Organic Acids and Bases

Organic acids and bases are acids & bases that contain a carbon skeleton  and also a number classes of neutral proton acids and bases (uncharged acids and bases)

Three parts of this study are

1.      Examines 3 main types of neutral organic proton to see why they are acids & why they have widely different acid strengths

2.      Look at the 2 main types of neutral organic bases

3.      Look at positively charged carbon acids & negatively charged carbon bases 

Three main types of neutral organic B-L acids

1.      Carboxylic acids

2.      Phenols

3.      Alcohols

Similarity of them is has an ^_OH group (acidic because of EN difference between O and H in O-H bond)  and the differences of them is in stability of the conjugate base.

Which One Is The Most Acidic...?

Carboxylic Acids

They are rather weak acids with pKa values of acetic acid is 4,8. Carboxylic acids are weaker than mineral acids but  they are the strongest acid of the neutral organic acids.

Such pKa HCl  =   -7,0 and HNO3    =   -5,2.

The reason are with see the strength of their conjugate base and resonance stabilized (Carboxylate ion resonance à reduces the energy of the anion and makes the carboxylic acid more acidic).

Phenols

pKa phenol = 10,0 (indicating that in water only very small portion of it ionizes). pKa carboxylic acid (=4,8) lower than pKa phenol (=10)  and phenol stabilization is less significant than its for carboxylic acid.

Two reason are the resonance stabilization of the phenolate ion disrupts the aromaticity of the aromatic ring. And the resonance stabilization in phenol places a negative charge on the C, which when compared to O, are not very EN.

Alcohols

are much less acidic than phenols with pKa typical alcohol = 15 – 18 in aqueous media

. Low acidity of alcohols à there is no resonance stabilization of the conjugate base.

Two Main Types Of Neutral Organic Bases

Amines

Ethers

Similarity : Contain one or more pairs of non bonding e, which available to :

(1)  donate to a Lewis acid or

(2)  accept a proton when the base is acting as a B-L base

Amines

•         derivates of ammonia (NH₃)

•         Most are weak bases in aqueous media

Why amines are stronger bases than other neutral organic bases ?

•         e non bonding pairs on N is more available than e non bonding pairs on others

•         Others : O, S or X

•         N holds its e less tightly than others

Ethers

•         weak bases in aqueous media

•         are good Lewis bases in non aqueous media, forming stable complexes with Lewis acids.

Positively Charged Carbon Acids (Carbocation / Carbonium Ion)

•         e deficient (contain a C without an octet of e)

•         very reactive reaction intermediates

•         a Lewis acid (reacts with the first available Lewis base)

•         a Hard acid (prefers a hard base)

•         1 empty p orbitals 

Negatively Charged Carbon Bases (Carbanion) 

•         1 pair of non bonding e

•         the structure is much like an amine

•         a strong base (C is not very EN)

referensi

 Organic Chemistry Chapter 5 Acid – Base Theory (2) by Richard F. Daley & Sally J. Daley

www.ochem4free.com