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.
Laws of Chemical Combination- In order to understand the composition of the compounds, it is necessary to have a theory which accounts for both qualitative and quantitative observations during chem
Choose the right answer from following.The relative lowering of the vapour pressure is equal to the ratio between the number of: (a) Solute moleules and solvent molecules (b) Solute molecules and the total molecules in the solution (c) Solvent molecules and the tota
Select the right answer of the question. Equimolar solutions in the same solvent have : (a)Same boiling point but different freezing point (b) Same freezing point but different boiling poin (c)Same boiling and same freezing points (d) Different boiling and differe
Superphosphate has the formula CaH4(PO4)2 H2O, what is the calculation to get the percentage of Phosphorus, I need to show the calculation. I know it is 30.9737622 u in weight and 2 atoms of the formula, but not sure how to work the calculation backwards.
the death cap mushroom based on your knowledge of the biochemistry of dna and rna
Illustrate the Lewis Dot Structure for the CH4O.
what is the molar conductance of chloropentaamminecobalt(III) chloride?
Give me answer of this question. A solution contains 25%H2O 25%C2H5OH , and 50% CH3 COOH by mass. The mole fraction of H2O would be: (a) 0.25 (b) 2.5 (c) 0.503 (d) 5.03.
For motion in one dimension, the distribution of the molecules over quantum states, speeds, and energies can be deduced.Here we show that the energy of a macroscopic gas sample can be described on the basis of our knowledge of the quantum states allowed to
Give me answer of this question. 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
18,76,764
1932216 Asked
3,689
Active Tutors
1458353
Questions Answered
Start Excelling in your courses, Ask an Expert and get answers for your homework and assignments!!