Nuggets of Knowledge for Chapter 13: Alkynes. Alkynes are hydrocarbons that contain a carbon-carbon triple bond.

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1 I. Intrductin t Alkynes Classifying alkynes Nuggets f Knwledge fr Chapter 13: Alkynes Alkynes are hydrcarbns that cntain a carbn-carbn triple bnd. Cmpunds that cntain a carbn-carbn triple bnd but are nt hydrcarbns are simply labeled as unsaturated. Alkynes with the triple bnd n the end are called terminal alkynes; if the triple bnd is in the middle, it is called an internal alkyne. Hybridizatin and bnd frmatin The tw carbns in the triple bnd have sp hybridizatin. One f the bnds is a sigma bnd, frmed by cmbining ne sp rbital frm each carbn atm. The ther tw bnds are pi bnds, frmed by cmbining a p rbital frm each carbn atm. The tw pi bnds are at right angles t each ther. A triple bnd is shrter than a duble r a single bnd. Gemetry The tw carbn atms have linear gemetry, s alkynes cannt be written ut in the zig zag line structure used fr alkanes and alkenes. When alkynes are frmed, the zig zag shape f the starting materials must be changed t shw the crrect gemetry in the alkyne prduct; likewise, when alkynes underg reactins, the linear shape must be changed t shw the crrect gemetry f the prducts. Cyclic alkynes can exist, but because f the linear gemetry, they must have at least 9 carbns in the ring t be stable at rm temperature. Sterechemistry N sterechemistry can exist at the tw triple bnd carbns, but chiral sterecenters may exist at ther carbn atms in an alkyne mlecule.

2 Mlecular frmula Simple alkynes have the mlecular frmula C n H 2n-2, since the triple bnd cntributes 2 units f unsaturatin. Alkynes with rings in their structure will lse 2 hydrgen atms fr each ring. Stabilizatin Alkyl substituents stabilize alkynes by hypercnjugatin, s internal alkynes are slightly mre stable than terminal nes because they have mre carbns attached. Reactivity Alkynes have tw sites f pssible reactivity at the pi bnds and the H s f terminal alkynes. Terminal alkynes are weakly acidic, and can be deprtnated by a suitable base. They can then participate in S N 2 and E2 reactins. Pi bnds are higher in energy than sigma bnds, making them smewhat reactive. Alkynes react as weak nuclephiles r bases, attacking strng acids r gd electrphiles. Alkynes underg sme f the same reactins that alkenes d, and prduce similar prducts with similar regiselectivity. Hwever, the initial prduct f an alkyne still has a duble bnd left, while prducts f alkenes have nly single bnds at the end. Alkynes can ften underg a secnd reactin with the remaining duble bnd. II. Natural Occurrences and Uses f Alkynes Alkynes themselves are rarely fund in nature. One interesting exceptin is diacetylene (HC---C-C---C-H), which has been fund in the carbn-rich atmspheres f several planets and mns in ur slar system. Arund 1000 unsaturated cmpunds cntaining a carbn-carbn triple bnd as well as ther functinal grups have been fund in varius plants and animals. These cmpunds are ften txic. Sme unsaturated cmpunds cntaining carbn-carbn triple bnds have been islated r synthesized fr use as drugs, including birth cntrl agents, analgesics, antihypertensives, an anticancer agents.

3 The smallest alkyne, acetylene, is the mst useful alkyne in industry. It is used in xyacetylene cutting and welding trches. It is made by heating calcium xide and carbn t a high temperature, then adding water, r by heating methane t a high temperature fr a fractin f a secnd. It is stred disslved in acetne under mdest pressure due t its explsive ptential. If heated r mechanically shcked under pressure, it will decmpse t its elements, carbn and hydrgen, releasing energy. Plyacetylene is a carbn chain with alternating duble and single bnds. It is useful in fil packaging fr cmputer cmpnents t dissipate static electric charge, and als fr light weight all-plymer batteries. When treated with idine, it cnducts electricity nearly as well as silver, the highest cnducting metal. III. Physical Prperties f Alkynes Alkynes are nnplar cmpunds, s they are insluble in water. They are als less dense than water, s an alkyne layer will flat n tp f a water layer. Alkynes have melting and biling pints similar t alkanes and alkenes. Alkynes with 4 carbns r less are gases at rm temperature; alkynes with 5 t 10 carbns are liquids; and alkynes with mre than 10 carbns are slids. Alkynes are flammable, and als plymerize easily, smetimes explsively. Liquid alkynes have a strng, pungent dr. IV. Spectrscpy f Alkynes IR Terminal alkynes have IR bands at cm -1 fr the carbn-carbn triple bnd, and cm -1 fr the C-H bnd next t the triple bnd. The triple bnd band is at a higher frequency than the duble bnd f alkenes because it is a shrter, strnger bnd, but it absrbs less light than a nitrile because it is less plar. The C-H band is als at a higher frequency than in alkenes because the bnd is shrter. Internal alkynes d nt have either f these tw bands, and lk pretty much like an alkane.

4 1 H NMR H s attached t the carbn-carbn triple bnd shw up n 1 H NMR spectra between 1.7 and 3.1 ppm. Lng range cupling can ccur between the H n the end f a terminal alkyne and the H s n the ther side, but the cupling cnstants are fairly small, s the splitting is ften t small t be seen. 13 C NMR C s in a carbn-carbn triple bnd shw up n 13 C NMR between ppm. V. Nmenclature f Alkynes Cmmn names Cmmn names f simple alkynes can be created by thinking f them as derivatives f acetylene. The substituents n the carbn-carbn triple bnd are given first, in alphabetical rder if there is mre than ne, fllwed by the ending acetylene, all as ne wrd. The prefix di is used if the substituents are the same. IUPAC names Naming an alkyne fllws the same steps as naming an alkane r alkene. 1. Identify and label the sterechemistry, if any. 2. Chse the principle chain and name it. 3. Number the principle chain. 4. Name and rder the substituents. Pririties fr chsing the chain: In chsing the principle chain, yu must first include the triple bnd, and if there are mre than ne, as many as pssible must be included. If there is als a duble bnd, it must als be included in the principle chain if pssible. If there is mre than ne pssible chain that includes all f the triple and duble bnds, chse the lngest chain. If there are tw different chains that are the same length, chse the ne with the mst substituents.

5 Changing the ending f the principle chain name: When the principle chain cntains ne triple bnd, change the ending frm ane t yne. If there are tw r mre triple bnds, change the ending t adiyne, atriyne, etc. If there is bth a duble and a triple bnd, change the ending t enyne. Pririties fr numbering the chain: When numbering the principle chain, give the lwest number t the triple bnd; if there are tw, give pririty t whichever cmes first. If there is a duble and a triple bnd, give pririty t whichever cmes first, unless the numbers are the same in this case, give pririty t the duble bnd. If there is still a chice, give pririty t the first substituent, and if all substituents have the same numbers, give pririty t the substituent that cmes first in the alphabet. Where t put the numbers fr the duble and triple bnds: List the numbers fr the duble and triple bnds at the beginning f the main chain name, unless there is bth a duble and a triple, in which case put the duble bnd number at the beginning, and the triple bnd number just befre the yne. Naming triple bnds as substituents: T name a substituent cntaining a triple bnd, add yn befre the yl f the substituent: ethyl becmes ethynyl. If needed, give the number f the triple bnd, cunting frm the pint f attachment. The cmmn name fr a three carbn substituent with the triple bnd n the end is prpargyl (just like allyl fr a duble bnd). VI. Acid-base prperties f Alkynes Unlike alkane and alkenes, alkynes are weakly acidic, with a pka f This is because f the sp hybridizatin f the C atms. The cnjugate base f acetylene is called an acetylide in; this name is als applied t cnjugate bases f ther alkynes.

6 Acetylide ins can be frmed by using NaH (sdium hydride) r NaNH 2 (sdium amide, ften disslved in NH 3 ), because they have cnjugate acid pka s that are strng enugh t favr the acetylide in. Any acid with a pka less than 26 will favr the alkyne, s acetylide ins will be prtnated in the presence f water r alchls. Acetylide ins react as gd nuclephiles and gd bases. They can participate in S N 2 reactins that frm terminal alkynes frm acetylene and an alkyl halide, r internal alkynes frm terminal alkynes and an alkyl halide; this is the mst cmmn way f making alkynes. In the first step, the acetylene r the alkyne is deprtnated; in the secnd step, the acetylide in reacts as a Nu with the alkyl halide. Hwever, nly 1 alkyl halides can be used; therwise E2 reactins may ccur. Aryl halides als cannt be used, as they d nt underg S N 2 reactins. T make an internal alkyne starting frm acetylene, tw separate reactins must be used t avid mixtures. VII. Synthesis f Alkynes by Eliminatin The mst cmmn way t make alkynes is by a substitutin reactin; hwever, there are sme alkynes that cannt be made this way. Fr these alkynes, eliminatin may be an ptin. Just as eliminatin may be used t make alkenes frm alkyl halides, duble eliminatin can be used t make alkynes frm alkyl dihalides. Either geminal r vicinal dihalides can be used. The first eliminatin is just like an E2 reactin. In the secnd step, hwever, a vinyl halide must underg eliminatin, and this requires a much strnger base sdium amide. Tw equivalents f base are needed. Cumulated dienes are higher in energy than alkynes they are nt frmed as final prducts f this reactin. When terminal alkynes are frmed, the strng base deprtnates them t frm acetylide ins. Treating the reactin with water at the end gives back the alkyne. Since vicinal dihalides can be made frm alkenes, a tw step synthesis allws an alkene t be made int an alkyne (as lng as there is nly ne substituent n bth sides, and rearrangements can be prevented). This reactin is als useful fr making alkynes that cannt be made using a substitutin reactin nes which wuld require a 2, 3, r phenyl halide.

7 VIII. Reductin f Alkynes t frm Alkenes and Alkanes Alkynes can underg additin f H 2 just as alkenes can. Additin f H s t a mlecule is called a reductin. By using different reagents, an alkyne can be reduced t an alkane, a cis alkene, r a trans alkene. When H 2 is used with a Pt, Pd, Rd, r Ni catalyst, the alkyne is reduced first t a cis alkene, then t an alkane. The reactin cannt be stpped at the alkene, because as sn as it is created, it reacts with hydrgen and catalyst. Syn additin is bserved, just as in the alkene reactin. This reactin was used t determine the difference in stability f terminal and internal alkynes. Internal alkynes have a lwer delta H fr this reactin than terminal alkynes d, and since they result in the same prduct, this shws that internal alkynes are mre stable t start ut. In rder t stp the reactin at the alkene stage, a less reactive r pisned catalyst must be used. The mst cmmn is called the Lindlar catalyst; it cntains palladium n calcium carbnate, deactivated with lead (II) acetate and quinline. It is pssible t stp at the alkene because the alkyne is mre reactive. This is the best way t make cis alkenes; unlike the E2 reactin, n cnstitutinal ismers r stereismers are frmed. The mechanism f this reactin is nt cmpletely understd, but invlves the crdinatin f the pi bnd with the surface f the catalyst, fllwed by the simultaneus transfer f bth hydrgens. Trans alkenes must be frmed using a very different reactin. The alkyne is treated with sdium metal disslved in liquid ammnia (which is a deep blue clr) this reactin is ften referred t as a disslving metal reactin. The net result is the additin f H 2 t the mlecule, but the H s cme frm the ammnia mlecule, while the sdium supplies the electrns. The mechanism invlves dnatin f an electrn frm sdium t frm a radical anin, which then takes a hydrgen frm ammnia. The sdium then dnates anther electrn, frming an alkenyl anin, which als takes a hydrgen frm ammnia.

8 If a terminal alkyne is used, a better surce f hydrgen must be added t prevent the alkyne frm being deprtnated, which wuld prevent it frm reacting. Ammnium sulfate fills this rle withut interfering with any ther step in the reactin. Since many alkynes can be created thrugh substitutin reactins, alkyne synthesis and reductin is a pwerful methd fr creating lng alkane chains as well as specific cis and trans alkenes. IX. Additin f HX and X 2 t Alkynes Additin f HX Either ne r tw equivalents f HX can be added t an alkyne. HCl, HBr, and HI can all be used. If ne equivalent is added, then a vinyl halide is btained. This prduct is much less reactive than the alkyne, s the reactin stps there if nly ne equivalent is used. If tw equivalents are added, then the vinyl halide reacts again t frm a geminal dihalide. The secnd halide always adds t the carbn that already had ne. Just as in alkenes, the halide ges t the mre substituted side in a terminal alkyne. In an internal alkyne, neither side is mre substituted, and unless the starting material is symmetrical, tw prducts will result. Just as in alkenes, the additin f HBr in the presence f perxides reverses the regiselectivity, and the brmide attaches t the less substituted side f the C=C. Additin f X 2 Either ne r tw equivalents f X 2 can be added t an alkyne. Cl 2 and Br 2 are mst cmmnly used. If ne equivalent is used, a geminal vinyl dihalide is frmed. Anti additin is bserved. If tw equivalents are used, a vicinal tetrahalide is frmed. X. Hydratin f Alkynes Acid- and mercury-catalyzed hydratin Water can be added acrss the carbn-carbn triple bnd in the presence f sulfuric acid and mercuric acetate.

9 This is rather like a cmbinatin f acid-catalyzed hydratin f alkenes and xymercuratin-reductin, except yu dn t need the NaBH 4 step. Only ne equivalent can be added, because the vinyl alchl (called an enl) that is frmed is unstable. It immediately reacts t frm an aldehyde r ketne. This reactin is called a tautmerizatin. We will discuss it further in the chapter n enls. On a terminal alkyne, the mre substituted side gets the OH, then becmes the C=O. Hydrbratin-xidatin f alkynes Water can als be added acrss the carbn-carbn triple bnd by using a brane reagent fllwed by hydrgen perxide and hydrxide. As in alkynes, the regiselectivity f this reactin is the reverse f the ne abve the OH ges t the less substituted side. The enl frmed by the reactin underges the same tautmerizatin as in the previus reactin. With a terminal alkyne, an aldehyde is frmed; with an internal alkyne, a ketne is frmed. Because f the C=C still present in the enl, there is the ptential fr it t react again with the brane, causing side prducts t be frmed. With internal alkynes, the steric hindrance f the alkyne itself prevents this, but with terminal alkynes, a sterically hindered brane reagent is needed. The mst cmmnly used brane reagents are disiamylbrane, and dicyclhexylbrane. XI. Oxidatins f Alkynes Alkynes will react with tw equivalents f ptassium permanganate (KMnO 4 ) t frm a mlecule with fur OH grups, which then reacts t frm tw C=O s plus tw water mlecules. It is imprtant t keep this reactin cl and neutral, r the diketne will react further. Alkynes will react with zne t frm tw carbxylic acids. This cleaves the triple bnd, giving tw mlecules. Unlike alkenes, n additinal reagents (such as hydrgen perxide r dimethyl sulfide) are needed.