Note that when two equivalents of HX are added to a terminal alkyne, both X s wind up on the same carbon!

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Milton Lambert
7 years ago
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1 Reactions of Alkynes: 1. Addition of X (Addn of, X, where X = Cl, ) Markovnikov 1x or 2x Types of Alkynes Terminal one end is substituted with R and one has an. The end with the R is more substituted, and therefore forms a more stable carbocation intermediate, thus Markovnikov can be easily distinguished: 1 equiv. 2 equiv. Note that when two equivalents of X are added to a terminal alkyne, both X s wind up on the same carbon! Internal Symmetrical Alkynes Both ends are equally substituted with the same R group so it doesn t matter which end the or X adds to, TE FIRST TIME This is not a regioselective reaction both ends are the same: 1 equiv. The SECND addition is problematic. nce the first addition has occurred, the two ends of the (now) alkene are not symmetrical. Each end still has one alkyl group. 1st equiv 2nd equiv??? 2 equiv. and That s a bad reaction makes two different products! 1

2 Internal Asymmetrical Alkynes In this version, the alkyne is equally substituted with a single R group on each end but the R groups are different so Markovnikov cannot be distinguished mixture of at least two products always results, even with only 1x addition of X. Bad reaction!! 1 equiv. and 1 equiv. Cl 2 equiv. 2. Addition of X 2 (X = Cl, ) ANTI 1x or 2x -Can only see ANTI for a single addition (1x) -ANTI is shown by the relative positions of the halides in the alkene product, NT by the use of wedges/dashes!! 1 equiv. Cl 2 2 equiv. Cl 2 More Examples: 1 equiv. Cl 2 1 equiv. 2 2

3 2 equiv. 2 An ldie: Cl 2 3. ydrogenation of alkynes (2X Addn of, ) SYN (can t see it but it IS happening!) 2x ALWAYS 2, Pd/C 2, Pd/C 2, Pd/C 2, Pd/C 2, Pd/C 4. Formation of CIS alkenes (1X Addn of, ) SYN 1X ALWAYS -Catalyst is poisoned with lead (Pb) and thus reacts slower than Pd/C. This reaction gives us some insight into the relative reactivity of alkenes and alkynes. Since only the starting alkynes continue to react without any of the product alkenes undergoing hydrogenation, we can see that alkynes ARE more reactive than alkenes in a hydrogenation. 3

4 2 Pd/CaC 3 /Pb 2 Pd/CaC 3 /Pb 2, Pd/CaC 3 /Pb 5. Formation of TRANS alkenes - (1X Addn of, ) -ANTI 1X ALWAYS Li or Na, N 3 Li, N 3 Li, N 3 The mechanism for this reaction is completely different than those in the past this reaction involves an electron transfer process. Both Na and Li are Group I elements that want to give up one electron in order to become Nobel Gas configurations. They dump an electron into the pi system, creating a radical anion, which immediately removes a proton ( + ) from the solvent, N 3. Then the process repeats itself: 4

5 Li Li + + 1e- N 2 Li Li + + 1e- N 2 The stereochemistry is determined when the second electron is transferred. Which is more stable? And why? The first one is more stable because it places the alkyl groups around the newly forming alkene farther away from each other (less steric interactions!). The next two reactions are the Markovnikov and non-markovnikov additions of and to an alkyne But you will not see alcohols form in this reaction! When and add to the alkyne, an enol is formed, which rearranges to form a carbonyl (C=) group: enol This particular type of rearrangement is called a tautomerization and thus forms aldehydes or ketones in these reactions: enol Tautomerization always favors the carbonyl form (look at the equilibrium arrows above) presumably because the electrons in the pi system are more stable closer to the electronegative oxygen atom. 6. Mercury-catalyzed hydration (1X Addition of, ) -Markovnikov 5

6 2 S 4, 2 gs 4 Addition of, in a Markovnikov fashion always produces an enol, which then undergoes tautomerization to form a ketone. These products always seem harder to draw. It may be easier for you if you draw the intermediate enol (physically draw or mentally note!), placing the on the more substituted end. When you tautomerize (to draw the final product), the carbonyl (C=) always forms where the was attached. 2 S 4, 2 gs 4 2 S 4, 2 gs 4 Terminal alkynes always produce methyl ketones (why are they called that?) and symmetrical alkynes will always produce the same ketones. Regiochemistry doesn t matter because symmetrical internal alkynes are equally substituted with the same R group. Markovnikov doesn t matter 2 S 4, 2 gs 4 As with the addition of X, asymmetrical alkynes are a poor choice for starting material. Because the two ends are equally substituted with one alkyl group, they are equally good at stabilizing the carbocation intermediate and Markovnikov cannot be easily discerned the reaction produces two different ketone products. Bad Reaction! gs 4, 2 2 S 4 6

7 More Examples: 2 S 4, 2 gs 4 2 S 4, 2 gs 4 2, 2 S 4 1. g(ac) 2, 2 2. NaB 4 7. ydroboration of Alkynes (1X Addn of, ) Non-Markovnikov This reaction commonly uses disiamylborane instead of B 3 to prevent multiple additions to the alkyne pi system (more sterics slow down addition process to the TW pi bonds of alkyne). B Addition of disiamylborane to a terminal alkyne in a Non-Markovnikov fashion always produces an enol that tautomerizes to form an aldehyde. 1. disiamylborane 2. Na, 2 2 Tautomerization of an enol occurs: 1. disiamylborane 2. Na, 2 2 7

8 For the addition to internal alkynes, we can use B 3 since there are already sterics in the molecule surrounding both ends of the alkyne. Symmetrical Internal Alkyne will always produce a single ketone product due to symmetry. Regiochemistry doesn t matter since its symmetrical and addition in either manner forms the same four-membered cyclic transition state. 1. B 3 2. Na, 2 2 As with the mercury-catalyzed hydration, hydroboration of an asymmetrical internal alkyne is problematic. Because both ends are equally substituted but not symmetrical, the reaction cannot distinguish the Non-Markovnikov transition state, resulting in two different ketone products. Bad Reaction! Try these: 1. B 3 1. B 3 2. Na, Na, Disiamylborane 2. Na, Disiamylborane 2. Na, B 3 2. Na, xidative Cleavage of Alkynes -Severs both pi bonds as well as the sigma bond -Always form Carboxylic Acids (or C 2 ) -Use either reagent and get SAME results: KMn 4, 2 or 3 + 8

9 R Zn, 3 + Internal Symmetrical both ends form same carboxylic acid Zn, 3 + Internal Asymmetrical forms two different carboxylic acids KMn 4, 3 + Terminal one end has only a hydrogen atom attached. In the oxidative cleavage process, that carbon will be oxidized to its full extent forming C 2 (or carbonic acid, if you like) Zn, 3 + KMn 4, Zn, Zn, 3 + KMn 4, 3 + And Finally: 9. Acetylide Anion Formation and Reaction Powerful C-C bond forming reaction increases length of carbon chain - uses Acetylene (-C C-) or any terminal alkyne Watch this general reaction: 9

10 1. Base 2. RX R In Chapter 9: Acetylide Anion Reaction 1. Base 2. RX R In Chapter 18: Williamson Ether Synthesis R' 1. Base 2. RX R' R In Chapter 21: R' C 1. Base 2. RX R' C R In Chapter 22: 1. Base 2. RX R onsted-lowry Acid-Base Theory: Acid + Base Conj Base + Conj Acid C pka = 25 + NaN 2 C Na + N 3 pka = 35 Alkene protons (vinylic protons) have a pka of about 45 and alkane protons have a pka of 60! Why are terminal alkynes acidic? (Why is the conjugate base stable?) C B: C + B The anion that forms is contained in an sp hybridized orbital, the shortest, roundest hybrid orbital of all the hybrids, and orbital that will hold the electron-pair closest to the positive carbon nucleus, stabilizing the anion. 10

11 Acetylide anions are nucleophiles and they react with electrophiles. The electrophiles of choice for this section are alkyl halides. Acetylide anions can be alkylated (addition of alkyl groups, either a methyl group, -C 3, or any primary alkyl group, -C 2 R.) R NaN 2 R Na + N 3 E + R E Alkyl halides are electrophiles, due to the polarity of the C-X bond: δ + δ - C Cl Reaction with alkyl halide will result in addition of carbon chain to alkyne, where the ydrogen atom once was: R C Na C 3 R C C 3 + Na 1. NaN 2, N 3 2. C 3 C 2 1. NaN 2, N 3 2. Cl 1. NaN 2, N 3 2. I Synthesis: 11

12 Ex: 12

Electrophilic Addition Reactions

Electrophilic Addition Reactions Electrophilic addition reactions are an important class of reactions that allow the interconversion of C=C and C C into a range of important functional groups. Conceptually,