Alkynes Nomenclature Physical Properties Synthesis Reactions.

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Unit 5 alkenes and alkynes

Alkynes Nomenclature Physical Properties Synthesis ReactionsUnit 5 - 4 Reactions of Alkynes111/12/2014AlkynesContain at least one carbon-carbon triple bond (CC).Also called acetylenes.

Properties of CC Bondssigma () bondpi () bonds CC consists of a bond and two bonds. CC is shorter than a C=C bond.

Properties of CC BondsThe pi bonds force the four atoms involved to be linear.The e- density of the pi bonds is a cylinder surrounding the two C atoms.

AlkynesThe pi bonds are relatively easy to break, which makes CC a functional group.The pi bonds block nucleophilic attack.Nomenclature of AlkynesWhen the chain contains more than three C atoms, use a number to give the location of the triple bond.Terminal alkynes have one H on the triple-bonded C. Internal alkynes do not.CH3CCCH2CH3 2-pentynepent-2-yneinternal alkyneCH3CCH propyneterminal alkyneUnit 5 - 4 Reactions of Alkynes11/12/20146IR of Alkynes1-octynea terminal alkyne

IR of Alkynes4-octynean internal alkyne

Nomenclature of AlkynesApply the same rules you learned for the alkanes.Use the root name of the longest chain containing the triple bond, but change -ane to -yne.In 8th edition of Wade, alkenes and alkynes are given equal priority, but -en is first alphabetically, soThe numbering starts at the end closer to the alkene, andthe order of naming is en-yne.

Nomenclature of AlkynesName the following:(E)-3-methylhept-2-en-4-yne


Nomenclature of AlkynesAlkynes as substituents are called alkynyl groups.

ethynylbenzeneUses and Synthesis of AcetyleneMain use is as a fuel for oxyacetylene welding. It is one of the cheapest organic elements.Synthesized fromcoal: 3C + CaO CaC2 +COCaC2+2H2O HCCH + Ca(OH)2natural gas: 2CH4 HCCH + 3H2driven by S, high T, and 0.01s heating timeStability of AcetyleneAcetylene (HCCH) is thermodynamically unstable:HCCH(g) 2C(s) + H2(g)This can happen to the compressed gas.produces a very hot flame when burned in pure oxygenPhysical Properties of AlkynesSimilar to alkanes and alkenes of comparable molecular weight.nonpolarvirtually insoluble in watersoluble in organic solventsPhysical Properties of AlkynesTerminal alkynes (-CC-H) have an acetylenic H that is more acidic than the Hs on other hydrocarbons due to the greater s character of the sp bond. The greater s character makes the C-H bond more polar and the acetylenic H more acidic.pKa of terminal acetylenes 25pKa of alkanes 50pKa of NH3 is 35 (so -:NH2 reacts with -CC-H)pKa of alcohols 16 (and alkoxides dont)Formation of Acetylide IonsFrom the reaction of a terminal alkyne with sodium amideCH3CH2CCH + NaNH2 CH3CH2CC:- Na+ + :NH3CH3CCCH3 + NaNH2 NRAcetylide ions are strong nucleophiles.OH- and RO- are not strong enough to remove the terminal H.Synthesis of Alkynesfrom acetylides (lengthens the C skeleton)an excellent way to make a more complex alkynealkylation of an acetylideaddition to carbonyl groupsby elimination reactionsUnit 5 - 4 Reactions of Alkynes1711/12/2014Synthesis of Alkynesfrom acetylides (lengthens the C skeleton) - an SN2 reactionalkylation of an acetylideif the halide is 2, there will also be the elimination productwhich would be what?

Unit 5 - 4 Reactions of Alkynes1811/12/2014Synthesis of AlkynesPredict the product:

Unit 5 - 4 Reactions of Alkynes1911/12/2014Synthesis of Alkynesfrom acetylides (lengthens the C skeleton)addition to carbonyl groupsthe acetylide is the nucleophileaddition to aldehydes gives 2 alcoholsaddition to ketones gives 3alcoholsUnit 5 - 4 Reactions of Alkynes2011/12/2014Synthesis of Alkynesfrom acetylide addition to a carbonyl group

an alkoxide iona 3 alcoholUnit 5 - 4 Reactions of Alkynes2111/12/2014Synthesis of AlkynesPredict the product:

Unit 5 - 4 Reactions of Alkynes2211/12/2014Synthesis of Alkynesby elimination reactionsA vicinal dihalide or a geminal dihalide can undergo a double dehydrohalogenation to form the alkyneThis requires STRONGLY BASIC conditions, and many compounds cant take it.KOH in a sealed tube heated to 200CNaNH2, 150CUnit 5 - 4 Reactions of Alkynes2311/12/2014Dehydrohalogenation with KOHThe heated base is so strong that the triple bond can migrate along the carbon chain to form the more stable internal alkyne.

terminal alkyne, will rearrangeinternal alkyne

Dehydrohalogenation with NaNH2NaNH2 is even stronger than fused KOH. It is so strong that it traps the terminal alkyne as the sodium salt, and no rearrangement occurs.

terminal alkyne, will not rearrangemajor componentSynthesis of AlkynesPredict the product:

Unit 5 - 4 Reactions of Alkynes2611/12/2014Reactions of AlkynesAdditionsreduction to alkanesreduction to alkenesaddition of halogensaddition of HXaddition of waterMarkovnikovanti-MarkovnikovOxidationsto diketonescleavageNucleophilic attack on electrophilescovered in synthesisReactions of AlkynesReduction to alkanes by hydrogen.second bond energy = 226 kJfirst bond energy = 264 kJAlkynes can undergo double additionsThese typically go all the way to the alkane.

R-CC-R + 2H2(g) RCH2CH2RPt, Pd, or NiReactions of AlkynesReduction to cis-alkenesthe syn addition of hydrogen can be stopped at the alkene stage if a poisoned catalyst is usedNi2BLindlars catalyst: Pd/BaSO4 poisoned with quinoline (in CH3OH to dissolve CC)

Reactions of AlkynesReduction to trans-alkenesNa/NH3(l) requiredNa + NH3(l) e- NH3 + Na+The Hs come from NH3.The solvated e- leads to a vinyl radical, which is more stable in the trans geometry.

Reactions of AlkynesAddition of halogensOne mole X2, syn or anti addition leading to cis- or trans-alkenesTwo moles X2 leads to the tetrahalides

Reactions of AlkynesAddition of HXReaction proceeds through a vinyl cation intermediate then a carbocation intermediate, so the addition is Markovnikov.If a peroxide is used with HBr, the anti-Markovnikov product will be formed. Why?

Reactions of AlkynesMarkovnikov addition of waterHgSO4/H2SO4 catalystproduct is a ketone, not an alcohol

Keto-enol conversionCan occur in acid or base, but the mechanisms are different.The mechanism shown is the conversion in acid.

Reactions of Alkynesanti-Markovnikov addition of waterhindered dialkylborane neededproduct is an aldehyde, not an alcohol

Reactions of AlkynesPermanganate oxidationsnearly neutral conditions neededproduct is an -diketone, not a diolterminal alkynes give keto acids

Reactions of AlkynesPermanganate oxidationswarm, basic conditions cause cleavageproducts are salts of carboxylic acids(A second step, acidification, is needed to produce the acids themselves.)

Reactions of AlkynesOzonolysis followed by hydrolysis

Reactions of AlkynesPredict the products

Reactions of AlkynesPredict the products