Organic Chemistry ReviewsChapter 3

Cindy Boulton

September 7, 2008

Types of Reactions Addition

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

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

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

Acids and Bases Acid

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

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

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

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

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

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

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

Curvy arrow notation Reaction mechanism Shows electron flow when bonds are breaking

in forming Always start at a pair of electrons

Isotopic labeling Deuterium

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