis as we go on this semester and it will be a very important part of what you learn in CH 232
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