Core Concepts
This article will explain a phenomenon in organic chemistry known as the inductive effect. This concept plays a significant role in determining the reactivity and stability of different molecules and compounds. After reading this article, you will be able to understand how the inductive effect plays a crucial role in the behavior of organic compounds like acids and bases.
Electronegativity
Electronegativity is a measure of the tendency of certain atoms to attract electrons toward themselves. The Pauling Scale is commonly used to describe this behavior. The greater an atom’s electronegativity, the stronger its tendency to attract electrons toward itself, thereby pulling electron density away from any atom with which it forms a bond. Fluorine is the most electronegative element with a value of 4.0, and francium is the least electronegative element with a value of 0.7.
The Inductive Effect
The inductive effect is a consequence of the presence of electronegative atoms in a molecule. It occurs when the electronegative atoms within a molecule cause a displacement of electron density along a sigma (σ) bond. The presence of these electron-withdrawing groups results in a dipole denoted by the letter 𝛿, where partially negative charges are denoted by δ-, and the less electronegative atom, with a partially positive charge, is indicated by δ+.
Examples of the Inductive Effect
In the pictures above you can observe the result of the inductive effect in different molecules. In hydrochloric acid, the chlorine (Cl) is more electronegative than hydrogen (H). Therefore, the Cl pulls electrons towards itself much strongly, which generates a partially negative pole around it. Similarly, the H is acting as an electron-donating group and a partially positive charge develops around it. In water, the Oxygen (O) is more electronegative than the hydrogens and it creates an electron-rich region denoted by 2δ-, while the hydrogens donate electrons and experience a decreased electron density. When functional groups like alkyl groups (such as methyl, ethyl, etc.) donate electrons they exhibit a positive inductive effect. When electronegative atoms or groups, such as halogens (fluorine, chlorine, etc.), displace the electron density towards themselves, they exhibit a negative inductive effect.
Inductive Effect- Acids and Bases
One of the most important applications of the inductive effect in organic chemistry relates to the acidity and basicity of molecules. The presence of electronegative elements affects the stability of different molecules, which also determines their acidity. Let’s look at an example:
Which one of these two molecules is more acidic?
The inductive effect could help us answer this question! As we can see, the difference between the molecule on the left and the one on the right is the presence of fluorine (F) on the right figure. As we discussed before, fluorine is very electronegative, and it will pull the electron density towards itself. This will be crucial when analyzing the conjugate bases of both of these compounds:
Since fluorine attracts the electron density, the conjugate base of fluoroacetic acid is more stable, compared to the conjugate base of acetic acid, which does not have any electronegative atoms pulling the electron density away from the negative charge. The fact that the conjugate base of fluoroacetic acid is more stable than the conjugate base of acetic acid, makes fluoroacetic acid a stronger acid. Similarly, when comparing bases, more stable bases are weaker bases. Thus, the acetate ion is more basic than the fluoroacetate ion. The pKa of acetic acid is around 7.4, while the pKa of fluoroacetic acid is around 2.7, which confirms that the latter is a stronger acid. More on this can be found in our article on Factors in Acid Strength.
Inductive Effect and Distance
The inductive effect is not as great when the electron-withdrawing group is far from the functional group we are considering. Let’s look at an example:
Which of these two molecules is more acidic?
In the left molecule, the bromine (Br) is closer to the acidic proton, and it will cause a stronger inductive effect compared to the bromine in the right molecule, which is farther from the acidic proton. This means that 2-bromopentanoic acid will produce a more stable conjugate base than 3-bromopentanoic acid. As a result, the former is more acidic.
The Inductive Effect and Aromatic Substitutions
In aromatic substitution reactions, the inductive effect plays a crucial role in determining both the reactivity of the aromatic ring and the orientation of the substituents (Directing Effects). As we discussed previously, the inductive effect refers to the electron-withdrawing or electron-donating influence of substituents transmitted through sigma bonds. The presence of electron-donating groups (EDGs), such as alkyl groups or groups with lone pairs like -OH and -OR, increase the electron density on the aromatic ring through their inductive effects, making the ring more reactive towards electrophiles and generally directing new substituents to the ortho and para positions.
Conversely, electron-withdrawing groups (EWGs), such as -NO2, -SO3H, and carbonyl-containing groups, decrease the electron density on the aromatic ring, making it less reactive towards electrophiles and typically directing new substituents to the meta position.
This phenomenon can be better understood by reading our article on Electrophilic Aromatic Substitution.