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University of Minnesota Twin Cities
CHEM 2331H
T.Andrew Taton

Chapter 9 Book Notes Alkynes 9.1) Introduction • Alkynes: hydrocarbons w/ C-C triple bonds; aka acetylenes b/c = derivatives of acetylene (simplest alkyne) • C=C triple bond chemistry = similar to double bond  mostly same rxns as alkenes (esp. additions and oxidation) o But some rxns specific to triple bond o unique characteristics of C=C triple bond or unusual acidity of acetylenic =C-H bond • Triple bond  4 fewer H’s than corresponding alkane o molecular formula: C H n 2n-2 o  contributes to 2 elements of unsaturation • Alkynes not = as common as alkenes in nature o some plants use alkynes to protect against disease/preditors  cicutoxin = toxic compound in water hemlock  capillin protects plant against fungal diseases o uncommon in drugs, but some drugs have it  parsalmide used as analgesic  ethynyl estradiol (synthetic female hormone) = common ingredient in birth control pills  Dynemicin A= antibacterial compound being tested as antitumor agent 9.2) Nomenclature ofAlkynes • IUPAC Names: Find longest continuous chain of C’s that includes triple bond, start #ing @ end closest to triple bond (use # of C @ triple bond before alkyne name), change –ane to –yne o use #s for substituent positions too • When additional different functional groups present, combine suffixes  alkenynes (double and triple bond), alkynols (triple bond and alcohol) o alcohol gets higher priority than alkene/alkyne, so start #ing @ end closer to there o alkene and alkyne have equal priority but if equidistant, double has priority b/c – ene comes before –yne alphabetically • Common names: describe alkynes as derivatives of acetylene o most molecules can be named as acetylene molecule + 1 or 2 alkyl substituents o like common nomenclature of ethers (name 2 alkyl groups bonded to O) • Terminal alkyne/terminal acetylene: triple bond’s @ end of chain (H-C=C aka acytelenic H) o many alkyne properties depend on if acytelenic H present o Internal alkyne/internal acetylene: if triple bond’s not @ end of chain 9.3) Physical Properties of Alkynes • Similar properties to alkanes/alkenes w/ similar molecular weights o Alkynes = relatively nonpolar, nearly insoluble in water, soluble in most organic solvents (acetone, ether, methylene chloride, chloroform, alcohols, etc.) o Have characteristic, mildly offensive odors o Ethyne, propyne, and butynes = gases @ room T (just like corresponding alkanes and alkenes) o Bp’s of alkynes nearly same as corresponding alkanes/alkenes 9.4) Commercial Importance ofAlkynes • 9.4A) Uses of Acetylene and Methylacetylene o Acetylene = most important commercial alkyne  Largest use = fuel for oxyacetylene welding torch  important industrial feedstock  Colorless, foul-smelling gas, burns in air w/ yellow, sooty flame   If apply flame to pure oxygen  light blue, flame T↑ dramatically  Exothermic, has more energy released per mole of product than ethane/ethane  better fuel for high T flame o Ethane might be ok for heating a house b/c most total heat produced per mole of gas burned, but for welding torch, want highest T of products (need to look @ heat per mole of product)  Oxyacetylene flame reaches T as high as 2800 C o o Acetylene initially considered dangerous, explosive when 1 used for welding  = thermodynamically unstable  when compressed gas subject to thermal/mechanical shock, decomposes into elements (releases 234 kJ/56 kcal) energy/mol   initial decomposition splits container  products (hydrogen & finely divided C) burn in air o Acetylene safely stored&handled in cylinders filled w/ crushed firebrick wet w/ acetone  Acetylene dissolves freely in acetone  dissolved gas not as prone to decomposition  Firebrick helps control decomposition by minimizing cylinder’s free volume, cools&controls decomposition before out of control o Methylacetylene also used in welding torches  doesn’t decompose as easily as acetylene, burns better in air (as opposed to pure oxygen)  well suited for household soldering & brazing that needs higher T than propane torches can reach  industrial synthesis of it  mixture of methylacetylene + its isomer propadiene (allene) •  mixture sold commercially as MAPP gas (MethylAcetylene- ProPadiene) • 9.4B) Manufacture of Acetylene o Acetylene = one of cheapest organic chemicals, made from coal/natural gas  synthesis from coal: heat lime + coke (roasted coal) in electric furnace  calcium carbide  water + calcium carbide  acetylene + hydrated lime o 2 rxn in synthesis once = light source in coal mines before battery-powered lights  let water slowly drip onto calcium carbide  acetylene generated, feeds small flame where gas burns in air w/ yellow flickering light  But flame ignites methane gas commonly in coal seams explosions  Battery-powered miner’s lamps give better light, ↓methane explosion danger o Synthesis from natural gas: natural gas mostly made up of methane  heat methane for short time  acetylene  endothermic, but 2*moles of products as reactants  more moles  more entropy  entropy term dominates free energy eqn @ high T 9.5) Electronic Structure of Alkynes • Acetylene has 3 electron pairs btwn C nuclei o Each C bonded to 2 other atoms, no nonbinding valence electrons  each C needs 2 hybrid orbitals o  2 sp hybrid orbitals, 180 degrees o sp orbitals overlap w/ each other and w/ s orbital from H  sigma bond framework • 2 remaining unhybridized p orbitals overlap on each C  2 pi bonds o orbitals overlap @ right angles to each other  1 pi bond has electron density above/below C, other has electron density in front/back of C o shape of pi bonds  blend  cylinder of electron density that encircles sigma bond btwn 2 C’s • C=C bond length = 1.20 Å, C-H bond length = 1.06 Å o Both bond lengths = shorter than corresponding bonds in ethane/ethane • Triple bond = shorter b/c of attractive overlap of 3 electron pairs and high s character of sp orbitals 2 o sp orbitals are about half s character (not 1/3 s character in sp or 1/4 s character in sp )  use more of closer/tightly held s orbital o C-H bond is also shorter b/c of sp orbital 9.6) Acidity ofAlkynes; Formation ofAcetylide Ions • Terminal alkynes = much more acidic than other hydrocarbons o removing acetylenic proton  acetylide ion: central role in alkyne chemistry • Acetylenic H = acidic b/c of sp hybrid =C-H bond o C-H bond acidity ↑ as s character of orbitals ↑ (sp < sp < sp) o b/c smaller pK vaaue  stronger acid 9 o acetylenic proton = 10 *more acidic than vinyl proton • Abstracting acetylenic proton  carbanion w/ lone pair electrons in sp hybrid orbital o electrons in sp orbital = closer to nucleus  less charge separation than in 3 2 carbanions w/ lone pair in sp or sp orbital- - o Acetylene can be deprotonated by amide (NH ) ion,2but not by alkoxide ion (OR) • Very strong bases (like sodium amide, NaNH ) d2protonate terminal acetylenes  acetylide ions/alkynide ions (type of carbanion) o Hydroxide/alkoxide ions aren’t strong enough bases to deprotonate alkynes o Internal alkynes don’t have acetylenic protons  don’t react • Sodium amide frequently used as base to form acetylide salts o mide ion = conjugate base of ammonia, which = base o ammonia = very weak acid, one of its H’s can be reduced by sodium metal  sodium salt of amide ion = very strong conj. base • Acetylide ions = strong nucleophiles o one of best methods for synthesizing substituted alkynes = nucleophillic attack by acetylide ion on unhindered alkyl halide 9.7) Synthesis of Alkynes fromAcetylides • 2 different common approaches to synthesize alkynes • 1) appropriate electrophile undergoes nucleophilic attack by acetylide ion o electrophile may be unhindered primary alkyl halide (undergoes S 2) N o or may be carbonyl compound (undergoes addition  alcohol) o either rxn joins 2 fragments  product w/ lengthened C skeleton o used in laboratory syntheses of alkynes • 2) double dehalogenation of dihalide o  doesn’t enlarge C skeleton o isomerization of triple bond may occur, so dehalogenation only useful when desired product has triple bond in thermodynamically favored position • 9.7A)Alkylation ofAcetylide Ions o Acetylide ion = strong base & powerful nucleophile (like other carbanions); can displace halide ion from suitable substrate  substituted acetylene o Need alkyl halide to be excellent S 2 Nubstrate for S 2 Nxn to produce good yield  methyl/primary  no bulky substituents/branches close to rxn center o If backside approach hindered, acetylide ion may abstract proton  E2 mechanism • 9.7B) Addition of Acetylide Ions to Carbonyl Groups o Acetylide ions can also add to carbonyl groups (C=O)  O = more electronegative than C  polarized double bond (equal amts partial – or + charges respectively) o +charged C = electrophilic, when attacked by nucleophile, O has –charge   alkoxide ion: strong base, conj. base of alcohol (weak acid)  adding water/dilute acid protonates alkoxide  alcohol o Acetylide ion = nucleophile, adds to carbonyl group  alkoxide ion  ad
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