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Question
Which compound in the following pair will react faster in SN2 reaction with OH−?
(CH3)3CCl or CH3Cl
Solution
\[\begin{array}{cc}
\phantom{....}\ce{CH3}\\
\phantom{..}|\\
\ce{CH3 - C - Cl}\\
\phantom{..}|\\
\phantom{....}\ce{CH3}\\
\end{array}\] or CH3 — Cl
The SN2 mechanism involves the attack of the nucleophile at the atom bearing the leaving group. But, in case of (CH3)3CCl, the attack of the nucleophile at the carbon atom is hindered because of the presence of bulky substituents on that carbon atom bearing the leaving group. On the other hand, there are no bulky substituents on the carbon atom bearing the leaving group in CH3Cl. Hence, CH3Cl reacts faster than (CH3)3CCl in SN2 reaction with OH−.
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RELATED QUESTIONS
Which alkyl halide from the following pair would you expect to react more rapidly by an SN2 mechanism? Explain your answer.
CH3CH2CH2CH2Br or \[\begin{array}{cc}
\ce{CH3CH2CHCH3}\\
\phantom{...}|\\
\phantom{....}\ce{Br}\
\end{array}\]
Write the structure of the major organic product in the following reaction:
\[\ce{CH3CH(Br)CH2CH3 + NaOH ->[water]}\]
Write the structure of the major organic product in the following reaction:
\[\ce{CH3CH2CH2OH + SOCl2 ->}\]
What happens when ethyl chloride is treated with aqueous KOH?
SN1 reactions are accompanied by racemization in optically active alkyl halides.
What is the action of the following on ethyl bromide:
silver acetate
AgCN reacts with haloalkanes to form isocyanide. Haloalkanes react with KCN to form alkyl cyanides as the main product. Why?
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 | \[\ce{R-O-N=O}\] |
The order of reactivity of the given haloalkanes towards nucleophile is:
Which of the following is the correct order of decreasing SN2 reactivity?
Which of the following undergoes nucleophilic substitution exclusively by SN1 mechanism?
The increasing order of nucleophilicity would be:
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).
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Cyanide ion acts as an ambident nucleophile. From which end it acts as a stronger nucleophile in aqueous medium? Give reason for your answer.
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The product formed during SN1 reaction is a racemic mixture.
In which reaction mechanism carbocation is formed?
In SN1 reactions, the correct order of reactivity for the following compounds:
CH3Cl, CH3CH2Cl, (CH3)2CHCl and (CH3)3CCl is ______.
Discuss the mechanism of alkaline hydrolysis of methyl bromide.
