1. Organic Chemistry Reviews Chapter 3
Cindy Boulton
September 7, 2008

2. Types of Reactions Addition Elimination Substitution Rearrangement
Two molecules become one
Elimination
Opposite of addition
One molecule loses elements that become a separate molecule
Substitution
One group replaces another
Rearrangement
A molecule undergoes reorganization
Forms a constitutional isomer

3. Homolytic and Heterolytic
Breaking of Covalent Bonds!
Homolytic
Bond broken and 1 electron goes to each fragment
Forms radicals
Contain one unpaired electron
Reactive and unstable
Single headed arrow
Heterolytic
Both electrons go to more electronegative fragment
Form ions
Cation- positive charge from loosing an electron
Anion- negative charge from gaining a lone pair of electrons
Double headed arrow

4. Carbocation and Carboanion
Heterolytic bond cleavage of a bond to a carbon
Unequal sharing of electrons in bond
Carbocations
Electron deficient
Positive charge
Loose electrons to more electronegative element
Carboanions
Extra electrons
Negative charge
Gain electrons from less electronegative element

5. Acids and Bases Acid Base Conjugate acid Conjugate base
Bronsted-Lowery: Substance that can donate or lose a proton
Lewis: electron pair acceptors
Base
Bronsted-Lowery: Substance that can accept or remove a proton
Lowery: electron pair donors
Conjugate acid
Ion that forms when a base accepts a proton
Conjugate base
Ion that forms when an acid loses its proton

6. Strength of Acids and Bases
pKa = -log(Ka)
pKa
Lower number, more acidic
How strongly it hold on to it proton H+ Ka
Higher number, more acidic
Tells strength and how readily the proton H+ breaks
pKa Chart
List of acids and their pKa value
Used to predict acid-base reactions

7. Structure and Acidity
Covalent bond between atoms with different electronegativity
Partial negative charge: δ-
Partial positive charge: δ+
Greater electronegative difference between two atoms bonded create greater partial charges
Electronegativity- how much an atom wants electrons
Increase left to right across periodic table
Increases bottom to top down periodic table
More electronegative -> more acidic -> stronger acid -> lower pKa value
Examples: CH4, NH3, H2O, HF

8. A troubling observation
With halogens the most electronegative element (F) is not the strongest acid
H-F most electronegative, pKa=3.2, weak acid, strong bond
H-I least electronegative, pKa= -11, strong acid, weak bond
Acidity increases left to right
Electronegativity effect
Acidity increases top to bottom
Bond strength effect
Anion stability increases due to size of atom

9. Inductive and Electronegative Effect
As the electronegativity of the atom the H is attached to increases, the acidicy also increases
By adding electronegative atoms, pKa value reduced and inductive effect increased
Inductive effect- transmission of a charge or electron- attraction ability through space and bonds of molecule
Inductive effect weakens due to distance or a spacer
Example

10. Why are Carboxylic Acids acidic?
Inductive Effect
Electron pulled away from H to both Oxygen and any other electronegative elements attached
Resonance Stabilization
The conjugate base or anion is stable
Anion is not reactive and won’t reverse the reaction
Resonance Hybrid
Negative charge is spread out over both Oxygen- partial negative chard δ-
Double bond and Single bond => 1 ½ bond

11. Using pKa values to predict reactions
Identify base, acid -> conjugate acid, conjugate base
Assign pKa values to Acid and Conjugate Acid
Reaction proceeds in direction of forming a weaker acid- pKa values increase
Δpka > 2, then 100% - reaction goes to completion
ΔpKa < 2, then equilibrium

12. Curvy arrow notation Reaction mechanism
Shows electron flow when bonds are breaking in forming
Always start at a pair of electrons

13. Isotopic labeling Deuterium Tritium Isotopes of Hydrogen
2 H - 2 neutrons, 1 proton D+
Tritium
3 H - 3 neutrons, 1 proton T+
Isotopes of Hydrogen
Protons source
Radioactive- used as tracer of Hydrogen