Transcript Slide 1 - Farmasi Unand

THEORIES OF ACIDS AND
BASES
The Arrhenius Theory of acids and bases
The theory
• Acids are substances which produce hydrogen ions in solution.
• Bases are substances which produce hydroxide ions in solution.
Neutralisation happens because hydrogen ions and hydroxide ions
react to produce water.
In the sodium hydroxide case, hydrogen ions from the acid are
reacting with hydroxide ions from the sodium hydroxide - in line
with the Arrhenius theory.
However, in the ammonia case, there don't appear to be any
hydroxide ions!
THEORIES OF ACIDS AND
BASES
The Bronsted-Lowry Theory of acids and bases
• An acid is a proton (hydrogen ion) donor.
• A base is a proton (hydrogen ion) acceptor
The relationship between the Bronsted-Lowry and Arrhenius
theory. The Bronsted-Lowry theory doesn't go against the
Arrhenius theory in any way - it just adds to it. Hydroxide ions
are still bases because they accept hydrogen ions from acids
and form water. An acid produces hydrogen ions in solution
because it reacts with the water molecules by giving a proton to
them.When hydrogen chloride gas dissolves in water to
produce hydrochloric acid, the hydrogen chloride molecule
gives a proton (a hydrogen ion) to a water molecule. A coordinate (dative covalent) bond is formed between one of the
lone pairs on the oxygen and the hydrogen from the HCl.
Hydroxonium ions, H3O+, are produced.
Conjugate pairs
When the acid, HA, loses a proton it forms a base, A-. When the
base, A-, accepts a proton back again, it obviously refoms the
acid, HA. These two are a conjugate pair.
Ammonia is a base because it is accepting hydrogen ions from the
water. The ammonium ion is its conjugate acid - it can release
that hydrogen ion again to reform the ammonia.The water is
acting as an acid, and its conjugate base is the hydroxide ion.
The hydroxide ion can accept a hydrogen ion to reform the
water.Looking at it from the other side, the ammonium ion is an
acid, and ammonia is its conjugate base. The hydroxide ion is a
base and water is its conjugate acid.
Amphoteric substances
A substance which can act as either an acid or a
base is described as being amphoteric.
The Lewis Theory of acids and
bases
The theory
• An acid is an electron pair
acceptor.
• A base is an electron pair
donor.
The relationship between the
Lewis theory and the
Bronsted-Lowry theory
Lewis bases
It is easiest to see the
relationship by looking at
exactly what BronstedLowry bases do when they
accept hydrogen ions.
Three Bronsted-Lowry
bases we've looked at are
hydroxide ions, ammonia
and water, and they are
typical of all the rest.
Conjugate pairs
Lewis acids
Lewis acids are electron pair acceptors. In the above example, the
BF3 is
acting as the Lewis acid by accepting the nitrogen's lone pair. On
the
Bronsted-Lowry theory, the BF3 has nothing remotely acidic about
it.
This is an extension of the term acid well beyond any common use.
A final comment on Lewis acids and bases:
• A Lewis acid is an electron pair acceptor.
• A Lewis base is an electron pair donor.
Acid/Base Chemistry
Fundamental Role in synthesis, analytical behavior, reactivity
including phsyiological behavior.
General Equation:
Strengths of Acids:
1. Inverse relationship rule
Acid/Base Chemistry
2. Periodic effects:
a. Acidity increases from left to right: CH < NH < OH < FH
b. Acidity increases from top to bottom: HI > HBr > HCl > HF; H2S >
H2O
Acid Strength depends on the ability of the conjugate base to
stabilize a negative charge.
a) Presence of electronegative elements:
Acid/Base Chemistry
2. Resonance stabilization:
Acid/Base Chemistry
Measure of Acid/Base Strength
Aqueous systems - pKa, pKb
Scale: 0 to 14, neutral = 7
Acid/Base Chemistry
Base strength is often reported in terms of pKa which is the
strength of the conjugate acid.
Example: Pyridine, pKa = 5.19 (Merck Index)
Rank the following compounds in order of relative basicity:
• Resonance stabilization Cyclohexylamine vs. aniline
Acid/Base Chemistry
Basicity trends for amines:
Amides are very weakly basic (pKa = -1)
Solvation effects:
Explanation: (CH3)3NH+ is less solvated in H2O
THE ACID-BASE BEHAVIOUR OF AMINO ACIDS
Amino acids as zwitterions
Zwitterions in simple amino acid solutions. An amino acid
has both a basic amine group and an acidic carboxylic
acid group.
THE ACID-BASE BEHAVIOUR OF AMINO ACIDS
Amino acids as zwitterions
Zwitterions in simple amino acid solutions. An amino acid
has both a basic amine group and an acidic carboxylic
acid group. There is an internal transfer of a hydrogen
ion from the -COOH group to the -NH2 group to leave an
ion with both a negative charge and a positive charge.
This is called a zwitterion.
THE ACID-BASE BEHAVIOUR OF AMINO ACIDS
Adding an alkali to an amino acid solution
If you increase the pH of a solution of an amino acid by
adding hydroxide ions, the hydrogen ion is removed
from the -NH3+ group.
You could show that the amino acid now existed as a negative ion
using electrophoresis.
THE ACID-BASE BEHAVIOUR OF AMINO ACIDS
Adding an acid to an amino acid solution
If you decrease the pH by adding an acid to a solution of an
amino acid, the -COO- part of the zwitterion picks up a
hydrogen ion.
This time, during electrophoresis, the amino acid would
move towards the cathode (the negative electrode).
THE ACID-BASE BEHAVIOUR OF AMINO ACIDS
Adding an acid to an amino acid solution
If you decrease the pH by adding an acid to a solution of an
amino acid, the -COO- part of the zwitterion picks up a
hydrogen ion.
This time, during electrophoresis, the amino acid would
move towards the cathode (the negative electrode). The
pH at which this lack of movement during
electrophoresis happens is known as the isoelectric
point of the amino acid. This pH varies from amino acid
to amino acid.
THE ACIDITY OF PHENOL
Why is phenol acidic?
Unlike alcohols (which also contain an -OH group) phenol is
a weak acid. A hydrogen ion can break away from the OH group and transfer to a base.
For example, in solution in water:
Phenol is a very weak acid and the position of equilibrium
lies well to the left. Phenol can lose a hydrogen ion
because the phenoxide ion formed is stabilised to some
extent. The negative charge on the oxygen atom is
delocalised around the ring. The more stable the ion is,
the more likely it is to form.
THE ACIDITY OF PHENOL
One of the lone pairs on the oxygen atom overlaps with the
delocalised electrons on the benzene ring.
THE ACIDITY OF PHENOL
This overlap leads to a delocalisation which extends from
the ring out over the oxygen atom. As a result, the
negative charge is no longer entirely localised on the
oxygen, but is spread out around the whole ion.