Chap-06 -Organic compounds containing Nitrogen

https://docs.google.com/document/d/1pQqh67ARnO9ZgPASNIXUHz_bcucCaKyg/edit?usp=sharing&ouid=109474854956598892099&rtpof=true&sd=true Organic Compounds Containing Nitrogen General methods of preparation, properties, reactions and uses. Amines : Nomenclature, classification, structure, basic character and identification of primary, secondary and tertiary amines and their basic character. Diazonium Salts: Importance in synthetic organic chemistry. NITRO COMPOUNDS Compound containing NO2 group are termed as nitro compounds. NO2 group is infact ambident group and is capable of getting attached to the carbon chain through nitrogen (e.g., RNO2) well as through oxygen (e.g., R–O–N=O alkyl nitrite). Electronic Structure of NO2 Group : Nitro alkane have high dipole moment. This suggest that nitro alkanes may be represented by following 2 canonical structures : C H A P T E R CHAPTER INCLUDES Nitro Compounds Amines Cyanides and Isocyanides O R—N + O O– R—N +O + O R—N O Benzene diazonium Chloride (B.D.C.) (ii) (iii) The resonance hybrid structure (iii) has both the N–O bond lengths equal, which is actually found. General Methods of Preparation By heating an alkyl halide with alcoholic solution of silver nitrite C2H5Br + AgNO2 ⎯⎯Δ→ C2H5NO2 + AgBr (Some amount of C2H5–ONO is also formed) By direct nitration of hydrocarbons : Nitration of alkanes is difficult in comparison with that of aromatic hydrocarbons. Alkanes undergo nitration with fuming HNO3 in the vapour phase at 423-673 K under pressure giving a mixture of nitroalkanes resulting through cleavage of C—C bonds. For example, CH3CH3 ⎯⎯Fum⎯in⎯g → CH3CH2NO2 + CH3 — NO2 Ethane HNO3 , 673 K Nitroethane Nitromethane Lower members can be nitrated by vapour phase nitrations. This reaction occurs by a free radical mechanism and the ease of substitution of hydrogens follows the following order. tertiary > secondary > primary Aromatic nitro compounds are prepared by nitration of aromatic compounds with nitric acid or nitrating mixture (mixture of conc. HNO3 with conc. H2SO4) NO2 +HNO3 Conc. H2SO4 333 K +H2O Benzene + Conc. HNO3 Nitrobenzene Conc.H2SO4 373K NO2 m-dinitrobenzene NO2 NO2 + Fuming HNO3 + fuming H2SO4 NO2 NO2 1,3,5–trinitrobenzene (T.N.B.) Chemical Properties Reactions due to α-H-atom Acidity : Due to stablisation of conjugate base formed, primary and secondary nitroalkances having hydrogen atom on the carbon atom directly attched with –NO2 are weak acidic, O ⊕ CH2—N | H O ⊕ ⊕ CH2—N –H O O (Stabilised conjugate base) Therefore, nitroalkanes having α-H show tautomerism. O CH3—N O O CH2 = N OH Nitro-form (Pseudo acid form) Aci-form (Nitrolic form) a strong acid Action of halogen : CH3NO2 ⎯⎯C⎯l2 → NaOH CCl3NO2 Nitrochloroform (chloropicrin) Action of nitrous acid : Primary nitroalkanes react with nitrous acid to form nitrolic acids, which dissolves in sodium hydroxide giving red solution. CH3CH2NO2 + O = NOH NOH NOH Ch3 – C – No2 + H2O Nitrolic acid NONa CH3 — C — NO2 + NaOH CH3 — C — NO2 Sod. Nitrolate (red) + H2O Secondary nitroalkanes react with nitrous acid to give colourless crystalline pseudonitroles which give blue colour in sodium hydroxide solution. H R — C — NO2 + HON = O N = O R — C — NO2 + H2O R R Pseudonitrol (Blue in NaOH) Tertiary nitroalkanes do not react with nitrous acid because they do not contain an α-hydrogen. Condensation with aldehydes : Primary and secondary nitroparaffins condense with aldehydes in the presence of alkali forming nitroalcohols. CH3CHO + CH3NO2 ⎯⎯→ CH3CH(OH)CH2NO2 β−Hydroxy nitropropane Reaction with Grignard reagent : RCH = N Hydrolysis : OH + CH3MgI O CH4 + RCH = N OMgI O CH3CH2NO2 + H2O ⎯⎯H⎯Cl → CH3COOH + NH2OH Secondary nitroalkanes on hydrolysis produce ketones. 2R2 CHNO2 ⎯⎯H⎯Cl → 2R2 CO + N2O + H2O boiling Tertiary nitroalkanes do not undergo hydrolysis. Halogenation : Primary and secondary nitroparaffins are readily halogenated in alkali solution (X2 + NaOH or NaOX). The halogen atom enters the α-position. Cl Cl | | CH3CH2NO2 ⎯⎯Cl2⎯+ N⎯aO⎯H → CH3 C HNO2 + CH3 C NO2 Nitroethane | (1° nitroparaffin) Cl Reduction : Various reduction stages of nitro group are given below — NO 2 +2⎯H→ — NO ⎯+⎯2H→ —NHOH Nitroso +⎯2H →— NH 2 1ºAmine Reduction in strongly acidic medium CH3CH2NO2 + 6 [H] ⎯⎯Sn /⎯H⎯Cl → CH3CH2NH2 + H2O OH− Ethylamine Reduction of Nitro benzene in different medium Acidic Medium NO2 + – NH3Cl – NH2 Sn|HCl OH or Fe|HCl Aniline Salt Neutral Medium NO2 NHOH Zn/NH4Cl H2O +ZnO N-Phenyl hydroxyl amine Reduction in alkaline medium Depending upon the nature of the reducing agent, nitro benzene forms different products. NO2 2 Zn/NaOH CH3OH [8H] [As2O3+NaOH] [6H] OR Na3AsO3+NaOH N = N Azo benzene O N = N Azoxy benzene +H2O +H2O H H Zn/NaOH (aq.) [10H] N – N +H2O Hydrazo benzene Selective Reduction (Zinin Reduction) Reduction of m-dinitrobenzene with ammonium sulphide or sodium poly sulphide reduces only one –NO2 group. NO2 NO2 Electrophilic substituion NH2 m-nitroaniline NO2 Nitrobenzene + Cl2 AlCl3 NO2 Cl m-Chloro nitrobenzene NO2 + HNO3(conc.) conc. H2SO4 NO2 Nitrobenzene 373 K NO2 m-Dinitrobenzene NO2 Nitrobenzene + H2SO4(fuming) 373 K NO2 SO3H m-Nitrobenzene sulphonic acid Nucleophilic Aromatic substitution amine act as nucleophile and attacks another molecule of alkyl halide. Cl + Nu + Cl Nu Cl Nu Nu . . + Cl N –O O N N O + O O O NO2 Stablisation of anion by resonance CLASIFICATION OF AMINES H CH3 CH3 CH3 — N — H Methylamine (Primary) CH3 — N — H Dimethylamine (Secondary) CH3 — N — CH3 Trimethylamine (Tertiary amine) General Methods of Preparation By the reaction of an alkyl halide with ammonia (Ammonolysis of alkylhalide) : C H I + HNH ⎯⎯→ CH CH –NH +I– ⎯⎯O⎯H → CH CH − NH 2 5 2 3 2 3 3 2 2 1º The reaction does not give only 1º amine as the product, because now ammine can act as nucleophile and attacks at other molecule of alkyl halide C H I + H NC H – ⎯⎯→ (CH –CH ) NH +I– ⎯⎯O⎯H →(CH3CH2 )2 − NH2 2 5 2 2 5 3 2 2 2 2º C2H5I + HN(C2H5 )2 ⎯⎯→ (CH CH ) NH+I– – ⎯⎯O⎯H →(CH3 − CH2 )3 N Ethyl iodide Diethylamine 3 2 3 3º C2H5I + N(C2H5 )3 → + (C2H5 )4 NI¯ Tetraethylammonium iodide (quaternary salt) By the reduction of nitro compounds : CH3CH2NO2 + 3H2 ⎯⎯Ran⎯ey⎯Ni → CH3CH2NH2 + 2H2O Nitroethane Pd - C Ethylamine C6H5NO2 + 3H2 ⎯⎯Ran⎯ey⎯Ni → C6H5NH2 + 2H2O Nitrobenzene or Pd - C Aniline with above reactions only 1º –amine can be formed. By the reduction of alkyl nitriles (or cyanides) and isonitriles (isocyanides) : CH3C ≡ N Acetonitrile ⎯⎯Ran⎯ey⎯Ni /⎯H⎯2 → LiAlH4 or Na / C2H5OH CH3CH2NH2 Ethylamine CH3NC ⎯⎯Ran⎯ey⎯Ni /⎯H⎯2 →CH3NHCH3 Or LiAlH4 2º amine By the reduction of amides : CH3CONH2 Acetamide ⎯⎯LiA⎯lH4⎯/eth⎯er → CH3CH2NH2 Ethylamine By the reduction of oximes : CH3CH = NOH + 4[H] ⎯⎯Na/⎯C2H⎯5O⎯H → CH3CH2NH2 + H2O Acetaldoxime By rearrangement reactions Ethylamine By Hofmann bromamide reaction : This is one of the most convenient method for the preparation of primary amines. It involvesaction of halogen (bromine or chlorine) and alkali (NaOH or KOH) on 1° amides to form amines with one carbon atom less. In this reaction molecular rearrangement takes place, in which alkyl group migrated over to N–atom. CH3CONH2 + Br2 + 4KOH → CH3NH2 + 2KBr + K 2CO3 + 2H2O C6H5CONH2 + Br2 + 4KOH → C6H5NH2 + K 2CO3 + 2KBr + 2H2O Benzamide Aniline Schmidt reaction : RCOOH + HN Conc. → R–NH + CO + N + H O 3 H2SO4 2 2 2 2 Curtius reaction : RCOCI + HN3 ⎯⎯Δ → R–NH2 + HCI + CO2 + N2 By Gabriel phthalimide reaction : This is a very convenient method for the preparation of pure aliphatic primary amines. Phthalimide is first of all treated with KOH to form potassium phthalimide which on heating with alkyl halide gives N-alkyl phthalimide. The latter is hydrolysed or hydrozinolysed to give primary amines. RX Pot. phthalimide Pot. phthalimide H2O COOH COOH O + RNH2 H2N – NH2 Aromatic amines cannot be prepared from this reaction. NH NH + R–NH2 O Secondary amines can be prepared by the hydrolysis of p-nitroso-dialkyl aniline with boiling alkali: NH2 N(CH3)2 2CH3I Aniline Dimethylaniline HONO N(CH3)2 OH NaOH + (CH ) NH NO NO p-Nitroso dimethylaniline p-Nitroso phenol Dimethylamine Tertiary amines are prepared by the decomposition of tetra-alkyl ammonium hydroxide : (C2H5 )4 N+I¯ Tatraethyl ammonium iodide + AgOH ⎯⎯→ (C2H5 )4 N+OH¯ Tatraethyl ammonium hydroxide AgI (C2H5 )4 N+OH¯ ⎯⎯he⎯at → (C2H5 )3 N + C2H4 + H2O Triethyl amine However, tetramethyl ammonium hydroxide also decomposes to give tertiary amine but in a different way. (CH3 )4 N+OH¯ ⎯⎯he⎯at → (CH3 )3 N + CH3OH Chemical Properties Nitrogen atom of all the three types of amines has a lone pair of electrons which is responsible for most of the reactions of amines. Reactions given by primary, secondary and tertiary amines : Basic character : The basic character of amines is due to the presence of unshared electron pair on nitrogen atom which accepts proton; the readiness with which the lone pair of electrons is available for co-ordination with a proton determine the relative basic strength of amines. H N H H Ammonia R N H H Primary amine R N R H Secondary amine Like ammonia, amines dissolve in water to form alkylammonium ion and hydroxide ion. + RNH2 + H2O RNH3 + OH– Aliphatic amines are stronger bases than ammonia and aryl amines. [RNH2 > NH3 > C6H5NH2] NH2 I II We have observed that aniline is a resonance hybrid of five structures (I to V) while the protonated aniline (anilinium ion) is a resonance hybrid of only two structures. Arylalkylamines Arylalkylamines are stronger bases than arylamines (e.g., aniline), but slightly weaker than the alkylamines. Alkylamines (e.g., CH3NH2) Arylalkylamines > (e.g., C6H5CH2NH2) Arylamines (e.g., C6H5NH2) Effect of substituents on the nitrogen atom of the group : The replacement of hydrogen atom of an amino group by an electron-releasing substituent (e.g., methyl group) increases the basic character. NH2 < Aniline H — N — CH3 < N-Methylaniline H3C — N — H3C N, N-Dimethylaniline On the other hand, replacement of hydrogen atom(s) of the amino group by electron-withdrawing phenyl group(s) decreases the basic character. NH2 > Aniline H — N — > Diphenylamine — H — N — Triphenylamine Reaction of water : NH3 H2O ⎯⎯→ NH4OH NH+ + OH¯ Ammonia Ammonium 4 hydroxide CH3NH2 + H2O ⎯⎯→ CH3NH3OH CH3NH+ + OH¯ Methyl amine (Pri.) (CH3 )2NH Methyl amm. hydroxide + H2O ⎯⎯→ (CH3 )2NH2OH 3 (CH3 )2NH+ + OH¯ Dimethyl amine Dimethyl amm. (Sec.) hydroxide Reaction with inorganic acids : C2H5NH2 + HCl ⎯⎯→ Ethyl amine C2H5NH3 +Cl– Ethylamine hydrochloride 2(C2H5 )2NH + H2SO4 ⎯⎯→ Diethyl amine Alkylation : [(C2H5 )2NH2 ]2.SO4 Diethylamine hydrogen sulphate CH NH ⎯⎯1. C⎯H⎯3I → (CH ) NH ⎯⎯1. C⎯H⎯3I → (CH ) N + ⎯⎯CH⎯3I → – 3 2 – 3 2 – 3 3 (CH3 )4 NI Methyl amine (primary amine) 2. OH Dimethyl amine (sec. amine) 2. OH Trimethyl amine (ter. amine) Tetramethylammine. iodide NH2 CH3I OH– NHCH3 CH3I OH– N(CH3)2 CH3I N+(CH ) I– Anline N-Methylaniline N, N-Dimethylaniline Trimethylanilinium iodide Hence, this reaction may be used for distinguishing the three types of amines. Dealkylation : (CH3 )3 N.HCl ⎯⎯he⎯at → (CH3 )2NH + CH3Cl Trimethyl amine hydrochloride Dimethyl amine (CH3 )2 NH.HCl ⎯⎯he⎯at → CH3NH2 + CH3Cl Dimethyl amine hydrochloride Methyl amine CH3NH2.HCl ⎯⎯he⎯at → Methyl amine hydrochloride CH3Cl Methylchloride NH3 Reaction with nitrous acid : Different types of amines form different products with nitrous acid (NaNO2 + HCl). An ice cold solution of a primary aromatic amine in hydrochloric acid reacts with an ice-cold aqueous solution of sodium nitrite, forming water-soluble compound known as diazonium salt. NaNO2 + HCl ⎯⎯→ HNO2 + NaCl C6H5NH2.HCl + HONO ⎯⎯⎯→ C6H5 N ≡ NCl + H2O Aniline hydrochloride 0–5°C Benzenediazonium chloride Aliphatic primary amines react with cold nitrous acid to give alcohols or sometimes alkenes with the quantitative evolution of nitrogen gas (test for aliphatic primary amines). CH3CH2NH2 ⎯⎯HO⎯NO⎯/H⎯Cl → [CH3CH2N+ ≡ NCl¯ ] ⎯⎯H2⎯O → CH3CH2OH + N2 273−278 K Ethanediazonium chloride Secondary amines (aliphatic as well as aromatic) react with nitrous acid to form N-nitrosoamines. (CH3 )2NH + HONO ⎯⎯→ (CH3 )2N — N = O + H2O Dimethylamine N−Nitrosodimethylamine Nitrosoamines are water-insoluble yellow oils and when warmed with a crystal of phenol and a few drops of conc. H2SO4 produce a green solution which turns blue on adding alkali. This reaciton is known as Libermann’s nitroso reaction and may be used as a test for secondary amines. Aliphatic tertiary amines dissolves in cold nitrous acid to form unstable nitrites which decompose on warming to give nitrosoamine and alcohol. (CH3 )3N Trimethylamine HNO2 ⎯⎯→ + (CH3 )3 NHNO2 Trimethylammoniumnitrite ⎯⎯he⎯at → (CH3 )3N.NO N−Nitrosodimethylamine CH3OH N(CH3)2 + HONO N, N-Dimethylaniline N(CH3)2 + H2O + NaCl NO p-Nitroso-N, N-Dimethylaniline Reactions given only by primary and secondary amines : Acylation : (Reaction with acetyl chloride) : Primary and secondary amines react with acid chlorides and acid anhydrides to form acyl derivatives or substituted amides. CH3NH2 + ClCOCH3 ⎯⎯→ CH3NHCOCH3 + HCl Acetylmethylamine (N−Methylacet amide) (CH3 )2NH + ClCOCH3 ⎯⎯→ (CH3 )2NCOCH3 Acetyldimethylamine (N, N−Dimethyl acetamide) HCl Like alkyl amines, aryl amines react with acid chlorides and acid anhydrides to form aryl substituted amides commonly called anilides. The reaction is best carried out in the presence of a base like pyridine. C2H5NH2 + CH3MgBr CH4 + Mg Br NHC2H5 (C H ) NH + C H MgBr C2H6 Br + Mg 2 5 2 2 5 N(C2H5)2 Reactions given only by primary amines : Carbylamine reaction : C2H5NH2 + CHCl3 + 3KOH ⎯⎯→ Ethylamine C2H5NC Ethyl isocyanide + 3KCl + 3H2O C6H5NH2 + CHCl3 + 3KOH ⎯⎯→ Aniline C6H5NC Phenyl isocyanide + 3KCl + 3H2O Hofmann’s mustard oil reaction : CH3NH2 + S = C = S S = C NHCH3 SH N–Methyl dithiocarbamic acid HgCl2 CH3N = C = S + HgS + 2HCl Methyl isothiocyanate (Mustard oil like smell) Black ppt. C6H5NH2 + S=C=S ⎯⎯HgC⎯l2 → Aniline C6H5N=C=S Phenyl isothiocyanate H2S + 2HCl Reaction with aldehydes and ketones : Primary amines react with carbonyl group to form anils or schiff’s bases C2H5NH2 + CH3CHO ⎯⎯→ C2H5N=CH.CH3 + H2O Ethylidenethylamine Reactions of the benzene nucleus : Electrophilic substitution reaction : Halogenation : NH2 Br + Br2 (water) NH2 Br Aniline Br 2, 4, 6-Tribromoaniline Nitration : Nitration reactions are never performed directly as they get connected to p-benzoquinone hence they are first acetylated and then nitrated. NH2 Aniline (CH3CO)2O HNO3/H2SO4 NHCOCH3 NO2 H O/H+ heat NH2 NO2 + CH3COOH Sulphonation : p-Nitroacetanilide p-Nitroaniline NH2 NHSO3H NH2 +H2SO4 (fuming) Aniline 453 K –H2O Salt 453 K 3 hours SO3H Sulphanilic acid Coupling reaction : + ≡ H N N NCl + 2 ⎯⎯(⎯−H⎯Cl) ⎯→ Weak acidic medium N = N.NH 313 K Benzenediazonium chloride Aniline Diazoaminobenzene (yellow dye) N = N NH2 p-Aminoazobenzene (brilliant orange red dye) Oxidation : Arylamines, unlike alkyl amines, are very susceptible to oxidation. This is because of the presence of high electron density on the ring of arylamines due to which electron removal (oxidation) becomes very easy. Thus arylamines (e.g., aniline) darken in colour even on standing in air at room temperature. More intense colour is obtained by stronger oxidising agents e.g., Oxidising agent Colour Bleaching powder Violet K2Cr2O7 + conc. H2SO4 Blue K2Cr2O7 + CuSO4 + dil. H2SO4 Black Na2Cr2O7 NH2 O O H2SO4 Aniline p-Benzoquinone (Oxidised product) CYANIDES AND ISOCYANIDES Cyanides and isocyanides, the two series of isomeric compounds, are derivatives of hydrogen cyanide (hydrocyanic acid or prussic acid) which exists in the following two tautomeric forms. H — C ≡ N H — N →= C General methods of preparation of Nitriles and Isonitriles : From alkyl halides : CH3Cl Chloromethane KCN → CH3CN Ethanenitrile KCl From arenediazonium salts : + N ≡ NCl CN + KCN CuCN or Cu powder + N2 + KCl Benzene diazonium chloride Benzonitrile From primary amines : Alkyl and aryl carbylamines are prepared by carbylamine reaction (heating of primary amine with chloroform and alcoholic potash). CH CH NH + CHCl + 3KOH(alc.) → CH CH N →= C + 3KCl + 3H O 3 2 2 3 3 2 2 Ethyl amine Ethyl isonitrile C6H5NH2 + CHCl3 + 3KOH(alc.) → Aniline C6H5NC Phenyl isonitrile + 3KCl + 3H2O Chemical Properties The two important chemical reactions of nitriles and isonitriles are hydrolysis and reduction. Hydrolysis : R — C ≡ N H+ or OH– (+H2O) O R — C — NH2 H+ or OH– +H2O RCOOH + NH3 Isonitriles, on the other hand, are hydrolysed by dilute acids but not by alkalies to form a primary amine and formic acid. R — N = C + H2O Alkyl isonitrile H+ (+ H2O) O R — NH — C — H H+ (+ H2O) R — NH2 + HCOOH 1° Amine Methanoic acid Reduction : Nitriles are fully reduced to primary amines by sodium and alcohol (Mendius reaction), lithium aluminium hydride or hydrogen in presence of Ni or Pt (catalytic reduction). R — C ≡ N + 4[H] ⎯⎯Na ⎯/ C2⎯H5O⎯H → RCH NH or LiAlH4 2 2 R — NC ⎯⎯Na|⎯C2H⎯5O⎯H → or LiAlH4 R — NH — CH3 R — C ≡ N + 2H2 ⎯⎯Pt o⎯r ⎯Ni → RCH2NH2 Stephen’s reduction : Partial reduction of nitrile with stannous chloride in presence of hydrochloric acid to form imine hydrochloride (Stephens reduction) is also possible. The imine hydrochloride may be decomposed easily with boiling water forming aldehyde as the final product. SnCl2 + 2HCl → SnCl4 + 2H CH3 — C ≡ N + 2[H] + HCl ether 290 K CH3CH = NH.HCl Acetaldimine hydrochloride boiling water CH3CHO + NH4Cl Ethanal Reaction with Grignard reagents : R — C ≡ N + R′MgX ether R′ R — C H OH N MgX H2 H+ R′ R — C ≡ O o Reaction may not stop at ketonic stage. May be ketone further is attacked by Grignard reagent to give 3 alcohol. Addition reaction : + − CH3 − N ≡ C : + Cl2 → CH3 − N = CCI2 Methyliminocarbonyl chloride + − CH3 − N ≡ C :+ S → CH3 − N = C = S Methyl isonitrile Methyl isothiocyanate Rearrangement : R− N→ C ⎯⎯Δ →R− C ≡ N Isonitrile Nitrile DIAZONIUM SALT Preparation of benzene diazonium chloride NH2 + NaNO + HCl 0–5ºC N2Cl + NaCl + H2O Properties of Benzene Diazonium Chloride (B.D.C.) Substitution Reaction : Benzene diazonium chloride is used for prepration of several organic compounds. + N2 + HCl + CH3CHO + N2 + HCl + H3PO3 Cl N2Cl + N2 + HCl Br OR + N≡NCl + N2 + HCl CN + N2 + NaCl OH I + N2 + KCl F + N2 + BF3 + HCl NaOH NHNH2 + NaCl + H2O biphenyl Coupling Reaction : Condensation of diazonium salt with electron rich aromatic compounds like phenols and amines to form azodyes. OH N=N OH p-Hydroxy azobenzene (orange dye) N2Cl N=N NH2 p-Amino azobenzene OH N=N OH Phenylazo-β-naphthol (Red dye) ❑ ❑ ❑