Advertisements
Advertisements
प्रश्न
Discuss the mechanism of alkaline hydrolysis of bromomethane.
उत्तर
a. Alkaline hydrolysis of bromomethane follows bimolecular nucleophilic substitution (SN2)mechanism. The hydrolysis reaction can be written as follows:
1. Approach of the nucleophile (Backside attack):
i. The nucleophile (OH-) slowly approaches the carbon atom from the opposite side of the C - Br bond.
ii. C – OH weak bond is formed, while the existing C – Br bond gradually weakens.
iii. It is a slow process and hence, it is the rate determining step (R.D.S.).
2. Transition state (Activated complex): With the approach of OH- group and the
gradual departure of Br, a stage comes where the central atom is attached to five substituents. This state is known as transition state of reaction.
At this stage, the three hydrogen atoms lie in a plane perpendicular to the HO – C – Br axis.
3. Stereochemistry of SN2 reaction:
The attack might take place from back as well as from front side.
i. If backside attack takes place:
As shown in the figure given below, the OH group occupies a position in the product which is opposite to the position of Br. Similarly the positions of H2 and
H3 in the reactant and in the product are on opposite sides i.e., inverted due to the back side attack. This is known as inversion of configuration. Thus, backside
attack of nucleophile leads to inversion of configuration.
ii. If front side attack takes place: In this situation, the OH- occupies the same position which was occupied by Br in the reactant and the position of H1, H2 and
H3 also remain the same. Therefore, the configuration of the carbon is retained. This is known as retention of configuration.
The product X is obtained with inversion of configuration and not Y, with retention of configuration (X and Y are enantiomers). Thus, in SN2 reaction, the nucleophile attacks from backside leading to the inversion of configuration.
APPEARS IN
संबंधित प्रश्न
Write the main products when methyl chloride is treated with AgCN.
Give reasons for the following:
(CH3)3C–O–CH3 on reaction with HI gives (CH3)3C–I and CH3–OH as the main products and not (CH3)3C–OH and CH3–I.
In the following pair of halogen compounds, which compound undergoes a faster SN1 reaction?
Write the isomers of the compound having the formula C4H9Br.
Which compound in the following pair will react faster in SN2 reaction with OH−?
CH3Br or CH3I
Which compound in the following pair will react faster in SN2 reaction with OH−?
(CH3)3CCl or CH3Cl
What happens when ethyl chloride is treated with aqueous KOH?
Which would undergo SN2 reaction faster in the following pair and why ?
CH3 – CH2 – Br and CH3 – CH2 – I
In the SN1 reaction, racemization takes place. It is due to:
SN2 mechanism proceeds through intervention of ____________.
Which of the following is the correct order of decreasing SN2 reactivity?
2-Bromopentane is heated with potassium ethoxide in ethanol. The major product obtained is ____________.
The reaction of C6H5–CH=CH–CH3 with HBr produces:
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 ______.
Ethylene chloride and ethylidene chloride are isomers. Identify the correct statements.
(i) Both the compounds form same product on treatment with alcoholic KOH.
(ii) Both the compounds form same product on treatment with aq.NaOH.
(iii) Both the compounds form same product on reduction.
(iv) Both the compounds are optically active.
Elimination reactions (especially β-elimination) are as common as the nucleophilic substitution reaction in case of alkyl halides. Specify the reagents used in both cases.
Why are aryl halides less reactive towards nucleophilic substitution reactions than alkyl halides? How can we enhance the reactivity of aryl halides?
Chlorination of alkanes is an example of
Which one is the correct order of nucleophilic strength (pKa) of following nucleophiles?
When CH3CH2CHCl2 is treated NaNH2 product formed is:-
CCl4 is insoluble in water because:-
Give reason for the following:
The product formed during SN1 reaction is a racemic mixture.
The number of chiral alcohol (s) with molecular formula C4H10O is ______.
Which of the following compounds will show retention in configuration on nucleophile substitution by OH− ion?
In SN1 reactions, the correct order of reactivity for the following compounds:
CH3Cl, CH3CH2Cl, (CH3)2CHCl and (CH3)3CCl is ______.
Optical activity of an enantiomeric mixture is +12.6° and the specific rotation of (+) isomer is +30°. The optical purity is ______ %.
Arrange the following compounds in increasing order of reactivity towards SN2 reaction.
2-Bromopentane, 1-Bromopentane, 2-Bromo-2-methylbutane
