Cowardly.14.5: Electrophilic substitution (2023)

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    So far we have seen an electrophilic addition and electrophilic isomerization - we now go to the third variation of electrophilia, which is electrophilic substitution. In an electrophilic substitution reaction, some electrons have been attacked first.

    Mechanism for the electrophilic substitution:

    Cowardly.14.5: Electrophilic substitution (2)

    Basic electrophilic reactions in isoprenoid biosynthesis

    The electrophilic substitution steps are very important in the biosynthetic pathways of isoprenoid compounds. In the early chain elongation reaction (EC of many isoprenoid pathways, the IPP and DMAPP building blocks combine to produce a so-called 10-carbon isoprenoid product grananas diphosphate (GPP):

    Cowardly.14.5: Electrophilic substitution (3)

    In the first step (step a below), the diphosphate group of DMAPP leaves the allyl carbocation.

    Cowardly.14.5: Electrophilic substitution (4)

    In step 1 \ (\ pi \) the electrons in the IPP attack the electrophilic carbocation of step A, which leads to a new carbon binding and a tertiary carbocation medium -sized.who started in Ipp.

    Exercise 14.5.1

    dmapis much more susceptible to spontaneous hydrolysis thanIppIf dissolved in water, explain why.

    Exercise 14.5.2

    Farnezyl Diphosphor(FPP) is synthesized by adding an additional five-core structural elementGeranyl Diphosphoran. What is this structure element?IppTheDMAPP? Draw the creation mechanismFPP.

    Cowardly.14.5: Electrophilic substitution (5)

    Exercise 14.5.3

    Propose the probable mechanism of the following transformation, which is the first phase of a slightly complex reaction in the synthesisIzoprenoidcompound in plants.(Science 1997,277, 1815)

    Cowardly.14.5: Electrophilic substitution (6)

    Exercise 14.5.4

    TheElektrophilkul mElectrophileSubstitution reaction is often aCarbokation, but it can also be a methyl groupS-Adenosylmethionin(Sam - see Section 8.8a). Suggest the likely mechanism for thisMethylationReaction.(Biochemistry 2012,51, 3003)

    Cowardly.14.5: Electrophilic substitution (7)

    Electrophilic aromatic substitution

    So far, we have mainly focused on the electrophilic reactions of alkenes.Remember section 2.2 that bonds \ (\ pi \) in aromatic rings are much less reactive than in alkenes.However, aromatic systems are actually subject to electrophilic substituents with a strong electrophilic such as carbocation, and if the intermediate carbocation is created, it can be sufficiently stabilized.

    Aromatic Electrophilic Mechanism (Friedel-Crafts Alcalation

    Cowardly.14.5: Electrophilic substitution (8)

    Organic chemists often refer to electrophilic aromatic substitution reactions with carbocation-electrophils asFriedel-Crafts alkylation reaction.

    Exercise 14.5.5

    Aromatic rings generally do not bend according to the electrophilsAdd onReactions.Why not?

    The following Friedel-Crafts reaction is part of the biosynthesis of vitamin K and related biomolecules.

    Cowardly.14.5: Electrophilic substitution (9)

    The loss of the diphosphate forms a strong carbocation electrophilic (step a), which attracts the electrons in the aromatic ring \ (\ pi \) in order to form a carbocation with a new carbon binding (step 1). The substitution ends by proton abstraction (step 2), by the aromatic sixteenth.

    An important point here is that an electrophile must be very electrophil -usually a carbocation, since aromatic \ (\ pi \) bonds are much less reactive than p -light bonds.aromatic electrons \ (\ pi \) is step 1 in the figure above).

    Keep in mind that stabilization of the intermediate formed in the rate-limiting step results in a lower activation energy for the step that speeds up the reaction.

    Organic chemists use the term ring activation to refer to the accelerating effect of electronic heteroatoma in electrophilic aromatic substitution reactions. Aromatic rings that the activating oxygen or nitrogen atoms are missing are less reactive compared to electrophilic substitution.

    An example of the ring-activating effect of a nitrogen atom on an aromatic ring can be found in the following Friedel-Crafts reaction (EC, which should be familiar from the introduction to this chapter:

    Cowardly.14.5: Electrophilic substitution (10)

    We would like to remind you that this is an important early step in the biosynthetic pathway for ergot alkaloids, which are thought to be the primary cause of the 17th century "imprisonment" of several young girls in Salem, Massachusetts. (

    Exercise 14.5.6

    Draw a possible mechanism of biosynthesisDimethylloallil Tryptophanincluding a resonance structure that shows howCarbokationInterproduct in relation to the speed -determining step is stabilized by the free electron pairs in ring nitrogen (in other words).activateRing).

    In addition to being important biochemical transformations, Friedel-Crafts reactions are often performed in the laboratory. It is instructive to look at a few examples to see how the same principles of structure and reactivity apply to both biochemical and laboratory reactions.

    Below you will find an example of a Friedel-Crafts alkylation reaction in the laboratory:

    Cowardly.14.5: Electrophilic substitution (11)

    We would like to remind you that a strong electrophile, as carbocation is required to get an electrophilic aromatic substitution. 2 -Chlorpropan -Reagez is electrophilic, but insufficiently electrophilic to react with benzene.Here is the tone trigeum catalyst: He reacts like Lewis --Lewis --LEWIS reactions -acidity with alkyl chloride that produces secondary carbokation:

    Cowardly.14.5: Electrophilic substitution (12)

    The carbocation generated in this way is sufficiently electrophilic to react with aromatic electrons \(\pi\) in a manner familiar to the biochemical examples discussed above:

    Cowardly.14.5: Electrophilic substitution (13)

    However, you may have found that an element of the biochemical reactions of Friedel -Craft is missing: There is no activating group to stabilize the intermediate carbocation.Measures the speed of the alkylation of Friedel -Craft.

    Cowardly.14.5: Electrophilic substitution (14)

    Note that two products are formed in the example shown above: one is ortho-discount from benzene and the other is para-discount. It should also be noted that no meta-discount product is formed.-Para-controlling effect referred to and you are guided in the following exercise to explain it.

    Exercise 14.5.7
    1. Draw the lowest energy resonances of the carbocation intermediates leading to the ortho and para products in the above reaction. Use resonance structures to illustrate how a methoxy substituent is a ring activating group.
    2. Draw a hypothetical carbocation intermediate in the reaction that leads to the formation of the meta-distubist product.Is this carbocation stabilized by metered by meter?Do you understand why there are no metas?
    Exercise 14.5.8
    1. a) As in the case of electrophilic aromatic substitutions, there is a ring group, which are also nebulous groups of the ring. For each of the benzene reagents studied, draw a carboc to indirectly lead to an indirect product for the ortho compensation product and decide whether the substituent activates the ring, whether it deactivates the ring in the reaction of Friedel -Crafts with 2 -Lloropropane and Alcl3 (in other words, which, which, what, which, which, which, which, which, which, which, which , which, which, which, which, which, which, which is which what, which, which, which, which, what, which, which, which, which, which, which, which, which, which, which, which, which, which, which, which, which, which would react faster than benzene, which would react slower?) Explain how the effect deactivates the ring.

    Cowardly.14.5: Electrophilic substitution (15)

    1. (Hard!) Ring deactivation by plants is usually meta-sedentary as well. Use one of your indirect carbocation drawings from part (a) of this exercise and the concept of resonance to explain this observation.
    2. (First part of answer (b)) Look again at the biosynthetic action of vitamin K and discuss the effects of ring activation/direction of two substrates on the substrate.


    Why does 14 benzene undergo electrophilic substitution reactions though it has three double bonds justify? ›

    Benzene is a planar molecule having delocalized electrons above and below the plane of the ring. Hence, it is electron-rich. As a result, it is highly attractive to electron-deficient species i.e., electrophiles. Therefore, it undergoes electrophilic substitution reactions very easily.

    What are the 5 electrophilic substitution reaction? ›

    There are six key electrophilic aromatic substitution reactions in most introductory organic chemistry courses: chlorination, bromination, nitration, sulfonation, Friedel-Crafts alkylation, and Friedel-Crafts acylation.

    What is an example of electrophile substitution? ›

    Basic examples of electrophilic substitution reaction of benzene are nitration, sulfonation, halogenation, Friedel Craft's alkylation and acylation, etc.

    Why does benzene undergo electrophilic substitution but not addition? ›

    The presence of the delocalised electrons makes benzene particularly stable. Benzene resists addition reactions because that would involve breaking the delocalisation and losing that stability.

    Why does benzene undergo electrophilic substitution with difficulty? ›

    Nucleophiles are electron-rich. Hence, they are repelled by benzene. Hence, benzene undergoes nucleophilic substitutions with difficulty.

    What is the order of electrophilic substitution in benzene? ›

    Therefore, the correct order of reactivity towards electrophilic substitution is Phenol > benzene > chlorobenzene > benzoic acid.

    What is electrophilic substitution reaction in benzene? ›

    The electrophilic substitution reaction of benzene nitration occurs when Benzene is combined with nitric acid and sulfuric acid. Benzene and sulfuric acid form an electrophile, which reacts with the nitric acid to form a resonance-stabilised structure containing a nitrate group.

    How do you know if a reaction is nucleophilic or electrophilic substitution? ›

    In electrophilic substitution reactions, the electrophile (a positive ion or partially positive center of polar compound) attacks the electrophilic center of the molecule, whereas in the nucleophilic substitution reaction, the nucleophile (a molecule rich in electrons) attacks the nucleophilic center by removing the ...

    What are the four most common electrophilic aromatic substitution reaction? ›

    Electrophilic Aromatic Substitution -Halogenation, Nitration, Sulfonation, Friedel-Crafts alkylation reaction, and their Mechanism with FAQs of Electrophilic Aromatic Substitution.

    What compounds undergo electrophilic substitution? ›

    substitution reaction

    Examples of electrophilic species are the hydronium ion (H3O+), the hydrogen halides (HCl, HBr, HI), the nitronium ion (NO2+), and sulfur trioxide (SO3). Substrates of nucleophiles are commonly alkyl halides, while aromatic compounds are among the most important substrates of electrophiles.

    How does electrophilic substitution work? ›

    In an electrophilic substitution reaction, a pair of π-bonded electrons first attacks an electrophile - usually a carbocation species - and a proton is then abstracted from an adjacent carbon to reestablish the double bond, either in the original position or with isomerization.

    How do you identify an electrophile? ›

    There are two requirements for a molecule to be considered a good electrophile. First, it must contain an electrophilic center or atom. Second, the electrophilic atom must be able to accommodate a new sigma bond. Please keep in mind the difference between electrophile and electrophilic center.

    What are 2 examples of electrophiles and nucleophiles? ›

    Electrophiles are electron deficient species and can accept an electron pair from electron rich species. Examples include carbocations and carbonyl compounds. A nucleophile is electron rich species and donates electron pairs to electron deficient species. Examples include carbanions, water , ammonia, cyanide ion etc.

    What are three electrophiles examples? ›

    Examples of electrophiles are hydronium ion (H3O+, from Brønsted acids), boron trifluoride (BF3), aluminum chloride (AlCl3), and the halogen molecules fluorine (F2), chlorine (Cl2), bromine (Br2), and iodine (I2).

    Which species will undergo electrophilic substitution reaction most easily? ›

    Thus, due to the electron-donating effect of the hydroxyl group in phenol, electron density increases at the ortho and para position of the aromatic ring. So, phenol undergoes electrophilic substitution more easily than benzene.

    Which of the compound undergoes electrophilic substitution most easily? ›

    +M effect groups increase electron density on o-and p-position in benzene ring, hence they favour the electrophilic substitution . This is the reason why aniline is most reactive among given compounds.

    Is a benzene ring a nucleophile or electrophile? ›

    Benzene is a nucleophile because of its delocalized electrons. The molecule has electron rich areas which allow it to donate them to electrophiles.

    What is the main problem encountered during electrophilic substitution? ›

    The main problem encountered during electrophilic substitution reactions of aromatic amines is that of their very high reactivity.

    Which is more prone to electrophilic substitution reaction? ›

    In series of activating group OH comes first then OCH3, hence, phenol is most reactive towards electrophilic substitution reaction.

    Why benzene Cannot undergo elimination reaction? ›

    Option(C): Elimination- There is no such evidence of elimination reaction in Benzene since it completely destroys the stability of Benzene.

    What are the 3 types of substitution reactions of benzene? ›

    Benzene can undergo substitution reactions such as alkylation, nitration, and sulfonation, and addition reactions including hydrogenation.

    How many are more reactive than benzene towards electrophilic substitution? ›

    Among the given compounds, only five compounds are more reactive than benzene towards electrophilic aromatic substitution reaction.

    Which is more reactive than benzene towards electrophilic substitution reaction? ›

    Asseration:Phenol is more reactive than benzene towards electrophilic substitution reaction.

    Which electrophilic substitution reaction of benzene is reversible? ›

    Among the aromatic electrophilic substitution reactions, Sulphonation is an example of a reversible reaction.

    What is the electrophile in the acylation of benzene? ›

    The electrophile is CH3CO+. It is formed by reaction between the ethanoyl chloride and the aluminium chloride catalyst.

    What is the role of catalyst in electrophilic substitution reaction in benzene? ›

    Benzene is a π electron rich system and is also resonance stabilized. It undergoes electrophilic substitution reactions to give monosubstituted benzene. Electrophilic substitution reactions are carried out in presence of a catalyst which helps in the formation of electrophiles which are electron-deficient species.

    What makes a reaction electrophilic? ›

    Electrophiles are electron-deficient species that are attracted to an electron-rich center. Electrophiles react by accepting an electron pair in order to form a bond to a nucleophile including the interactions of a proton and a base.

    Which compounds show nucleophilic substitution reaction? ›

    The p-chloronitrobenzene can easily undergo the nucleophilic substitution reaction.

    What is the difference between electrophilic substitution and nucleophilic substitution? ›

    Electrophilic Aromatic Substitution is the reaction in which an electrophile substitutes hydrogen in the aromatic ring. In contrast, Nucleophilic Aromatic Substitution is the reaction in which a nucleophile substitutes a leaving group in the aromatic ring.

    What is an example of electrophilic aromatic substitution? ›

    Nitration and sulfonation of benzene are two examples of electrophilic aromatic substitution. The nitronium ion (NO2+) and sulfur trioxide (SO3) are the electrophiles and individually react with benzene to give nitrobenzene and benzenesulfonic acid respectively.

    What is the order of electrophilic aromatic substitution? ›

    Hence, the correct order of reactivity towards electrophilic substitution is: C6H5−OH>C6H6>C6H5−Cl>C6H5−COOH.

    Where does electrophilic substitution in usually occur? ›

    Electrophilic substitution usually occurs preferentially in the aryl group (Figure 3). In compounds containing both an aryl group and a fused benzene ring, electrophiles usually attack the aryl group exclusively.

    Why are electrophilic substitution important? ›

    Electrophilic aromatic substitution is one of the most important reactions in synthetic organic chemistry. Such reactions are used for the synthesis of important intermediates that can be used as precursors for the production of pharmaceutical , agrochemical and industrial products.

    What is the catalyst for electrophilic substitution reaction? ›

    The electrophilic substitution reaction between benzene and chlorine or bromine. Benzene reacts with chlorine or bromine in an electrophilic substitution reaction, but only in the presence of a catalyst. The catalyst is either aluminum chloride (or aluminum bromide if you are reacting benzene with bromine) or iron.

    Which molecule is most electrophilic? ›

    So, carbon 1 is the most electrophilic one.

    Are all electrophiles positive? ›

    Most electrophiles are positively charged, have an atom that carries a partial positive charge, or have an atom that does not have an octet of electrons. Electrophiles mainly interact with nucleophiles through addition and substitution reactions.

    Which is most electrophilic? ›

    Therefore, o-hydroxy toluene is most reactive towards electrophilic reagent.

    What are 3 common nucleophiles? ›

    In both laboratory and biological organic chemistry, the most common nucleophilic atoms are oxygen, nitrogen, and sulfur, and the most common nucleophilic compounds and functional groups are water/hydroxide ion, alcohols, phenols, amines, thiols, and sometimes carboxylates.

    Is H2O a nucleophile or electrophile? ›

    H2O can act as both, i.e., nucleophile and electrophile.

    Is water an electrophile? ›

    Water can act as both electrophile and nucleophile. But, the nucleophilic character of water is more than electrophilic character due to the ready availability of lone pair of electrons on oxygen atoms. Due to the high value of electronegativity of oxygen it easily attracts electrophiles.

    Which is strongest nucleophile? ›

    CH3−O− is the strongest nucleophile which is capable of acting as donar of electon pair.

    Which of the following is not an electrophile? ›

    hence, Option(C) H3O+ is not an electrophile.

    What acts as an electrophile? ›

    The species which are electron deficient and accept a pair of electrons are called electrophile. Hence, SO3 is a electrophile as it contains an electron deficient center.

    Why does benzene undergo substitution reaction rather than addition though it contains three double bonds? ›

    In benzene, the π-electrons are delocalised and makes the structure more stable. Delocalization of π electron is called resonance. Thus, benzene does not give addition reactions because of resonance stabilisation.

    Why is benzene stable though it contains three double bonds? ›

    The presence of three double bonds does not make the benzene stable, it is stable because of the three double bonds that are actually delocalized pi-electrons that are found to be in resonance. The bonds are not fixed like a normal double bond, they move around the structure to create several resonating structures.

    Why does benzene have 3 double bonds? ›

    Because each of the six carbon atoms and their corresponding p orbitals are equivalent, it is impossible for them to only overlap with one adjacent p orbital to create three defined double bonds.

    Why do Arenes undergo substitution reactions even though they contain double bonds? ›

    Arenes contain double bonds just like alkenes but they do not undergo electrophilic addition because these would result to their loss of ring aromaticity. In electrophilic aromatic substitution reactions, a carbocation is generated while in nucleophilic aromatic substitutions, a carbo anion is generated.

    Why does benzene not undergo addition reactions like alkenes? ›

    Although benzene rings have pi bonds like alkenes and alkynes, they cannot undergo addition reactions because of their stability and instead do Aromatic Reactions.

    What is the difference between addition and substitution reactions of benzene? ›

    Benzene does not undergo electrophilic addition reactions. Instead, it undergoes electrophilic substitution reactions which leave the electron ring unchanged. Electrons are attracted to the electrophile. Electrons from the delocalised benzene ring are attracted to the electrophile.

    Why double bond is more stable than triple bond? ›

    In a triple bond, a total of6 electrons are shared. In a double bond, 4 electrons are shared and in a single bond, only 2 electrons are shared. Thus, the bond strength is as follows: Triple bond > Double bond > Single bond.

    Why are double and triple bonds less stable? ›

    It is depicted by two horizontal lines between two atoms in a molecule. This type of bond is much stronger than a single bond, but less stable; this is due to its greater amount of reactivity compared to a single bond.

    Does benzene contain 3 double bonds that alternate with single bonds? ›

    Benzene- C6H6 has alternate carbon-carbon single and double bonds with 9 single bonds and 3 double bonds as shown below.

    What are 3 benzene rings together called? ›

    Anthracene is a solid polycyclic aromatic hydrocarbon (PAH) of formula C14H10 consisting of three fused benzene rings. It is a component of coal tar. Anthracene is used in the production of the red dye alizarin and other dyes.

    What is it called when there are 3 double bonds? ›

    If more than one double bond is present, indicate their position by using the number of the first carbon of each double bond and use the suffix -diene (for 2 double bonds), -triene (for 3 double bonds), -tetraene (for 4 double bonds), etc. H2C.

    What is the unusual stability of benzene? ›

    Benzene, however, is an extraordinary 36 kcal/mole more stable than expected. This sort of stability enhancement is now accepted as a characteristic of all aromatic compounds. A molecular orbital description of benzene provides a more satisfying and more general treatment of "aromaticity".

    Why does benzene have alternating single and double bonds? ›

    Unlike cyclohexane, benzene only contains six hydrogen atoms, giving the impression that the ring is unsaturated and each carbon atom participates in one double bond. Two different structures with alternating single and double bonds around the ring can be written for benzene.

    Do all benzene rings have double bonds? ›

    We explored some potential structures for benzene in Aromatic Chemistry, each containing three C=C double bonds. But in actual fact, we know that benzene doesn't contain any double bonds at all. Instead, all of its carbon-carbon bonds are identical intermediates - halfway between a single and a double bond in length.

    Do all aromatics have double bonds? ›

    Aromatic compounds are cyclic compounds containing double bonds so that their three-dimensional structure is planar.


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