Attacking reagents Questions in English

(D) An electrophile is an electron-deficient species that accepts an electron pair.
$AlCl_3$,$BF_3$,and $(CH_3)_3C^{+}$ are all electron-deficient and act as electrophiles.
$NH_3$ contains a lone pair of electrons on the nitrogen atom,which it can donate to an electrophile,making it a nucleophile.
Therefore,$NH_3$ is not an electrophile.

(A) Electrophiles are electron-deficient species that accept electron pairs. According to the Lewis theory,species that accept electron pairs are defined as $Lewis \ acids$. Therefore,electrophiles are $Lewis \ acids$.

(B) nucleophile is a species that donates an electron pair to form a chemical bond.
$NH_3$ contains a lone pair of electrons on the nitrogen atom,which it can donate,making it a nucleophile.
$BF_3$ and $BeCl_2$ are electron-deficient species (Lewis acids),and while $H_2O$ is also a nucleophile,$NH_3$ is a classic example of a nitrogen-based nucleophile.

(B) An electrophile is a reagent that can accept an electron pair in a chemical reaction.
$H_2O$,$NH_3$,and $ROR$ (ethers) contain atoms with lone pairs of electrons,making them nucleophiles.
$SO_3$ (sulfur trioxide) has an electron-deficient sulfur atom because the electronegative oxygen atoms pull electron density away from it,allowing it to accept an electron pair.
Therefore,$SO_3$ is an electrophile.

(D) . The process of nitration takes place as follows:
$HONO_2 + 2H_2SO_4 \rightleftharpoons H_3O^{+} + 2HSO_4^- + NO_2^+$
The electrophile responsible for nitration is the nitronium ion,$NO_2^+$.

(A) An electrophile is an electron-deficient species that can accept an electron pair.
In $BCl_3$,the central Boron atom has only $6$ electrons in its valence shell,making it electron-deficient (incomplete octet).
Therefore,$BCl_3$ acts as an electrophile.
$NH_3$ and $CH_3OH$ have lone pairs and act as nucleophiles,while $AlCl_4^-$ has a complete octet and acts as a nucleophile or spectator ion.

(A) An electrophile is an electron-deficient species that accepts an electron pair.
$AlCl_3$ is a Lewis acid because the central $Al$ atom has an incomplete octet (only $6$ electrons in its valence shell).
Therefore,$AlCl_3$ acts as an electrophile.
$CN^{-}$,$NH_3$,and $CH_3OH$ possess lone pairs of electrons and act as nucleophiles.

(D) nucleophile is a chemical species that donates an electron pair to form a chemical bond in a reaction.
$CH_3NH_2$ has a lone pair on the nitrogen atom.
$RO^{-}$ is an anion with lone pairs on the oxygen atom.
$CH_3MgBr$ contains a carbanion-like character $(CH_3^{-})$ due to the polar $C-Mg$ bond,making it a strong nucleophile.
Therefore,all of the given species act as nucleophiles.

(C) Nucleophilicity is the ability of a species to donate an electron pair to an electrophile.
In a period of the periodic table,nucleophilicity decreases as electronegativity increases.
The electronegativity order of the atoms bearing the negative charge is $C < N < O < F$.
Since $CH_3^-$ has the least electronegative atom $(C)$,it holds its lone pair the least tightly and is therefore the best nucleophile.
Thus,the order of nucleophilicity is $CH_3^- > NH_2^- > OH^- > F^-$.

(C) In $CH_3CN$ (acetonitrile),the nitrogen atom has a lone pair of electrons,which allows it to act as a nucleophile.
Additionally,the carbon atom of the cyano group $(-CN)$ is bonded to a highly electronegative nitrogen atom,making the carbon atom electron-deficient and susceptible to nucleophilic attack,thus allowing it to act as an electrophile.
Therefore,$CH_3CN$ exhibits both nucleophilic and electrophilic character.

(B) In the nitration of benzene,the nitrating mixture (concentrated $HNO_3$ and $H_2SO_4$) reacts to produce the nitronium ion $(NO_2^+)$,which acts as the electrophile.
The reaction is: $HNO_3 + 2H_2SO_4 \rightarrow NO_2^+ + 2HSO_4^- + H_3O^+$

(B) In the sulphonation of benzene,concentrated sulphuric acid or fuming sulphuric acid $(H_2SO_4 + SO_3)$ is used.
Sulphur trioxide $(SO_3)$ acts as the active electrophilic species.
It is an electron-deficient molecule that accepts a pair of $\pi$ electrons from the benzene ring to initiate the electrophilic aromatic substitution reaction.

(A) The cyanide ion $(CN^-)$ contains a carbon atom with a lone pair of electrons and a negative charge.
Because it can donate this lone pair to an electrophile to form a new chemical bond,it acts as a nucleophile.
Therefore,the cyanide ion is nucleophilic.

(A) The basic strength of an ion is inversely proportional to the stability of its conjugate acid.
$1$. The conjugate acids are: $H_2O$,$NH_3$,$HC \equiv CH$,and $CH_3CH_3$.
$2$. The acidity order of these conjugate acids is: $H_2O > NH_3 > HC \equiv CH > CH_3CH_3$.
$3$. Since the basic strength is the reverse of the acidity of the conjugate acid,the order of basic strength is: $CH_3CH_2^- > NH_2^- > HC \equiv C^- > OH^-$.
Thus,the correct option is $A$.

(B) An electrophile is an electron-deficient species that seeks electrons to complete its octet or to form a bond.
$SO_3$ (sulfur trioxide) acts as an electrophile because the sulfur atom is bonded to three highly electronegative oxygen atoms,making it electron-deficient (it has a partial positive charge).
$RNH_2$ and $ROH$ contain lone pairs on nitrogen and oxygen respectively,making them nucleophiles.
$NO_3^-$ is an anion and acts as a nucleophile.
Therefore,$SO_3$ is the correct electrophile.

(A) The basic strength of a species is inversely proportional to the stability of its conjugate acid.
$1$. The conjugate acids are: $H_2O, NH_3, HC \equiv CH, CH_3-CH_3$.
$2$. The acidity order of these conjugate acids is: $H_2O > NH_3 > HC \equiv CH > CH_3-CH_3$ (based on electronegativity of the atom bearing the negative charge: $O > N > sp-C > sp^3-C$).
$3$. Therefore,the basic strength order is the reverse: $CH_3 - \mathop C\limits^\Theta H_2 > HC \equiv \mathop C\limits^\Theta > \mathop N\limits^\Theta H_2 > \mathop O\limits^\Theta H$.
Note: The provided options do not contain the correct order derived. However,based on standard chemical principles,the correct order is $CH_3 - \mathop C\limits^\Theta H_2 > HC \equiv \mathop C\limits^\Theta > \mathop N\limits^\Theta H_2 > \mathop O\limits^\Theta H$. Since this is not listed,we select the option that best represents the trend of increasing basicity with decreasing electronegativity.

(D) Nucleophilicity is determined by the electron density on the nucleophilic atom and the stability of the conjugate base.
$1$. $\text{CH}_3\text{O}^-$ is a strong nucleophile due to the $+I$ effect of the methyl group,which increases electron density on the oxygen atom.
$2$. $\text{OH}^-$ is a strong nucleophile but slightly less than $\text{CH}_3\text{O}^-$ due to the absence of the $+I$ effect.
$3$. $\text{C}_6\text{H}_5\text{O}^-$ (phenoxide ion) is a weaker nucleophile because the negative charge is delocalized into the benzene ring via resonance.
$4$. $\text{CH}_3\text{COO}^-$ (acetate ion) is the weakest nucleophile among these because the negative charge is delocalized over two oxygen atoms via resonance,making it very stable and less reactive.
Thus,the correct order is $\text{CH}_3\text{O}^- > \text{OH}^- > \text{C}_6\text{H}_5\text{O}^- > \text{CH}_3\text{COO}^-$.

(A) nucleophile is a species that donates an electron pair to form a chemical bond.
$CH_3OH$ has lone pairs on oxygen,$CH_3OCH_3$ has lone pairs on oxygen,and $NH_3$ has a lone pair on nitrogen,so they all act as nucleophiles.
$CH_3CH_2NO_2$ (nitroethane) does not have a lone pair available for donation in a way that allows it to act as a typical nucleophile in standard organic reactions,as the nitrogen atom is already bonded to oxygen atoms with a formal positive charge,making it electron-deficient.

(C) An electrophile is an electron-deficient species that accepts an electron pair to form a chemical bond.
$H_2O$,$NH_3$,and $C_2H_5NH_2$ all possess lone pairs of electrons on the central atom ($O$ or $N$),making them nucleophiles.
$AlCl_3$ is an electron-deficient compound because the central $Al$ atom has only $6$ electrons in its valence shell (an incomplete octet).
Therefore,$AlCl_3$ acts as a Lewis acid and is an electrophile.

(A) $(1)$ Free radicals are electrically neutral species containing an odd number of electrons (unpaired electron). Thus,$(1)-(B)-(E)$.
$(2)$ Electrophiles are electron-deficient species,often having an incomplete octet,and act as Lewis acids. Thus,$(2)-(D)-(F)$.
$(3)$ Nucleophiles are electron-rich species with a complete octet (lone pairs) and act as Lewis bases. Thus,$(3)-(A)-(C)$.
Therefore,the correct matching is $(1)-(B)-(E), (2)-(D)-(F), (3)-(A)-(C)$.

(A) Nucleophilicity is inversely proportional to the electronegativity $(EN)$ of the atom carrying the negative charge.
Since the electronegativity of the atoms decreases in the order $F > O > N > C$,the nucleophilicity increases in the order $F^{-} < RO^{-} < R_2N^{-} < R_3C^{-}$.
Therefore,the decreasing order of nucleophilicity is $R_3C^{-} > R_2N^{-} > RO^{-} > F^{-}$.

(A) The basicity of an anion is inversely proportional to the acidic strength of its conjugate acid.
Conjugate acids are: $CH_3CH_3$ $(sp^3)$,$CH_2 = CH_2$ $(sp^2)$,$HC \equiv CH$ $(sp)$,and $H_2O$.
The acidic strength order of these conjugate acids is: $H_2O > HC \equiv CH > CH_2 = CH_2 > CH_3CH_3$.
Therefore,the basicity order of their conjugate bases is: $CH_3CH_2^- > CH_2 = CH^- > HC \equiv C^- > OH^-$.

(B) An electrophile is an electron-deficient species that can accept an electron pair.
$NO_2^+$,$H^+$,and $BF_3$ are all electron-deficient and act as electrophiles.
$Na^+$ is a stable cation with a complete octet configuration $(1s^2 2s^2 2p^6)$. It does not have a tendency to accept electron pairs in typical organic reactions,hence it is not considered an electrophile.

(C) Nucleophilicity depends on the availability of the lone pair and the stability of the negative charge.
$1$. $CH_3O^-$ is a strong nucleophile because the negative charge is localized on the oxygen atom.
$2$. $CN^-$ is a strong nucleophile due to the high polarizability of the carbon atom.
$3$. $CH_3COO^-$ has the negative charge delocalized over two oxygen atoms via resonance,making it less nucleophilic than $CH_3O^-$.
$4$. $CH_3C_6H_4SO_3^-$ (tosylate ion) has the negative charge highly delocalized over three oxygen atoms,making it a very weak nucleophile.
Comparing these,the order is: $CH_3O^- > CN^- > CH_3COO^- > CH_3C_6H_4SO_3^-$.
Thus,the correct order is $(ii) > (iii) > (i) > (iv)$.

(C) An electrophile is an electron-deficient species that can be either neutral (e.g.,$BF_3$,$AlCl_3$) or positively charged (e.g.,$H^+$,$NO_2^+$).
It acts as a Lewis acid and forms a chemical bond by accepting a pair of electrons from a nucleophile.

(D) nucleophile is an electron-rich species that donates an electron pair to an electrophile.
Since it donates an electron pair,it acts as a Lewis base,not a Lewis acid.
Therefore,the statement that a nucleophile is a Lewis acid is incorrect.

(C) Electrophiles are electron-deficient species that act as Lewis acids by accepting an electron pair.
$Cl^{\oplus}$,$BH_3$,and $NO_2^{\oplus}$ are electron-deficient and act as electrophiles.
In $H_3O^{\oplus}$,the oxygen atom has a complete octet and a lone pair of electrons,which it can donate.
Therefore,$H_3O^{\oplus}$ is not electron-deficient and does not behave as an electrophile.

(C) nucleophile is a species that donates an electron pair to an electrophile.
In general,a negatively charged species is a better nucleophile than its conjugate acid because the negative charge increases the electron density on the donor atom.
Comparing $H_2O$ and $OH^{\Theta}$,$OH^{\Theta}$ has a negative charge,making it a stronger nucleophile.
Note: Options $B$ and $C$ are identical ($HO^{\Theta}$ and $OH^{\Theta}$ are the same species). Assuming the question intends to compare different species,$OH^{\Theta}$ is the correct choice.

(D) An electrophile is an electron-deficient species that can accept an electron pair.
$CH_3-CH_2-O^{\oplus}H_2$ is an electrophile because the oxygen atom carries a positive charge.
$CH_3-Br$ is an electrophile because the carbon atom is bonded to a more electronegative bromine atom,creating a partial positive charge on the carbon.
$Br_2$ is an electrophile because it can undergo heterolytic cleavage to form $Br^{\oplus}$ and $Br^{\ominus}$.
$CH_3-CH=CH_2$ is a nucleophile because it contains a $\pi$-bond,which is a region of high electron density,making it capable of donating an electron pair.

(C) Electrophiles are electron-deficient species that have a tendency to accept electrons.
They can be neutral or positively charged.
$AlCl_3$ is an electron-deficient molecule (incomplete octet),$SO_3$ has an electron-deficient sulfur atom due to electronegative oxygen atoms,and $NO_2^{\oplus}$ is a positively charged species.
Therefore,$AlCl_3, SO_3, NO_2^{\oplus}$ are all electrophiles.
$H_2O$ and $NH_3$ act as nucleophiles due to the presence of lone pairs.

(B) Nucleophilicity is the tendency of a species to donate an electron pair to an electrophile.
For species in the same period,nucleophilicity decreases as electronegativity increases.
The electronegativity order of the donor atoms is $C < N < O$.
Thus,the basicity and nucleophilicity order is $CH_3^{\ominus} > NH_2^{\ominus} > CH_3O^{\ominus}$.
$CH_3COO^{\ominus}$ is the least nucleophilic due to resonance stabilization of the negative charge.
Therefore,the correct order is $CH_3^{\ominus} > NH_2^{\ominus} > CH_3O^{\ominus} > CH_3COO^{\ominus}$.

(A) $4$-pyrone contains a carbonyl group $(C=O)$ and an ether oxygen atom in the ring. When it reacts with an acid $(H^+)$,the lone pair on the carbonyl oxygen atom attacks the proton because the resulting cation is stabilized by resonance,leading to an aromatic pyrylium ion structure. This makes the carbonyl oxygen the site of protonation.

(D) For group $(I)$: $H_3C-S^-$ is a better nucleophile than alkoxides ($H_3C-O^-$ and $CH_3CH_2O^-$) because $S$ is larger and more polarizable than $O$,and it is in a protic solvent $(CH_3OH)$. Thus,$(3)$ is the strongest.
For group $(II)$: In a polar aprotic solvent like $DMF$,nucleophilicity follows basicity. $NH_2^-$ is a stronger base than $OH^-$ and $H_2O$,making it the strongest nucleophile. Thus,$(3)$ is the strongest.
For group $(III)$: In a polar aprotic solvent like $DMSO$,nucleophilicity increases as steric hindrance decreases. $CH_3O^-$ $(3)$ is less sterically hindered than $(CH_3)_2CH-O^-$ $(1)$,making it a stronger nucleophile. Thus,$(3)$ is the strongest.
Comparing the options provided,the correct sequence for the strongest nucleophiles is $I-3, II-3, III-3$. Since this specific combination is not explicitly listed in the options,we re-evaluate the provided choices. Given the structure of the question,option $(d)$ $I-3; II-1; III-3$ is likely a typo in the source,but based on chemical principles,the strongest nucleophiles are $(3)$,$(3)$,and $(3)$ respectively.

(C) The basicity of an ion is determined by the stability of its conjugate acid. Since $H_2S$ is a stronger acid than $H_2O$,the conjugate base $CH_3CH_2S^{-}$ is a weaker base than $CH_3CH_2O^{-}$.
Nucleophilicity is influenced by polarizability and solvation. Sulfur is larger and more polarizable than oxygen,and it is less solvated in protic solvents,making $CH_3CH_2S^{-}$ a stronger nucleophile than $CH_3CH_2O^{-}$.
Therefore,$CH_3CH_2S^{-}$ is a weaker base but a more nucleophilic species than $CH_3CH_2O^{-}$.

(B) The sulfite ion $(SO_3^{2-})$ is an ambident nucleophile.
In the reaction with a proton $(H^{+})$,which is a hard electrophile,the attack occurs at the oxygen atom (harder center) to form the bisulfite ion $(O^{-}-S(=O)-OH)$.
In the reaction with an alkylating agent like $CH_3I$ ($CH_3^{+}$ is a softer electrophile),the attack occurs at the sulfur atom (softer center) to form the methylsulfonate ion $(CH_3-S(=O)_2-O^{-})$.

(C) nucleophile is a species that donates an electron pair to form a chemical bond.
$NH_3$,$OH^-$,and $CN^-$ all possess lone pairs of electrons and can act as nucleophiles.
$:CCl_2$ (dichlorocarbene) has an incomplete octet (only $6$ electrons in the valence shell of carbon),which makes it electron-deficient. Therefore,it acts as an electrophile rather than a nucleophile.

(A) An electrophile is an electron-deficient species that can accept an electron pair.
$CO_2$ has a central carbon atom bonded to two highly electronegative oxygen atoms via double bonds.
Due to the high electronegativity of oxygen,the carbon atom in $CO_2$ becomes electron-deficient (partial positive charge),making it susceptible to nucleophilic attack.
Therefore,$CO_2$ acts as an electrophile.
$H_3O^{+}$ is a protonated species but acts as an acid; $CH_4$ is a nucleophile/alkane; and $AlCl_4^{-}$ is a stable anion with a complete octet.

(C) Nucleophilicity depends on the ability of an atom to donate its lone pair of electrons to an electrophile.
In a group,nucleophilicity increases down the group because the larger atom is more polarizable and its electron cloud is less tightly held.
Comparing $C_6H_5-O^-$ and $C_6H_5-S^-$,sulfur is larger and more polarizable than oxygen,making $C_6H_5-S^-$ a stronger nucleophile.
Comparing $C_6H_5-CH_2^-$ (a carbanion) with the others,the negative charge on carbon is less stable than on oxygen or sulfur due to lower electronegativity,making it highly reactive and a very strong nucleophile.
However,in standard comparative contexts for these specific species,$C_6H_5-CH_2^-$ is generally considered the strongest nucleophile among the given options due to the high basicity and reactivity of the carbanion.

(B) Nucleophilicity is defined by the ability of an atom to donate an electron pair.
For atoms in the same period,nucleophilicity decreases as electronegativity increases.
The electronegativity order of the donor atoms is $C < N < O$.
Therefore,the nucleophilicity order is $CH_3^{-} > NH_2^{-} > CH_3O^{-}$.
$CH_3COO^{-}$ is the least nucleophilic due to resonance stabilization of the negative charge.
Thus,the correct order is $CH_3^{-} > NH_2^{-} > CH_3O^{-} > CH_3COO^{-}$.

(D) Electrophiles are electron-deficient species that can accept an electron pair.
$AlCl_3$ and $BF_3$ are Lewis acids with an incomplete octet,making them electrophiles.
$\overset{\oplus}{N}O_2$ is a positively charged species,which is also an electrophile.
$H_3O^{\oplus}$ (hydronium ion) has a complete octet and a lone pair of electrons on the oxygen atom,which it can donate. Therefore,it acts as a nucleophile or a Bronsted acid,not an electrophile.

(N/A) Nucleophiles are electron-rich species that can donate an electron pair. These include: $HS^{-}, C_{2}H_{5}O^{-}, (CH_{3})_{3}N:, H_{2}N:^{-}$. These species possess at least one lone pair of electrons available for donation to an electrophile.
Electrophiles are electron-deficient species that can accept an electron pair. These include: $BF_{3}, \overset{+}{Cl}, CH_{3}-\overset{+}{C}=O, \overset{+}{N}O_{2}$. These species either have an incomplete octet (like $BF_{3}$) or a positive charge/reactive site that can accept an electron pair from a nucleophile.

(B) An electrophilic center is an atom that is electron-deficient and can accept an electron pair.
In $CH_3-CH=O$,the carbonyl carbon is electrophilic due to the high electronegativity of oxygen.
In $CH_3-C \equiv N$,the nitrile carbon is electrophilic due to the high electronegativity of nitrogen.
In $CH_3-I$,the carbon atom attached to iodine is electrophilic because iodine is more electronegative than carbon,creating a partial positive charge on the carbon atom.

(N/A)

Nucleophiles and Nucleophilic reaction	Electrophiles and Electrophilic reaction
$(i)$ $A$ reagent that donates an electron pair is called a nucleophile $(Nu:)$ i.e.,nucleus seeking,and the reaction is called a nucleophilic reaction.	$(i)$ $A$ reagent that accepts an electron pair is called an electrophile $(E^{+})$ i.e.,electron seeking,and the reaction is called an electrophilic reaction.
$(ii)$ During a polar organic reaction,a nucleophile attacks an electrophilic centre of the substrate.	$(ii)$ During a polar organic reaction,an electrophile attacks a nucleophilic centre of the substrate.
$(iii)$ The electrophilic centre of the substrate is the specific atom or part of the substrate that is electron-deficient.	$(iii)$ The nucleophilic centre of the substrate is that specific atom or part of the substrate that is electron-rich.
$(iv)$ Examples of nucleophiles include negatively charged ions with lone pair electrons such as hydroxide ion $(OH^{-})$,cyanide $(CN^{-})$,carbanions $(R_{3}C^{-})$,and neutral molecules such as $H_{2}O:$,$RNH_{2}$,$R_{2}NH$,$R_{3}N$,etc.	$(iv)$ Examples of electrophiles include positively charged ions such as carbocations ($CH_{3}^{+}$,$CH_{3}CH_{2}^{+}$),and neutral molecules having functional groups like the carbonyl group $(>C=O)$ or alkyl halides ($R-X$,where $X$ is a halogen atom).
$(v)$ These species act as nucleophilic reagents due to the lone pair electrons present on the atom.	$(v)$ The carbon atom in carbocations has a sextet configuration; hence,it is electron-deficient and acts as an electrophile.
$(vi)$ In neutral molecules such as alkyl halides,due to the polarity of the $C-X$ bond,a partial positive charge is generated on the carbon atom,making it an electrophilic centre where a nucleophile can attack.	$(vi)$ During a reaction,an electrophilic reagent can receive a pair of electrons from a nucleophile,and an electrophilic reaction occurs.

Electrophiles are electron-deficient species and can accept an electron pair. Nucleophiles are electron-rich species and can donate an electron pair.
$(a) \ CH_{3}COOH + HO^{-} \to CH_{3}COO^{-} + H_{2}O$
Here,$HO^{-}$ acts as a nucleophile because it is an electron-rich species.
$(b) \ CH_{3}COCH_{3} + CN^{-} \to (CH_{3})_{2}C(CN)OH$
Here,$CN^{-}$ acts as a nucleophile because it is an electron-rich species.
$(c) \ C_{6}H_{6} + CH_{3}C^{+}=O \to C_{6}H_{5}COCH_{3} + H^{+}$
Here,$CH_{3}C^{+}=O$ (acylium ion) acts as an electrophile because it is an electron-deficient species.

(N/A) Ambident nucleophiles are nucleophiles that possess two nucleophilic centers.
They can attack through either of the two sites.
Examples include cyanide $(CN^-)$ and nitrite $(NO_2^-)$ ions.
$1.$ Cyanide ion $(:C \equiv N:^-)$ can attack through the carbon atom to form alkyl cyanides $(R-CN)$ or through the nitrogen atom to form alkyl isocyanides $(R-NC)$.
- Reaction with $KCN$ (ionic) favors attack through $C$ to form $R-CN$.
- Reaction with $AgCN$ (covalent) favors attack through $N$ to form $R-NC$.
$2.$ Nitrite ion $(:O-N=O:^-)$ can attack through the oxygen atom to form alkyl nitrites $(R-ONO)$ or through the nitrogen atom to form nitroalkanes $(R-NO_2)$.
- Reaction with $NaNO_2$ (ionic) favors attack through $O$ to form $R-ONO$.
- Reaction with $AgNO_2$ (covalent) favors attack through $N$ to form $R-NO_2$.

(N/A) The cyanide ion $(CN^-)$ acts as a stronger nucleophile from the carbon end.
This is because the formation of a $C-C$ bond is more stable than the formation of a $C-N$ bond.
In an aqueous medium,the carbon atom carries the negative charge and has a higher electron density,making it more nucleophilic compared to the nitrogen atom.

(N/A) The electrophilic reagent in the nitration of benzene is the nitronium ion,denoted as $NO_2^+$.
It is formed by the reaction between concentrated nitric acid $(HNO_3)$ and concentrated sulfuric acid $(H_2SO_4)$.
The reaction mechanism is as follows:
$HNO_3 + H_2SO_4 \rightleftharpoons H_2NO_3^+ + HSO_4^-$
$H_2NO_3^+ \rightarrow NO_2^+ + H_2O$
Overall reaction:
$HNO_3 + 2H_2SO_4 \rightarrow NO_2^+ + H_3O^+ + 2HSO_4^-$

(N/A) $(i)$ Reagent: $A$ reagent is a substance that attacks the substrate to initiate a chemical reaction. It is usually the smaller molecule.
$(ii)$ Substrate: The substrate is the organic molecule upon which the reagent acts to form the product.
Difference: The substrate provides the carbon skeleton,while the reagent provides the attacking group.
$(b)$ $(i)$ Reaction mechanism: It is the step-by-step description of the sequence of elementary reactions by which the overall chemical change occurs.
$(ii)$ Uses: It helps in understanding the path of the reaction,predicting the product,and controlling the reaction conditions.
$(c)$ General reaction: $\text{Substrate} + \text{Reagent}$ $\rightarrow \text{Intermediate}$ $\rightarrow \text{Product} + \text{By-product}$

(N/A) nucleophile is a species that donates an electron pair to form a chemical bond,while an electrophile is a species that accepts an electron pair.
$1$. $NO_2^+$: Electrophile (contains a positive charge and an electron-deficient nitrogen atom).
$2$. $OH^{-}$: Nucleophile (contains a lone pair of electrons and a negative charge).
$3$. $CH_3NH_2$: Nucleophile (nitrogen atom has a lone pair of electrons).
$4$. $NH_3$: Nucleophile (nitrogen atom has a lone pair of electrons).
$5$. $Br^{+}$: Electrophile (contains a positive charge).
$6$. $CH_3-CO^{-}-CH_3$: Nucleophile (the enolate ion has a negative charge and lone pairs on the oxygen atom).

(N/A) Nucleophiles are electron-rich species that possess at least one lone pair of electrons and are capable of donating them to an electron-deficient center.
Nucleophiles form a covalent bond with the electron-deficient center. In contrast,bases specifically donate an electron pair to $H^+$.
Example of base behavior: $HO^- + H^+ \rightleftharpoons H_2O$
Example of nucleophilic behavior: $HO^- + CH_3-Cl \longrightarrow HO-CH_3 + Cl^-$
Examples of nucleophiles:
$(i)$ Charged nucleophiles: $CN^-, I^-, Br^-, Cl^-, OH^-, -SH, -OR, -CH_3, -NH_2, -H, -C \equiv CH$ etc.
$(ii)$ Neutral nucleophiles: $H_2\ddot{O}:, H_2\ddot{S}:, CH_3\ddot{O}H, (CH_3)_2\ddot{O}:$ etc.
Nucleophilicity is the kinetic measure of a nucleophile's ability to react with an electron-deficient center,whereas basicity is a thermodynamic measure.
Ambident nucleophiles are species that possess two or more nucleophilic sites. For example,$CN^-$ can link through carbon to form alkyl cyanides or through nitrogen to form isocyanides. Similarly,the nitrite ion $[^-O-\ddot{N}=\ddot{O}:]$ can link through oxygen to form alkyl nitrites or through nitrogen to form nitroalkanes.
Factors affecting nucleophilicity include:
$(i)$ Size of the nucleophile
$(ii)$ Electronegativity of the donor atom
$(iii)$ Electron density on the donor atom
$(iv)$ Nature of the solvent