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2444-101411 - Strong Nucleophiles cyanide ion 1...

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Strong Nucleophiles: cyanide ion ? 1 • Cyclopentanone ( 9 ) reacts with potassium cyanide to give a low yield of 10 , consistent with cyanide as a weak nucleophile. • Compounds that have both a CN unit and an OH unit are called cyanohydrins . • The direct acyl addition of cyanide to 9 gives a poor yield of 10 , suggesting that the reaction may be reversible or that cyanide is a relatively poor electron donor, or both. • On the other hand, 10 is formed, so cyanide must have some ability to react as a nucleophile, which means that cyanide is a stronger nucleophile than chloride ion. O K :C N: K O C ! N + 9 10
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Strong Nucleophiles: cyanide ion ? 2 • Cyanide ion is a moderately strong nucleophile and acyl addition is facilitated by the presence of an acid catalyst. O H 2 O ; H 2 SO 4 K CN HO C ! N + 9 11 0°C , overnight 96% O K :C N: K O C ! N + 9 10
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Strong Nucleophiles: cyanide ion ? 3 • 9 is treated with KCN and sulfuric acid at 0°C to give 1-hydroxycyclopentane- carbonitrile, 11 , isolated in 96% yield. • The acid-mediated reaction clearly proceeds to product in good yield, so there is a difference in the direct nucleophilic reaction versus the same reaction in an acid medium. • It is known that a carbonyl reacts with an acid to give an oxocarbenium ion (see 2 and 12 ). • Once formed, it is quite reasonable that an oxocarbenium ion will react with cyanide ion to give 11 . • In other words, the positive carbon of oxocarbenium ion 12 reacts with the nucleophile to give 11 . O H 2 O ; H 2 SO 4 K CN HO C ! N + 9 11 0°C , overnight 96% O K :C N: HO C ! N 11 9 H–OSO 3 H OH 12
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Strong Nucleophiles: alkyne anions 4 • The hydrogen atom of a terminal alkyne (marked in red in 13 ) is weakly acidic, and it reacts with a suitable base to generate a conjugate base (the alkyne anion, 14 ). • Alkyne anions are classified as strong nucleophiles. A terminal alkyne (i.e., -C ! C- H ) is a carbon-acid . • Carbon acids are very important because removal of the acidic proton with a suitably strong base gives a carbanion (a carbon nucleophile), in this case an alkyne anion; -C ! C-H " -C ! C: . Carbon acids give carbanion conjugate bases that are also carbon nucleophiles . • Alkyne anions are strong nucleophiles in acyl addition reactions, generating a carbon-carbon bond irreversibly. C C Me H Na :NH 2 C C: Me Na H :NH 2 13 14 +
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Strong Nucleophiles: alkyne anions 5 • A specialized base called sodium hydride (NaH) is sometimes used for reaction of alkynes since the conjugate acid is hydrogen gas. • An example is the reaction of 15 with sodium hydride, which gives alkyne anion 16 and one-half a mole of hydrogen gas. • An aprotic organic solvent like diethyl ether can be used. • This is a particularly useful reagent since the by-product (H 2 ) escapes from the medium, driving the reaction to the right.
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