Lecture 7 - Relationship of Structure & Acidity

Question 1

the 5 factors that contribute the acidity of an organic molecule:

Answer 1

(1) the charge effect

(2) the element effect

(3) the hybridisation effect

(4) the resonance effect

(5) the polar effect

Question 2

the charge effect:

Answer 2

positively charged compounds attract electrons better than neutral ones hence making them more acidic

Question 3

the element effect:

Answer 3

evaluate the atom attached to the proton

as you go left along the a period the acidity will increase due to increased electronegativity of the atom attracting more negative charge and more easily releasing protons (dissociating)

increase in acidity also takes place down a group due to increasing atomic radii, resulting in weaker bonds (due to the longer bond distance) that allow protons to dissociate more easily

Question 4

the hybridisation effect:

Answer 4

bronsted acidity depends on the hybridisation state of the atom bonded to the acidic H

increasing s character = increasing acidity

Question 5

the resonance effect:

Answer 5

resonance lowers the energy of a compound

stabilisation of the conjugate base increases acidity

Question 6

the polar effect:

Answer 6

carboxylic acids illustrate the polar effect - the conjugate base is resonance-stabilised

electrostatic interactions can be stabilising or unstabilising

electronegativity substituents increase the acidity of carboxylic acids (inductive effect)

Halogens and other electronegative groups exert an electron-withdrawing polar effect that lowers the pKa of carboxylic acids

•Other groups can exert an electron-donating polar effect

•This raises the pKa of carboxylic acids