Advertisements
Advertisements
Question
What is the action of the following on ethyl bromide
alcoholic solution of potassium hydroxide.
Advertisements
Solution
\[\ce{\underset{\text{ethylbromide}}{C2H5Br} + alc KOH ->\underset{\text{ethene}}{CH2} = CH2 + KBr + H2O}\]
APPEARS IN
RELATED QUESTIONS
The treatment of alkyl chlorides with aqueous KOH leads to the formation of alcohols but in the presence of alcoholic KOH, alkenes are major products. Explain.
How the following conversion can be carried out?
Ethyl chloride to propanoic acid.
C–Cl bond length in chlorobenzene is shorter than C–Cl bond length in CH3–Cl.
The stability order for carbocation is _______.
(A) 2° > 3° > 1°
(B) 3° > 2° > 1°
(C) 3° > 1° > 2°
(D) 1° > 3° > 2°
What is the action of the following on ethyl bromide:
moist silver oxide
What is the action of the following on ethyl bromide?
moist silver oxide
Halogenation of alkanes is ____________.
Which one is most reactive towards SN1 reaction?
Which one of the following halogen compounds is difficult to be hydrolysed by SN1 mechanism?
Isopropyl chloride undergoes hydrolysis by:
Which of the following is the correct order of decreasing SN2 reactivity?
Which of the following is an optically active compound?
Which of the following compounds is optically active?
Which of the following undergoes nucleophilic substitution exclusively by SN1 mechanism?
Which of the following compounds will give a racemic mixture on nucleophilic substitution by OH ion?
1-Bromoethane, 1-Bromopropane, 1-Bromobutane, Bromobenzene
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).
SN1 reaction will be fastest in which of the following solvents?
A primary alkyl halide would prefer to undergo ______.
Cyanide ion acts as an ambident nucleophile. From which end it acts as a stronger nucleophile in aqueous medium? Give reason for your answer.
Match the reactions given in Column I with the types of reactions given in Column II.
| Column I | Column II | |
| (i) |  |
(a) Nucleophilic aromatic substitution |
| (ii) | \[\begin{array}{cc} \ce{CH3 - CH = CH2 + HBr -> CH3 - CH - CH3}\\ \phantom{............................}|\phantom{}\\ \phantom{.............................}\ce{Br}\phantom{} \end{array}\] |
(b) Electrophilic aromatic substitution |
| (iii) |  |
(c) Saytzeff elimination |
| (iv) |  |
(d) Electrophilic addition |
| (v) | \[\begin{array}{cc} \ce{CH3 CH2 CH CH3 ->[alc.KOH] CH3 CH = CH CH3}\\ \phantom{}|\phantom{..........................}\\ \phantom{}\ce{Br}\phantom{........................} \end{array}\] |
(e) Nucleophilic substitution (SN1) |
Which of the following is the definition of chirality?
When CH3CH2CHCl2 is treated NaNH2 product formed is:-
Assertion (A) : Nucleophilic substitution of iodoethane is easier than chloroethane.
Reason (R) : Bond enthalpy of C-I bond is less than that of C-Cl bond.
Complete the reaction with the main product formed:
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
Discuss SN2 mechanism of methyl bromide using aqueous KOH.
Acetic anhydride from acetic acid
Discuss the mechanism of alkaline hydrolysis of methyl bromide.
Consider the reactions,
(i) \[\begin{array}{cc}
\phantom{}\ce{\underset{}{(CH3)2CH - CH2Br} ->[C2H5OH] \underset{}{(CH3)2CH - CH2OC2H5 + HBr}}\\
\end{array}\]
(ii) \[\begin{array}{cc}
\phantom{}\ce{\underset{}{(CH3)2CH - CH2Br} ->[C2H5O-] \underset{}{(CH3)2CH - CH2OC2H5 + Br-}}\\
\end{array}\]
The mechanisms of reactions (i) and (ii) are respectively:
