Attacking reagents Questions in English

(D) An electrophilic center is an atom that is electron-deficient and can accept an electron pair. In the given molecule,we identify the following electrophilic centers:
$1$. The carbonyl carbon $(C=O)$ is electrophilic due to the high electronegativity of oxygen,which creates a partial positive charge on the carbon.
$2$. The $\beta$-carbon of the $\alpha,\beta$-unsaturated carbonyl system is electrophilic due to resonance,which delocalizes the positive charge from the carbonyl carbon to the $\beta$-carbon.
$3$. The carbon atom of the nitrile group $(-CN)$ is electrophilic because the nitrogen atom is more electronegative than carbon,creating a partial positive charge on the carbon.
Thus,there are $3$ electrophilic centers in the molecule.

(B) An electrophile is an electron-deficient species that accepts an electron pair to form a chemical bond.
$BF_{3}$ (Boron trifluoride) has an incomplete octet around the boron atom (only $6$ electrons),making it an electron-deficient molecule and thus an electrophile.
$RO^{-}$,$NH_{3}$,and $ROH$ all possess lone pairs of electrons,which makes them nucleophiles.

(D) nucleophile is a chemical species that donates an electron pair to form a chemical bond.
Species with at least one lone pair of electrons or a $\pi-$bond can act as nucleophiles.
In option $D$,$CN^{-}$ has a lone pair on carbon,$H_{2}O$ has two lone pairs on oxygen,and $ROH$ has two lone pairs on oxygen.
Therefore,$CN^{-}, H_{2}O, ROH$ are all nucleophiles.

(D) Electrophiles are electron-loving chemical species. These may be either positively charged or electrically neutral species with an incomplete octet or vacant orbitals.
$H_{2}O$,$NH_{3}$,and $C_{2}H_{5}OH$ are nucleophiles because they possess lone pairs of electrons.
$SO_{3}$ is an electrophile because the sulfur atom is bonded to three highly electronegative oxygen atoms,making it electron-deficient. It can be represented as a resonance hybrid of the following structures:
$O=S(=O)-O^{-} \leftrightarrow -O-S(=O)=O \leftrightarrow -O-S(=O)=O$ (with a positive charge on $S$ in each resonance structure).
Due to the partial positive charge on the $S$ atom,it acts as an electrophile.

(B) Nucleophiles (nucleus-loving species) are electron-rich species that possess at least one lone pair of electrons available for donation,acting as Lewis bases.
$BH_{3}$ and $AlCl_{3}$ are electron-deficient molecules (incomplete octet),which makes them electrophiles.
$NH_{3}$ contains a lone pair of electrons on the nitrogen atom,which it can donate,making it a nucleophile.

(A) Nucleophiles are electron-rich species that can donate an electron pair to an electrophile.
$CH_3NH_2$ contains a nitrogen atom with a lone pair.
$CH_3SH$ contains a sulfur atom with a lone pair.
$CH_3CHO$ is an electrophile due to the polar $C=O$ bond.
$C_2H_4$ is a nucleophile due to the electron-rich $\pi$-bond.
Therefore,$CH_3NH_2$,$CH_3SH$,and $C_2H_4$ are nucleophiles.
The total number of nucleophiles is $3$.

(D) An ambident nucleophile is a species that has two or more nucleophilic sites through which it can attack an electrophile.
In the cyanide ion $(CN^-)$,both the carbon atom (which carries a negative charge) and the nitrogen atom (which has a lone pair of electrons) can act as nucleophilic sites.
Therefore,the cyanide ion is an example of an ambident nucleophile.

(A) An electrophile is a species that accepts an electron pair to form a bond with a nucleophile. Since electrophiles accept electrons,they act as Lewis acids.
$CN^{-}$ is a nucleophile because it is electron-rich and carries a negative charge.
$BF_3$ is a neutral electrophile (Lewis acid) because it is electron-deficient.
$NO_2^{+}$ is a positively charged electrophile.
$AlCl_3$ is an electrophile because it is electron-deficient.
Therefore,$CN^{-}$ is the only species listed that is not an electrophile.

(A) nucleophile is a species that can donate an electron pair to form a chemical bond.
In $BH_{4}^{-}$,the boron $(B)$ atom has a formal negative charge but it has no lone pair of electrons available for donation.
In $CH_{3}MgBr$,the carbon $(C)$ atom is nucleophilic due to the $C-Mg$ bond polarity.
In $CH_{3}OH$,the oxygen $(O)$ atom has two lone pairs.
In $CH_{3}NH_{2}$,the nitrogen $(N)$ atom has one lone pair.
Therefore,the $B$ atom in $BH_{4}^{-}$ is not a nucleophilic site.
Hence,the correct option is $A$.

(A) Electrophiles are electron-deficient species (neutral or cationic).
They attack electron-rich $C$-atoms.