CH 320M 1st Edition Exam 3 Study Guide Chapter 7 Structure of Alkynes Alkynes contain one or more carbon carbon triple bonds The triple bond is a combination of one s bond formed by the overlap of sp hybrid orbitals and two p bonds formed by the overlap of two sets of parallel 2p orbitals Nomenclature of Alkynes According to IUPAC nomenclature the infix yn is used to show the presence of a carbon carbon triple bond We name the main chain as the longest one that contains the triple bond and assign numbers from the end that gives the carbon atoms of the triple bond the lower set of numbers The IUPAC system retains the name acetylene Common names are derived by adding a prefix for the substituent to the word acetylene Physical Properties of Alkynes The physical properties of alkynes are similar to those of alkanes and alkenes of comparable carbon skeletons Acidity of 1 Alkynes The pKa values of terminal alkynes are approximately 25 they are less acidic than water and alcohols but more acidic than alkanes alkenes and ammonia The hydrogen atom bonded to a carbon carbon triple bond is sufficiently acidic so that it can be removed by a strong base most commonly sodium amide NaNH2 sodium hydride NaH or lithium diisopropylamide LDA Preparation of Alkynes Alkynes can be prepared through alkylation of acetylide anions with methyl or primary halides This reaction is important because it creates a carbon carbon bond Starting with acetylene even unsymmetrical alkynes can be made using two sequential deprotonation alkylation reactions Alkynes can be made from alkenes through a two step sequence of halogenation with X2 followed by double dehydrohalogenation Allenes are sometimes seen as a by product of the double dehydrohalogenation Electrophilic Addition to Alkynes Alkynes add Br2 to give dibromoalkenes via anti addition stereochemistry A second mole of Br2 can be added to give a tetrabromoalkane Addition of Cl2 occurs with less stereoselectivity than the addition of Br2 Alkynes add two moles of HCl or HBr with both halogens adding to the same carbon For terminal alkynes Markovnikov s rule is followed and the hydrogen ends up on the terminal carbon atom Hydration of Alkynes to Aldehydes and Ketones Alkynes react with borane BH3 or sia 2BH followed by basic peroxide to give aldehydes or ketones The sterically hindered sia 2BH gives regioselective reaction with 1 alkynes leading to aldehydes The reaction initially produces an enol The functional group of an enol is an OH group on a carbon atom of an alkene The enol form is in equilibrium with the keto form namely an aldehyde or ketone and the equilibrium almost always lies far on the side of the keto form Tautomers are constitutional isomers that are in equilibrium with each other but differ in the location of a hydrogen and a double bond relative to a heteroatom most commonly O N and S Keto enol tautomerism is the most common type of tautomerism we encounter in this course Alkynes react with water in the presence of H2SO4 and Hg II salts to give ketones or acetaldehyde in the case of acetylene via an enol intermediate Reduction of Alkynes Alkynes react with two moles of H2 in the presence of a transition metal catalyst to give alkanes in a reaction that does not ordinarily stop at the alkene stage The Lindlar catalyst is a deactivated hydrogenation catalyst that stops alkyne hydrogenation at the alkene stage allowing conversion of alkynes to cis alkenes Hydroboration followed by an acid workup instead of basic peroxide also gives cis alkenes from alkynes Hydroboration protonolysis is an alternative way to prepare a cis alkene from an alkyne Dissolving metal reduction gives trans alkenes from alkynes Chapter 8 Organic Synthesis When planning an organic synthesis involving several reactions it is best to use the following systematic approach 1 Count the carbons in the products versus the starting material so that you know what fragments must be added or subtracted 2 Analyze the functional groups 3 Work backward retrosynthesis A functional group often reacts in the same fashion whether it is in a relatively simple molecule or a more complex molecule that contains numerous other functional groups Although they do not react directly other functional groups often have an influence over the regiochemistry or stereochemistry of a reaction Haloalkanes contain a halogen covalently bonded to an sp3 hybridized carbon Nomenclature In the IUPAC system halogen atoms are named fluoro chloro bromo and iodoand are listed in alphabetical order with other substituents In the common system they are named alkyl halides Haloalkenes contain a halogen covalently bonded to an sp2 hybridized carbon of an alkene In the common system they are named alkenyl or vinylic halides Haloarenes contain a halogen atom covalently bonded to a benzene ring Physical Properties of Haloalkanes The van der Waals radius of fluorine is only slightly greater than that of hydrogen and among the other halogens only iodine has a larger van der Waals radius than methyl Among alkanes and chloro bromo and iodoalkanes of comparable size and shape the haloalkanes have the higher boiling points predominantly because of the greater polarizability of the unshared electrons of the halogen atom Polarizability refers to the distortion of the distribution of electron density around an atom that is interacting with another atom or ion The electron density on larger less electronegative atoms is more polarizable than that of electrons on more electronegative atoms with smaller atomic radii Boiling points of fluoroalkanes are generally comparable to those of alkanes of similar size and shape because of the uniquely low polarizability of the valence electrons of fluorine The density of liquid haloalkanes is greater than that of hydrocarbons of comparable molecular weight because of the halogen s larger mass to volume ratio Preparation of Haloalkanes by Halogenation of Alkanes Free radical halogenation of alkanes uses Cl2 or Br2 and light or heat to produce haloalkanes Free radical halogenation is regioselective and replaces H atoms with the halogen in the order allylic 3 2 1 methyl This order of reactivity can be predicted on the basis of radical intermediate stabilities which follow the same order Radical intermediates are analogous to carbocations in the sense that they are stabilized by the same interactions namely resonance delocalization and hyperconjugation with attached alkyl groups Regioselectivity of