Cyanid-Ion

As an illustrative example, consider the vibrational energy relaxation of the cyanide ion in water [45], The mechanisms for relaxation are particularly difficult to assess when the solute is strongly coupled to the solvent, and the solvent itself is an associating liquid. Therefore, precise experimental measurements are extremely usefiil. By using a diatomic solute molecule, this system is free from complications due to coupling... 

The complexes of manganese(III) include [Mn(CN)g] (formed when manganesefll) salts are oxidised in presence of cyanide ions), and [Mnp5(H20)] , formed when a manganese(II) salt is oxidised by a manganate(VII) in presence of hydrofluoric acid ... 

Here, effectively, the Co " (aq) is being oxidised by the nitrite ion and the latter (in excess) is simultaneously acting as a ligand to form the hexanitrocobaltate(III) anion. In presence of cyanide ion CN. cobalt(II) salts actually reduce water to hydrogen since... 

Addition of aqueous cyanide ion to a copper(II) solution gives a brown precipitate of copper(II) cyanide, soluble in excess cyanide to give the tetracyanocuprate(II) complex [Cu(CN)4] . However, copper(II) cyanide rapidly decomposes at room temperature, to give copper(I) cyanide and cyanogen(CN)2 (cf. the similar decomposition of copper(II) iodide, below) excess cyanide then gives the tetracyanocuprate(I) [Cu(CN)4] . 

Silver is formed in nature as argentite. AgjS and horn silver. AgCl. The extraction of silver depends upon the fact that it very readily forms a dicyanoargentate(I) complex, [Ag(CN)2] (linear), and treatment of a silver ore with aqueous cyanide ion CN extracts the silver as this complex. The silver is then displaced from the complex by zinc ... 

Silver has little tendency to formally lose more than one electron its chemistry is therefore almost entirely restricted to the + 1 oxidation state. Silver itself is resistant to chemical attack, though aqueous cyanide ion slowly attacks it, as does sulphur or a sulphide (to give black Ag S). hence the tarnishing of silver by the atmosphere or other sulphur-containing materials. It dissolves in concentrated nitric acid to give a solution of silver(I) nitrate. AgNOj. 

Figure 3-22 shows a nucleophilic aliphatic substitution with cyanide ion as a nucleophile, i his reaction is assumed to proceed according to the S f2 mechanism with an inversion in the stereochemistry at the carbon atom of the reaction center. We have to assign a stereochemical mechanistic factor to this reaction, and, clearly, it is desirable to assign a mechanistic factor of (-i-1) to a reaction with retention of configuration and (-1) to a reaction with inversion of configuration. Thus, we want to calculate the parity of the product, of 3 reaction from the parity of the... 

The formation of ethyl isopropylidene cyanoacetate is an example of the Knoevenagel reaction (see Discussion before Section IV,123). With higher ketones a mixture of ammonium acetate and acetic acid is an effective catalyst the water formed is removed by azeotropic distillation with benzene. The essential step in the reaction with aqueous potassium cyanide is the addition of the cyanide ion to the p-end of the ap-double bond ... 

The reaction depends upon the catalytic influence of the cyanide ion, the mechanism being probably as follows ... 

Furfural undergoes condensation to furoin under the catal3rtic influence of cyanide ions in aqueous alcohol solution (compare Benzoin, Section IV,125) ... 

It is essential to use an excess of sodium, otherwise if sulphur and nitrogen are both present sodium thiocyanate, NaCNS, may be produced in the test for nitrogen it may give a red coloration with ferric iron but no Prussian blue since there will be no free cyanide ions. With excess of sodium the thiocyanate, if formed, will be decomposed ... 

Sulphur, as sulphide ion, is detected by precipitation as black lead sulphide with lead acetate solution and acetic acid or with sodium plumbite solution (an alkaLine solution of lead acetate). Halogens are detected as the characteristic silver halides by the addition of silver nitrate solution and dilute nitric acid the interfering influence of sulphide and cyanide ions in the latter tests are discussed under the individual elements. 

Strangely enough, cyanide ion is also involved in one special reaction giving an a-hydroxy-ketone. Can you show how the adduct A of benzaldehyde and cyanide ion can give a stable carbanion ... 

The product is called benzoin and the reaction is known therefore as the benzoin condensation. No base is needed other than cyanide ion. 

Clearly other combinations of logical and illogical synthons could be used to make 1,4-dioxygenated compounds. How could you use cyanide ion (as the CO2H synthon) to make a y-keto acid such as... 

Both are possible but we are more used to (1) since we can use a Michael acceptor and cyanide ion for the two synthons. How would you actually do a s mthesis this way ... 

Cyanide ion ( C = N ) The negatively charged carbon atom of cyanide ion IS usually the site of its nucleophilic character Use of cyanide ion as a nucleophile permits the extension of a carbon chain by carbon-carbon bond formation The product is an alkyl cyanide or nitrile... 

A second factor that can tip the balance m favor of substitution is weak basicity of the nucleophile Nucleophiles that are less basic than hydroxide react with both pri mary and secondary alkyl halides to give the product of nucleophilic substitution m high yield To illustrate cyanide ion is much less basic than hydroxide and reacts with 2 chlorooctane to give the corresponding alkyl cyanide as the major product... 

The mechanism of this reaction is outlined m Figure 17 8 It is analogous to the mech anism of base catalyzed hydration m that the nucleophile (cyanide ion) attacks the car bonyl carbon m the first step of the reaction followed by proton transfer to the carbonyl oxygen in the second step... 

The addition of hydrogen cyanide is catalyzed by cyanide ion but HCN is too weak an acid to provide enough C=N for the reaction to proceed at a reasonable rate Cyanohydrins are therefore normally prepared by adding an acid to a solution containing the carbonyl compound and sodium or potassium cyanide This procedure ensures that free cyanide ion is always present m amounts sufficient to increase the rate of the reaction... 

Step 1 Nucleophilic attack by the negatively charged carbon of cyanide ion at the... 

Reaction is catalyzed by cyanide ion Cy anohydrins are useful synthetic inter mediates cyano group can be hydro lyzed to —CO2H or reduced to —CH2NH2... 

Section 19 12 Nitnles which can be prepared from primary and secondary alkyl halides by nucleophilic substitution with cyanide ion can be converted to car boxyhc acids by hydrolysis... 

Section 20 18 Nitnles are prepared by nucleophilic substitution (8 2) of alkyl halides with cyanide ion by converting aldehydes or ketones to cyanohydrins (Table 20 6) or by dehydration of amides... 

The benzyltnmethylammonium ion migrates to the butyl bromide phase carrying a cyanide ion along with it... 

Once m the organic phase cyanide ion is only weakly solvated and is far more reactive than It IS m water or ethanol where it is strongly solvated by hydrogen bonding Nude ophilic substitution takes place rapidly... 

In the Strecker synthesis an aldehyde is converted to an a ammo acid with one more carbon atom by a two stage procedure m which an a ammo nitrile is an mterme diate The a ammo nitrile is formed by reaction of the aldehyde with ammonia or an ammonium salt and a source of cyanide ion Hydrolysis of the nitrile group to a car boxylic acid function completes the synthesis... 

Aldehydes by reaction with ammonia and cyanide ion (the Strecker synthesis)... 

The reaction of tert butyl chloride with cyanide ion proceeds by elimination rather than substitu tion... 

Probably the most extensively applied masking agent is cyanide ion. In alkaline solution, cyanide forms strong cyano complexes with the following ions and masks their action toward EDTA Ag, Cd, Co(ll), Cu(ll), Fe(ll), Hg(ll), Ni, Pd(ll), Pt(ll), Tl(lll), and Zn. The alkaline earths, Mn(ll), Pb, and the rare earths are virtually unaffected hence, these latter ions may be titrated with EDTA with the former ions masked by cyanide. Iron(lll) is also masked by cyanide. However, as the hexacy-anoferrate(lll) ion oxidizes many indicators, ascorbic acid is added to form hexacyanoferrate(ll) ion. Moreover, since the addition of cyanide to an acidic solution results in the formation of deadly... 

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