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