El efecto inductivo se relaciona con el efecto que ejerce los sustituyentes sobre la molécula, influenciando en su acidez y basicidad. Este efecto es ampliamente observable en los compuestos orgánicos ya que estos en su mayoría presentan enlaces covalentes simples. Una característica predominante de dicho fenómeno es el desplazamiento electrónico lo que permite estabilizar o desestabilizar la carga de las moléculas. Esto nos proporciona la posibilidad de relacionar la reactividad con la acidez de la molécula.
Los sustituyentes pueden tener como objetivo en la molécula, de actuar como activadores o desactivadores. Los sustituyentes que introducen electrones son los activadores; estos desestabilizan la carga y disminuyen la acidez de las moléculas. Por su parte, los sustituyentes que extraen electrones son los desactivadores; estos estabilizan la carga y aumentan la acidez, como se observa en la siguiente ilustración:
Existen dos tipos de efectos inductivo, y estos dependerán de la dirección de dicho efecto y su relación con las electronegatividades de los elementos. Si un átomo o grupo de ellos son más electronegativos que el hidrógeno, esto indica que estamos en presencia de un efecto inductivo negativo (-I). Al contrario, si el o los átomos son menos electronegativos que el hidrógeno y atrae los electrones hacia él, nos indica que es un efecto inductivo positivo (+I).
Sustituyentes activadores y desactivadores
Activadores
Fuertes: -OH; -NH2; -SH
Moderados: -O-R; -S-R; -S-S-R
Débiles: -R
Desactivadores
Fuertes: -COOH; -CO-H; -CO-R; -COO-R; -NO2; -C≡N; -SOO-OH y todos los grupos polarizables.
Débiles: -F,- -Cl, -Br, -I (extraen electrones por orden de electronegatividad)
ejercicios
- Ordenar en forma creciente a su acidez, a los siguientes compuestos: ácido propiónico, ácido valérico, ácido acético y ácido butírico.
En este ejercicio se puede observar que estamos en presencia de grupos alquilos, que son activadores débiles, por lo tanto al aumentar la cantidad de activadores, disminuirá la acidez de las moléculas. Entonces, el orden seria el siguiente: b<d<a<c.
La molécula más ácida en el ácido acético y la menos ácida es el ácido valérico.
- Ordenar en forma decreciente a su acidez, a los siguientes compuestos: ácido acético, ácido dicloroacético, ácido tricloroacético y ácido cloroacético.
En este tipo de ejercicios se observa el efecto inductivo aceptor, también llamado efecto inductivo negativo, ya que el átomo de cloro es más electronegativo que el hidrógeno, lo que permite que actúe como desactivador (atrae electrones hacia él) e induce la carga positiva en los otros átomos. En palabras más simples, a mayor cantidad de átomos de cloro que estén presentes en la molécula, mayor acidez tendrá la misma. El orden seria el siguiente: IV> III>II>I.
La molécula más ácida es el ácido tricloroacético y la menos ácida es el ácido acético.
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