Post on 20-Apr-2017
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Alkynes: Unsaturated Hydrocarbon
Jully TanSchool of Engineering
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Alkynes
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Naming AlkynesStep 1: Find the longest chain containing the triple bond.Step 2: Change –ene or -ane ending to -yne.Step 3: Number the chain, starting at the end closest to the triple bond.Step 4: Give branches or other substituents a number to locate their
position.
Example
CH3 C CH
propyne
CH3 C C CH2 CH2 Br
5-bromo-2-pentyne
CH3 CH
CH3
CH2 C C CH
CH3
CH3
2,6-dimethyl-3-heptyne
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Step 5: Substitutive nomenclature: As a substituent, an alkyne is named alkynyl.
Step 6: The double bond has a priority over the triple bond when numbered (Follow whichever that nearest to the end of the chain in determines the direction of numbering). The lowest numbers are given to the multiple bonds Whether double / triple. When there is a choice, HOWEVER, The double bond takes precedence!!!
CCH CH 2 CH CH2
1-Pentene-4-yne
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There are no cis - trans -isomers, because the triple bond is linear
C C H CH2 C C HEthynyl 2-propynyl
CH2 CH CH2 CH
CH3
C CH
4-methyl-1-hexen-5-yne
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Example of Naming Alkynes
The lowest numbers are given to the multiple bondsWhether double / triple. When there is a choice, HOWEVER, The double bond takes precedence!!!
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Carbon-carbon triple bond results from sp orbital on each C forming a sigma bond and unhybridized pX and py orbitals forming π bonds.
The remaining sp orbitals form bonds to other atoms at 180º to C-C triple bond. The bond is shorter and stronger than single or double Breaking a π bond in acetylene (HCCH) requires 318 kJ/mole (in ethylene it is 268 kJ/mole)
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CH3 CH3 CH2 CH2 CH CH
1.54 Å 1.33 Å 1.20 Å
1. Electronic Structure Of Alkynes
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Alkynes show three types of isomerism, Chain isomerism Position isomerism Functional isomerism.
Chain isomerism It is due to the different arrangement of carbon atoms in the chain i.e., straight chain or a branched-chain.
Position isomerism It is due to the difference in the location of the triple bond
2. Isomerism in Alkynes
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Functional isomerism Alkynes are isomeric with alkadienes both being represented by the general formula CnH2n-2. So there can be a difference in the nature of the bond (triple to double) giving different functional properties.
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Boiling points of alkynes are close to the boiling points of alkenes and alkanes.
3. Boiling Point of Alkynes
Alkyne have lower densities, than water and they are insoluble in water.
4. Density & Solubility
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Synthesis of Alkynes &
Reaction of Alkynes
Additional to Triple Bond
Hydrogen Halogen Hydrogen Halide
(HX) HydrationOxidation of triple bond
Synthesis Dehydration of alkyl
dihalides Reaction with 1° alkyl
halides
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Synthesis of Alkynes Method 1: Dehydration of alkyl dihalides using a strong base
1. Vicinal dihalides (compounds with halogen atoms on adjacent carbon atoms)2. Treatment of vicinal dihalides with strong bases, alcoholic KOH and sodium amide
results in alkynes.3. Vicinal dihalides obtained from addition of halogen to alkenes.
NaBr
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Method 2: Reaction of sodium acetylide with 1° alkyl halides
1. Acetylene & monosubstituted acetylenes contains a H atom attached to triple bonded C atom: Acetylenic hydrogen
2. Acetylenic H are acidic and replaced by certain metals forming salts: metal acetylides.
R C CH Na R C C Na+
R C C Na+ + R'CH2 X R C C CH2R' + NaX
+ H2
TerminalAlkyne
Methylor
PrimaryAlkylHalide
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Show how to synthesis 3-decyne from ethyne and any necessary alkyl halide
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Addition Reaction (i) Hydrogenation
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Conversion of Alkynes to cis-Alkenes Addition of H2 using chemically deactivated palladium on calcium carbonate as a
catalyst (the Lindlar catalyst) produces a cis alkene The two hydrogens add syn (from the same side of the triple bond)
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Conversion of Alkynes to trans-Alkenes Anhydrous ammonia (NH3) is a liquid below -33 ºC
alkaline metals dissolve in liquid ammonia and function as reducing agents Alkynes are reduced to trans alkenes with sodium or lithium in liquid ammonia The reaction involves a radical anion intermediate
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Addition Reaction (ii) Halogenation
Alkynes reacts with equivalent of halogens forming tetrahalides.
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Electrophilic addition to triple bonds proceeds slower, than addition to double bonds and often requires a catalyst. The Markovnikov’s rule is as valid as for the addition to double bonds. HBr in the presence of peroxides adds against the rule.
2 Step involved: Step 1: producing of haloalkene stageStep 2: Producing gem-dihalide stage (twin =both halogen at the same
Addition Reaction (iii) Hydrohalogenation
CH3
C CH3
Br
Br
HBr CH3
C
CH2
Br
CH3
C CH HBrperoxides CH3
C
CHBr
H
74%
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Addition Reaction (iv) HydrationDifferences from hydration of alkenes:1. A Hg2+ catalyst is required2. The product of addition is unstable and isomerizes.3. Water adds and loses a proton.4.A proton from aqueous acid replaces Hg(II)
RC CH OH2
HgSO4, H2OC C
R
OH
H
HC C
R
O
H
HH+
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Mechanism of Mercury(II)-Catalyzed Hydration of Alkynes
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Oxidative Cleavage of Alkynes Strong oxidizing reagents (O3 or KMnO4) cleave internal alkynes, producing two carboxylic
acids Terminal alkynes are oxidized to a carboxylic acid and carbon dioxide Neither process is useful in modern synthesis – were used to elucidate structures because the
products indicate the structure of the alkyne precursor
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Visual Test1. Bromine test2. Rxn with H2SO4 (to differentiate alkanes and alkynes)
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