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.
Relative humidity is the ratio of the partial pressure of water in air to the partial pressure of water in air saturated with water at the same temperature, stated as a percentage: Relative = Q : Units of Measurement Unit of Unit of measurement- These are also some systems for units: (1) C.G.S.
Unit of measurement- These are also some systems for units: (1) C.G.S.
The infrared spectrum of gas samples shows the effect of rotational-energy changes along with the vibrational energy change.As we know from the interpretations given to thermodynamic properties of gases, gas molecules are simultaneously rotating and vibrating. It follows that an absor
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
give all physical aspects in the production of alcohol
Describe the number of electrons in the benzene? Write a short note on it?
The quantum mechanical methods, illustrated previously by the Schrödinger equation, are extended by the use of operators. Or, w
Two tanks which contain water are connected to each other through a valve. The initial conditions are as shown (at equilibrium):
Describe all the kinds of insulators which are present?
we need 10 examples for the polyhalogen compounds....please help me....need it urgently...
18,76,764
1927423 Asked
3,689
Active Tutors
1457044
Questions Answered
Start Excelling in your courses, Ask an Expert and get answers for your homework and assignments!!