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.
Can someone please help me in getting through this problem. 1.0 gm of pure calcium carbonate was found to require 50 ml of dilute HCL for complete reaction. The strength of the HCL solution is given by: (a) 4 N (b) 2 N (c) 0.4 N (d) 0.2 N
Select the right answer of the question. Molecular weight of urea is 60. A solution of urea containing 6g urea in one litre is : (a)1 molar (b)1.5 molar (c) 0.1 molar (d) 0.01 molar
Write a short note on the formula of diesel, petrol and also CNG?
Give me answer of this question. Provide solution of this question. Which one of the following is the expression of Raoult's law: (a) P-P1/P = n/n+N (b) P1-P/P = N/ N+n (c)P-P2/P1= N/ N-n (d) P1-P/P2= N-n/N
Aryl halides are much less reactive towards nucleophilic substitution reactions than haloalkanes. The less reactivity of aryl halides can be described
The volume of water to be added to 100cm3 of 0.5 N N H2SO4 to get decinormal concentration is : (a) 400 cm3 (b) 500cm3 (c) 450cm3 (d)100cm3
arrange in order of basicity,pyridine,pipyridineand pyorine
The weight of pure NaOH needed to made 250cm3 of 0.1 N solution is: (a) 4g (b) 1g (c) 2g (d) 10g Choose the right answer from above.
Benzene and toluene form nearly ideal solutions. At 20°C, the vapour pressure of benzene is 75 torr and that of toluene is 22 torr. The parial vapour pressure of benzene at 20°C for a solution containing 78g of benzene and 46g of toluene in torr is: (a) 50 (b)
The ionic radii of Rb+ and I- respectively are 1.46 Å and 2.16Å. The very most probable type of structure exhibited by it is: (a) CsCl type (b) ZnS type (c) Nacl type (d) CaF2 type Discover Q & A Leading Solution Library Avail More Than 1442759 Solved problems, classrooms assignments, textbook's solutions, for quick Downloads No hassle, Instant Access Start Discovering 18,76,764 1949988 Asked 3,689 Active Tutors 1442759 Questions Answered Start Excelling in your courses, Ask an Expert and get answers for your homework and assignments!! Submit Assignment
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