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https://docs.google.com/document/d/1Deduyy8NfNhXFpZLklsejkpU7lJ8gTjm/edit?usp=share_link&ouid=109474854956598892099&rtpof=true&sd=true Organic Compounds Containing Halogen General methods of preparation, properties and reactions; Nature of C-X bond; Mechanisms of substitution reactions. Uses; Environmental effects of chloroform, iodoform, freons and DDT. C H A P T E R ALKYL HALIDES General Methods of Preparation From Alcohols By the action of halogen acids Alkyl chlorides are obtained by passing dry hydrogen chloride gas into the alcohol in presence of anhydrous ZnCl2. C2H5OH + HCl(g) ⎯⎯Anh⎯yd.⎯ZnC⎯l2 →C2H5Cl + H2O CHAPTER INCLUDES Alkyl Halides Preparation Chemical reactions Ethyl alcohol (Ethanol) Ethyl chloride (Chloroethane) SN2 SN1 The order of rate of reaction is 3° alcohol > 2° alcohol > 1° alcohol. By the action of phosphorus halides Alkyl chlorides can be prepared by refluxing alcohols with phosphorus pentachloride or phosphorus trichloride. Aryl Halides Preparation Chemical reactions ROH +PCl5 → Alcohol RCl Alkyl chloride HCl + POCl3 Phosphoryl chloride or Phosphorus oxychloride Alkyl bromides or alkyl iodides are prepared by the action of phosphorus tribromide or tri-iodides on alcohols. 3ROH +PBr3 → 3RBr + H3PO3 Alcohol Alkyl chloride Phosphorus acid By the action of thionyl chloride (SOCl2)- Alkyl chlorides can be prepared by heating alcohol (1° or 2°) and thionyl chloride in the presence of pyridine (base). (Darzen's Reaction) ROH + SOCl2 ⎯⎯Pyr⎯idin⎯e → RCl + SO2 ↑ + HCl↑ Alcohol (1° or 2°) Thionyl chloride Alkyl chloride Gaseous by product From Alkenes Alkyl halides can be prepared by the addition of halogen acids to alkenes. H2C = CH2 + HX → CH3 − CH2 − X Ethylene Ethyl halide From Alkanes Alkyl halides can be prepared by the direct halogenation of alkanes in the presence of light or heating at least at 250 – 300°C or a suitable catalyst, e.g., CH4 Methane Cl2 ⎯⎯Ligh⎯t ⎯or → 250– 400°C CH3Cl Methyl chloride (Chloromethane) HCl C2H6 + Br2 ⎯⎯Ligh⎯t ⎯or → C2H5Br HBr Ethane 250−400°C Ethyl bromide (Bromoethane) The mechanism is a free radical reaction mechanism. The reactivity of halogen depends upon the nature of halogen used to abstract hydrogen hence F2 > Cl2 > Br2 >I2 The reaction with the iodine being reversible can take place in the presence of an oxidizing agent like iodic acid (HIO3), nitric acid (HNO3) or mercuric oxide (HgO). R – H + I2 ⎯⎯HIO⎯3 o⎯r H⎯NO⎯3 → R – I + HI Fluorination of alkanes is carried out by heating suitable halo alkanes with inorganic fluorides, such as AsF3, SbF3, Hg2F2 etc. 2CH CH – Cl + Hg F → 2CH CH – F + Hg Cl From silver salts of fatty acids (Borodine - Hundsdiecker Reaction) C6H5COOAg+ Br2 ⎯⎯CC⎯l4 →C6H5Br+ AgBr + CO2 Silver benzoate Reflux Phenyl bromide By halogen exchange (a) C2H5Br Ethyl bromide NaI ⎯⎯CH⎯3OH⎯or⎯Ac⎯eto⎯n⎯e → C2H5I Ethyl iodide NaBr The reaction is possible because sodium iodide is soluble in methanol and acetone whereas sodium chloride and sodium bromide are insoluble (b) Reaction of Grignard's reagent with I2. RMgX + I2 (where X=Cl or Br) → R −I + MgXI From Alkyl hydrogen sulphate Alkyl iodides can be prepared by treating alkyl hydrogen sulphates with an aqueous solution of potassium iodide. C2H5HSO 4 + KI → Ethyl hydrogen sulphate C2H5I Ethyl iodide KHSO 4 CHEMICAL REACTIONS Halogen derivatives of alkanes are highly reactive as the halogen atoms are easily replaced. These derivatives especially the alkyl halides are widely used in the synthesis of many organic compounds. The Chemical reactions of alkyl halides may be classified into three types : Nucleophilic substitution reactions. Elimination reactions. Miscellaneous reactions. Nucleophilic substitution reactions Alkyl halides are highly reactive in nature. This is due to the fact that halogen atom is good leaving group. I is best and F is worst leaving group among halogen. Therefore for a given alkyl group, the order of reactivity for SN reactions is, iodides > bromides > chlorides. Nucleophilic substitution reaction can occur through two type of mechanisms SN2 (Nucleophilic Substitution, Bimolecular) mechanism. SN1 (Nucleophilic Substitution, Unimolecular) mechanism. Kinetics of nucleophilic substitution For the given Nucleophilic substitution reactions : CH3Br Methyl bromide KOH(aq) → CH3OH Methyl alcohol KBr (CH3 )3 CBr Tert- butyl - bromide KOH(aq) → (CH3 )3 COH + KBr Tert–butyl alcohol The rate of the first reaction (calculated by experimental methods) is Rate = k[CH3Br] [KOH] i.e. it follows SN2 mechanism In second reaction, the rate is dependent on the concentration of alkyl halide only and is independent of nucleophile concentration Rate = k[(CH3)3 C–Br] i.e, It follows SN1 mechanism. Hence first reaction follows second order kinetics and second reaction follows first order kinetics. Mechanism of SN2 (Substitution nucleophilic bimolecular) In this the nucleophile (OH¯ ) collides with the reactant (CH3Br) molecule at the face most remote (Back side attack) from the halogen atom and possesses sufficient energy to break the C – Br bond to form C – OH bond. Thus a complete inversion of configuration takes place. H H H +δ –δ HO¯ + H – C – Br H → HO ....... C Br H → HO – C – H + Br¯ H Transition State sp2 hybridised Transition State of a SN2 reaction Since this reaction involves the formation of only one transition state and no intermediates between the reactants and the products. So there will only one activation energy of the reaction as shown in the potential energy of the graph. The reactants are shown to be slightly higher in energy than products since the reaction is exothermic. H H δ– –δ H–O....... C Br (Transition state) (trigonal bipyramidal geometry) H Ea ΔH CH3–Br+OH CH OH + Br– Progress of the reaction Potential energy diagram of an SN2 reaction Mechanism of SN1 Reaction : It is two steps reaction R' | Ist Step : R − C − X + OH− | R'' ⎯⎯Slo⎯w⎯→ R' | R − C⊕ | R'' XO R' | IInd Step : R − C⊕ + OHO | R'' R' | Rate = k [R −C | ⎯F⎯a⎯s⎯t → X] R' | R − C | R'' OH R'' Stereochemistry of SN1 reaction : Partial racemisation in case of lesser stable carbocation and complete racemisation in case of more stable carbocation takes place in SN1 reaction. Factors affecting the rate of SN1 and SN2 reactions : Structure of Substrate : for S 1 : Systems giving stable carbocation or having bulkier groups at β position. X > (CH3)3C—X > (CH3)2CH—X > CH3CH2X > CH3X for SN2 : Greater the positive charge on the carbon of C – X bond, lesser is the steric hinderance easier will be the attack of the nucleophile on it. The order of reactivity of RX in SN2 reactions is CH3X > primary alkyl halide > secondary alkyl halide > tertiary alkyl halides > neopentyl halides. Strength and concentration of nucleophile : for SN1 : As nucleophile does not participate in rate determining step, therefore there is no effect on rate of concentration and strength of nucleophile. for SN2 : Increasing the concentration of nucleophile increases the rate for SN2, a better nucleophile will yield better products. Solvent Effect : Polar protic solvent favours SN1 whereas polar aprotic solvent favours SN2 mechanism. Nature of leaving group : Good leaving group always increases rate of nucleophilic substitution reactions for the same reason the order of reactivity for SN reactions of halides follow the given trend : RI > RBr > RCl > RF Miscellaneous reactions Reaction with magnesium (formation of Grignard's reagent). R – X + Mg ⎯⎯Dry⎯e⎯th⎯er → RMgX For a given alkyl group, the ease of formation of Grignard’s reagent is of the order : iodide > bromide > chloride Reduction (formation of alkanes) Alkyl halides are reduced to alkanes by any of the following reducing agents : H2 in the presence of Ni, Pt or Pd (catalytic hydrogenation). Lithium-aluminium hydride (LiAlH4). Nascent hydrogen obtained from Zn-Cu couple and alcohol or Zn and HCl or Sn and HCl or Na and alcohol. Friedel Craft’s reaction Formation of Alkyl benzene C6H6 + CH3CH2 − Cl ⎯⎯Anh⎯ydr⎯ous⎯A⎯lC⎯l3 →C6H5 − CH2CH3 + HCl Ethyl benzene Vinylic halides and aryl halides do not give a silver halide precipitate, when treated with alc. AgNO3 because vinylic and phenyl cations are very unstable and therefore, do not form readily. ARYL HALIDES General methods of Preparation From arenes by direct halogenation C6H6 + Cl2 ⎯⎯FeC⎯l3 ,⎯298⎯K →C6H5Cl + HCl Benzene Chlorobenzene C6H6 + Br2 ⎯⎯FeB⎯r3⎯,Δ →C6H5Br + HBr Benzene Bromobenzene If excess of halogen is used, dihaloderivatives are formed. Chlorination of toluene in the presence of iron (using equimolar quantities) gives a mixture of o- and p- chlorotoluenes. CH3 CH3 Cl CH3 + Cl2 Fe + Cl Preparation of aryl iodides Aryl iodides cannot be prepared by direct iodination because the reaction is reversible and hydriodic acid formed being a strong reducing agent reduces C6H5I to C6H6. C6H6 + I2 C6H5I + HI To overcome this difficulty, iodination is carried out in the presence of oxidizing agent such as nitric acid, mercuric oxide (HgO) or iodic acid, which oxidizes the hydroiodic acid to iodine and thus the reaction proceeds in the forward direction. Aryl fluorides cannot be prepared by this method because fluorine is highly reactive and the reaction is very violent and uncontrolable. Direct Halogenation. When calculated amount of chlorine is passed through boiling toluene in presence of sunlight or ultra – violet light and in the absence of halogen carrier, benzyl chloride is formed. CH3 Toluene + Cl ⎯⎯Sun⎯lig⎯ht → 2 Boil CH2Cl Benzyl chloride + HCl With NBS When toluene is treated with NBS (N-bromosuccinimide) in the presence of peroxides, benzyl bromide is formed. C H – CH + CH2CO Peroxides C H – CH Br CH2CO + 6 5 3 NBr ⎯⎯⎯ ⎯→ 6 5 2 NH Toluene CH2CO Benzyl bromide CH2CO NBS Suiccinimide From Diazonium salts Sandmeyer’s reaction Preparation of chlorobenzene + N2 Cl Benzenediazonium chloride ⎯⎯CuC⎯l / H⎯Cl → Cl Chlorobenzene + N2 Preparation of bromobenzene – CuBr / HBr C6H5 N2 Cl Bnzenediazonium chloride ⎯⎯⎯⎯⎯→ C6H5Br Bromobenzene HCl + N2 Preparation of iodobenzene + – Δ C6H5 N2 Cl + KI ⎯⎯→ Bnzenediazonium chloride C6H5I Iodobenzen e HCl + N2 Preparation of fluorobenzene (Balzschiemann reaction). + N2 Cl Benzenediazonium chloride ⎯⎯HBF⎯4 → −HCl + N2 BF4 Benzenediazonium fluroborate ⎯⎯Δ→ C6H5F Fluorobenzene N2 + BF3 By Raschig Process: On commercial scale, chlorobenzene is prepared by passing a mixture of benzene vapours, air and hydrogen chloride gas over cupric chloride (catalyst) at 500 K. 2C H + 2HCl + O ⎯⎯CuC⎯⎯l2 → 2C H Cl + 2H O 6 6 2 500 K 6 5 2 By Hunsdiecker Reaction : COOAg + Br2 Distillation CCl4, 350 K Br + AgBr + CO2 From phenol C6H5OH+ PCl5 → Phenol C6H5Cl Chlorobenzene POCl3 + HCl Here the yields are very poor due to the formation of triaryl phosphate. Physical Properties They are colourless stable liquids. Insoluble in water but soluble inorganic solvent. Boiling and melting points : Their boiling and melting point is higher than alkyl halides. Boiling point increases with increasing size of halogen. F Cl Br I < < < Boiling points of isomeric dihalo benzenes are very nearly the same but the melting point of para isomers is higher than ortho and meta isomer. Chemical Properties Haloarene gives SN reaction rarely only under drastic conditions becuase of partial double bond character between C–X bond. i.e., at high temp. & pressure and in presence of certain catalyst. Replacement by hydroxyl group (Formation of phenols) (Dow's process) Cl + NaOH ⎯⎯623⎯K → ONa ⎯⎯H⎯Cl → OH + NaCl aqueous 300 atm Chlorobenzene Sod. Phenoxide Phenol Replacement by amino group (Formation of aryl amines) + 3NH 2 + Cu O ⎯⎯473⎯K → NH2 + Cu Cl + H O Chlorobenzene 3 2 60 atom Aniline 2 2 2 C H Cl + + & Liquid NH3 6 5 K NH2 ⎯⎯⎯⎯⎯→ C6H5NH2 + KBr Bromobenzene 240 K Aniline Replacement by cyanide group (Formation of aryl cyanides) 2C6H5Br Cu2 (CN)2 ⎯⎯Pyr⎯idin⎯e → 2C6H5CN + 2CuBr Bromobenzene 470 K Cyanobenzene (Benzonitrile) Replacement by methoxy group C6H5Cl + CH3ONa ⎯⎯47⎯3 →C6H5OCH3 + NaCl Sod. methoxide Cu salts Anisole Activation of halogen atoms Presence of electron-withdrawing groups e.g., – NO2, –CN, SO3 H, –CHO, –COOH, etc. especially at the o- or p- position but not at m-position w.r.t. the halogen increases the ease of replacement of halogen by nucleophile. While electron donating groups like –NH2, –OH, –OR etc. at o- or p- position with respect to halogen atom donates electron due to resonance and thus reduces positive charge on the C-atom of C–Cl bond and thus decreases the reactivity of aryl halides. Reactions due to benzene nucleus (electrophilic substitutions) Aryl halides undergo electrophilic substitution reactions to give o- and p-substituted derivatives. Halogenation Cl + Cl2 ⎯⎯FeC⎯l3 → −HCl Cl o- p- Dichloro benzene Nitration Cl + HNO3 ⎯⎯H2S⎯O4⎯,Δ → −H2O Cl NO2 + Cl NO2 o- p- Nitrochlorobenzene Friedel - craft alkylation reaction Cl + CH3Cl ⎯⎯AlC⎯l3 → −HCl Cl CH3 + Cl CH3 o- p- Chloro toluene or Tolyl chloride Friedel Craft acylation reaction Cl + CH3COCl Acetyl chloride ⎯⎯AlC⎯l3 → −HCl Cl O C – CH3 + Cl COCH3 o- p- Chloro acetophenone Some Important Halogen Compounds DDT : On heating with chloral in the presence of concentrated sulphuric acid, chlorobenzene forms (p, p' - dichlorodiphenyl trichloroethane (commonly known as DDT-a powerful insecticide.) But it is not biodegradable because it contains halogen atoms attached to the benzene rings. Cl H Cl Cl H Cl Cl – C – C = Cl Chloral Cl Chlorobenzene (2 molecules) ⎯⎯Con⎯c. ⎯H2S⎯O⎯4 → −H2O Cl – C – C Cl Cl DDT 2, 2–bis–(p–chlorophenyl) 1, 1, 1– trichloroethane Chloroform, Trichloromethane, CHCl3 Preparation : From ethyl alcohol (or acetone) and bleaching powder. C2H5OH + CaOCl2 → CHCl3 + (HCOO)2Ca Mechanism : CaOCl2 Bleaching powder + H2O ⎯⎯→ Ca(OH)2 + Cl2 CH3CH2OH + Cl2 ⎯⎯→ CH3CHO + 2HCl Ethyl alcohol Acetaldehyde CH3CHO + 3Cl2 ⎯⎯→ CCl3.CHO + 3HCl Acetaldehyde Chloral O H Ca + O H CCl3 CHO CCl3 CHO OOCH Ca + 2 CHCl3 OOCH Lime Chloral (2 moles) Cal. formate Chloroform Similarly, reactions taking place in case of acetone are as below: CH3CO.CH3 + 3Cl2 ⎯⎯→ CCl3.CO.CH3 + 3HCl Acetone Trichloroacetone Ca O H CCl3 COCH3 + Ca O H CCl3 COCH3 OOC.CH3 OOC.CH3 + 2 CHCl3 Lime Trichloroacetone (2 moles) Cal. acetate Chloroform From methane (Industrial method): Chlorination of methane in presence of sunlight gives a mixture of different compounds CH4 ⎯⎯C⎯l2 →CH3Cl ⎯⎯→ CH2Cl2 ⎯⎯→ CHCl3 ⎯⎯→ CCl4 Chloroform is separated from other possible products by fractional distillation. From carbon tetrachloride: (industrial method): Chloroform is produced by the partial reduction of CCl4 with iron fillings and water. CCl4 + 2[H] ⎯⎯F⎯e → CHCl3 + HCl H2O From chloral : Pure chloroform, required for anaesthetic use is prepared by the action of aqueous sodium hydroxide on chloral or chloral hydrate. CCl3 .CHO + NaOH ⎯⎯→ CHCl3 + HCOONa Chloral Chloroform Sod.formate CCl3 .CH(OH)2 + NaOH ⎯⎯→ CHCl3 + HCOONa + H2O Chloral hydrate Physical Properties Chloroform is a heavy colourless liquid (b.p.61°C) with a characteristic sweet smell and sweet taste. It is almost insoluble in water but soluble in alcohol and ether. It itself is a good solvent for oils, fats, resins, waxes, halogens, etc. Inhalation of its vapours produces unconsciousness and hence it finds use as a general anaesthetic. However, owing to its bad effect on heart it has now been replaced by better anaesthetic like ether, ethylene and ethylene chloride. Under ordinary conditions, it is not inflammable but its vapours when ignited burn with a green-edged flame. Chemical Properties Oxidation: When exposed to air and sunlight, chloroform is slowly oxidised to carbonyl chloride (phosgene), a highly poisonous substance. CHCl3 1 O 2 2 ⎯⎯lig⎯ht → COCl2 HCl Chloroform Phosgene To avoid the oxidation of CHCl3 to phosgene, chloroform (especially that meant for anesthetic purposes) is al- ways stored in dark coloured bottles to cut off light. The bottles are filled to the neck and well stoppered to exclude air. A small amount of alcohol is also added to prevent the oxidation of chloroform and to convert phos- gene (formed if any) to harmless ethyl carbonate. 2C2H5OH + COCl2 ⎯⎯→ (C2H5 )2 CO3 + 2HCl Ethyl carbonate Reduction: Different products are formed under different conditions. CHCl3 + 2[H] ⎯⎯Zn /⎯H⎯Cl → CH2Cl2 + 2HCl (acidic medium) Methylene chloride CHCl3 + 4[H] ⎯⎯Zn+⎯alc⎯.H⎯Cl → CH3Cl Methyl chloride 2HCl CHCl3 + 6[H] ⎯⎯Zn +⎯H2⎯O → (neutral medium) CH4 Methane 3HCl Hydrolysis (action of aqueous KOH): CHCl3 + 3KOH ⎯⎯−3K⎯Cl →CH(OH)3 ⎯⎯−H2⎯O →HCOOH ⎯⎯KO⎯H →HCOOK Chloroform Unstable Formic acid (−H2O) Pot.formate Chlorination: CHCl3 + Cl2 (gas) ⎯⎯diffu⎯se⎯d → CCl4 HCl Chloroform sunlight Carbon tetrachloride Nitration (action of conc. HNO3). Cl3CH + HO.NO2 ⎯⎯→Cl3CNO2 + H2O Chloroform Chloropicrin Chloropicrin or nitrochloroform is a liquid and used as insecticide. Carbylamine reaction (action of alcoholic potash and primary amine). CHCl3 + C2H5NH2 + 3KOH ⎯⎯→ C2H5NC + 3KCl + 3H2O CHCl3 + C6H5NH2 + 3KOH ⎯⎯→ C6H5NC + 3KCl+ 3H2O Aniline Phenyl isocyanide (having news eating odour) Condensation with ketones, e.g., acetone CH3 CH3 C = O + HCCl3 NaOH CH3 CH3 OH C CCl3 Acetone Chloroform Chloretone Chloretone, a colourless solid, is used as hypnotic. Heating with silver powder (dehalogenation) HCCl3 + 6Ag + Cl3CH ⎯⎯he⎯at → CH ≡ CH+ 6AgCl Acetylene Reaction with phenol and alkali (Reimer-Tiemann reaction) C6H5OH + CHCl3 + 3NaOH ⎯⎯→ OH CHO + 3NaCl + 3H2O Chloroform is used : as an anaesthetic as a solvent for fats, oils, waxes, rubbers, etc. in laboratory for testing primary amines in the preparation of chloropicrin (an insecticide) and chloretone (a hypnotic) as preservative for anatomical specimens IODOFORM Preparation Iodoform reasembles chloroform in methods of preparation Laboratory Preparation : C2H5OH + I2 ⎯⎯→ CH3CHO + 2HO CH3CHO + 3I2 ⎯⎯→ CI3CHO + 3HI CI3CHO + OH− ⎯⎯→ CHI3 ↓+ HCOO− Iodoform (ppt.) Industrial preparation : In industries CHI3 is prepared by electrolysis of ethanol, sodium carbonate and potassium iodide. KI K+ + I– Cathode K+ + e– → K 2I–→ I + 2e 2K + 2H2O → 2KOH + H2 KOH is neutralised by CO2 C2H5OH + 4I2 + 3Na2CO3 → CHI3 + HCOONa + 5NaI + 3CO2 + 2H2O Physical Properties Yellow crystalline solid Pigment characteristic odour Insoluble in water Hydrolysis CHI3 + KOH Reduction ⎯⎯→ HCOOK CHI3 + Red P/H2 ⎯⎯→ CH2I2 (methylene iodide) Carbylamine Reaction NH2 + CHI3 + KOH(alc) ⎯⎯→ C6H5NC (Phenyl isocyanide) (Characteristic odour) (This is a characteristic reaction of ºI Heating Alone CHI3 ⎯⎯O⎯2 →CO + I2 ↑ +2H2O Δ amines) Iodoform Test Formation of iodoform is commonly used as a test for detecting the presence of following compounds/ groupings. CH3CH2OH; – CHOH.CH3 ; CH3CHO ; – CO.CH3 The test is carried out by warming the above type of compound with iodine and sodium hydroxide (sodium hypoiodite, NaOI) when a yellow crystalline precipitate of iodoform is formed. RCOCH3 + 3I2 + 4NaOH ⎯⎯→ CHI3 ↓+ RCOONa + 3NaI + 3H2O ( yellow ) ❑ ❑ ❑