ACID – BASE

Chemical reactions in Organic Chemistry nearly all of them can be classify in to acid – base reactions. So Key to understanding organic chemical reaction knowledge of acids & bases

THREE SIGNIFICANT MOLECULAR CHARACTERISTICS EFFECT ACIDITY & BASICITY :
• The compound’s primary functional group
• The inductive effect caused by the presence of additional functional group
• The delocalization / resonance effects of the e in a molecule

Arrhenius’s Acid
A substance that when added to water, increases the concentration of hydronium ions,H3O+ or hydrogen ions, H+ (proton) and source of H+ ion.

Arrhenius’s Base
A substance that when added to water, increases the concentration of hydroxide ions, -OH
and source of -OH ion.

Acid-Base of Bronsted-Lowry
• Acid :
any molecules or ion that donates a proton to another molecules or ion (= proton donor)
• Base :
any molecules or ion that receives that proton (= proton acceptor)

An equilibium reactions
When an acid & a base react with each other ≈ the reactants & products are in equilibrium with each other. It means the reactants on the left side of the equation are reacting & forming product and the products on the right side are also reacting & forming the starting reactants.

Lewis’s Acid-Base
• Acid : a molecule that forms a covalent bond by accepting a pair of e (= e pairs acceptor)
• Base : a molecule that forms a covalent bond by donating a pair of e (= e pairs donor)

Electrophile & Nucleophile
Electrophile = lover of e (seeks electrons (- charge)
Nucleophile = lover of nuclei (seeks a nucleus (+ charge))

Acid and Base Strength
Depends on the extent to which it ionizes in water. Chemists use the AUTOIONIZATION of pure water to determine the values for the concentration of acidic and basic solutions.


Autoionization

The reaction of 2 molecules of water with each other to give a hydronium ion and a hydroxide ion.The amount of autoionization at 25oC = 10-7 M. Chemist call this a neutral solution. It means If u add a compound that is more acidic than water, u increase conc. of H30+ and make the solution acidic. If u add a compound that is more base than water, u increase conc. of –OH and make the solution basic.
Strong-Weak Acid & Base

To determine the relative strength of an acid or a base, you need to find out how much the acid or the base ionizes or dissociates in water at equilibrium. Strong acids & bases à ionize completely in water. Weak acids & bases à ionize only partly in water.

The position of equilibrium is on the side of the weaker member of the acid conjugate base pair or base conjugate acid pair.

Hard Soft Acids and Bases (HSAB)

Direction of the reaction & the stability of the products, depend on : the strength of acid & base and the hardness or softness of acid & base (qualitative measure).

Acid

Soft
 e pair acceptor atoms are large
 Have a low positive charge density
 Contain unshared pairs of e in their valence shell (in the p or d orbitals)
 Have a high polarizability
 Have a low EN
Ex. halogens, phosporus & sulfur compounds

Hard
 e pair acceptor atoms are small
 Have a high positive charge density
 Contain no unshared pairs of e in their valence shell
 Have a low polarizability
 Have a high EN
Ex. hydrogen ion

BASE

Soft
 The donor atoms are large (hold their valence e loosely)
 Have a high polarizability
 Have a low negative charge density
 Have a low EN
Ex. cyanide ion, iodide ion

Hard
 The donor atoms are small (hold their valence e tightly)
 Have a low polarizability
 Have a high negative charge density
 Have a high EN
Ex. hydroxide ion

HSAB Principle

Hard acids prefer to bond with hard basesSoft acids prefer to bond with soft bases
. helps to predict the outcome of an acid-base reaction.

Base vs Nucleophile

HSAB Principle à help to determine whether a particular compound will act as a base or as a nuclepohile
A soft base is a good nucleophile
A hard base is a better base
A Nucleophile : generally reacts with a positive or partially positive carbon
A Base : generally reacts with a positive or partially positive hydrogen




referensi
by Richard F. Daley & Sally J. Daley. www.ochem4free.com