Cyanides and Isocyanides (Cyan Compounds) PDF

# Cyanides and Isocyanides (Cyan Compounds) The cyan compounds are characterized by presence of a cyan group (CN). They are considered to be derivatives of hydrogen cyanide (HCN). Since the cyanide ion (CEN:) has unshared pair of electrons, a proton can attach itself to either of these atoms. Thus...

# Cyanides and Isocyanides (Cyan Compounds) The cyan compounds are characterized by presence of a cyan group (CN). They are considered to be derivatives of hydrogen cyanide (HCN). Since the cyanide ion (CEN:) has unshared pair of electrons, a proton can attach itself to either of these atoms. Thus hydrogen cyanide exists as a tautomeric equilibrium mixture of the cyanide form (I) and the isocyanide form (II). As shown by spectroscopic examination the cyanide form dominates. | Form | Structure | Name | |---|---|---| | I | H-CEN | Cyanide | | II | CEN-H | Isocyanide | The organic compounds derived from the cyanide form are called Alkyl Cyanides or Nitriles, while those derived from isocyanide form are called Alkyl isocyanides or Isisonitriles. | Form | Structure | Name | |---|---|---| | I | H-CEN -> R-CEN | Alkyl Cyanides or RCN | | II | H-N≡C: -> R-N≡C: | Alkyl Isocyanides or RNC | ## Alkyl Cyanides or Nitriles Alkyl cyanides or nitriles are alkyl derivatives of hydrogen cyanide (or butyrocyanic acid) in which the substituent R is linked to the carbon of the cyan group - CN. They are often referred to as nitriles (containing nitrogen). Alkyl cyanides or nitriles have the general formula R-CEN; where -CEN is the functional group. ### Nomenclature of alkyl cyanides Alkyl Cyanides are named based on the following three systems i. Cyanide system - In this system, they are named as if they were salts of hydrogen cyanide. The name of an individual member is derived by putting down name of the alkyl group and adding the word "cyanide". ii. Common System - Their common names are derived from the corresponding acids to which they are hydrolysed. The ending -acid is chopped and the suffix -onitrile is added. For example, CH₃CN upon hydrolysis yields acetic acid and is therefore named acetonitrile. CH₃-C≡N + H₂O → CH₃COOH + NH₃ acetonitrile acetic acid iii. IUPAC System - In this system, the longest carbon chain containing the cyan group (CN) as the terminal group determines the parent name. The IUPAC name of an individual nitrile is derived by adding the name of the parent hydrocarbon on the suffix -nitrile. | Structure | IUPAC Name | |---|---| | CH₃-CH₂-CN | Ethanenitrile | | CH₃CH₂CH₂ -CN | Propanenitrile | | CH₃CH₂-CH₂CN | Butanenitrile | | CH₃-CH-CH₂CN | 2-methylpropanenitrile | | | | CH₃ | ## Methods of Preparation Alkyl cyanides or nitriles can be obtained by many methods of which the more important ones are given below ### (1) Action of Metal Cyanide on Alkyl halides Nitriles can be prepared by heating an alkyl halide with sodium or potassium cyanide in aqueous ethanolic solution. R-X + NaCN → R-CN + NaX Alkyl halide Nitrile e.g CH₃CH₂- Br + NaCN → CH₃CH₂-CN + NaBr ethyl bromide propanenitrile Small amounts of alkyl isocyanide are formed which can be removed by partial hydrolysis because it is hydrolysed more rapidly than the alkyl cyanide. Note that nitriles are prepared from alkyl halides by this method, an extra carbon atom is introduced. This is the key step in ascending the series of carboxylic acids (or 1^o alcohols). RCOOH + CH₃OH → RCOOCH₃ → RCH₂OH → RCH₂Br → RCH₂CN H_2O LiAlH₄ PBr₃ KCN Esterification reduction ### (2) Dehydration of Amides or Ammonium salts of carboxylic acids Simple amides on drastic dehydration by heating with phosphoric pentoxide yield nitriles. R-C≡N + H₂O Amide Nitrile e.g. CH₃CH₂-C-NH₂ → CH₃CH₂-CN + H₂O P₂O₅ propane amide propanenitrile Higher molecular weight amides may be dehydrated by heating alone. Ammonium salts of carboxylic acids dehydrate first to form amide which on loss of a molecule of water yields nitrile - Thus where carboxylic acid vapours mixed with ammonia are passed over heated alumina at 500^oC, a nitrile results (industrial). R-C=OH + NH₃ → R-C=ONH₄ → R-C-NH₂ → R-C≡N H₂O Ammonium salt of Carboxylic acid amide nitrile ### (3) Dehydrogenation of Higher Amines Higher amines get dehydrogenated by passing their vapours over Cu or Ni catalyst at high temperature. Cu R-C=N → R-CEN + 2H₂ 300^oC ### (4) Action of Grignard Reagents with Cyanogen chloride Grignard reagents react with cyanogen chloride (Cl-CEN) to form nitriles. RMgX + Cl-CEN → ether R-CEN + MgXCl Grignard reagent nitrile e.g. C₂H₅MgBr + Cl-CEN → ether C₂H₅-CEN + MgBrCl ethyl magnesium bromide propanenitrile ### (5) Dehydration of Aldoximes Aldoximes upon dehydration with acetic anhydride or thionyl chloride yield nitriles. R-C≡N-OH → R-CEN (CH_₃CO)₂O -H₂O nitrile e.g. CH₃-C≡N-OH → (CH_₃CO)₂O CH₃-CEN -H₂O acetaldoxime ethanenitrile ### (6) Addition of HCN to Alkenes Hydrogen cyanide adds to terminal alkenes in the gaseous phase at 350^oK in the presence of alumina to give nitriles. CH₃C=CH₂ + HCN → Al₂O₃ CH₃-C-CN 2-methylpropene 300^oC 2,2-dimethylpropanenitrile | CH₃ ### (7) Aminoxidation of Alkanes or Aldehydes An alkane having a terminal methyl group is oxidized at elevated temperature (500-600^oC) over a catalyst in the presence of ammonia to form nitrile. R-CH₃ + NH₃ + 3/2O₂ → R-C≡N + 3H₂O alkane nitrile e.g. CH₃CH₂CH₃ + NH₃ + 3/2O₂ → CH₃CH₂-C≡N + 3H₂O propane propanenitrile An aldehyde gives a similar reaction in presence of sodium methoxide (NaOCH₃) and Cu⁺ ion at 30^oC. CH₃-CHO + NH₃ + O₂ → CH₃-C≡N + 2H₂O ethanal ethanenitrile ## Physical properties of Alkyl Nitriles (1) Lower molecular weight (up to C₁₄) aliphatic nitriles are colourless liquids at room temperature. The higher members are crystalline solids. (2) They have fairly pleasant smells, resembling that of bitter almonds. (3) Nitriles are dipolar compounds and are highly associated molecules. Their boiling points are therefore, abnormally higher. (4) The lower members are soluble in water with which they form hydrogen bond. The solubility decreases as the hydrocarbon group gets larger. (5) They are moderately poisonous in contrast to hydrogen cyanide which is a deadly poison. ## Chemical properties of Nitriles The cyan group (CN) in nitriles is strongly polarised by the high dipole moment of 4.0D. Nitriles can be represented as a resonance hybrid of two canonical forms, in which there is a high contribution from the dipolar form. [R-CEN: → R-C≡N: or R-C≡N:]^- I II resonance hybrid Because of the dipole, a nitrile is subject to electrophilic attack on nitrogen and to nucleophilic attack on carbon. The cyan group being electron attracting activates the α hydrogen which can easily be removed as a proton by strong bases. The following are some of the reactions of nitriles. ### (i) Hydrolysis On boiling with aqueous mineral acid or alkali, nitriles are hydrolysed to a carboxylic acid and ammonia. R-CEN + H₂O → R-C-NH₂ → R-C-OH + NH₃ H⁺ or OH^- H₂O nitrile amide carboxylic acid e.g. CH₃-C≡N + H₂O → CH₃-C-NH₂ → CH₃-C-OH + NH₃ H⁺ or OH^- H₂O acetonitrile acetamide acetic acid ### (ii) Alcoholysis Nitriles on boiling with excess of alcohol in the presence of Conc. H₂SO₄ or HCl form esters. R-CEN + R’OH + H₂O → R-C-OR’ + NH₄ H⁺ H₂O nitrile alcohol ester e.g. CH₃CH₂-CEN + CH₃OH → CH₃CH₂-C-OCH₃ + NH₄ propanenitrile methanol methyl propionate ### (iii) Addition of Grignard Reagents Nitriles react with Grignard reagents to form ketimines which on hydrolysis yield ketones. CH₃-CEN + CH₃MgBr → CH₃-C=NMgBr → CH₃-C=O + H₂O acetonitrile methyl magnesium bromide acetone ### (iv) Reduction (a) Partial reduction - When nitriles are reduced with lithium triethoxyaluminum hydride, LiAlH(OCH₂CH₃)₃, in ether, aldimine is first produced. This on subsequent hydrolysis yields an aldehyde. R-CEN + LiAlH(OCH₂CH₃)₃ → ether R-CH=NH → H⁺ RCHO mitrile aldimine aldehyde (b) Complete Reduction to primary amines - On treatment with lithium aluminium hydride LiAlH₄, nitriles undergo complete reduction to yield primary amines. R-CEN + 4H → LiAlH₄ R-CH₂-NH₂ nitriles primary amine ### (v) Alkylation Nitriles having α-hydrogen atoms are alkylated readily when heated with an alkyl halide in the presence of finely divided sodium amide (NaNH₂). R-C≡N + R-X → NaNH₂ R-C-CEN + HX alkyl cyanide R’ 2^o Cyanide R-C-CEN + R-X → NaNH₂ R-C-CEN + HX 2^o cyanide R’’ 3^o cyanide **Note**: This reaction provides a procedure for preparing tertiary amines. ### (vi) Thorpe Nitrile Condensation Nitriles undergo condensation in the presence of sodium amide when only the α-hydrogen atoms are involved. Two molecules of nitriles under Claisen type condensation as follows; CH₃-CH₂-C≡N + CH₃-C≡N → CH₃-CH₂-C-CH-C≡N propane nitrile | CH₃ 3-imino-2-methylpentanenitrile # Alkyl Isocyanides or Isonitriles As the name implies, the isocyanides or Isonitriles are structural isomers of the cyanides or nitriles. They are regarded as alkyl derivatives of the isocyanide form of hydrogen cyanide (H-N≡C). They have the general formula R-N≡C: where -N≡C: is the functional group. Structurally, isonitriles are best represented as a resonance hybrid of three canonical forms. R-N≡C: <-> R-N=C: The dipotar form I makes a higher contribution. | Formula | Common Name | IUPAC Name | |---|---|---| | CH₃-NC | Methyl isocyanide | methyl carbylamine | | CH₃CH₂-NC | Ethyl isocyanide | ethyl carbylamine | | CH₃CH₂CH₂-NC | Propyl Isocyanide | propyl carbylamine | ## Methods of Preparation (1) By heating an alkyl iodide with silver cyanide (Ag-C≡N:) in aqueous ethanolic solution. R-I + Ag-C≡N: → R-N≡C: + AgI alkyl iodide alkyl isocyanide A small amount of the isomeric cyanide is also produced. ### (2) By heating a primary amine and chloroform (CHCl₃) with ethanolic potassium hydroxide (Hofmann Carbylamine reaction). R-NH₂ + CHCl₃ + 3 KOH → R-N≡C: + 3KCl + 3H₂O amine chloroform alkyl isocyanide ### (3) By heating isocyanates (RNCO) with alkyl phosphites. R-N=C=O + (C₂H₅)₃P → 150-198^oC R-N≡C: + (C₂H₅O)₃PO alkyl isocyanate triethyl phosphite alkyl isocyanide trialkyl phosphate ### (4) By heating N-alkyl formamides with phosphorous chloride in pyridine solution. R-N-C-H + POCl₃ → Pyridine R-N≡C: + H₂O || alkyl isocyanide O N-alkyl formamide ## Physical Properties of Alkyl Isocyanides (1) They highly unpleasant smelling liquids. (2) They are almost insoluble in water. The nitrogen atom is not having a lone pair of electrons for hydrogen bonding. (3) They have lower boiling points than respective isomeric nitriles (C₂H₅CN by 21^oC; R-NC bp 68^oC). This is attributed to their lower dipole moments (3.0D) compared to that of nitriles (4.0D). ## Chemical Properties of Alkyl Isocyanides Alkyl isocyanides are a very reactive class of compounds. Their reactions usually involve nucleophilic attack on the reagent (A-B) followed by the nucleophilic attack by the fragment B on the carbon atom of the isocyanide. Thus, both fragments of the attacking reagents are added to the isocyanide carbon. R-N≡C: + A-B → R-N≡C-A + B → R-N≡C-A n Second addition product First addition product Some of the common reactions of isocyanides are given below. ### (1) Isomerization to ### Isonitriles on heating with NaCN or KCN rearrange ### to more stable cyanides. R-N≡C: + KCN → R-CEN alkyl isocyanide alkyl cyanide **Mechanism**: Here the cyanide ion brings about a nucleophilic displacement reaction. [N≡C:]^- + R-N≡C: → N≡C-R + N≡C: Cyanide ion alkyl isocyanide Alkyl cyanide (from NaCN or KCN) (more stable) ### (2) Addition Reactions Isocyanides react with hydrogen sulfide, mercuric chloride, Grignard reagents, amines, alcohols etc. to form addition compounds. For example, R-N≡C: + Cl₂ → R-N=C=Cl alkyl isocyanide alkylimino carbonyl chloride R-N≡C: + S → R-N=C=S alkyl isothiocyanate R-N≡C: + H₂O → R-N=C=O alkyl isocyanate ### (3) Hydrolysis Alkyl Isocyanides are hydrolysed by dilute acids to form a primary amine and formic acid but are not hydrolysed by alkali. R-N≡C + 2H₂O → Acid R-NH₂ + HCOOH alkyl isocyanide 1^o amine formic acid Note the difference in hydrolysis of nitriles and isonitriles. The nitriles give carboxylic acids and ammonia whereas the isonitriles give primary amines and formic acids on hydrolysis. ### (4) Reduction Isonitriles are reduced to secondary amines either by Sodium and ethanol or by catalytic hydrogenation (Pt, H₂). R-N≡C: + 2H₂ → Na, C₂H₅OH R-NH-CH₃ alkyl isocyanide 2^o amine e.g. CH₃CH₂-N≡C: + 2H₂ → C₂H₅OH CH₃CH₂-NH-CH₃propylonitrile ethyl methyl amine Remember that reduction of nitriles with sodium and ethanol will form primary amines. ## VINYL CYANIDE (Acrylonitrile, CH₂=CH-CN) It is commercially the most important nitrile. ### Preparation (1) By catalytic addition of hydrogen cyanide to acetylene. HC≡C-H + H-C≡N → Cu₂Cl₂, NH₄Cl → HC≡C-CN or CH₂=CH-CN acetylene 90% acrylonitrile (2) From ethylene and propylene derived from petroleum sources. CH₂=CH₂ → O₂ → Cat. CH₂-CH₂ → HCN, OH → -H₂O CH₂=CH-CN ethylene ethylene oxide acrylonitrile (3) From propylene. 2 CH₂=CH-CH₃ + 2NH₃ + 3O₂ → Cat. 450^oC 2 CH₂=CH-CN + 6H₂O Propylene acrylonitrile ## Physical | Chemical Properties of Acrylonitrile Acrylonitrile is a colorless liquid, bp 77.3^oC. It polymerizes in the presence catalyst (organic peroxides and other oxidizing agents) to form polyacrylonitrile. x (CH₂=CH-CN) → Cat. [-CH₂-CH-]_x acrylonitrile CN polyacrylonitrile Polyacrylonitrile is used for making synthetic fibers such as Orlon, Acrilan, Crestan and Zefran.