Inductive Effect

Inductive effect is a concept of general organic chemistry or GOC.

Now, you might wonder what GOC is.

Ever wondered why molecules behave differently? Some are acidic, some basic, and some surprise us with unexpected reactions. The answer involves a hidden tug-of-war within the molecule known as the inductive effect. It's not a physical fight between tiny atoms, but rather a subtle shift in the electron cloud, which can have significant consequences.

Imagine you and a friend are holding a rope, but your friend is much stronger. As they pull the rope, it naturally shifts towards them. Similarly, in a molecule, when atoms with different strengths in their "tug-of-war" (electronegativities) are connected, the shared electrons get pulled toward the stronger one. This creates a permanent shift in electron density, influencing the bonds throughout the molecule, and that's the inductive effect in action.

Now, let's talk about GOC, which stands for general organic chemistry. It's like the ABCs of organic chemistry and helps us understand basic things like the order of acidity or basicity in reactions, the likelihood of intermediates reacting, and the inductive effect.

What is The Inductive Effect?

The inductive effect is one category of the electronic displacement effect. 

An electron displacement effect is an effect that electrons displace from their position.

Inductive Effect

Inductive effect is a partial shifting/displacement of σ electrons towards a more electronegative atom of σ bond.

This means σ  (or single bond electrons) shifts towards a more electronegative atom. Because of this, partial charges develop.

Let’s take inductive effect examples:

1. C-C-C-X

We can see a σ-bond between C and X, both sharing one electron.

Assume electronegativity of X > electronegativity of C. Then,

Both the electrons in this bond wish to shift towards X as it can pull more electrons towards itself. This process occurs in the following manner:

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Here, a partial negative charge develops on X, i.e., δ-, and a partial δ+ on one C-atom, and then proceeding to the left, on the second C-atom, the partial charge is δδ+, then on the third C-atom, it is δδδ+ and so on. 

As we can see, the magnitude of the partial charge decreases as we go to the left, i.e., away from X.

Since there was an electronegativity difference, that’s why the shifting occurred.

Here, X is an electron-withdrawing group or - I group because it withdraws electrons from Carbon.

This effect is also known as a negative inductive effect (- I effect) or electron-withdrawing inductive effect.

Let’s take the example of CH3Cl.

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Here, Cl is more electronegative than C, so partial δ- is formed on Cl and a partial δ+ on C.

2. C-C-C-Y

Here, Y is less electronegative than C-atom.

Here, C pulls σ-electrons towards itself, and on carbon, a partial negative charge appears as δ-, while δ+ on Y-atom.

Here, Y is an electron-donating group or + I group because it donates electrons to Carbon.

This effect is also known as a positive inductive effect (+ I effect) or electron-donating inductive effect.

Note: Inductive effect considers the partial shifting of σ-electrons or a single bond only.

Order of Inductive Effect

Two types of order of inductive effect are:

1. - I order

We know that neutral species are more stable than charged species.

We also need to know one thing, the more the charge distribution occurs, the more stable is the species. 

This means, if the charge remains static, the species remains unstable. Therefore, species must have a charge flow for it to remain stable.

Let’s talk about - I order for the other species:

NF3+  > NR+ > NH3+  > NO2 > - C ≡ N (cyanide group) > - CHO (aldehyde) > -RC (= O) R’ > - R - COOH > F > Cl > Br > I > - O - R > - O  H > - C ≡ CH > - NH2  ⋍ ⏣ > CH = CH2 > H

Points to Ponder:

Here, R stands for an alkyl group.

NF3+, NR+,  and  NH3+  are highly electronegative or high - I effect showing groups.

• C ≡ N → Cyanide group

• CHO →  Aldehyde group

• RC(= O)R’ → Ketone group

• R - COOH → Carboxylic acid group

• O - R → Epoxy group

• O - H → Alcohol group

• The order of - NH2  is approximately equal to Benzene (⏣).

• I effect of H  ⋍ zero.

Let’s determine the order of stability of the following species given below:

1. F - CH2 - COO- 

2. Cl - CH2 - COO-

3. Br - CH2 - COO-

4. I - CH2 - COO-

We know that F, Cl, Br, I are electronegative elements, and they all show - I effect.

Like in F - CH2 - COO- 

An electron from the electron cloud of COO- distributes to C in CH2, i.e., 

F ← CH2 ← COO-. 

Since the order of electronegativity is F > Cl > Br > I, this means Fluorine is the most electronegative element.

So, the maximum distribution of electrons will occur in F - CH2 - COO-.

Therefore, the stability order is:

(F - CH2 - COO- ) > (Cl - CH2 - COO-) > (Br - CH2 - COO-)  > (I - CH2 - COO-)

However, if the species has a positive charge like F - CH2 - COO+. Here, F tries to pull the electron from COO+ as it doesn’t have electrons for distribution. Therefore, 

F - CH2 - COO+ becomes unstable.

Let’s take another example of stability order:

NO2  - CH2  - COO- > F - CH2  - COO- >  CH = CH - COO-.

Note:

More the - I effect in the species is, more is its acidic order.

F - CH2 - COOH on releasing  H- becomes F - CH2 - COO-.

Similarly, the other species on releasing H- becomes Cl - CH2 - COO-, Br - CH2 - COO-, I - CH2 - COO-

We already know that F - CH2 - COOH is the most stable species. So, the acidic order will be F - CH2 - COOH > Cl - CH2 - COOH > Br - CH2 - COOH > I - CH2 - COOH.

2. + I order

CHΘ > NHΘ> OΘ> O > -COO > - C(CH3)3 > - H - C - (CH3)2 > -CH2 - CH3 > - CH3

Where,

• C(CH3)3 is 3° alkyl group

• H - C - (CH3)2 is 2° alkyl group

• CH2 - CH3 is 1° alkyl group

• CH3 is a methyl group

∴ More the alkyl groups, more are the + I effect.

Inductive Effect on Stability of Molecules

The stability of a molecule depends on the charge of individual atoms and groups bonded to them, known as the inductive effect. For example, if a group with a -I effect is attached to a positively charged atom, the resulting molecule becomes less stable due to an increase in positive charge. Conversely, if a negatively charged atom is introduced to a group with a -I effect, the resulting molecule becomes more stable.

When a -I group is attached to a molecule, its electron density decreases, making it more likely to accept electrons and increasing its acidity. On the other hand, a +I group increases the electron density of a molecule, making it more basic by enhancing its ability to donate electrons.

Applications of the Inductive Effect

Illustration 1: Structures with more covalent bonds are more stable. 

(I) $\mathrm{H}_2 \mathrm{C} = \stackrel{\oplus}{\mathrm{N}} = \stackrel{\ominus}{\mathrm{N}}$

(II) $\mathrm{H}_2 \stackrel{\oplus}{\mathrm{C}} - \mathrm{N} = \stackrel{\oplus}{\mathrm{N}}$

(III) $\mathrm{H}_2 \stackrel{\ominus}{\mathbf{C}} - \stackrel{\oplus}{\mathrm{N}} \equiv \mathrm{N}$

(IV) $\mathrm{H}_2 \stackrel{\ominus}{\mathrm{C}} - \mathrm{N} = \stackrel{\oplus}{\mathrm{N}}$

The order of stability is I > III > II > IV.

Illustration 2: Demonstrates that electron-withdrawing groups (EWG) increase acidity, while electron-donating groups (EDG) decrease acidity. 

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The order of acidity in this case is d > c > e > a > b.

Illustration 3: 

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The order of acidity of protons is -COOH > -OH (Nitro substituted) > -OH > acetylenic proton.

Illustration 4: Shows the order of acidity as I > II > III > IV, with meta isomer being more acidic due to the -I effect.

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Illustration 5: Presents the order of basicity as I > III > II > IV, with I being the most basic due to the presence of an oxygen atom in III, which decreases basicity by the –I effect.

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Inductive Effect vs Electromeric Effect

The key differences between the electromeric and the inductive effects are:

Conclusion

The Inductive Effect plays a crucial role in organic chemistry, especially in the context of JEE Main. It primarily influences sigma bonds, imparting a permanent character to its impact. This effect is notable for its consistent nature, not relying on any attacking reagent for manifestation. Understanding the Inductive Effect is paramount for JEE Main aspirants, as it forms the foundation for comprehending various reactions and mechanisms in organic chemistry. Mastery of this concept enhances problem-solving skills and enables students to navigate complex questions with confidence.