Depto. Química Orgánica

Acids and Bases

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15/07/2017
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Vídeo ácidos y bases

WE ALL KNOW THEM: THE ACIDS

Acids are very familiar substances.  They are characterized by having hydrogen atoms (red) bonded to very electronegative atoms such as oxygen (blue) o el cloro (green).

CH3-COOH (HOOC-CH2)2C(OH)COOH
HClCH3CH(OH)COOH
Acetic acid is in vinegarCitric acid is in lemon or orange juicesChlorhydric acid is in the gastric juiceLactic acid is in milk
They have a characteristic flavor. Some are too strong and dangerous to try. Hydrogen atoms bonded to electronegative atoms break away easily. That is the chemical characteristic that makes an acid so.
Image Simulated acid burning

A bond between two atoms of a molecule can be broken in two ways:

HOMOLYTIC CLEAVAGE


The two electrons of the bond are divided between the two initially bonded atoms. This cleavage is called homolytic and very difficult.

HETEROLYTIC CLEAVAGE


The two electrons in the bond stay on the side of the more electronegative atom of the two that formerly form the bond. This is the most common way of bond cleavage.

Image
When hydrogen is bonded to an atom of moderate electronegativity, the cleavage is homolytic (usually very difficult).
Image
When hydrogen is bonded to an atom of high electronegativity, the cleavage is heterolytic (usually much easier). 
A bond that is broken very easily in a heterolytic way is the one that unites the electronegative atoms of a molecule with hydrogen. The electronegative atom keeps the electrons (heterolytic cleavage) and a "proton" is released, that is, a hydrogen atom "naked" of electrons. Molecules that lose "protons" easily belong to substances called acids.
Image Model of sulfuric acid (H2SO4) losing a proton
Image Sulfuric acid (H2SO4) is a very strong acid that loses a "proton" easily. When the molecule "keeps" the two electrons from the bond, it acquires a negative charge that offsets the positive charge of the "proton."

WE ALL KNOW THEM: BASES

Bases are also very familiar substances.  They are characterized by having very electronegative atoms such as oxygen (red) and nitrogen (blue) that can accept protons, even if they are already have some of them.

NaOHNH3
NaCO3HCaCO3
Sodium hydroxide or "soda" is used used to unclog pipesAmmonia is a cleaning product se usa como producto de limpiezaSodium bicarbonate is a stomach antacidCalcium carbonate is the main constituent of marble
A bond that forms very easily is the one established between a "proton" and an atom, of a molecule, with unshared pairs of electrons. Molecules that accept "protons" easily belong to substances called bases.
It is logical to think that acids and bases react easily with each other, mutually transferring a "proton."
Acids react with bases by transferring a proton. The product of the reaction is a salt, formed by an anion (negatively charged molecule) and a cation (positively charged molecule).
Image The acid loses a proton and the acid molecule (originally neutral) becomes negatively charged and becomes a base.
The base accepts a proton and the base molecule (originally neutral) becomes positively charged and becomes an acid.
The combination of anion and cation forms the resulting salt.