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Ch 4 : Analyzing Organic Reactions

Terms in this set (31)

Lewis Acid - Electron Acceptor
An atom, ion, or molecule that accepts an electron pair to form a covalent bond

Tend to be electrophiles
Lewis Base - Electron Donor
An atom, ion, or molecule that donates an electron pair to form a covalent bond

Tend to be nucleophiles
Coordinate Covelent Bonds
Covalent bonds in which both electrons in the bond come from the same starting atom
Bronsted-Lowry Acid
a molecule or ion that is a proton donor
Bronsted-Lowry Base
a molecule or ion that is a proton acceptor
Amphoteric
Molecules that can act as both a Bronsted-Lowry Acid or Base

Ex : H20

The acidity and basicity of a molecule is dependent on its properties of the solution the molecule is in

Water can only act as a base in acidic solutions and as an acid in basic solutions
Acid Dissociation Constant - Ka
Measures the strength of an acid in solution


Ratio of the concentration of the dissociated form of an acid to the undissociated form

Stronger acids have larger K(a) values than weaker acids
pKa
Ionization constant of a chemical compound.
the pH at which half of the molecules of that species are deprotonated

Stronger acids have smaller pKa values than weaker acids, pKa = -logKa
Alpha-Carbon
The carbon adjacent to a carbonyl;

Because enol forms of carbonyl-containing carbanions are stabilized by resonance acidic hydorgens are easily lost
Alpha-Hydrogen
Hydrogen attached to the alpha-carbon

Due to the resonance stability of the carbonyl carbon, these hydrogns are easily lost and are typically the first to deprotonate
Acidity of Common Functional Groups
Alkanes < Alkenes < H2 < Amines < Alkynes < Ketones < Alcohols < Water < Carboxylic acids < Hydronium Ion

Alcohols, Aldehydes, Ketones, Carboxylic Acids, and Carboxylic Acid derivatives tend to act as acids - Are easier to target with nucleophilic reactants


Amines and Amides tend to act as bases - the Nitrogen of these compounds tend to form coordinate covalent bonds by donating a lone pair of electrons to the Lewis Acid
Nucleophiles
Nucleus loving species with either lone pairs or pi bonds that are used to form bonds with electrophiles

Tend to be electron rich and act similar to bases

Good nucleophiles tend to be good bases

Nucleophilicity is determined by reactions rates with common electrophiles - kinetic property

Basicity is dependent on the equilibrium position of the reaction - thermodynamic property


Examples -
Anions (Br, OH, CN ions)
Alkenes and alkynes
Atoms with lone pairs (Water, and Ammonia)
Factors of Nucleophilicity
Charge - Nucleophilicity increases with increasing electron density, negative charge

Electronegativity - Nucleophilicity decreases with increasing electronegativity as atoms are less likely to share electrons

Steric Hinderance - Nucleophilicity decreases with larger bulkier molecules

Solvent - protic solvents decrease nucleophilicity, by protonating the nucleophile or through hydrogen bonding
Solvent Effects ???
In polar protic solvents, nucleophilicity increases down the periodic table
.

Common protic solvents are carboxylic acids, ammonia/amines, and water/alcohols. In polar aprotic solvents

In aprotic solutions, nucleophilicity increases up the periodic table. Common aprotic solvents are DMF, DMSO, and Acetone.
Electrophiles
Electron loving species with a positive charge or positively polarized atom.

Accepts an electron pair when forming new bonds with a nucleophile

Greater degree of positive charge results in greater electrophilicity

Good Electrophiles : Carbocations, carbonyls, then alcohols

Electrophilicity and Acidity are identical properties
Leaving Groups
Molecular fragments that retain electrons after heterolysis

The best leaving groups will be able to stabilize the extra electrons.


Weak bases are more stable with an extra set of electrons and therefore make good leaving groups.

Conjugate bases of strong acids make good leaving groups.

Alkanes and hydrogens almost never serve as leaving groups - form very reactive basic ions
Heterolytic Reactions
Essential opposite of coordinate covalent bond formation - when the bond is broken both electrons are given to 1 of the 2 products
Nucleophilic Substitution Reactions
Occurs when the nucleophile forms a bond with a substrate carbon and the leaving group leaves

SN1 and SN2 Reactions
SN1 Reaction
Nucleophilic substitution reaction

Has 2 Steps :

1. Leaving group leaves generating positively charged carbocation
- Rate limiting step
2. Nucleophile attacks the carbocation resulting in the substitution product
Carbocation
An organic ion in which a carbon atom has a positive charge

Forms when the leaving group leaves the molecule


The formation and stabilization of this determines all other aspects of SN1 reactions
Unimolecular Nucleophilic Substitution
(SN1) Reactions
Bimolecular Nucleophilic Substitution
(SN2) Reactions
SN2 Reactions
Nucleophilic substitution reaction

Has 1 Step :

Nucleophile attacks the compound at the same time as the leaving group leaves
Single rate-limiting step involved two molecules
Concerted Reaction
SN2 reactions

Nucleophile attacks the compound at the same time as the leaving group leaves.

Single rate-limiting step involved two molecules.
Backside Attack
Nucleophile displaces the leaving group this way.
Nucleophile must be strong and the substrate can't be sterically hindered.
Oxidizing Agents and Reactions
element or compound in redox reactions that accepts electron from another species
because it gains electrons, it is reduced
good oxidizing agents:

high affinity for electrons or have high oxidation states ; O2, O3, Cl2, MnO4-, CrO42-, dichromate and PCC; often contain a metal and a large # of oxygen atoms
Reducing Agents and Reactions
element or compound in redox reactions that loses electron from another species
because it loses electrons, it is oxidized
good reducing agents:

have low EN and ionization energies or contain a hydride ion (H-); often contain a metal and a large # of hydrides; examples are sodium, magnesium, aluminum, zinc, sodium hydride (NaH), calcium dihydride (CaH2), lithium aluminum (LiAlH4) and sodium borohydride (NaBH4)
Chemoselectivity
Preference of one functional group over other functional groups
Steric Hinderance / Prevention
Prevention of reactions at a location.

Protects the reaction of synthesis formation.
Protecting Group
Convert certain functional groups to prevent reactions from occurring.
Steps to Problem Solving
1. Nomenclature
2. Identify functional groups
3. Identify reagents
4. Identify most reactive functional groups
5. Identify first step of the reaction
6. Consider steroselectivity
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