How haloalkanes are prepared from alcohols?
This is the common method for preparing haloalkanes in laboratory. Alcohols can be converted to haloalkanes by substitution of - OH group with a halogen atom. Different reagents can be used to get haloalkanes from alcohols as described below: Reaction by Halogen Acids: alcohols can be converted into haloalkanes by action with halogen acids. The reaction, in general, can be represented as Chlorolakanes are obtained by treating alcohols with HCl in the presence of anhydrous zinc chloride. The anhydrous ZnCl2 acts as dehydrating agent and thus favours the forward reaction (Le Chateliar principle). The mixture of HCl and anhydrous ZnCl2 is known as Lucas reagent. Bromoalkanes can also be obtained in an identical manner by heating alcohols with KBr or NaBr and conc. H2SO4.HBr is generated in situ (during the reaction) by reaction of KBr or NaBr with conc. H2SO4. KBr + H2SO4 KHSO4 + HBrC2H5OH + HBr C2H5Br + H2OThe reaction can also be carried out with conc. Solution (48%) of hydrobromic acid. Iodoalkanes are obtained by heating alcohols withKI and 95% H3PO4 (phosphoric acid). C2H5OH + HI C2H5I + H2OThe reaction can also be carried out with conc. solution (57%) of hydroiodic acid.Reactivity of halogen acids towards this reaction is: HI > HBr > HClIt is because of the fact that the bond dissociation energy of HI is less than that of HBr which in turn is less than that of HCl.Reactivity of alcohols towards this particular reaction is: tertiary > secondary > primaryIt is because of the fact that greater the number electron releasing groups on -carbon atom of alcohol more is the polarity of C-OH bond. Consequently greater is the ease with which it cleaves. Reaction by phosphorus halides: Chlorolakanes are obtained by reaction of alcohols with PCl3 or PCl5. Bromo and iodo alkanes are obtained by reaction of alcohols with a mixture of red phosphorus and Br2 or I2. The function of red phosphorus and Br2 or I2 is to get PBr3 and PI3 during the course of the reaction. PBr3 and PI3 being relatively unstable have to be prepared either in situ or just before use. They cannot be accumulated for long periods. For example, P4 + 6Br2 4PBr3 P4 + 6I2 4PI3 The alkyl halides are obtained in excellent yield (80%) in the reaction. The phosphorus halide method is better than halogen acid method because Certain alcohols such as, secondary and tertiary alcohols tend to dehydrate in the presence of halogen acids. Intermediate carbocation formed can undergo rearrangement in the presence of acids. Reaction by Thionyl Chloride: in this method the alcohol is refluxed with thionyl chloride in the presence of a small amount of pyridine. Refluxing is the process of heating a volatile liquid in a flask fitted with condenser so that the vapours are condensed back into the flask.The reaction of straight chain primary alcohols in the presence or absence of pyridine is called Darzen's procedure. Pyridine is used in the reaction to neutralize hydrogen chloride in the reaction.The above methods cannot be applied for conversion of phenols to aryl halides because carbon-oxygen bond in phenols has partial double bond character due to resonance and hence is stronger and difficult to break.
This is the common method for preparing haloalkanes in laboratory. Alcohols can be converted to haloalkanes by substitution of - OH group with a halogen atom.
Different reagents can be used to get haloalkanes from alcohols as described below: Reaction by Halogen Acids: alcohols can be converted into haloalkanes by action with halogen acids. The reaction, in general, can be represented as Chlorolakanes are obtained by treating alcohols with HCl in the presence of anhydrous zinc chloride. The anhydrous ZnCl2 acts as dehydrating agent and thus favours the forward reaction (Le Chateliar principle). The mixture of HCl and anhydrous ZnCl2 is known as Lucas reagent. Bromoalkanes can also be obtained in an identical manner by heating alcohols with KBr or NaBr and conc. H2SO4.HBr is generated in situ (during the reaction) by reaction of KBr or NaBr with conc. H2SO4. KBr + H2SO4 KHSO4 + HBrC2H5OH + HBr C2H5Br + H2OThe reaction can also be carried out with conc. Solution (48%) of hydrobromic acid. Iodoalkanes are obtained by heating alcohols withKI and 95% H3PO4 (phosphoric acid). C2H5OH + HI C2H5I + H2OThe reaction can also be carried out with conc. solution (57%) of hydroiodic acid.Reactivity of halogen acids towards this reaction is: HI > HBr > HClIt is because of the fact that the bond dissociation energy of HI is less than that of HBr which in turn is less than that of HCl.Reactivity of alcohols towards this particular reaction is: tertiary > secondary > primaryIt is because of the fact that greater the number electron releasing groups on -carbon atom of alcohol more is the polarity of C-OH bond. Consequently greater is the ease with which it cleaves. Reaction by phosphorus halides: Chlorolakanes are obtained by reaction of alcohols with PCl3 or PCl5. Bromo and iodo alkanes are obtained by reaction of alcohols with a mixture of red phosphorus and Br2 or I2. The function of red phosphorus and Br2 or I2 is to get PBr3 and PI3 during the course of the reaction. PBr3 and PI3 being relatively unstable have to be prepared either in situ or just before use. They cannot be accumulated for long periods. For example, P4 + 6Br2 4PBr3 P4 + 6I2 4PI3 The alkyl halides are obtained in excellent yield (80%) in the reaction. The phosphorus halide method is better than halogen acid method because Certain alcohols such as, secondary and tertiary alcohols tend to dehydrate in the presence of halogen acids. Intermediate carbocation formed can undergo rearrangement in the presence of acids. Reaction by Thionyl Chloride: in this method the alcohol is refluxed with thionyl chloride in the presence of a small amount of pyridine. Refluxing is the process of heating a volatile liquid in a flask fitted with condenser so that the vapours are condensed back into the flask.The reaction of straight chain primary alcohols in the presence or absence of pyridine is called Darzen's procedure. Pyridine is used in the reaction to neutralize hydrogen chloride in the reaction.The above methods cannot be applied for conversion of phenols to aryl halides because carbon-oxygen bond in phenols has partial double bond character due to resonance and hence is stronger and difficult to break.
Unit of measurement- These are also some systems for units: (1)
i) Show that the equilibrium constant Kp for the reaction CaCo3(s) ↔ CaO(s) +CO2(g)is about unity (i.e. = 1.0) at 895 °C.ii) If two grams of calcium carbonate are pl
The accuracy of your written English will be taken into account in marking. 1. (a) Identify the spectator ions in the following equation &nb
Provide solution of this question. According to Raoult's law the relative lowering of vapour pressure of a solution of volatile substance is equal to: (a) Mole fraction of the solvent (b) Mole fraction of the solute (c) Weight percentage of a solute (d) Weight perc
The electric charge acquired by macromolecules affects the equilibrium set up across a semipermeable membrane.Laboratory studies of macromolecule solutions as in osmotic pressure and dialysis studies confine the macromolecules to one compartment while allo
What do you mean by the term hydra? Briefly define it.
Alkyl halides can be prepared from alkanes through substitution and from alkenes through addition of halogen acids or through allylic substitution. From alkanesWhen alkanes are treated with halogens, chlo
The normality of 10 lit. volume hydrogen peroxide is: (a) 0.176 (b) 3.52 (c) 1.78 (d) 0.88 (e)17.8
25 ml of a solution of barium hydroxide on titration with 0.1 molar solution of the hydrochloric acid provide a litre value of 35 ml. The molarity of barium hydroxide solution will be: (i) 0.07 (ii) 0.14 (iii) 0.28 (iv) 0.35
Describe how dipole attractions, London dispersion forces and the hydrogen bonding identical?
18,76,764
1927005 Asked
3,689
Active Tutors
1411847
Questions Answered
Start Excelling in your courses, Ask an Expert and get answers for your homework and assignments!!