1 University of Massachusetts Chemistry 261 Organic Chemistry Study Guide Alkynes This guide is a concise study guide on lecture notes the text Organic Chemistry by John McMurray and other texts to give you what you need to know as an Organic Chemisty Student in the chapter on Alkynes Alkynes are hydrocarbons with atleast one triple bond between carbon atoms They will be called as terminal alkynes if the triple bond is present in the end of the carbon chain They have the general formula of CnH2n 2 Alkynes are unsaturated because the valency of carbon is not satisfied and they can undergo addition reactions to become saturated and the triple bonds will become single bonds Hence they are highly reactive and their properties are different that of alkenes and alkanes What are Alkynes In organic chemistry an alkyne is an unsaturated hydrocarbon containing at least one carbon carbon triple bond between two carbon atoms 1 The simplest acyclic alkynes with only one triple bond and no other functional groups form a homologous series with the general chemical formula CnH2n 2 Alkynes are traditionally known as acetylenes although the name acetylene also refers specifically to C2H2 known formally as ethyne using IUPAC nomenclature Like other hydrocarbons alkynes are generally hydrophobic but tend to be more reactive What are the chemical Properties 1Chemical properties Alkynes are characteristically more unsaturated than alkenes Thus they add two equivalents of bromine whereas an alkene adds only one equivalent in the reaction Other reactions are listed below In some reactions alkynes are less reactive than alkenes For example in a molecule with an ene and an yne group addition occurs preferentially at the ene 2 Possible explanations involve the two bonds in the alkyne delocalising which would reduce the energy of the system citation needed or the stability of the intermediates during the reaction They show greater tendency to polymerize or oligomerize than alkenes do The resulting polymers called polyacetylenes which do not contain alkyne units are conjugated and can exhibit semiconducting properties 2Naming alkynes Alkynes are named with the greek prefix system without any additional letters Examples include ethyne or octyne In parent chains with 4 or more carbons it is necessary to say where the triple bond is located For octyne one can either write 3 octyne or oct 3 yne when the bond starts at the 3rd carbon The lowest number possible is given to the triple bond When no superior functional groups are present the parent chain must include the triple bond even if it is not the longest possible carbon chain in the molecule Ethyne gas is usually called acetylene gas 3Synthesis Commercially the dominant alkyne is acetylene itself which is used as a fuel and a precursor to other compounds e g acrylates Hundreds of millions of kilograms are produced annually by partial oxidation of natural gas 4 2 CH4 3 2 O2 HC CH 3 H2O Propyne also industrially useful is also prepared by thermal cracking of hydrocarbons Most other industrially useful alkyne derivatives are prepared from acetylene e g via condensation with formaldehyde 4 Reactions alkynes participate in many organic reactions 3 1Addition of hydrogen halogens and related reagents edit Alkynes characteristically undergo reactions that show that they are doubly unsaturated meaning that each alkyne unit is capable of adding two equivalents of H2 halogens or related HX reagents X halide pseudohalide etc Depending on catalysts and conditions alkynes add one or two equivalents of hydrogen Hydrogenation to the alkene is usually more desirable since alkanes are less useful RC CR H2 cis RCH CR H The largest scale application of this technology is the conversion of acetylene to ethylene in refineries The steam cracking of alkanes yields a few percent acetylene which is selectively hydrogenated in the presence of a palladium silver catalyst For more complex alkynes the Lindlar catalyst is widely recommended to avoid formation of the alkane for example in the conversion of phenylacetylene to styrene Similarly halogenation of alkynes gives the vinyl dihalides or alkyl tetrahalides RC CR 2 Br2 RCBr2CRBr2 The addition of nonpolar E H bonds across C C is general for silanes boranes and related hydrides The hydroboration of alkynes gives vinylic boranes which oxidize to the corresponding aldehyde or ketone In the thiol yne reaction the substrate is a thiol Hydrohalogenation gives the corresponding vinyl halides or alkyl dihalides again depending on the number of equivalents of HX added The addition of water to alkynes is a related reaction except the initial enol intermediate converts to the ketone or aldehyde Illustrative is the hydration of phenylacetylene gives acetophenone 7 and the Ph3P AuCH3 catalyzed hydration of 1 8 nonadiyne to 2 8 nonanedione 8 Practical Application 4Alkynes in nature and medicine According to Ferdinand Bohlmann the first naturally occurring acetylenic compound dehydromatricaria ester was isolated from an Artemisia species in 1826 In the nearly two centuries that have followed well over a thousand naturally occurring acetylenes have been discovered and reported Polyynes a subset of this class of natural products have been isolated from a wide variety of plant species cultures of higher fungi bacteria marine sponges and corals 9 Some acids like tariric acid contains an alkyne group Diynes and triynes species with the linkage RC C C CR and RC C C C C CR respectively occur in certain plants Ichthyothere Chrysanthemum Cicuta Oenanthe and other members of the Asteraceae and Apiaceae families Some examples are cicutoxin oenanthotoxin falcarinol and carotatoxin These compounds are highly bioactive e g as nematocides 10 1 Phenylhepta 1 3 5 triyne is illustrative of a naturally occurring triyne Alkynes occur in some pharmaceuticals including the contraceptive norethynodrel A carbon carbon triple bond is also present in marketed drugs such as the antiretroviral Efavirenz and the antifungal Terbinafine Molecules called ene diynes feature a ring containing an alkene ene between two alkyne groups diyne These compounds e g calicheamicin are some of the most aggressive antitumor drugs known so much so that the ene diyne subunit is sometimes referred to as a warhead Ene diynes undergo rearrangement via the Bergman cyclization generating highly reactive radical intermediates that attack DNA within the tumor 11 5More on Naming Alkynes The alkynes can