Question

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Rank the following structures in order of decreasing electrophile strength.

Answer 1

General guidance

Concepts and reason

Electrophile:

In organic chemistry, the electrophilic atom is usually a carbon atom which is 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 moves along sigma bonds within a molecule. It leads to different arrangements of electrons for the same molecules. It is called a 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 C=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 C=O bond less electrophilic. For example,

Step-by-step

Step 1 of 2

Partial Charge separations:

Resonance:

Explanation | Common mistakes | Hint for next step

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.

Step 2 of 2

Descending order of electrophilic strength:

The protonated carbonyl group is the most electrophilic in nature because the carbon atom has full positive charge on it. So, it ranks the 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.

Answer

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