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Halogen Derivatives

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Halogen Derivatives Dr. Anil Kumar Ojha, Assistant Professor, Department of Chemistry B.Sc Part -1 ( Honour’s and Subsidiary) Halogen derivatives Compounds derived from hydrocarbon by the replacement of one or more hydrogen atom (s) by the halogen atom (s) are known as halogen derivatives. The halogen derivatives of alkanes, alkenes, alkynes and arenes are known as alkyl halides (haloalkanes), alkenyl halides (haloalkenes), alkynyl halides (haloalkynes) and aryl halides (halobenzenes) respectively. Halogen derivatives may be classified as follows : Organic Compounds Containing Halogen (S) Alkyl halide Aryl halide Mono haloalkanes Poly haloalkanes AlkylPrimary Halides alkyl halide. Secondary alkyl halide Tertiary alkyl halide (1) Monohaloalkanes : These are halogen-substituted alkanes. The general formula is C n H 2n+1 X , where X is any halogen. Alkyl halides have been further classified, on the basis of nature of the carbon atom to which halogen is attached into following categories : (i) Primary alkyl halide : The halogen is attached to primary carbon atom, i.e. RCH 2 X . R (ii) Secondary alkyl halide : The halogen is attached to secondary carbon atom, i.e. CHX . R R (iii) Tertiary alkyl halide : The halogen is attached to tertiary carbon atom, i.e. R C X . R Preparation of alkyl halides : (1) By free radical halogenation of alkanes, e.g. UV light CH 4 + X 2 ⎯ ⎯ ⎯⎯ →CH 3 X + HX (where, X = Cl, Br only) Excess Note: ❑ The formation of di, tri, tetra-halides etc. (as side products) also takes place. ❑ Iodination of alkanes is a reversible process, therefore, formation of iodoalkanes is possible only in the presence of oxidising agents such as HIO 3 , conc. HNO 3 etc., which oxidises the HI produced. Iodination with methane does not take place at all. ❑ Fluorination of alkanes takes place with rupture of C – C bonds in higher alkanes. Therefore alkyl fluorides are generally prepared by halide exchange reactions. Alkyl fluorides are, therefore, prepared indirectly by heating alkyl chlorides with inorganic fluorides e.g. AsF3 , SbF 3 , AgF, Hg 2 F2 etc. (Swarts reaction) 2CH 3 CH 2 Cl + Hg 2 F2 → 2CH 3 CH 2 F + Hg 2 Cl 2 (2) By reaction of halogen acids (HX) to alkenes, e.g. RCH = CH 2 + HX → R C H − CH 3 | X (3) By reaction of PCl5 or PCl 3 or SOCl 2 with alcohol : RCH 2 OH + PCl5 → RCH 2 Cl + POCl 3 + HCl ; 3 RCH 2 OH + PCl 3 → 3RCH 2 Cl + H 3 PO3 Pydrine RCH 2 OH + SOCl 2 ⎯ ⎯ ⎯⎯ → RCH 2 Cl + HCl + SO 2 ZnCl 2 (4) By reaction of alcohols with halogen acids: RCH 2 OH + HX ⎯ ⎯ ⎯ → RCH 2 X + H 2 O (5) By Hunsdiecker reaction: RCOOAg + X 2 → RX + AgX + CO 2 Note: ❑ This reaction proceeds through free radical mechanism. ❑ The yield of alkyl halide is 1o 2 o 3 o . ❑ This reaction is used to reduce the length of carbon chain. ❑ Only bromides are obtained in good yield in this reaction. The chlorides can also be obtained by this reaction but the yield is very low. Iodides however cannot be obtained because these form esters with silver salts. (Birnbaurn Simonini reaction). 2RCO 2 Ag + I2 → RCO 2 R + CO 2 + 2AgI Properties : Physical properties: (1) Lower members like CH 3 Cl, CH 3 Br, C 2 H 5 Cl are colourless gas while some of the higher members are colourless sweet smelling liquids. Higher homologues are odourless solids. (2) The boiling point and density is in the order R − F R − Cl R − Br R − I Example CH 3 F CH 3 Cl CH 3 Br CH 3 I B.P. (o C) –78.4 – 23.8 3.6 42.5 Density (gm ml −1 ) 0.84 0.92 1.73 2.28 (3) They have very low solubilities in water but miscible with non-polar solvents. (4) These have a cumulative toxicity and are carcinogenic. Chemical properties : (1) Alkyl halides undergo nucleophilic substitution reaction : In general, RCH 2 − X + Y → RCH 2 − Y + X Where, Y = Cl , Br ,OH , RO , RS ,CH 3 COO ,CN , R , NH 2 , N 3 , NO 2 Similarly alkyl halide reacts with compound with general formula ‘HY’ as below RX + HY → RY + HX , Where, HY = H 2 O, ROH, RCOOH, NH 3 The general reactions are as follows : R – OH (Alcohol) R – OR' (Ether) R – SH (Thiol) R – CN (NItrile) R – X , Pri or Sec) R – SR' (Thioether) (X = Cl, Br or I) (Alkyne) (Ester) (quaternary ammonium halide) PCl5 Note: ❑ Reactivity of halides towards SN 1 –mechanism is 3 o 2 o 1 o and SN 2 –mechanism is 1 o 2 o 3 o . ❑ Polar solvents favour while non polar solvents favour mechanism. ❑ High concentration of nucleophiles favour while low concentration mechanism. ❑ Rate of reaction in mechanism do not depend upon the nature of the attacking nucleophile. However, in , rate depend on the strength of the attacking nucleophile. (2) Alkyl halides undergo dehydrohalogenation which is a -elimination reaction in which halogen is lost from -carbon atom while H is lost from -carbon atom. This reaction is governed by Saytzeff’s rule. Note: ❑ Ease of dehydrohalogenation among halides : . Br CH 3 | | (3) Isomerization : / anhy. AlCl3 CH 3 CH 2 CH 2 CH 2 Br ⎯ ⎯ ⎯ ⎯ ⎯⎯ →CH 3 CH 2 − CH − CH 3 + CH 3 − C− Br | CH 3 (2) Dihaloalkanes : These have been classified in two categories namely Gem-dihalide in which the two halogens are attached to same carbon atom and vic-dihalide in which the two halogens are attached to Cl CH 2 Cl adjacent carbon atom, e.g. CH CH ; 3 Cl | Ethylidene chloride (gem-dihalide) CH 2 Cl Ethylene chloride (vic-dihalide) The vic-dihalides are prepared by reaction of halogens to alkene where as gem-dihalide are prepared by CH 2 Cl reaction of to aldehyde. ; CH 3 CH = O + PCl5 → CH 3 CHCl 2 + POCl 3 + HCl CH 2 = CH 2 + Cl 2 → | CH 2 Cl The gem and vic-dihalides can be distinguished by reaction with aq. KOH : CH 2 Cl CH 2 OH | + aq. KOH → | ; CH 2 Cl CH 2 OH diol OH CH CHCl + aq. KOH → CH CH ⎯ ⎯−H⎯2O →CH CHO 3 2 3 OH 3 (3) Trihaloalkanes (i) Chloroform (CHCl 3 ) : It can be prepared by reaction of acetone or ethyl alcohol with bleaching powder. 2CH 3 CH 2 OH + 8Cl 2 + Ca(OH )2 → 2CHCl 3 + (HCOO )2 Ca + 10 HCl The important properties of CHCl 3 (sweet smelling liquid) are as follows : sun light (a) On oxidation it yields phosgene gas which is very poisonous: CHCl 3 + [O] ⎯ ⎯ ⎯⎯ → COCl 2 + HCl air Phosgene For medical purpose, the chloroform is filled in coloured bottles and kept in dark to avoid sunlight. Further the bottles are filled upto neck to avoid oxygen (air) and 1% C 2 H 5 OH is added to convert phosgene into harmless ethyl carbonate. (b) Upon alkaline hydrolysis with KOH, chloroform gives potassium formate. OH KOH CHCl 3 + 3KOH → CH OH → HCOOH ⎯ ⎯ ⎯ → HCOOK + H 2 O OH (c) With acetone it forms chloretone which is used as a hypnotic. (d) When chloroform is warmed with a primary amine and alc. KOH, it forms isocyanide. The reaction is called carbylamine reaction, RNH 2 + CHCl 3 + 3KOH → RNC + 3KCl + 3H 2 O (R = alkyl or aryl) The carbene is the reaction intermediate. (e) When is treated with phenol in the presence of NaOH, salicyladehyde is formed. The reaction is called Reimer Tiemann reaction. OH OH OH CHO + CHCl 3 + 3NaOH ⎯⎯ → + 3NaCl + Major product CHO O || (ii) Iodoform : When methyl ketones or alcohols having following structure reacts with I2 and − C− CH 3 o alkali (aq. NaOH or aq. Na 2 CO 3 ), a yellow solid of melting point 119 C is obtained. The solid is called H iodoform and the reaction is called Iodoformic. | (Where, R = H, alkyl, aryl) R − C − CH 3 | OH As a rule all methyl ketones and alkanol-2 give +ve iodoform test, e.g. Alcohols and ketones which gives +ve iodoform test –ve Iodoform test (due to absence of − CH 3 gp.) H CH 3 CH 2 OH | H − C − OH | H CH 3 − CH − CH 3 H | | CH 3 CH 2 C − OH OH | H C6 H 5 COC 6 H 5 , C6 H 5 COC 6 H 11 The reaction is as follows : C2 H5OH + 4 I2 + 6 NaOH → CHI 3 + HCOONa + 5 NaI + H 2O H | R The mechanism is as follows : R − C − CH 3 + NaOI → C = O + NaI + H 2 O | CH 3 OH R CHCl 3 R C = O + 3NaOI → R C = O + 3NaOH ; C = O + NaOH → CHI + RCOONa CH I C 3 3 3 I3 C Yellow ppt. (4) Tetrahaloalkanes (i) Freons is trade name of fluorochloromethanes having general formula CFX Cl y , where (x + y = 4). (ii) The prefix per means that all H atoms of the hydrocarbons are replaced by fluorine atoms, e.g. perfluorooctane CF3 (CF2 )6 CF3 . These are chemically inert due to electrostatic force of attraction between + on C and − on F. (iii) When CCl instead of is used in Reimer Tiemann reaction the product is salicylic acid. PCl4 5 OH OH OH + CCl 4 + 4 NaOH ⎯⎯ → COOH + Salicylic acid (Major product) Aryl Halides. COOH These are organic compounds in which halogen is directly attached to benzene ring, e.g. Cl These can be prepared by reaction of halogen to aromatic hydrocarbon. Cl + + HCl chlorobenzene (2) By Sandmeyer’s reaction : When acid solution of diazonium salt is added to a solution of cuprous halide dissolved in corresponding halogen acid, aryl halides are obtained, e.g. Cl I Cu 2Cl2 ⎯ ⎯ ⎯⎯ → + N 2 ; + KI ⎯⎯ → + N 2 + KCl HCl Br + HBr ⎯ ⎯Cu 2⎯Br⎯2 → + HCl + OH Cl PCl5 (3) By reaction of phenol with , ⎯ ⎯⎯ → + POCl 3 + HCl Properties Physical properties : Aryl halides are generally colourless liquid or crystalline solid.
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