Hydrogen cyanides

Hydrogen cyanide (hydrocyanic acid) is a colorless liquid (b.p. 25.6°C) that is miscible with water, producing a weakly acidic solution. It is a highly toxic compound, but a very useful chemical intermediate with high reactivity. It is used in the synthesis of acrylonitrile and adiponitrile, which are important monomers for plastic and synthetic fiber production. 

Hydrogen cyanide is produced via the Andrussaw process using ammonia and methane in presence of air. The reaction is exothermic, and the released heat is used to supplement the required catalyst-bed energy  

A platinum-rhodium ahoy is used as a catalyst at 1100°C. Approximately equal amounts of ammonia and methane with 75 vol % air are introduced to the preheated reactor. The catalyst has several layers of wire gauze with a special mesh size (approximately 100 mesh). 

The Degussa process, on the other hand, reacts ammonia with methane in absence of air using a platinum, aluminum-ruthenium ahoy as a catalyst at approximately 1200°C. The reaction produces hydrogen cyanide and hydrogen, and the yield is over 90%. The reaction is endothermic and requires 251 KJ/mol. 

Hydrogen cyanide may also be produced by the reaction of ammonia and methanol in presence of oxygen  

Hydrogen cyanide (melting point -14°C, boiling point 26°C) is manufactured by the reaction of natural gas (methane), ammonia, and air over a platinum or platinum-rhodium catalyst at elevated temperature (the Andrussow process). 

Hydrogen cyanide is also available as a by-product from acrylonitrile manufacture by ammoxidation. 

Hydrogen cyanide is used for the production of methyl methacrylate, adiponitrile, cyanuric chloride, and chelating agents. 

Hydrogen cyanide (HCN) is a colorless gas that is extremely poisonous. It has the odor of bitter almond, and its presence normally goes unnoticed. Hydrogen cyanide is used for rodent control, fumigation of ships for pest control, and 

Hydrogen cyanide is absorbed in an NaOH solution to form NaCN, which reacts with chloramine-T (iV-chloro-p-toluenesulphonamide), forming chlo-rocyane 

The cyanopyridine cation is obtained by the addition of chlorocyane to pyridine. During its hydrolysis, the pyridine ring is opened, forming gluta-cone aldehyde 0CH-CH=CH-CH2-CH0. This reacts with barbituric acid to form a colour whose intensity is proportional to the hydrogen cyanide concentration [18]. 

Anhydrous hydrogen cyanide is a colorless or pale yellow liquid witli a mild odor similar to that of bitter almonds. Tlie liquid boils at 78.3°F and 1.0 atm and forms a colorless, flanunablc, toxic gas. Hydrogen cyanide is completely 

Three chemical properties of hydrogen cyanide contribute to tlie potential for an accidental release of the chemical. 

Hydrogen cyanide is flanunable in air at concentrations from 6 to 41% hydrogen cyanide. 

The addition of alkaline chemicals, water, and/or heat may promote self-polymerization and decomposition of hydrogen cyanide. The self-polj ineriziUion reaction is exothermic, and the heat released will promote further polymerization. The heat generation will also result in the decomposition of hydrogen cyanide into anunonia and formate. Tlie pressure rise from polymerization or decomposition reactions can become explosive. Small amounts of acid, such as sulfuric or phosphoric, will help to stabilize tlie hydrogen cyanide against polymerization. 

The addition of large quantities of acid ( 15wt% of concentrated sulfuric acid) ctm cause rapid, and liiglily exothennic decomposition of hydrogen cyanide. When sulfuric acid is involved, tlie decomposition by-products will be sulfur dioxide and carbon dioxide. 

Three eheinieal properties of hydrogen cyanide eontribute to tlie potential for an aeeidental release of the eheinieal.  

Hydrogen eyanide is flanuiiable in air at eoneenlrations from 6 to 41% hydrogen eyanide. 

Process Applications TABLE 8.4.1 Exposure Limits for Hydrogen Cyanide 265 

Nomnicrobial processes for destruction of cyanide wastes have been extensively investigated and are well known. Parga et al. (2003) have reported several oxidation methods for treating cyanide solutions. Such methods include (1) destruction of cyanide by oxidation with chlorine dioxide in a gas-sparged hydrocyclone reactor, (2) destruction by ozone in a stirred batch reactor, and 

Incineration and landfill disposal are commonly used to destroy or dispose cyanide wastes. Some common laboratory methods of destruction of cyanide are briefly outlined below under the compounds in the following sections. 

Classified Hazardous Waste, RCRA Waste Number P063 DOT Label Poison A and Flammable Gas, UN 1614, 5% solution or more, UN 1613 Formula HCN MW 27.03 CAS [74-90-8] Structure H—C=N, linear triply bonded Synonyms hydrocyanic acid prussic acid formonitrile 

Firefighters chance a great risk to the exposure to HCN, which is a known fire-effluent gas. Materials such as polyurethane foam, silk, wool, polyacrylonitrile, and nylon fibers bum fo produce HCN (Sakai and Okukubo 1979 Yamamoto 1979 Morikawa 1988 Levin el al. 1987 Sumi and Tsuchiya 1976) along wilh CO, acrolein, CO2, formaldehyde, and other gases. Emissions of these toxic gases take place primarily under the conditions of oxygen deficiency, and when Ihe air supply is plentiful the emissions are decreased considerably (Hoschke et al. 1981). 

Bertol et al. (1983) determined that 1 g of polyacrylonitrile generated 1500 ppm of HCN. Thus a lethal concentration of HCN could be obtained by burning 2 kg of polyacrylonitrile in an average-sized living room. 

The reported demand for hydrogen cyanide in the U.S. A. is now over 550 kt per annum. Possibly one quarter is by-product from acrylonitrile manufacture the remainder is produced by the oxidation of methane/ ammonia mixtures over platinum at about 1100°C. The major use (40-45%) is in DuPont s adiponitrile production, with some 30-35% used for methyl methacrylate (MMA) (methyl 2-methylpropenoate) manufacture and 10% for sodium cyanide. 

Acrylonitrile manufacture probably contributes a major proportion of the (much smaller) demand in western Europe for MMA and NaCN production, though first-intent production is practised for example Degussa dehydrogenate methane/ammonia mixtures over platinum at 1200-1300°C. 

LABORATORY CHEMICAL SAFETY SUMMARY HYDROGEN CYANIDE  

Substance Hydrogen cyanide (Hydrocyanic acid prussic acid) CAS 74-90-8 

Physical Properties Colorless or pale blue liquid or gas bp 26 °C, mp -13 °C Miscible in water in all proportions 

Odor Bitter almond odor detectable at 1 to 5 ppm however, 20 to 60% of the population are reported to be unable to detect the odor of HCN 

Toxicity Data Approx LD oral (rat) 10 mg/kg Approx LD skin -1500 mg/kg (rabbit) LC50 inhal (rat) 63 ppm (40 min) PEL (OSHA) 10 ppm (11 mg/m )—skin TLV-TWA (ACGIH) Ceiling 10 ppm (11 mg/m )—skin 

It may be noted that the free halogens will not react with the respective liquid hydrogen halides. Not even iodine reacts with hydrogen iodide, where the formation oi a polyiodide anion might be expected. On the other hand iodine monochloride will react with chloride ion donors in hydrogen chloride just as well as in the absence of a solvent32 

There are some indications that hydrogen bromide and hydrogen iodide can act as acids in liquid hydrogen chloride. 

Triphenylcarbinol is solvolysed to the ionized species (C6H5)3C+HCl2  

Hydroxonium chloride is nearly insoluble, just as water is scarcely soluble in liquid hydrogen chloride. 

Many of the reactions which have been mentioned are not specific for the solvent-systems under consideration and are expected also to take place in other acceptor so vents. 

At the battle of the Somme in July 1916 French artillery fired shells filled with hydrogen cyanide (CWS symbol, AC). The compound had 

First United States Army, Report of Operations 2 Feb-8 May 1945, Annex No. 9, p. 192. Intel Div, CWS, Theater Service Forces, ETOUSA, German Chemical Warfare, World War II, Sep 45, p. 39. Hereafter cited as German Chemical Warfare. 

Hydrogen cyanide is also known as hydrocyanic add and prussic acid. 

The French had some difficulty in using hydrogen cyanide as an agent because AC vapor is light and therefore has a tendency to diffuse instead of lying close to the ground. Also, AC has a tendency to decompose— sometimes so violently that the container exploded.  

In an attempt to cut down the rate of diffusion the French mixed AC with stannic chloride. To prevent AC from decomposing the French added arsenic trichloride. To keep the mixture from crystallizing and to make soldiers more susceptible to the agent they added chloroform. The addition of these compounds diluted the AC so much that the final mixture contained only 50 percent of the cyanide. This meant that twice as many shells, or shells with twice the capacity, were needed to deliver the same weight of the cyanide—a rather wasteful procedure. 

Very little has been reported on the photodissociation dyamics of HCN leading to the production of the CN X and A state fragments, despite the fact that this is the simplest triatomic cyanide compound. This is probably because the absorption above 150 nm is very weak and structured so that a laser in the VUV region is needed. Nevertheless, some results from a recent study on the photodissociation dynamics of HCN at 193 and 157 nm have been presented at the Sixteenth Informal Conference on Photochemistry (131). 

When the photolysis was done at 157 nm, a much larger signal could be obtained, so that the experimental conditions could be adjusted over a wider range. They again noted that a large amount of vibrational and rotational energy was present in 

Hydrocyanation of alkenes usually gives anti-Markovnikov products. Interestingly, however, addition of HCN to styrene yields mostly the branched (Marko-vnikov) adduct. This was suggested to result from stabilization of the branched alkylnickel cyanide intermediate by interaction of nickel with the aromatic ring.176 

Hydrocyanation of dienes, a process of industrial importance (see Section 6.2.4), yields 1,4-addition products when conjugated dienes are reacted. The addition involves ri3-allyl intermediates (19)  

The stereochemistry of hydrocyanation of 1,3-cyclohexadiene was shown to occur with syn stereoselectivity, indicating that cis migration of the cyanide anion follows the formation of the 7t-allylnickel complex.177 

Nonconjugated dienes (1,4-pentadiene, 1,5-hexadiene) are transformed mainly to products originating from conjugated dienes formed by isomerization.178 In contrast, 1,7-octadiene in which the double bonds are separated by four methylene groups preventing isomerization to conjugated dienes, yields mainly isomeric mononitriles. 

HCN adds more readily to alkynes than to alkenes.179 The addition of HCN to acetylene catalyzed by Cu+ ions was once a major industrial process to manufacture acrylonitrile carried out in the presence of copper(I) chloride, NH4CI, and HC1180 (see Section 6.2.4). Zerovalent Ni and Pd complexes are effective catalysts 

Methane accounts for approximately 85 percent of the composition of natural gas with heavier hydrocarbons, nitrogen, and, in some regions, helium accounting for the other 15 percent [1]. Purification of methane is carried out at ambient or low temperature absorption (5—10 thousand ppm and 1—2 thousand ppm, respectively) and low-temperature fractionation (100 ppm) [2]. Impurities in the methane, such as heavier hydrocarbons, promote undesirable side reaction. Methane is also produced in an increasing number of organic waste-disposal plants [3]. Methane is used as feedstock to produce many chemicals, including hydrogen cyanide, carbon disulfide, and chlorinated methanes. 

The reaction takes place over a platinum or platinum-rhodium catalyst at temperatures of 1000—1500°C and atmospheric pressure [2,4—6]. 

Methane must be essentially pure to prevent the formation of soot and poisoning the catalysts. The removal of higher hydrocarbons and desulfurization of the methane is necessary [9]. This process has been adapted by 

American Cyanamide, DuPont, Goodrich, ICI, Mitsubishi, Monsanto, Monte-catini, Nippon Soda, and Rohm Haas [10], 

The Degussa BMA (Blausaure-Methan-Ammoniak, or hydrocyanic acid-methane-ammonia) process also is used in the production of hydrogen cyanide from methane. The difference between the Andrussow process and the Degussa process is that the latter does not use air in the synthesis of hydrogen cyanide. The reaction is as follows  

Hummel and Janssen suggested the following mechanism to explain the formation of the azulmine polymer 

Reaction (48) is about 50 kcal endothermic, and certainly could not take place without solvation of the CN ion. On the other hand, there is spectroscopic 

The anionic mechanism is similar to that postulated to explain the thermal polymerisation. The apparent general similarity of the polymers produced by the two methods of initiation justifies this. Cross-linking involves additions to the free CN groups, and regular networks with two or more interconnected polymer chains are possible. Thus the structure of the azulmine is highly complex. Termination must occur by reactions of the growing anions with H2CN+ ions formed in the reaction 

This reaction has been shown to be very rapid77. Sulphuric and acetic acids sup press the polymerisation. Evidently their anions are ineffective as initiators, and the enhanced proton concentration provided by them must reduce the chain lifetime. The slight retarding effect of oxygen could be due to electron scavenging. However, the authors suggest that there may be a small free radical component of the chain reaction, which is inhibited in the presence of oxygen. 

When the HCN contained 1-2 % mole of water the yields tended to increase exponentially with dose. This was due to the occurrence of a superimposed thermal reaction, which became faster as the radiation products accumulated and persisted after irradiation ceased. Hummel and Janssen attributed it to the hydrolysis of imino and cyano groups on the polymer chain. They suggested that the ammonia generated in this process reacted with HCN to form cyanide ions, which then initiated the polymerisation. 

lARC Monographs on the Evaluation of the Carcinogenic Risk of Chemicals to Humans, Vol 54, Occupational exposures to mists and vapours from sulfuric acid and other strong inorganic acids, pp 189-211. Lyon, International Agency for Research on Cancer, 1992 

Sellakumar AR, Snyder CA, Solomon JJ, et ah Carcinogenicity of formaldehyde and hydrogen chloride in rats. Toxicol Appl Pharmacol 81A01- 06, 1985 

Synonyms Hydroq anic acid aero liquid HCN prussic acid formonitrile 

Physical Form. Colorless gas liquefying at 26°C (may be found in the workplace both as a liquid and a gas) 

Rodenticide and insecticide fumigant chemical intermediate for the manufacture of synthetic fibers, plastics, and nitrites 

EXTREMELY POISONOUS GAS AND LIQUID, POISONOUS BY SKIN ABSORPTION, HIGHLY FLAMMABLE 

Colorless liquid or gas characteristic odor of bitter almonds bp, 26°C.1 

Flash point, -18°C explosive limits, 6-41% ignition temperature, 538°C. If possible, shut off flow of gas and remove cylinder quickly from area in which a fire has developed. Breathing apparatus must be worn during these operations. Use dry chemical foam or carbon dioxide to extinguish.2 

Very soluble in water, the solution being only weakly acidic (does not redden litmus).1 

Stable at or below room temperature in the presence of 0.1 % acid polymerizes explosively above 184°C or in the presence of alkali.3 

This reaction, although it involves esters of MA, appears synthetically very useful. Dialkyl esters of MA react with HCN to yield different products depending on the reaction conditions. Michael and Weiner observed that on allowing a 10% aqueous methanol solution of dimethyl fumarate and KCN to react at room temperature, three products were obtained 73, 74, and 75. 

In anhydrous methanol, on the other hand, two main products obtained were 74 and a cyclopentanone derivative 76 after 6 days at room temperature. 

The crude yield of 76 could be increased to 75% by refluxing the mixture for 2 h. Obviously, the reaction is initiated by a nucleophilic attack by CN on the fumarate, leading to the products 75 and 76 according to the scheme below. 

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It is readily oxidized by air to benzoic acid. With aqueous KOH gives benzyl alcohol and benzoic acid. Gives addition products with hydrogen cyanide and sodium hydrogen sulphite. 

CHjiCH-CN. Volatile liquid b.p. 78"C. Manufactured by the catalytic dehydration of ethylene cyanhydrin, by the addition of hydrogen cyanide to ethyne in the presence of CuCI or the reaction of propene, ammonia and air in the presence of a molybdenum-based catalyst. 

Legon A 0, Millen D J and Mjdberg P J 1977 The hydrogen cyanide dimer identification and structure from microwave spectroscopy Chem. Phys. Lett. 47 589... 

Dulmage W J and Lipscomb W N 1951 The crystal structures of hydrogen cyanide, HON Acta Crystallogr. 4 330... 

By (he direct addition of hydrogen cyanide to aldehydes and ketones, giving cyanhydrins ... 

C6H5CH(0H)CN I HCOCgHi - C6H5CH(OH)COC H, - HCN unchanged benzaldehyde, giving benzoin and regenerating the hydrogen cyanide... 

To 2 ml. of the ester, add 2--3 drops of a saturated freshly prepared solution of scdium bisulphite. On shaking, a gelatinous precipitate of the bisulphite addition product (D) of the keto form separates, and on standing for 5-10 minutes usually crystallises out. This is a normal reaction of a ketone (see p. 344) hydrogen cyanide adds on similarly to give a cyanhydrin. 

Hydrogen cyanide (inhaled) or alkali oanides (taken by mouth, rf. p. IQ2) inhale amyl nitrite from freshly opened capsules. Obtain medical attention urgently. 

If the reaction is allowed to become too vigorous, hydrogen cyanide is liberated and some glycolate is formed. 

By interaction of hydrogen cyanide and hydrogen chloride with an anxnatic compound (hydrocarbon, phenol or phenol ether) in the presence of aluminium chloride (or zinc chloride). This is known as the Gattermann... 

Mandelic acid. This preparation is an example of the synthesis of an a-hydroxy acid by the cyanohydrin method. To avoid the use of the very volatile and extremely poisonous hquid hydrogen cyanide, the cyanohydrin (mandelonitrile) is prepared by treatment of the so um bisulphite addition compound of benzaldehj de (not isolated) with sodium cyanide ... 

This operation should be conducted in a fume cupboard (hood) as hydrogen cyanide may be evolved. 

Tertiary amines capable of eliminating a secondary amine to form a conjugated system can react with hydrogen cyanide to form y-keto nitriles by amine replacement. Thus (I) yields p-benzoylpropionitrile (IV) ... 

Cool the filtrate (A) to 5-10° and add concentrated hydrochloric acid dropwise and with vigorous stirring (FUME CUPBOARD hydrogen cyanide is evolved) to a pH of 1-2 (about 50 ml.) a crude, slightly pink 3-indoleacetic acid is precipitated. The yield of crude acid, m.p. 159-161°, is 20 g. Recrystallise from ethylene dichloride containing a small amount of ethanol 17 -5 g. of pure 3 indoleacetic acid, m.p. 167-168°, are obtained. 

Nitrogen and sulphur present. Just acidify 2-3 ml. of the fusion solution with dilute nitric acid, and evaporate to half the original volume in order to expel hydrogen cyanide and/or hydrogen sulphide which may be present. Dilute with an equal volume of water. If only one halogen is present, proceed as in tests (i) or (iii). If one or more halogens may be present, use tests (ii), (iii) or (iv). 

The conversion of primary alcohols and aldehydes into carboxylic acids is generally possible with all strong oxidants. Silver(II) oxide in THF/water is particularly useful as a neutral oxidant (E.J. Corey, 1968 A). The direct conversion of primary alcohols into carboxylic esters is achieved with MnOj in the presence of hydrogen cyanide and alcohols (E.J. Corey, 1968 A,D). The remarkably smooth oxidation of ethers to esters by ruthenium tetroxide has been employed quite often (D.G. Lee, 1973). Dibutyl ether affords butyl butanoate, and tetra-hydrofuran yields butyrolactone almost quantitatively. More complex educts also give acceptable yields (M.E. Wolff, 1963). 

The maximum yield of 2-alkylseIenazole is 25%. In this way. 4-methylselenazole (7) was obtained starting from hydrogen cyanide, hydrogen selenide and chloroacetone. It is the only known selenazole not substituted in the 2-position. The yield relative to chloroacetone is very low (2.5%) (Scheme 2). 

Lewis s concept of shared electron parr bonds allows for four electron double bonds and SIX electron triple bonds Carbon dioxide (CO2) has two carbon-oxygen double bonds and the octet rule is satisfied for both carbon and oxygen Similarly the most stable Lewis structure for hydrogen cyanide (HCN) has a carbon-nitrogen triple bond... 

With this as background let us now examine how the principles of nucleophilic addition apply to the characteristic reactions of aldehydes and ketones We 11 begin with the addition of hydrogen cyanide... 

The product of addition of hydrogen cyanide to an aldehyde or a ketone contains both a hydroxyl group and a cyano group bonded to the same carbon Compounds of this type are called cyanohydrins... 

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... 

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