How do polar solvents help in the first step in `S_(N)1` mechanism?
How do polar solvents help in the first step in `S_(N)1` mechanism?
Text Solution
Verified by Experts
In the `S_(N^(1))` mechanism, a carbocation intermediate is formed in the first step which is a slow step. Polar solvents such as aqueous KOH help in stabilising the carbocation intermediate by solvation. Thus, they favour `S_(N^(1))` mechanism.
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The following will react faster by S_N1 mechanism
Read the passage given below and answer the following questions: Nucleophilic substitution reaction of haloalkane can be conducted according to both S_(N)^(1) and S_(N)^(2) mechanisms. However, which mechanism it is based on is related to such factors as the structure of haloalkane, and properties of leaving group, nucleophilic reagent and solvent. Influences of halogen : No matter which mechanism the nucleophilic substitution reaction is based on, the leaving group always leave the central carbon atom with electron pair. This is just the opposite of the situation that nucleophilic reagent attacks the central carbon atom with electron pair. Therefore, the weaker the alkalinity of leaving group is , the more stable the anion formed is and it will be more easier for the leaving group to leave the central carbon atom, that is to say, the reactant is more easier to be substituted. The alkalinity order of halogen ion is I^(-) lt Br^(-) lt Cl^(-) lt F^(-) and the order of their leaving tendency should be I^(-) gt Br^(-) gt Cl^(-) gt F^(-) . Therefore, in four halides with the same alkyl and different halogens, the order of substitution reaction rate is RI gt RBr gt RCl gt RF . In addition, if the leaving group is very easy to leave, many carbocation intermediates are generated in the reaction and the reaction is based on S_(N)^(1) mechanism. If the leaving group is not easy to leave, the reaction is based on S_(N)^(2) mechanism. Influences of solvent polarity: In S_(N)^(1) reaction, the polarity of the system increases from the reactant to the transition state, because polar solvent has a greater stabilizing effect on the transition state than the reactant, thereby reduce activation energy and accelerate the reaction. In S_(N)^(2) reaction, the polarity of the system generally does not change from the reactant to the transition state and only charge dispersion occurs. At this time, polar solvent has a great stabilizing effect on Nu than the transition state, thereby increasing activation energy and slow down the reaction rate. For example, the decomposition rate (S_(N)^(1)) of tertiary chlorobutane in 25^(@)C water (dielectric constant 79) is 300000 times faster than in ethanol (dielectric constant 24). The reaction rate (S_(N)^(2)) of 2-bromopropane and NaOH in ethanol containing 40% water is twice slower than in absolute ethanol. In a word, the level of solvent polarity has influence on both S_(N)^(1) and S_(N)^(2) reactions, but with different results. Generally speaking, weak polar solvent is favorable for S_(N)^(2) reaction, while strong polar solvent is favorable for S_N^(1) reaction, because only under the action of polar solvent can halogenated hydrocarbon dissociate into carbocation and halogen ion and solvents with a strong polarity is favorable for solvation of carbocation, increasing its stability. Generally speaking, the substitution reaction of tertiary haloalkane is based on S_(N)^(1) mechanism in solvents with a strong polarity (for example, ethanol containing water). (Ding, Y. (2013). A Brief Discussion on Nucleophilic Substitution Reaction on Saturated Carbon Atom. In Applied Mechanics and Materials (Vol. 312, pp. 433-437). Trans Tech Publications Ltd.) S_(N)^(1) mechanism is favoured in which of the following solvents:
Read the passage given below and answer the following questions: Nucleophilic substitution reaction of haloalkane can be conducted according to both S_(N)^(1) and S_(N)^(2) mechanisms. However, which mechanism it is based on is related to such factors as the structure of haloalkane, and properties of leaving group, nucleophilic reagent and solvent. Influences of halogen : No matter which mechanism the nucleophilic substitution reaction is based on, the leaving group always leave the central carbon atom with electron pair. This is just the opposite of the situation that nucleophilic reagent attacks the central carbon atom with electron pair. Therefore, the weaker the alkalinity of leaving group is , the more stable the anion formed is and it will be more easier for the leaving group to leave the central carbon atom, that is to say, the reactant is more easier to be substituted. The alkalinity order of halogen ion is I^(-) lt Br^(-) lt Cl^(-) lt F^(-) and the order of their leaving tendency should be I^(-) gt Br^(-) gt Cl^(-) gt F^(-) . Therefore, in four halides with the same alkyl and different halogens, the order of substitution reaction rate is RI gt RBr gt RCl gt RF . In addition, if the leaving group is very easy to leave, many carbocation intermediates are generated in the reaction and the reaction is based on S_(N)^(1) mechanism. If the leaving group is not easy to leave, the reaction is based on S_(N)^(2) mechanism. Influences of solvent polarity: In S_(N)^(1) reaction, the polarity of the system increases from the reactant to the transition state, because polar solvent has a greater stabilizing effect on the transition state than the reactant, thereby reduce activation energy and accelerate the reaction. In S_(N)^(2) reaction, the polarity of the system generally does not change from the reactant to the transition state and only charge dispersion occurs. At this time, polar solvent has a great stabilizing effect on Nu than the transition state, thereby increasing activation energy and slow down the reaction rate. For example, the decomposition rate (S_(N)^(1)) of tertiary chlorobutane in 25^(@)C water (dielectric constant 79) is 300000 times faster than in ethanol (dielectric constant 24). The reaction rate (S_(N)^(2)) of 2-bromopropane and NaOH in ethanol containing 40% water is twice slower than in absolute ethanol. In a word, the level of solvent polarity has influence on both S_(N)^(1) and S_(N)^(2) reactions, but with different results. Generally speaking, weak polar solvent is favorable for S_(N)^(2) reaction, while strong polar solvent is favorable for S_N^(1) reaction, because only under the action of polar solvent can halogenated hydrocarbon dissociate into carbocation and halogen ion and solvents with a strong polarity is favorable for solvation of carbocation, increasing its stability. Generally speaking, the substitution reaction of tertiary haloalkane is based on S_(N)^(1) mechanism in solvents with a strong polarity (for example, ethanol containing water). (Ding, Y. (2013). A Brief Discussion on Nucleophilic Substitution Reaction on Saturated Carbon Atom. In Applied Mechanics and Materials (Vol. 312, pp. 433-437). Trans Tech Publications Ltd.) Nucleophilic substitution will be fastest in case of:
Read the passage given below and answer the following questions: Nucleophilic substitution reaction of haloalkane can be conducted according to both S_(N)^(1) and S_(N)^(2) mechanisms. However, which mechanism it is based on is related to such factors as the structure of haloalkane, and properties of leaving group, nucleophilic reagent and solvent. Influences of halogen : No matter which mechanism the nucleophilic substitution reaction is based on, the leaving group always leave the central carbon atom with electron pair. This is just the opposite of the situation that nucleophilic reagent attacks the central carbon atom with electron pair. Therefore, the weaker the alkalinity of leaving group is , the more stable the anion formed is and it will be more easier for the leaving group to leave the central carbon atom, that is to say, the reactant is more easier to be substituted. The alkalinity order of halogen ion is I^(-) lt Br^(-) lt Cl^(-) lt F^(-) and the order of their leaving tendency should be I^(-) gt Br^(-) gt Cl^(-) gt F^(-) . Therefore, in four halides with the same alkyl and different halogens, the order of substitution reaction rate is RI gt RBr gt RCl gt RF . In addition, if the leaving group is very easy to leave, many carbocation intermediates are generated in the reaction and the reaction is based on S_(N)^(1) mechanism. If the leaving group is not easy to leave, the reaction is based on S_(N)^(2) mechanism. Influences of solvent polarity: In S_(N)^(1) reaction, the polarity of the system increases from the reactant to the transition state, because polar solvent has a greater stabilizing effect on the transition state than the reactant, thereby reduce activation energy and accelerate the reaction. In S_(N)^(2) reaction, the polarity of the system generally does not change from the reactant to the transition state and only charge dispersion occurs. At this time, polar solvent has a great stabilizing effect on Nu than the transition state, thereby increasing activation energy and slow down the reaction rate. For example, the decomposition rate (S_(N)^(1)) of tertiary chlorobutane in 25^(@)C water (dielectric constant 79) is 300000 times faster than in ethanol (dielectric constant 24). The reaction rate (S_(N)^(2)) of 2-bromopropane and NaOH in ethanol containing 40% water is twice slower than in absolute ethanol. In a word, the level of solvent polarity has influence on both S_(N)^(1) and S_(N)^(2) reactions, but with different results. Generally speaking, weak polar solvent is favorable for S_(N)^(2) reaction, while strong polar solvent is favorable for S_N^(1) reaction, because only under the action of polar solvent can halogenated hydrocarbon dissociate into carbocation and halogen ion and solvents with a strong polarity is favorable for solvation of carbocation, increasing its stability. Generally speaking, the substitution reaction of tertiary haloalkane is based on S_(N)^(1) mechanism in solvents with a strong polarity (for example, ethanol containing water). (Ding, Y. (2013). A Brief Discussion on Nucleophilic Substitution Reaction on Saturated Carbon Atom. In Applied Mechanics and Materials (Vol. 312, pp. 433-437). Trans Tech Publications Ltd.) S_(N)^(1) reaction will be fastest in which of the following solvents?
Read the passage given below and answer the following questions: Nucleophilic substitution reaction of haloalkane can be conducted according to both S_(N)^(1) and S_(N)^(2) mechanisms. However, which mechanism it is based on is related to such factors as the structure of haloalkane, and properties of leaving group, nucleophilic reagent and solvent. Influences of halogen : No matter which mechanism the nucleophilic substitution reaction is based on, the leaving group always leave the central carbon atom with electron pair. This is just the opposite of the situation that nucleophilic reagent attacks the central carbon atom with electron pair. Therefore, the weaker the alkalinity of leaving group is , the more stable the anion formed is and it will be more easier for the leaving group to leave the central carbon atom, that is to say, the reactant is more easier to be substituted. The alkalinity order of halogen ion is I^(-) lt Br^(-) lt Cl^(-) lt F^(-) and the order of their leaving tendency should be I^(-) gt Br^(-) gt Cl^(-) gt F^(-) . Therefore, in four halides with the same alkyl and different halogens, the order of substitution reaction rate is RI gt RBr gt RCl gt RF . In addition, if the leaving group is very easy to leave, many carbocation intermediates are generated in the reaction and the reaction is based on S_(N)^(1) mechanism. If the leaving group is not easy to leave, the reaction is based on S_(N)^(2) mechanism. Influences of solvent polarity: In S_(N)^(1) reaction, the polarity of the system increases from the reactant to the transition state, because polar solvent has a greater stabilizing effect on the transition state than the reactant, thereby reduce activation energy and accelerate the reaction. In S_(N)^(2) reaction, the polarity of the system generally does not change from the reactant to the transition state and only charge dispersion occurs. At this time, polar solvent has a great stabilizing effect on Nu than the transition state, thereby increasing activation energy and slow down the reaction rate. For example, the decomposition rate (S_(N)^(1)) of tertiary chlorobutane in 25^(@)C water (dielectric constant 79) is 300000 times faster than in ethanol (dielectric constant 24). The reaction rate (S_(N)^(2)) of 2-bromopropane and NaOH in ethanol containing 40% water is twice slower than in absolute ethanol. In a word, the level of solvent polarity has influence on both S_(N)^(1) and S_(N)^(2) reactions, but with different results. Generally speaking, weak polar solvent is favorable for S_(N)^(2) reaction, while strong polar solvent is favorable for S_N^(1) reaction, because only under the action of polar solvent can halogenated hydrocarbon dissociate into carbocation and halogen ion and solvents with a strong polarity is favorable for solvation of carbocation, increasing its stability. Generally speaking, the substitution reaction of tertiary haloalkane is based on S_(N)^(1) mechanism in solvents with a strong polarity (for example, ethanol containing water). (Ding, Y. (2013). A Brief Discussion on Nucleophilic Substitution Reaction on Saturated Carbon Atom. In Applied Mechanics and Materials (Vol. 312, pp. 433-437). Trans Tech Publications Ltd.) Polar solvents make the reaction faster as they:
Read the passage given below and answer the following questions: Nucleophilic substitution reaction of haloalkane can be conducted according to both S_(N)^(1) and S_(N)^(2) mechanisms. However, which mechanism it is based on is related to such factors as the structure of haloalkane, and properties of leaving group, nucleophilic reagent and solvent. Influences of halogen : No matter which mechanism the nucleophilic substitution reaction is based on, the leaving group always leave the central carbon atom with electron pair. This is just the opposite of the situation that nucleophilic reagent attacks the central carbon atom with electron pair. Therefore, the weaker the alkalinity of leaving group is , the more stable the anion formed is and it will be more easier for the leaving group to leave the central carbon atom, that is to say, the reactant is more easier to be substituted. The alkalinity order of halogen ion is I^(-) lt Br^(-) lt Cl^(-) lt F^(-) and the order of their leaving tendency should be I^(-) gt Br^(-) gt Cl^(-) gt F^(-) . Therefore, in four halides with the same alkyl and different halogens, the order of substitution reaction rate is RI gt RBr gt RCl gt RF . In addition, if the leaving group is very easy to leave, many carbocation intermediates are generated in the reaction and the reaction is based on S_(N)^(1) mechanism. If the leaving group is not easy to leave, the reaction is based on S_(N)^(2) mechanism. Influences of solvent polarity: In S_(N)^(1) reaction, the polarity of the system increases from the reactant to the transition state, because polar solvent has a greater stabilizing effect on the transition state than the reactant, thereby reduce activation energy and accelerate the reaction. In S_(N)^(2) reaction, the polarity of the system generally does not change from the reactant to the transition state and only charge dispersion occurs. At this time, polar solvent has a great stabilizing effect on Nu than the transition state, thereby increasing activation energy and slow down the reaction rate. For example, the decomposition rate (S_(N)^(1)) of tertiary chlorobutane in 25^(@)C water (dielectric constant 79) is 300000 times faster than in ethanol (dielectric constant 24). The reaction rate (S_(N)^(2)) of 2-bromopropane and NaOH in ethanol containing 40% water is twice slower than in absolute ethanol. In a word, the level of solvent polarity has influence on both S_(N)^(1) and S_(N)^(2) reactions, but with different results. Generally speaking, weak polar solvent is favorable for S_(N)^(2) reaction, while strong polar solvent is favorable for S_N^(1) reaction, because only under the action of polar solvent can halogenated hydrocarbon dissociate into carbocation and halogen ion and solvents with a strong polarity is favorable for solvation of carbocation, increasing its stability. Generally speaking, the substitution reaction of tertiary haloalkane is based on S_(N)^(1) mechanism in solvents with a strong polarity (for example, ethanol containing water). (Ding, Y. (2013). A Brief Discussion on Nucleophilic Substitution Reaction on Saturated Carbon Atom. In Applied Mechanics and Materials (Vol. 312, pp. 433-437). Trans Tech Publications Ltd.) S_(N)^(1) reaction will be fastest in case of:
Which of the following compound is most rapidly hydrolysed by S_(N)1 mechanism?
Which one hydrolysis at a faster rate by s_(N^(1)) mechanism?
Knowledge Check
Read the passage given below and answer the following questions: Nucleophilic substitution reaction of haloalkane can be conducted according to both S_(N)^(1) and S_(N)^(2) mechanisms. However, which mechanism it is based on is related to such factors as the structure of haloalkane, and properties of leaving group, nucleophilic reagent and solvent. Influences of halogen : No matter which mechanism the nucleophilic substitution reaction is based on, the leaving group always leave the central carbon atom with electron pair. This is just the opposite of the situation that nucleophilic reagent attacks the central carbon atom with electron pair. Therefore, the weaker the alkalinity of leaving group is , the more stable the anion formed is and it will be more easier for the leaving group to leave the central carbon atom, that is to say, the reactant is more easier to be substituted. The alkalinity order of halogen ion is I^(-) lt Br^(-) lt Cl^(-) lt F^(-) and the order of their leaving tendency should be I^(-) gt Br^(-) gt Cl^(-) gt F^(-) . Therefore, in four halides with the same alkyl and different halogens, the order of substitution reaction rate is RI gt RBr gt RCl gt RF . In addition, if the leaving group is very easy to leave, many carbocation intermediates are generated in the reaction and the reaction is based on S_(N)^(1) mechanism. If the leaving group is not easy to leave, the reaction is based on S_(N)^(2) mechanism. Influences of solvent polarity: In S_(N)^(1) reaction, the polarity of the system increases from the reactant to the transition state, because polar solvent has a greater stabilizing effect on the transition state than the reactant, thereby reduce activation energy and accelerate the reaction. In S_(N)^(2) reaction, the polarity of the system generally does not change from the reactant to the transition state and only charge dispersion occurs. At this time, polar solvent has a great stabilizing effect on Nu than the transition state, thereby increasing activation energy and slow down the reaction rate. For example, the decomposition rate (S_(N)^(1)) of tertiary chlorobutane in 25^(@)C water (dielectric constant 79) is 300000 times faster than in ethanol (dielectric constant 24). The reaction rate (S_(N)^(2)) of 2-bromopropane and NaOH in ethanol containing 40% water is twice slower than in absolute ethanol. In a word, the level of solvent polarity has influence on both S_(N)^(1) and S_(N)^(2) reactions, but with different results. Generally speaking, weak polar solvent is favorable for S_(N)^(2) reaction, while strong polar solvent is favorable for S_N^(1) reaction, because only under the action of polar solvent can halogenated hydrocarbon dissociate into carbocation and halogen ion and solvents with a strong polarity is favorable for solvation of carbocation, increasing its stability. Generally speaking, the substitution reaction of tertiary haloalkane is based on S_(N)^(1) mechanism in solvents with a strong polarity (for example, ethanol containing water). (Ding, Y. (2013). A Brief Discussion on Nucleophilic Substitution Reaction on Saturated Carbon Atom. In Applied Mechanics and Materials (Vol. 312, pp. 433-437). Trans Tech Publications Ltd.) S_(N)^(1) mechanism is favoured in which of the following solvents:
Read the passage given below and answer the following questions: Nucleophilic substitution reaction of haloalkane can be conducted according to both S_(N)^(1) and S_(N)^(2) mechanisms. However, which mechanism it is based on is related to such factors as the structure of haloalkane, and properties of leaving group, nucleophilic reagent and solvent. Influences of halogen : No matter which mechanism the nucleophilic substitution reaction is based on, the leaving group always leave the central carbon atom with electron pair. This is just the opposite of the situation that nucleophilic reagent attacks the central carbon atom with electron pair. Therefore, the weaker the alkalinity of leaving group is , the more stable the anion formed is and it will be more easier for the leaving group to leave the central carbon atom, that is to say, the reactant is more easier to be substituted. The alkalinity order of halogen ion is I^(-) lt Br^(-) lt Cl^(-) lt F^(-) and the order of their leaving tendency should be I^(-) gt Br^(-) gt Cl^(-) gt F^(-) . Therefore, in four halides with the same alkyl and different halogens, the order of substitution reaction rate is RI gt RBr gt RCl gt RF . In addition, if the leaving group is very easy to leave, many carbocation intermediates are generated in the reaction and the reaction is based on S_(N)^(1) mechanism. If the leaving group is not easy to leave, the reaction is based on S_(N)^(2) mechanism. Influences of solvent polarity: In S_(N)^(1) reaction, the polarity of the system increases from the reactant to the transition state, because polar solvent has a greater stabilizing effect on the transition state than the reactant, thereby reduce activation energy and accelerate the reaction. In S_(N)^(2) reaction, the polarity of the system generally does not change from the reactant to the transition state and only charge dispersion occurs. At this time, polar solvent has a great stabilizing effect on Nu than the transition state, thereby increasing activation energy and slow down the reaction rate. For example, the decomposition rate (S_(N)^(1)) of tertiary chlorobutane in 25^(@)C water (dielectric constant 79) is 300000 times faster than in ethanol (dielectric constant 24). The reaction rate (S_(N)^(2)) of 2-bromopropane and NaOH in ethanol containing 40% water is twice slower than in absolute ethanol. In a word, the level of solvent polarity has influence on both S_(N)^(1) and S_(N)^(2) reactions, but with different results. Generally speaking, weak polar solvent is favorable for S_(N)^(2) reaction, while strong polar solvent is favorable for S_N^(1) reaction, because only under the action of polar solvent can halogenated hydrocarbon dissociate into carbocation and halogen ion and solvents with a strong polarity is favorable for solvation of carbocation, increasing its stability. Generally speaking, the substitution reaction of tertiary haloalkane is based on S_(N)^(1) mechanism in solvents with a strong polarity (for example, ethanol containing water). (Ding, Y. (2013). A Brief Discussion on Nucleophilic Substitution Reaction on Saturated Carbon Atom. In Applied Mechanics and Materials (Vol. 312, pp. 433-437). Trans Tech Publications Ltd.) S_(N)^(1) mechanism is favoured in which of the following solvents:
A
benzene
B
carbon tetrachloride
C
acetic acid
D
carbon disulphide
Read the passage given below and answer the following questions: Nucleophilic substitution reaction of haloalkane can be conducted according to both S_(N)^(1) and S_(N)^(2) mechanisms. However, which mechanism it is based on is related to such factors as the structure of haloalkane, and properties of leaving group, nucleophilic reagent and solvent. Influences of halogen : No matter which mechanism the nucleophilic substitution reaction is based on, the leaving group always leave the central carbon atom with electron pair. This is just the opposite of the situation that nucleophilic reagent attacks the central carbon atom with electron pair. Therefore, the weaker the alkalinity of leaving group is , the more stable the anion formed is and it will be more easier for the leaving group to leave the central carbon atom, that is to say, the reactant is more easier to be substituted. The alkalinity order of halogen ion is I^(-) lt Br^(-) lt Cl^(-) lt F^(-) and the order of their leaving tendency should be I^(-) gt Br^(-) gt Cl^(-) gt F^(-) . Therefore, in four halides with the same alkyl and different halogens, the order of substitution reaction rate is RI gt RBr gt RCl gt RF . In addition, if the leaving group is very easy to leave, many carbocation intermediates are generated in the reaction and the reaction is based on S_(N)^(1) mechanism. If the leaving group is not easy to leave, the reaction is based on S_(N)^(2) mechanism. Influences of solvent polarity: In S_(N)^(1) reaction, the polarity of the system increases from the reactant to the transition state, because polar solvent has a greater stabilizing effect on the transition state than the reactant, thereby reduce activation energy and accelerate the reaction. In S_(N)^(2) reaction, the polarity of the system generally does not change from the reactant to the transition state and only charge dispersion occurs. At this time, polar solvent has a great stabilizing effect on Nu than the transition state, thereby increasing activation energy and slow down the reaction rate. For example, the decomposition rate (S_(N)^(1)) of tertiary chlorobutane in 25^(@)C water (dielectric constant 79) is 300000 times faster than in ethanol (dielectric constant 24). The reaction rate (S_(N)^(2)) of 2-bromopropane and NaOH in ethanol containing 40% water is twice slower than in absolute ethanol. In a word, the level of solvent polarity has influence on both S_(N)^(1) and S_(N)^(2) reactions, but with different results. Generally speaking, weak polar solvent is favorable for S_(N)^(2) reaction, while strong polar solvent is favorable for S_N^(1) reaction, because only under the action of polar solvent can halogenated hydrocarbon dissociate into carbocation and halogen ion and solvents with a strong polarity is favorable for solvation of carbocation, increasing its stability. Generally speaking, the substitution reaction of tertiary haloalkane is based on S_(N)^(1) mechanism in solvents with a strong polarity (for example, ethanol containing water). (Ding, Y. (2013). A Brief Discussion on Nucleophilic Substitution Reaction on Saturated Carbon Atom. In Applied Mechanics and Materials (Vol. 312, pp. 433-437). Trans Tech Publications Ltd.) Nucleophilic substitution will be fastest in case of:
Read the passage given below and answer the following questions: Nucleophilic substitution reaction of haloalkane can be conducted according to both S_(N)^(1) and S_(N)^(2) mechanisms. However, which mechanism it is based on is related to such factors as the structure of haloalkane, and properties of leaving group, nucleophilic reagent and solvent. Influences of halogen : No matter which mechanism the nucleophilic substitution reaction is based on, the leaving group always leave the central carbon atom with electron pair. This is just the opposite of the situation that nucleophilic reagent attacks the central carbon atom with electron pair. Therefore, the weaker the alkalinity of leaving group is , the more stable the anion formed is and it will be more easier for the leaving group to leave the central carbon atom, that is to say, the reactant is more easier to be substituted. The alkalinity order of halogen ion is I^(-) lt Br^(-) lt Cl^(-) lt F^(-) and the order of their leaving tendency should be I^(-) gt Br^(-) gt Cl^(-) gt F^(-) . Therefore, in four halides with the same alkyl and different halogens, the order of substitution reaction rate is RI gt RBr gt RCl gt RF . In addition, if the leaving group is very easy to leave, many carbocation intermediates are generated in the reaction and the reaction is based on S_(N)^(1) mechanism. If the leaving group is not easy to leave, the reaction is based on S_(N)^(2) mechanism. Influences of solvent polarity: In S_(N)^(1) reaction, the polarity of the system increases from the reactant to the transition state, because polar solvent has a greater stabilizing effect on the transition state than the reactant, thereby reduce activation energy and accelerate the reaction. In S_(N)^(2) reaction, the polarity of the system generally does not change from the reactant to the transition state and only charge dispersion occurs. At this time, polar solvent has a great stabilizing effect on Nu than the transition state, thereby increasing activation energy and slow down the reaction rate. For example, the decomposition rate (S_(N)^(1)) of tertiary chlorobutane in 25^(@)C water (dielectric constant 79) is 300000 times faster than in ethanol (dielectric constant 24). The reaction rate (S_(N)^(2)) of 2-bromopropane and NaOH in ethanol containing 40% water is twice slower than in absolute ethanol. In a word, the level of solvent polarity has influence on both S_(N)^(1) and S_(N)^(2) reactions, but with different results. Generally speaking, weak polar solvent is favorable for S_(N)^(2) reaction, while strong polar solvent is favorable for S_N^(1) reaction, because only under the action of polar solvent can halogenated hydrocarbon dissociate into carbocation and halogen ion and solvents with a strong polarity is favorable for solvation of carbocation, increasing its stability. Generally speaking, the substitution reaction of tertiary haloalkane is based on S_(N)^(1) mechanism in solvents with a strong polarity (for example, ethanol containing water). (Ding, Y. (2013). A Brief Discussion on Nucleophilic Substitution Reaction on Saturated Carbon Atom. In Applied Mechanics and Materials (Vol. 312, pp. 433-437). Trans Tech Publications Ltd.) Nucleophilic substitution will be fastest in case of:
A
1-Chloro-2,2-dimethyl propane
B
1-Iodo-2,2-dimethyl propane
C
1-Bromo-2,2-dimethyl propane
D
1-Fluoro-2,2-dimethyl propane
Read the passage given below and answer the following questions: Nucleophilic substitution reaction of haloalkane can be conducted according to both S_(N)^(1) and S_(N)^(2) mechanisms. However, which mechanism it is based on is related to such factors as the structure of haloalkane, and properties of leaving group, nucleophilic reagent and solvent. Influences of halogen : No matter which mechanism the nucleophilic substitution reaction is based on, the leaving group always leave the central carbon atom with electron pair. This is just the opposite of the situation that nucleophilic reagent attacks the central carbon atom with electron pair. Therefore, the weaker the alkalinity of leaving group is , the more stable the anion formed is and it will be more easier for the leaving group to leave the central carbon atom, that is to say, the reactant is more easier to be substituted. The alkalinity order of halogen ion is I^(-) lt Br^(-) lt Cl^(-) lt F^(-) and the order of their leaving tendency should be I^(-) gt Br^(-) gt Cl^(-) gt F^(-) . Therefore, in four halides with the same alkyl and different halogens, the order of substitution reaction rate is RI gt RBr gt RCl gt RF . In addition, if the leaving group is very easy to leave, many carbocation intermediates are generated in the reaction and the reaction is based on S_(N)^(1) mechanism. If the leaving group is not easy to leave, the reaction is based on S_(N)^(2) mechanism. Influences of solvent polarity: In S_(N)^(1) reaction, the polarity of the system increases from the reactant to the transition state, because polar solvent has a greater stabilizing effect on the transition state than the reactant, thereby reduce activation energy and accelerate the reaction. In S_(N)^(2) reaction, the polarity of the system generally does not change from the reactant to the transition state and only charge dispersion occurs. At this time, polar solvent has a great stabilizing effect on Nu than the transition state, thereby increasing activation energy and slow down the reaction rate. For example, the decomposition rate (S_(N)^(1)) of tertiary chlorobutane in 25^(@)C water (dielectric constant 79) is 300000 times faster than in ethanol (dielectric constant 24). The reaction rate (S_(N)^(2)) of 2-bromopropane and NaOH in ethanol containing 40% water is twice slower than in absolute ethanol. In a word, the level of solvent polarity has influence on both S_(N)^(1) and S_(N)^(2) reactions, but with different results. Generally speaking, weak polar solvent is favorable for S_(N)^(2) reaction, while strong polar solvent is favorable for S_N^(1) reaction, because only under the action of polar solvent can halogenated hydrocarbon dissociate into carbocation and halogen ion and solvents with a strong polarity is favorable for solvation of carbocation, increasing its stability. Generally speaking, the substitution reaction of tertiary haloalkane is based on S_(N)^(1) mechanism in solvents with a strong polarity (for example, ethanol containing water). (Ding, Y. (2013). A Brief Discussion on Nucleophilic Substitution Reaction on Saturated Carbon Atom. In Applied Mechanics and Materials (Vol. 312, pp. 433-437). Trans Tech Publications Ltd.) S_(N)^(1) reaction will be fastest in which of the following solvents?
Read the passage given below and answer the following questions: Nucleophilic substitution reaction of haloalkane can be conducted according to both S_(N)^(1) and S_(N)^(2) mechanisms. However, which mechanism it is based on is related to such factors as the structure of haloalkane, and properties of leaving group, nucleophilic reagent and solvent. Influences of halogen : No matter which mechanism the nucleophilic substitution reaction is based on, the leaving group always leave the central carbon atom with electron pair. This is just the opposite of the situation that nucleophilic reagent attacks the central carbon atom with electron pair. Therefore, the weaker the alkalinity of leaving group is , the more stable the anion formed is and it will be more easier for the leaving group to leave the central carbon atom, that is to say, the reactant is more easier to be substituted. The alkalinity order of halogen ion is I^(-) lt Br^(-) lt Cl^(-) lt F^(-) and the order of their leaving tendency should be I^(-) gt Br^(-) gt Cl^(-) gt F^(-) . Therefore, in four halides with the same alkyl and different halogens, the order of substitution reaction rate is RI gt RBr gt RCl gt RF . In addition, if the leaving group is very easy to leave, many carbocation intermediates are generated in the reaction and the reaction is based on S_(N)^(1) mechanism. If the leaving group is not easy to leave, the reaction is based on S_(N)^(2) mechanism. Influences of solvent polarity: In S_(N)^(1) reaction, the polarity of the system increases from the reactant to the transition state, because polar solvent has a greater stabilizing effect on the transition state than the reactant, thereby reduce activation energy and accelerate the reaction. In S_(N)^(2) reaction, the polarity of the system generally does not change from the reactant to the transition state and only charge dispersion occurs. At this time, polar solvent has a great stabilizing effect on Nu than the transition state, thereby increasing activation energy and slow down the reaction rate. For example, the decomposition rate (S_(N)^(1)) of tertiary chlorobutane in 25^(@)C water (dielectric constant 79) is 300000 times faster than in ethanol (dielectric constant 24). The reaction rate (S_(N)^(2)) of 2-bromopropane and NaOH in ethanol containing 40% water is twice slower than in absolute ethanol. In a word, the level of solvent polarity has influence on both S_(N)^(1) and S_(N)^(2) reactions, but with different results. Generally speaking, weak polar solvent is favorable for S_(N)^(2) reaction, while strong polar solvent is favorable for S_N^(1) reaction, because only under the action of polar solvent can halogenated hydrocarbon dissociate into carbocation and halogen ion and solvents with a strong polarity is favorable for solvation of carbocation, increasing its stability. Generally speaking, the substitution reaction of tertiary haloalkane is based on S_(N)^(1) mechanism in solvents with a strong polarity (for example, ethanol containing water). (Ding, Y. (2013). A Brief Discussion on Nucleophilic Substitution Reaction on Saturated Carbon Atom. In Applied Mechanics and Materials (Vol. 312, pp. 433-437). Trans Tech Publications Ltd.) S_(N)^(1) reaction will be fastest in which of the following solvents?
A
Acetone (dielectric constant 21)
B
Ethanol (dielectric constant 24)
C
Methanol (dielectric constant 32)
D
Chloroform (dielectric constant 5)
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