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प्रश्न
Write the mechanism of the following reaction:
\[\ce{{n}BuBr + KCN ->[EtOH-H2O] {n}BuCN}\]
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उत्तर १
This reaction is a first-order nucleophilic substitution (SN1). The mechanism can be stated as:
Step 1: Generation of nucleophile:
\[\ce{KCN ->[EtOH-H2O]K+ + \overset{-}{C} ≡ N}\]
Step 2: Nucleophilic attack and formation of the transition state:
Step 3: Generation of product:
Therefore, we have
\[\ce{CH3 - CH2 - CH2 - CH2 - Br + KCN ->[EtOH/H2O]CH3CH2CH2CH2CN + KBr}\]
उत्तर २
KCN is the resonating hybrid of the following structures:
\[\ce{K^+ [^- ^{\bullet}_{\bullet}C ≡ N^{\bullet}_{\bullet} <-> ^{\bullet}_{\bullet}C = \overset{\bullet\bullet}{N}^{\bullet}_{\bullet}^-]}\]
Therefore, CN− acts as an ambident nucleophile. It can attack the carbon atom of the C–Br bond in n-BuBr through either the carbon (C) or nitrogen (N) atom. Since the C–N bond is weaker than the C–C bond, the attack occurs at the carbon atom, leading to the formation of n-butyl cyanide.
\[\ce{K^+CN^- + \underset{n-butyl bromide}{CH3CH2CH2\overset{\delta+}{C}H2 - \overset{\delta-}{B}r} -> \underset{n-butyl cyanide}{CH3CH2CH2CH2CN} + KBr}\]
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संबंधित प्रश्न
Which alkyl halide from the following pair would you expect to react more rapidly by an SN2 mechanism? Explain your answer.
CH3CH2CH2CH2Br or \[\begin{array}{cc}
\ce{CH3CH2CHCH3}\\
\phantom{.....}|\\
\phantom{.......}\ce{Br}\
\end{array}\]
In the following pair of halogen compounds, which compound undergoes a faster SN1 reaction?
Write the isomers of the compound having the formula C4H9Br.
Arrange the compounds of the following set in order of reactivity towards SN2 displacement:
2-Bromo-2-methylbutane, 1-Bromopentane, 2-Bromopentane
The stability order for carbocation is _______.
(A) 2° > 3° > 1°
(B) 3° > 2° > 1°
(C) 3° > 1° > 2°
(D) 1° > 3° > 2°
Which of the following pairs is/are correctly matched?
| Reaction | Product | |
| I | RX + AgCN | RNC |
| II | RX + KCN | RCN |
| III | RX + KNO2 | \[\begin{array}{cc} \phantom{.......}\ce{O}\\ \phantom{.....}/\\ \ce{R - N}\phantom{....}\\ \phantom{.....}\backslash\backslash\\ \phantom{.......}\ce{O} \end{array}\] |
| IV | RX + AgNO2 | \[\ce{R-O-N=O}\] |
Halogenation of alkanes is ____________.
Which one is most reactive towards SN1 reaction?
Optically active isomers but not mirror images are called ____________.
The process of separation of a racemic modification into d and l-enantiomers is called ____________.
The increasing order of nucleophilicity would be:
The correct order of increasing the reactivity of C–X bond towards nucleophile in following compounds.
(I)
(II)
(CH3)3CCl
(III)
(CH3)2CHCl
(IV)
Which of the following alkyl halides will undergo SN1 reaction most readily?
Read the passage given below and answer the following question:
Nucleophilic substitution reaction of haloalkane can be conducted according to both SN1 and SN2 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− < Br− < Cl− < F− and the order of their leaving tendency should be I− > Br− > Cl− > F−. Therefore, in four halides with the same alkyl and different halogens, the order of substitution reaction rate is RI > RBr > RCl > 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 SN1 mechanism. If the leaving group is not easy to leave, the reaction is based on SN2 a mechanism.
Influences of solvent polarity: In SN1 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 SN2 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 (SN1) of tertiary chlorobutane in 25℃ water (dielectric constant 79) is 300000 times faster than in ethanol (dielectric constant 24). The reaction rate (SN2) 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 SN1 and SN2 reactions, but with different results. Generally speaking, weak polar solvent is favorable for SN2 reaction, while strong polar solvent is favorable for SN1 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 SN1 mechanism in solvents with a strong polarity (for example, ethanol containing water).
Nucleophilic substitution will be fastest in case of ______.
Which of the following is the definition of chirality?
Which one of the following compounds is more reactive towards SN1 reaction?
Which of the following compounds will show retention in configuration on nucleophile substitution by OH− ion?
The following questions are case-based questions. Read the passage carefully and answer the questions that follow:
|
Nucleophilic Substitution: Influences of solvent polarity: The reaction rate (SN2) of 2-bromopropane and NaOH in ethanol containing 40% water is twice slower than in absolute ethanol. Hence the level of solvent polarity has an influence on both SN1 and SN2 reactions but with different results. Generally speaking, a weak polar solvent is favourable for SN2 reaction, while a strong polar solvent is favourable for SN1. Generally speaking, the substitution reaction of tertiary haloalkane is based on SN1 mechanism in solvents with a strong polarity (for example ethanol containing water). |
Answer the following questions:
(a) Why racemisation occurs in SN1? (1)
(b) Why is ethanol less polar than water? (1)
(c) Which one of, the following in each pair is more reactive towards SN2 reaction? (2)
(i) CH3 – CH2 – I or CH3CH2 – Cl
(ii)
OR
(c) Arrange the following in the increasing order of their reactivity towards SN1 reactions: (2)
(i) 2-Bromo-2-methylbutane, 1-Bromo-pentane, 2-Bromo-pentane
(ii) 1-Bromo-3-methylbutane, 2-Bromo-2-methylbutane, 2-Bromo-3- methylbutane
Which of the following reactions is an example of nucleophilic substitution reaction?
