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| Names | |
|---|---|
| Preferred IUPAC name Carbononitridic bromide[3] | |
| Other names | |
| Identifiers | |
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3D model (JSmol) | |
| 1697296 | |
| ChemSpider |
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| ECHA InfoCard | 100.007.320 |
| EC Number |
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| MeSH | Cyanogen+Bromide |
| RTECS number |
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| UNII | |
| UN number | 1889 |
| |
| |
| Properties | |
| BrCN | |
| Molar mass | 105.922 g·mol−1 |
| Appearance | Colorless solid |
| Density | 2.015 g/cm3 |
| Melting point | 50 to 53 °C (122 to 127 °F; 323 to 326 K) |
| Boiling point | 61 to 62 °C (142 to 144 °F; 334 to 335 K) |
| Reacts | |
| Vapor pressure | 16.2 kPa |
| Thermochemistry | |
Std enthalpy of formation(ΔfH⦵298) | 136.1–144.7 kJ/mol |
| Hazards | |
| GHS labelling: | |
   | |
| Danger | |
| H300,H310,H314,H330,H410 | |
| P260,P273,P280,P284,P302+P350 | |
| NFPA 704 (fire diamond) | |
| NIOSH (US health exposure limits): | |
PEL (Permissible) | 5 mg/m3 |
| Related compounds | |
Related alkanenitriles | |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Cyanogen bromide is theinorganic compound with theformula BrCN. It is a colorless solid that is widely used to modifybiopolymers, fragmentproteins andpeptides (cuts the C-terminus of methionine), and synthesize other compounds. The compound is classified as apseudohalogen.
Thecarbon atom in cyanogen bromide is bonded tobromine by a single bond and tonitrogen by atriple bond (i.e.Br−C≡N). The compound is linear and polar, but it does not spontaneously ionize in water. It dissolves in both water and polarorganic solvents.
Cyanogen bromide can be prepared byoxidation ofsodium cyanide withbromine, which proceeds in two steps via the intermediatecyanogen ((CN)2):
When refrigerated the material has an extended shelflife. Like some other cyanogen compounds, cyanogen bromide undergoes an exothermic trimerisation tocyanuric bromide ((BrCN)3). This reaction is catalyzed by traces of bromine, metal salts, acids and bases. For this reason, experimentalists avoid brownish samples.[4]
Cyanogen bromide ishydrolyzed to formisocyanic acid andhydrobromic acid:
The main uses of cyanogen bromide are to immobilize proteins, fragment proteins by cleavingpeptide bonds, and synthesizecyanamides and other molecules.
Cyanogen bromide is often used to immobilize proteins by coupling them toreagents such asagarose foraffinity chromatography.[5] Because of its simplicity and mildpH conditions, cyanogen bromide activation is the most common method for preparing affinity gels. Cyanogen bromide is also often used because it reacts with thehydroxyl groups on agarose to formcyanateesters andimidocarbonates. These groups are reacted withprimary amines in order to couple the protein onto the agarose matrix, as shown in the figure. Because cyanate esters are more reactive than are cyclic imidocarbonates, the amine will react mostly with the ester, yieldingisourea derivatives, and partially with the less reactive imidocarbonate, yielding substituted imidocarbonates.[6]
The disadvantages of this approach include the toxicity of cyanogen bromide and its sensitivity to oxidation. Also, cyanogen bromide activation involves the attachment of aligand to agarose by an isourea bond, which is positively charged at neutral pH and thus unstable. Consequently, isourea derivatives may act as weakanion exchangers.[6][dead link]
Cyanogen bromide hydrolyzespeptide bonds at the C-terminus ofmethionine residues. This reaction is used to reduce the size ofpolypeptide segments for identification andsequencing.
Theelectron density in cyanogen bromide is shifted away from the carbon atom, making it unusuallyelectrophilic, and towards the moreelectronegative bromine and nitrogen. This leaves the carbon particularly vulnerable to attack by anucleophile, and the cleavage reaction begins with a substitution reaction in which bromine is ultimately replaced by the sulfur in methionine. This attack is followed by the formation of a five-membered ring as opposed to a six-membered ring, which would entail the formation of adouble bond in the ring between nitrogen and carbon. This double bond would result in a rigid ring conformation, thereby destabilizing the molecule. Thus, the five-membered ring is formed so that the double bond is outside the ring, as shown in the figure.
Although the nucleophilic sulfur in methionine is responsible for attacking BrCN, the sulfur incysteine does not behave similarly. If the sulfur in cysteine attacked cyanogen bromide, the bromide ion would deprotonate the cyanideadduct, leaving the sulfur uncharged and the beta carbon of the cysteine not electrophilic. The strongest electrophile would then be the cyanide carbon, which, if attacked by water, would yieldcyanic acid and the original cysteine.
Cleaving proteins with BrCN requires using abuffer such as 0.1M HCl (hydrochloric acid) or 70% (formic acid).[7] These are the most common buffers for cleavage. An advantage to HCl is that formic acid causes the formation of formyl esters, which complicates protein characterization. However, formic is still often used because it dissolves most proteins. Also, the oxidation of methionine tomethionine sulfoxide, which is inert to BrCN attack, occurs more readily in HCl than in formic acid, possibly because formic acid is a reducing acid. Alternative buffers for cleavage includeguanidine orurea in HCl because of their ability tounfold proteins, thereby making methionine more accessible to BrCN.[8]
Water is required for normal peptide bond cleavage of theiminolactone intermediate. In formic acid, cleavage of Met-Ser and Met-Thr bonds is enhanced with increased water concentration because these conditions favor the addition of water across theimine rather than reaction of the side chain hydroxyl with the imine. Lowered pH tends to increase cleavage rates by inhibiting methionine side chain oxidation.[8]
When methionine is followed byserine orthreonine, side reactions can occur that destroy the methionine without peptidebond cleavage. Normally, once the iminolactone is formed (refer to figure), water and acid can react with the imine to cleave the peptide bond, forming ahomoserine lactone and new C-terminal peptide. However, if the adjacent amino acid to methionine has ahydroxyl orsulfhydryl group, this group can react with the imine to form a homoserine without peptide bond cleavage.[8] These two cases are shown in the figure.
Cyanogen bromide is a common reagent inorganic synthesis. In most reactions, it acts as a source of electrophiliccyanogen and nucleophilicbromide; carbocations preferentially attack the nitrogen atom.[4] In the presence of a Lewis acid, it cyanidatesarenes.[9]
BrCN convertsalcohols tocyanates;amines tocyanamides ordicyanamides.[4] Excess BrCN continues the reaction toguanidines;hydroxylamines yieldhydroxyguanidines similarly.[9]
The cyanamides so formedumpole the original amine, and tends to eliminate alkyl substituents. In thevon Braun reaction, tertiary amines react with cyanogen bromide to yield disubstituted cyanamides and an alkyl bromide.[9] That net reaction is similar to thePolonovski elimination, but does not require N-oxidation.[4]
In bromocyanation, BrCN adds across multiple bonds to give a vicinal cyanobromide. Bromocyanatedenols spontaneously undergo aDarzens-like elimination to an epoxynitrile.[4]
Cyanogen bromide is also a dehydrating agent, hydrolyzing tohydrogen bromide andcyanic acid.[9]
The compound is used in the synthesis of the pharmaceuticals4-methylaminorex andviroxime.
Cyanogen bromide can be stored under dry conditions at 2 to 8 °C for extended periods.[6]
Cyanogen bromide is volatile, and readily absorbed through theskin orgastrointestinal tract. Therefore, toxic exposure may occur by inhalation, physical contact, or ingestion. It is acutely toxic, causing a variety ofnonspecific symptoms. Exposure to even small amounts may cause fatal injury. LD50 orally in rats is reported as 25–50 mg/kg.[10]
Cyanogen bromide was used as achemical weapon in theWorld War I byAustro-Hungarian forces, either asbenzene solution or mixture with 25%bromoacetone and 50% benzene.[11]
The recommended method to deactivate cyanogen bromide is withsodium hydroxide andbleach.[12] The aqueous alkali hydroxide instantly hydrolyzes BrCN to alkali cyanide and bromide. The cyanide can then be oxidized bysodium orcalcium hypochlorite to the less toxic cyanate ion. Deactivation is extremelyexothermic and may be explosive.[10]
