Question

Rank the following structures in order of decreasing electrophile strength.

Answer 1

Concepts and reason

Electrophile:

In organic chemistry, the electrophilic atom is usually a carbon atom, bonded to electronegative atoms such as nitrogen, oxygen, and fluorine. In these kinds of molecules, separation of charges (polarization) occurs. As a result of polarization, the carbon atom contains a partial positive charge while the electronegative atom contains a partial negative charge, and the molecule is known as an electrophile. Resonance:

In a conjugated molecule, the lone/pi-bond pair of electrons move along sigma bonds within a molecule. It leads to different arrangements of electrons for the same molecules. It is called resonance.

Fundamentals

Inductive effect:

The introduction of electronegative atoms (nitrogen, oxygen, or fluorine) causes a negative inductive effect, which is the pulling of bond pair electrons through sigma bonds, making the carbon in a \(\mathrm{C}=\mathrm{O}\) bond more electrophilic. For example,

 

The introduction of alkyl groups causes a positive inductive effect, which is the donation of electrons through sigma bonds, making carbon in a \(\mathrm{C}=\mathrm{O}\) bond less electrophilic. For example,

 

Partial Charge separations:

Resonance:

In a carbonyl functional group, the oxygen atom pulls the electrons away from the carbon atom, making the carbon atom more electrophilic in nature because the oxygen atom is more electronegative than the carbon atom. The carbon atom in a protonated carbonyl group gets a positive charge due to its resonance structures.

 

Descending order of electrophilic strength:

The protonated carbonyl group is the most electrophilic because the carbon atom has a full positive charge. So, it ranks first in the descending order of electrophilic strength.

 

The carbon atom in fluorine substituted carbonyls is more electrophilic in nature due to the negative inductive effect. Hence, it ranks second in the descending order of electrophilic strength.

The alkyl-substituted carbonyls are less electrophilic in nature due to the positive inductive effect. So, these molecules are ranked by increasing carbon chain length with the descending order of electrophilic strength.