Chapter 8: Chemistry of Alkynes (C n H 2n-2 )

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1 hapter 8: hemistry of Alkynes ( n 2n-2 ) Bonding & hybridization Both are sp-hybridized Bond angles = 180 o 1 σ + 2 π bonds Linear around lassification R R R' σ bond energy: 88 kcal/mol π bond energy: 56 kcal/mol No conformers No geometric isomerism While cycloalkanes and alkenes are common, alkynes seldom appear in a ring. Why? terminal alkyne internal alkyne Nomenclature IUPA: Same rules apply as for alkenes, except that names end in yne. The triple bond takes precedence in the numbering of carbons in the main chain. ommon naming: = acetylene Many alkynes are named as derivatives of acetylene by adding a prefix indicating the alkyl groups attached on each side of the = Ex: 3 3 = dimethylacetylene Molecule shape and effect on physical properties: Linearity of alkynes increases van der Waals contact, strengthens London forces Boiling points of alkynes are slightly higher than alkenes of similar MW Solubility properties are similar to other hydrocarbons

2 Preparation of Alkenes and Alkynes Alkenes: 1) Thermal cracking of alkanes: used to produce small alkenes likeethylene 900 o = Disadvantage: Not useful for preparing larger alkenes with specific structures 2) Base-catalyzed elimination from RX: Position of double bond is predictable KO 3 2 O 3 3 3) Acid-catalyzed dehydration of alcohols 3 + K + 2 O Zaitsev s rule: The most substituted alkene is formed preferentially Alkynes 3 O 3 PO 4 heat major O minor 1) Dehydrohalogenation of a vicinal dihalide: 3 3 2KO 3 2 O ) Dehydrohalogenation of vinylic alkyl halides: l 3 3 1) 2 NaN 2 2) 3 O N 3 + Nal

3 Reactivity of alkynes toward electrophiles: Like alkenes, alkynes have π bonds that act as nucleophiles, therefore electrophilic addition is a major reaction pathway. Some aspects of alkyne reactivity and mechanisms can be explained by comparing the electronegativities of carbon atoms involved in hybridization: sp > sp 2 > sp 3 increasing electronegativity Why? The closer the atom holds its electrons, the greater its electronegativity. This in turn affects the mechanism and type of intermediate that forms during electrophilic addition. Electrophilic addition to alkynes may involve a vinylic cation, or a more complex arrangement where the positive charge is spread out: -l 2 +l - l -l δ l - l l The unsubstituted vinylic cation is unstable, due to the + charge on an sphybridized (would be an sp 2 -hybridized in regular carbocations) Substituted vinylic carbocations are more stable due to e- donation arbocation stability (revisited): 3 o > 2 o > 1 o = 2 o vinylic >> 1º vinylic

4 1. Electrophilic additions to alkynes A. ydrohalogenation and halogenation reactions: Follow Markovnikov orientation Proceed with anti stereochemistry of addition The electrophile and the nucleophile add from opposite sides of the carbon-carbon bond (trans orientation); one geometric isomer predominates Ex: l 2 3 l 2 3 l (E)-2,3-dichloro-2-pentene anti and syn addition of reagents results in different isomers! Will continue with addition of a second equivalent to the newly-formed alkene when an excess of electrophile is present: Ex: l l 3 l 2 3 A geminal dichloride l

5 B. ydration reactions of alkynes 1. Addition of water to an alkyne might produce an alcohol, as for alkenes: O O Internal alkyne An alkene with O group on = : an enol owever, enols are not very stable. The pi electrons prefer the O bond, especially in acid solution. Keto-enol tautomerism occurs to produce =O: O O Enol Ketone Tautomers = isomers which differ in placement of the double bond and atom The keto form is more stable and will be the major product formed in a hydration. Terminal alkynes do not easily undergo acid-catalyzed hydration. 2. Mercury-catalyzed hydration (similar to oxymercuration of alkenes) will produce ketones, even with terminal alkynes 3 2 O, + gso 4 3 O 2 3 O 3 Markovnikov 2-propanone (a ketone) 3. ydroboration-oxidation: Anti-Markovnikov orientation results in formation of aldehydes. With terminal alkynes, a bulky alkylborane may be used 3 1) B 3 /TF or disamylborane 2) O-, 2 O 2, 2 O 3 2 O Anti-Markovnikov Propanal (an aldehyde)

6 . Addition of hydrogen to produce alkenes or alkanes (You may have noticed a theme: what alkenes can do, alkynes can do twice) ydrogenation: When regular Pt catalyst is used, two additions occur 1equivalent 3 3 2,Pt 3 3 1moreequiv 2,Pt Alkyne Alkene (syn-addition) Alkane 2- butyne cis-2-butene butane or (Z)-2-butene To produce alkenes, addition of the second equivalent of 2 can be avoided by using a partly inactivated metal catalyst ( poisoned palladium ) to only deliver 1 equiv. 2 : Lindlar s catalyst = Pd on ao 3, treated with lead acetate and quinoline Lindlar's catalyst 3 3 atalytic hydrogenation only produces cis isomers; trans-alkenes require a different approach: 3 3 Na or Li Liquid N 3 Sodium reacts more readily with alkyne than = (no second addition to the alkene), producing a vinylic anion (R 2 = R:-) The vinylic anion can attack N 3 Since the trans form of vinylic anion is more stable; the major product is trans. D. Oxidative cleavage: Oxidation of alkynes: bond is cleaved, yielding carboxylic acids or O 2 : 3 3 Internal: Terminal: R R' R O O KMnO 4 or O 3 + KMnO 4 or O 3 R R O O O + R' O O O

7 arbonyl compounds from alkynes:

8 Acidity of terminal alkynes Recall that electronegativities of atoms follow this trend: sp > sp 2 > sp 3 ow does this affect the acidity of a hydrogen atom bonded to the alkyne? > > 3 3 pka = 25 pka = 44 pka = 50 In the grand scheme, acetylenes are more acidic than ammonia but less than water. 3 3 < 2 = 2 < N 3 < = < 2 O < F Increasing acidity Increasing basicity > 2 =- > N 2 - > =:- > O- > F- In the presence of a very strong base, a terminal alkyne forms a conjugate base: an acetylide ion: NaN 2 N Na + +N 3 3 Acid, pka = 25 conjugate base, pka = 36 The base must be stronger than the acetylide ion (itself a pretty strong base) Amide ion is used in the form of sodium amide in liquid ammonia: NaN 2 /N 3 Note: The weak acidity of the bond in terminal alkynes is unique among hydrocarbons in alkenes, alkanes and internal alkynes do not behave as acids.

9 New concept: Building a arbon Skeleton A chain-lengthening alkylation reaction Most of the reactions of hydrocarbons with pi bonds that we have encountered so far mainly result in new functional groups: --Electrophilic addition reactions of alkenes or alkynes which produce alkyl halides --ydration reactions involving addition of water or other oxygen-containing reagents to produce alcohols --ydrogenation reactions which saturate alkenes & alkynes (adding more ) Acetylide ions can undergo reactions in which new carbon-carbon bonds form, resulting in a longer carbon chain. The mechanism is not addition, but a nucleophilic substitution in which the acetylide ion replaces the halide: R Na + + R' R R' +Na Acetylide ion Primary alkyl halide Larger alkyne Alkynes are extremely useful starting points for synthesis because you can use them to build a longer chain/transform to another functional group by preparing acetylides: 3 2 NaN 2 2 liq N 3 3 Na l

10 Multistep synthesis: Figuring out how to prepare a compound step by step Points to keep in mind when designing the synthesis of a compound: 1. There are probably several pathways to get you to your goal. (In the lab, that means the shortest pathway with the most product yield and cheapest reagents!) 2. Reactions in the textbook are grouped by reactant, not necessarily by product. You may have to consider several different classes of compounds (functional groups) as reactants. It helps to compile your own list, adding reactions as you learn them. 3. A retrosynthetic approach is most effective: Think of each reaction you learned in reverse: the product becomes the reactant! Start each problem by figuring out what to do last. 4. Work your way backwards to plan each step, focusing on making the necessary functional group or other changes in the molecule until you get to the suggested (or cheapest) starting reagents. 5. Next, work back through your proposed steps in the forward direction, taking into account both major and minor products. Your desired product in each step should be the major one. 6. Keep mechanisms in mind, especially where rearrangements are possible or regioselectivity is the rule. Practice makes perfect: the more problems you solve whether working in the forward or reverse direction, the easier it is to see patterns of reactivity and use them.

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