Title PMT_Chem_Haloalkanes_and_Haloarenes_refresher.pdf ✅

10 CHAPTER Haloalkanes Haloalkanes are the halogen deivatives of hydrocarbons and these are of the following types depending upon the number of hydrogen atoms present in them. Monohalides (R–X): Monohalides are the mono- halogen derivatives of alkanes having a general formula Cn H2n+1 X and are known as alkyl halides. R–X may be of three types: 1. Primary R–CH2 X 2. Secondary R2 ⋅ CH.X 3. Tertiary R3 ⋅ C.X Dihalides (Cn H2n X2 ): Dihalides are the di-halogen de- rivatives of alkanes and are of geminal and vicinal types. Geminal Vicinal C X X C C X C X CH3 CHBr2 BrCH2 CH2 Br Ethylidine dibromide Ethylene dibromide α, ω or terminal dihalides. For example, BrCH2 –CH2 –CH2 –CH2 Br 1, 4-dibromobutane Trihalides (Cn H2n – 1 X3 ) or haloforms: Trihalides are the tri-halogen derivatives of alkanes and represented by a general formula CHX3 (haloforms). Tetrahalides: Tetrahalides are the tetra-halogen deriva- tives of alkanes and represented by a general formula CX4 or CX2 Y2 . Monohalides Monohalides are called haloalkanes, for example, the IUPAC name of C2 H5 Br is bromoethane. These may show chain, position and optical isomerisms. Methods of preparation By direct halogenation of alkanes R–H + X–X hv R–X + HX For example, CH4 + Cl2 hv or 520–650K CH3 Cl + CH2 Cl2 + CHCl3 + CCl4 • The reactivity order is F2 > Cl2 > Br2 > I2 R3 CH > R2 CH2 > RCH3 > CH4 t s p Details are given in GOC II and hydrocarbons. From alcohols • R–OH + PCl5 R–Cl + POCl3 + HCl • 3R–OH + PCl3 3R–Cl + H3 PO3 • R–OH + SOCl2 Pyridine R–Cl + SO2 + HCl Haloalkanes and Haloarenes ■ Haloalkanes ■ CCl4 Or Pyrene (Carbontetra-Chloride) ■ Teflon (–CF2 –CF2 –)n ■ Chlorobenzene Chapter Outline
10.2 ■ Chapter 10 It is called Darzen’s method. Here, pyridine is used to reflux gaseous by-products. • R–H + SO2 Cl2 Pyridine R–Cl + HCl + SO2 Here Pyridine also reacts with HCl N + HCl HCl Pyridine N Hydrogen chloride By Groove’s method • R–OH + HX Anhy. ZnCl2 R–X + H2 O + • R–OH H+ R–OH2 R+ + H2 O R+ X– fast R–X (SN1) [or R–O+ H2 X– Slow R–X + H2 O] SN2 • The order of reactivity of HX with alcohols is HI > HBr > HCl • The order of reactivity of alcohols towards HX is tertiary (t) > secondary (s) > primary (p). (That is decided by stability of carbocation formed). For example, (CH3 ) 3 C–OH + HCl (CH3 ) 3 C–Cl + H2 O Tert. butyl alcohol Tert. butyl chloride • In case of tertiary alcohols, the reaction may take place even in the absence of ZnCl2 . • The function of ZnCl2 is make cleavage of –C–O– bond by co-ordination with O-atom. Bromoalkanes C2 H5 OH +KBr +H2 SO4 C2 H5 Br+ KHSO4 + H2 O Iodoalkanes CH3 CH2 OH KI/H2 SO4 CH3 CH2 I + KHSO4 + H2 O CH3 –CH2 OH + HI H3 PO4 CH3 –CH2 I + H2 O C2 H5 OH + KI + H3 PO4 C2 H5 I + KH2 PO4 + H2 O Here 57% HI can also be used in place of KI. Reaction of R–OH with P + I2 or Br2 6R–OH + 2P + 3I2 6R–I + 2H3 PO3 6C2 H5 OH + 2P + 3Br2 6C2 H5 Br + H3 PO3 3C2 H5 OH + PBr3 OR 3C2 H5 Br + H3 PO3 By finkelstein reaction It is specially used for iodo alkanes preparation using SN2 mechanism. CH3 –CH2 Br + NaI CH3 CH2 I + NaBr As NaCl, NaBr are insoluble in Acetone so they are removed by filteration. Fluoroalkane CH3 Cl + AgF CH3 F + AgCl 2CH3 Cl + Hg2 F2 2CH3 F + Hg2 Cl2 3CH3 Cl + AsF3 3CH3 F + AsCl3 This reaction is called Swart reaction. Borodine–Hunsdiecker reaction: It is used mainly for preparing bromoalkanes from silver salts of acids as follows: RCOOAg +X2 CCl4 R–X + CO2 + AgX It follows a free radical mechanism and reduces one C– atom number. The yield of R–X decreases as 1° > 2° > 3 °. • In case of iodine, an ester is formed and the reaction is called Birnbaum–Simonini reaction. 2RCOOAg + I2 R–COOR + 2CO2 + 2Ag2 I From ethers R–O–R + PCl5 2R–Cl + POCl3 R–O–R′ + PCl5 R–Cl + R′–Cl + POCl3 From alkenes: HX reacts with alkenes according to Mar- kovnikov’s or anti-Markovnikov’s rule to give haloalkanes as follows: • According to Markovnikov’s rule: Br | CH3 –CH2 CH2 +HBr CH3 –CH–CH3 Concn H2 SO4 must not be used in case of so , to , and neo alcohols as they undergo dehydration with it. (CH3 ) 3 C–CH2 OH + HBr H2 SO4 Facts to Remember
Haloalkanes and Haloarenes ■ 10.3 In case carbocation formed is less stable, one time rear- rangement through 1, 2 shift of either hydride or methyl is made for e.g., • According to anti-Markovnikov’s rule: CH3 –CH CH2 +HBr Benzoylperoxide CH3 –CH2 –CH2 Br From primary amines R–NH2 + NOCl R–Cl + N2 + H2 O Nitrosyl chloride Physical properties of monohalides • Lower members (CH3 X, C2 H5 X) are colourless gases while higher members are colourless liquid up to C18. C18 onwards, members are colourless solids. • Boiling point and density increase with the increase in molecular weight. C4 H9 Cl > C3 H7 Cl > C2 H5 Cl > CH3 Cl RI > RBr > RCl > RF ... Like in alkanes bronching also decrease B.P. • The trend followed for the dipole moment of various compounds is RCl > RF > RBr > RI CH3 Cl > CH3 F > CH3 Br > CH3 I m → 1.86D 1.847D 1.83D 1.636D • The stability of compounds vary in the following sequence as bond length increases and bond energy decreases. RF > RBr > RCl > RI Solubility: These are less soluble in water as less energy is released when new attractions are setup between the haloalkane and the mater molecules as these are not as strong as the original H-bonds in water. Their solubility in organic solvents is quite good due to almost same interactions. Solubilities α 1 Mol.wt. . CH3 X > C2 H5 X > C3 H7 X > C4 H9 X Chemical properties of monohalides +d–d • Since the C–X– is polar, these are reactive compounds. • The reactivity order is as follows: RI > RBr > RCl R3 CX > R2 CHX > RCH2 X CH3 X > C2 H5 HX > C3 H7 X > .... Nucleophilic substitution reactions: Alkyl halides un- dergo nucleophilic substitution reactions as follows: +δ –δ R–X + Z– R–Z + X– Strong nucleophile This reactions occurs by either SN1 or SN2 mechanism (For details see Goc II Reaction mechanism part). For example, • R– X Aq. KOH R– OH + HX • R–X AgOH R – OH + AgX • R –X Alc. KCN R – CN + KX R–CN is the major product as KCN being ionic, provides CN– ions. Therefore, the attack occurs from the carbon atom side. • R–X AgCN R–NC + AgX The major product is R–NC as AgCN being covalent cannot furnish CN– ions. Therefore, the attack occurs from nitrogen atom (lp e– side) to give R–NC. • R–X AgNO2 R–NO2 + AgX Nitroalkane • R–X KNO2 R–O–N=O + KX Alkyl nitrite • R–X NaN3 R–N3 + NaX Azides • R–X NASH R–SH + NaX Thiols • R–X R'COOAg R′ COOH + AgX • R–X R'ONa R′ –O–R + NaX The above reaction is called Williumson synthesis. It is the best method to prepare all kind of ethers. (R–X must be a po or so halide). • R–X R'SNa C2 H5 OH/H2 O R′ –S–R + NaX Thioether • R–X Na2 S R–S–R + 2NaX • R–X C6 H6 Anhyd. AlCl3 C6 H5 –R + HX The above reaction is called Friedel–Crafts reaction. R–X Moist Ag2 O R–OH