Alkynes: Properties, Naming, Preparation, and Reactions - Prof. Peter B. Balanda, Study notes of Organic Chemistry

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Alkynes 2 CH4 >1150 o C HC CH + 3 H2 steam H2C CH2 >1150 o C HC CH + H2 steam Petroleum ―cracking‖ Ethyne (Acetylene), the smallest alkyne Naming Alkynes • General hydrocarbon rules apply with ―-yne‖ as a suffix indicating an alkyne • Numbering of chain with triple bond is set so that the smallest number possible include the triple bond Diynes, Enynes, and Triynes • A compound with two triple bonds is a diyne – An enyne has a double bond and triple bond – A triyne has three triple bonds • Number from chain end nearest a double or triple bond – double bonds have priority if both are present in the same relative position Alkynes as substituents are called “alkynyl” CH,CH,CH,CH, > CH,CH,CH = CH CH,;CH,C=C+ Butyl 1-Butenyl 1-Butynyl (an alkyl group) (a vinylic group) (an alkynyl group) ©2004 Thomson - Brooks/Cole Br H OH OH Preparation of Alkynes by Dehydrohalogenation Alkyne forms through a vinylic halide intermediate The ability of vinylic halides to undergo elimination has been demonstrated Why 2 equivalents of the strong base NaNH2? Ans. The 1st eq. removes the hydroxyl proton Reactions of Alkynes: • Addition reactions of alkynes are similar to those of alkenes • Intermediate alkene reacts further with excess reagent • Regiospecificity according to Markovnikov C C HR1 E Nu C C R1 Nu E H E Nu C C Nu Nu R1 E E H Usually gives Anti Addition (Br and Br trans to each other) Electrophilic Addition – Halogenation Addition of H2 Suggestive of a bridging bromonium ion? C C Br Br ? ? Addition of HX to Alkynes Probably Involves Vinylic Carbocations C=C R/ OH H H H H H H An alkene An alkyl carbocation An alkyl bromide H Br H [| LF) XR Rc=co 2. | r—c=c + 6 \ / H R H An alkyne A vinylic carbocation A vinylic bromide ©2004 Thomson - Brooks/Cole – Secondary vinyl carbocations are about as stable as, or a bit less stable than, primary alkyl carbocations – Primary vinyl carbocations probably do not form at all E NuC C R H H H + C CR Nu C C HR E Nu + C C H R E Nu H H H H Eact Eact E The activation energy is within reach Keto-enol Tautomerism • Isomeric compounds that can rapidily interconvert by the movement of a proton are called tautomers and the phenomenon is called tautomerism • Enols rearrange to the isomeric ketone by the rapid transfer of a proton from the hydroxyl to the alkene carbon • The keto form is usually so stable compared to the enol that only the keto form can be observed Mechanism for Mercury(II)- Catalyzed Hydration of Alkynes See next 3 slides Hydration of Unsymmetrical Alkynes • If the alkyl groups at either end of the C-C triple bond are not the same, both products can form and this is not normally useful • If the triple bond is at the first carbon of the chain (then H is what is attached to one side) this is called a terminal alkyne • Hydration of a terminal always gives the methyl ketone, which is useful Hydroboration/Oxidation of Alkynes • BH3 (borane) adds to alkynes to give a vinylic borane • Oxidation with H2O2 produces an enol that converts to the ketone or aldehyde • Process converts alkyne to ketone or aldehyde with orientation opposite to mercuric ion catalyzed hydration Problem 8.6 What alkyne would you start with to prepare each of the following compounds by a hydroboration/oxidation reaction? C CH C C CHCH3 CH3 CH3CH CH3 Comparison of Hydration of Terminal Alkynes • Hydroboration/oxidation converts terminal alkynes to aldehydes because addition of water is non-Markovnikov • The product from the mercury(II) catalyzed hydration converts terminal alkynes to methyl ketones Conversion of Alkynes to trans-Alkenes • Anhydrous ammonia (NH3) is a liquid below -33 ºC – Alkali 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 (see Figure 8-4, next slide) mechanism Lithium metal donates an electron to the alkyne to give an anion radical... ... which abstracts a proton from ammonia solvent to yield a vinylic radical, The vinylic radical accepts another electron from a second lithium atom to produce a vinylic anion ... ... which abstracts another proton from ammonia solvent to yield the final trans alkene product. ©2004 Thomson - Brooks/Cole R—C=G—R’ + Lit | ili R ey me R—C=C +:NH,- \ H H PR \o=¢ LX R H Example: ©2004 Thomson - Brooks/Cole CH,OH 15 7-cis-Retinol (7-cis-vitamin A; vitamin A has a trans double bond at C7) C CH KMnO4 Predict the products of the following reactions. (a) COH CO2 O + O3 (b) O OH HO O O OH O HO Alkyne Acidity: Formation of Acetylide Anions • Terminal alkynes are weak Brønsted acids (alkenes and alkanes are much less acidic (pKa ~ 25). See Table 8.1, next slide, for comparisons) • Reaction of strong anhydrous bases with a terminal acetylene produces an acetylide ion • The sp-hydbridization at carbon holds negative charge relatively close to the positive nucleus (see figure 8-5) TABLE 8.1 Acidity of Simple Hydrocarbons Type Example K, pK, Alkyne HC=CH Ones 25 Stronger acid Alkene H,C=CH, 10-44 44 Alkane CH, ~10-8 60 Weaker acid ©2004 Thomson - Brooks/Cole NH, pK, 35 The nucleophilic acetylide anion uses its electron lone pair to form a bond to the positively polarized, electro- philic carbon atom of bromomethane. As the new C-C bond begins to form, the C—Br bond begins to break in the transition state. The new C-C bond is fully formed and the old C-Br bond is fully broken at the end of the reaction. This is called an Sy2 mechanism © 2004 Thomson/Brooks Cole H s- | H—C=C:---(---Br + Na* i\ HH Transition state | H / H—C=c—C. “H H + NaBr Alkylation of Acetylide Anions • Reaction with a primary alkyl halide produces a hydrocarbon that contains carbons from both partners, providing a general route to larger alkynes Limitations of Alkyation of Acetylide Ions • Reactions only are efficient with 1º alkyl bromides and alkyl iodides • Acetylide anions can behave as bases as well as nucelophiles • Reactions with 2º and 3º alkyl halides gives dehydrohalogenation, converting alkyl halide to alkene