Question

What are the `-"I"` and `+"I"` inductive effects?

Answer 1

WARNING! Long answer! Inductive effects are the effects on rates or positions of equilibrium caused by the polarity of the bond to a substituent group.

Explanation:

A -I effect or negative inductive effect occurs when the substituent withdraws electrons.

A +I effect or positive inductive effect occurs when the substituent donates electrons.

Inductive effects

Consider a `"C-F"` bond.

The highly electronegative `"F"` atom will draw the electrons in the `"C-F"` bond more closely toward itself.

The bond will be polarized, with the `"F"` atom getting a partial negative (`δ^"-"`) charge and the α-carbon atom getting a partial positive (`δ^"+"`) charge.

`underbrace(stackrelcolor(blue)(δδδδ^"+")("C"))_color(red)(δ)-underbrace(stackrelcolor(blue)(δδδ^"+")("C"))_color(red)(γ)-underbrace(stackrelcolor(blue)(δδ^"+")("C"))_color(red)(β)-underbrace(stackrelcolor(blue)(δ^"+")("C"))_color(red)(α)-stackrelcolor(blue)(δ^"-")("F")`

The α-carbon will in turn withdraw some electron density from the β-carbon, giving it a smaller partial positive (`δδ^"+"`) charge.

The inductive removal of electron density is passed with diminishing effect through the chain of `"C-C"` σ-bonds until it is almost negligible at the δ-carbon.

`"C"` is less electronegative than `"H"`, so alkyl groups are electron releasing.

`underbrace(stackrelcolor(blue)(δδδδ^"-")("C"))_color(red)(δ)-underbrace(stackrelcolor(blue)(δδδ^"-")("C"))_color(red)(γ)-underbrace(stackrelcolor(blue)(δδ^"-")("C"))_color(red)(β)-underbrace(stackrelcolor(blue)(δ^"-")("C"))_color(red)(α)-stackrelcolor(blue)(δ^"+")("R")`

Again, the effect is passed along the chain of carbon atoms but it dies out rapidly with distance.

-I effect

The strength of a carboxylic acid depends on the extent of its ionization: the more ionized it is, the stronger it is.

As an acid becomes stronger, the numerical value of its `"p"K_text(a)` drops.

Thus, for example. the order of acidity as shown by the `"p"K_text(a)` values is

`underbrace("Br-CH"_2"CH"_2"CH"_2"COO-H")_color(red)(4.59) < underbrace("Br-CH"_2"CH"_2"COO-H")_color(red)(4.01) < underbrace("Br-CH"_2"COO-H")_color(red)(2.86)`

The electronegative `"Br"` atom removes electron density from the atoms next to it, eventually weakening the `"O-H"` bond at the other end of the chain and making the compound more acidic.

Since this effect is caused by the inductive removal of electrons, it is called
a -I effect

+I effect

In the same way, an electron-donating alkyl group decreases the acidity of a carboxylic acid.

Thus, for example. the order of acidity as shown by the `"p"K_text(a)` values is

`underbrace("CH"_3"-CH"_2"COO-H")_color(red)(4.87) < underbrace("CH"_3"-COO-H")_color(red)(4.76) < underbrace("H-COO-H")_color(red)(3.74)`

Thus, acetic acid is weaker than formic acid, and propionic acid is weaker than acetic acid.

Since this effect is caused by the inductive donation of electrons, it is called
a +I effect