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Principles of Organic Chemistry
1. LEWIS STRUCTURES, LEWIS ACIDS AND
BASES,
AND LA/LB REACTIONS
LEWIS STURUCTURE
Lewis structure
a diagram that shows the bonding between atoms and
the lone pairs of electrons of a molecule
* Writing Lewis structures
1.Determine # of valence electrons (neutral molecule, cation, anion)
2.Connect the atoms with only single bonds
3.Place a sufficient number of non-bonded electron pairs around
each atom (octet) - use double or triple bonds if needed.
4.
Determine the formal charge on each atom
Ex) C2H4 (ethylene)
Ex) C2H2 (acetylene)
FORMAL CHARGE
* Formal charge
The charge which is assigned to an atom in a molecule, assuming that
electrons in a chemical bond are shared equally between atoms,
regardless of relative electronegativity.
Formal charge = X – Y - Z
X = # of valence electrons
Y = # of non-bonded electrons
Z = half # of bonded electrons
PRACTICE
formal charge
PRACTICE
nitrosyl chloride
formal charge and resonance
PRACTICE
PRACTICE
Common
Functional Groups
LEWIS ACIDS AND BASES
Lewis acid: electron pair acceptor
Lewis base: electron pair donor
Definition of Acid and Base
Arrhenius :
H+/ OHBrønsted-Lowry: H+ donor/acceptor
Lewis:
e- pair acceptor/ donor
- Amines are stronger LB than alcohols and ethers. Why?
hint: check the electronegativity of N and O
- Aliphatic amines are stronger LB than some aromatic amines. Why?
In aromatic amines the non-bonding e- pair on N is delocalized over
the aromatic ring.
LEWIS ACID/BASE REACTIONS
1. Writing the Lewis structures of LA and LB
2. Using a curved arrow to show the electron pair movement
3. Determining the formal charges on the two atoms
Ex)
LEWIS ACID/BASE REACTIONS- EXAMPLES
Strong acids are fully ionized in solution. Represent them as ions.
LEWIS ACID/BASE REACTIONS- EXAMPLES
Carbonyl carbon (δ+) as a Lewis acid
Nucleophile in SN2 reaction as a Lewis base