Organic Chemistry I - Alkynes - Quick Guides

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Study GuideOrganic Chemistry I–Alkynes1.Alkynes: NomenclatureWhat is Alkyne Nomenclature?The naming of alkynes follows rules from theInternational Union of Pure and Applied Chemistry(IUPAC). These rules are similar to the naming rules for alkenes, except for the suffix used to indicatethe presence of atriple bond.Steps for Naming Alkynes:1.Identify the Longest Chain with the Triple BondThe parent name of the alkyne comes from theIUPAC name of the alkanewith the same number ofcarbon atoms, but the-aneending is replaced with-yneto show the presence of the triple bond.Example: If the longest chain with a triple bond has five carbon atoms, the compound ispentyne.2.Number the Carbon Atoms in the ChainNumber the carbon atoms starting from the endclosest to the triple bond. This ensures that thetriple bond is placed between the second and third carbon atoms. If you number from the wrong end,the triple bond would be placed incorrectly.Example:oThe correct way to number is from right to left in2-pentyne.3.Indicate the Position of the Triple BondThe position of the triple bond is shown by placing thelower of the two numbersassigned to thetriple-bonded carbons in front of the name.Example: The compound shown is2-pentyne, with the triple bond located between the second andthird carbon atoms.

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Study Guide4.Substituent Atoms or GroupsIf there are any substituent atoms or groups, their positions and names are indicated in the name. Theposition of the substituent is numbered based on the longest chain.Example:oFor the compound shown with a chlorine atom, the name is5-chloro-2-hexyne.Key Takeaway•Identify the longest chainwith the triple bond and replace the-aneending with-yne.•Number the chainfrom the end closest to the triple bond.•Theposition of the triple bondis indicated by the numbers of the carbon atoms in the bond.•Thelocation of any substituentatoms or groups is added to the name.2.Alkynes: Physical PropertiesWhat Are the Physical Properties of Alkynes?Alkynes have physical properties similar to alkenes. Like alkenes, alkynes are generallynonpolarmolecules, meaning they have little solubility inpolar solvents, such as water. However, alkynes aresoluble innonpolar solventslike ether and acetone.•Alkynes with four or fewer carbon atomstend to be gases, just like alkanes and alkenes.Substituted Alkynes and Dipole MomentsWhat is a Dipole Moment?

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Study GuideSubstituted alkynes havesmall dipole moments. This is because theelectronegativity differencebetween thetriple-bonded carbon atomsand thesingle-bonded carbon atomscreates anunequal distribution of electrons.•Thetriple-bonded carbon atomsaresp hybridized(with a highers character), makingthem more electronegative.•Thesingle-bonded carbon atomsaresp³ hybridizedand less electronegative.This difference inelectronegativitybetween the carbon atoms results in anasymmetrical electrondistributionin the molecule. This asymmetry creates adipole moment.Key Takeaway•Alkynes arenonpolar moleculeswith little solubility in polar solvents.•Alkynes withfour or fewer carbon atomsare gases.•Substituted alkyneshave small dipole moments due to differences inelectronegativitybetweenspandsp³hybridized carbon atoms.3.Alkynes: AcidityWhat are Terminal Alkynes?Alkynes with the general structure ofR-C≡C-Hare referred to asterminal alkynes. These types ofalkynes areweakly acidic. When exposed to a strong base, such assodium amide (NaNH₂), theyundergo an acid-base reaction.3.1Why Are Terminal Alkynes Acidic?The acidity of terminal alkynes is due to thehigh level of s characterin thesp hybrid orbital. Thecarbon atom in the triple bond issp hybridized, meaning it has more s character than the sp³hybridized carbon in alkanes. This causes the electron cloud of the bond to shift closer to the carbonatom, making the hydrogen atom slightlypositive. This slight positive charge on the hydrogen makesit a weak proton, which can be removed by a strong base.

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Study Guide3.2Comparison with Alkanes and AlkenesInalkanesandalkenes, the s character in the hybridized carbon bonds is less. This means there is aless electronegative shiftin the overlap region of theσbond, making the hydrogen atomslesselectron-deficientand thereforeless acidic. In these cases, muchstronger basesare needed toremove the hydrogen atoms as protons.3.3Acetylide Ion FormationWhen a terminal alkyne reacts with a strong base, it forms anacetylide ion (R-C≡C⁻). The reaction isreversible, meaning the base may not form an acid stronger than the starting alkyne. The new acidcould alsoreprotonatethe acetylide ion.3.4Hydrolysis of the Acetylide IonIn the presence of water (H₂O), the acetylide ion can bereprotonated, resulting in the formation of aterminal alkyne and the hydroxide ion (OH⁻).Key Takeaway•Terminal alkynesare weakly acidic and can undergo acid-base reactions.•The acidity is due to thehigh s characterin thesp hybridized carbonbond.•The acetylide ion forms in an acid-base reaction and can bereprotonated.•The process ofhydrolysisof the acetylide ion can regenerate a terminal alkyne.

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Study Guide4.Alkynes: PreparationsHow Are Alkynes Prepared?The preparation of alkynes is very similar to that of alkenes. The main method of preparing alkynesinvolves theelimination of groups or ionsfrom molecules, leading to the formation ofπbonds.4.1DehydrohalogenationDehydrohalogenationis one common method. This involves theloss of a hydrogen atom and ahalogen atomfrom adjacent carbon atoms in an alkane. This process first forms analkene. Then,the loss of additional hydrogen and halogen atoms from the double-bonded carbon atoms leads to theformation of an alkyne.•The halogen atoms may be located on thesame carbon(ageminal dihalide) or onadjacent carbons(avicinal dihalide).4.2Second Step of DehydrohalogenationIn the second step of dehydrohalogenation,high temperaturesand anextremely strong basicsolutionare required.DehalogenationDehalogenationis another method.Vicinal tetrahaloalkanes(compounds with halogens onadjacent carbons) can undergodehalogenationwithzinc metalin anorganometallic reactiontoform alkynes.

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