ochem exam 2

Types of diastereomers

• Z: the higher priority groups are on the same side of the diastereomer

• E: the higher priority groups are on the opposite side of the diastereomer

Nucleophiles

• Nucleophilic centers are capable of donating a pair of electrons, and react with a positive charge

• Lone pairs and pi bonds are nucleophiles

Electrophiles

• Electrophilic centers are capable of accepting a pair of electrons, and react with a negative charge

• Carbocations/empty p orbitals are electrophiles

Meso compounds

Exhibits reflectional symmetry with chiral centers, and does not have an enantiomer

Fischer projections

• Convey the configuration of chiral centers

• All horizontal lines are wedges and all vertical lines are dashes

Alkene substitutions

• Mon-, di-, tri- and tetra- refers to how many alkyl groups are attached to the double bond in an alkene

• The stability of an alkene increases with the number of substituents

Determining number of reaction steps from energy diagram

Count the number of peaks/transition states

Exothermic reaction

ΔH° (bond dissociation energy) is negative

Endothermic reaction

ΔH° (bond dissociation energy) is positive

Energy of activation

The energy barrier between the reactants and the products that represents the minimum amount of energy required for a reaction to occur between two colliding reactants

Intermediates

Represented by the local minima/valleys on an energy diagram

Transition states

Represented by the local maxima/peaks on an energy diagram

Spontaneous reaction

ΔStot (total entropy) is positive and ΔG is negative

Nonspontaneous reaction

ΔStot is negative and ΔG is positive

Good leaving groups

• OTS

• H2O+

• I

• Br

• Cl

Base only

• H-

• NH2-

Nucleophile only

• H2S

• HS-

• I-

• Br-

• Cl-

• F-

Weak nucleophile/weak base

• H2O

• R-OH

Strong nucleophile/strong base

• HO-

• RO-

SN1

• Unimolecular

• First order

• Better with more substituents

• Needs a weak nucleophile

• Faster in polar protic solvents (hydrogen bonding)

• Hindered substrates react quickly

• Inverts and retains configuration

• Begins with the loss of a leaving group to give a carbocation intermediate, then a nucleophilic attack to the front or back

SN2

• Bimolecular

• Second order

• Better with less substituents

• Needs a strong nucleophile

• Faster in polar aprotic solvent (non-hydrogen bonding)

• Unhindered substrates react quickly

• Inverts configuration

• A nucleophile attacks the alkyl halide from the backside, causing the loss of a leaving group in a concerted/simultaneous fashion

E1

• Unimolecular

• First order

• Better with less substituents

• Needs a weak base

• Faster in polar protic solvents (hydrogen bonding)

• Hindered substrates react quickly

• Begins with the loss of a leaving group to give a carbocation intermediate, then deprotonation by the solvent to give an alkene in a stepwise fashion

E2

• Bimolecular

• Second order

• Better with more substituents

• Needs a strong base

• Faster in plar aprotic solvents (non-hydrogen bonding)

• A base removes a proton, causing the loss of a leaving group in a concerted/simultaneous fashion

• More substituted (more carbon attached) and trans alkene is the Zaitsev/major product, and the less substituted (less carbon attached) and cis alkene is the Hofmann/minor product

Heat favors…

Elimination reactions

Substitution/elimination processes can only occur…

When a leaving group is present

Good leaving groups are…

Conjugate bases of strong acids

The more carbon atoms that are directly attached to the carbocation…

The more stable it is

To predict if a carbocation will rearrange…

Identify if a more stable carbocation can be formed by a shift of a hydrogen (hydride shift) or an alkyl group (alkyl shift) from an adjacent carbon

Nucleophile only mechanisms

• 1º: SN2

• 2º: SN2

• 3º: SN1

Base only mechanisms

• 1º: E2

• 2º: E2

• 3º: E2

Strong nucleophile/strong base mechanisms

• 1º: SN2 major + E2 minor

• 2º: E2 major + SN2 minor

• 3º: E2

Weak nucleophile/weak base mechanisms

• 1º: SN2 + E2

• 2º: SN2 + SN1 + E2 + E1

• 3º: SN1 + E1

SN1 outcome

• Regiochemical outcome: the nucleophile attacks the carbocation, which is generally where the leaving group was originally connected unless a carbocation rearrangement took place

• Stereochemical outcome: the nucleophile replaces the leaving group with racemization

SN2 outcome

• Regiochemical outcome: the nucleophile attacks the a position where the leaving group is connected

• Stereochemical outcome: the nucleophile replaces the leaving group with inversion of configuration

E1 outcome

• Regiochemical outcome: the Zaitsev product is always favored over the Hofmann product

• Stereochemical outcome: the process is stereoselective; when applicable, a trans disubstituted alkene will be favored over a cis disubstituted alkene

E2 outcome

• Regiochemical outcome: the Zaitsev product is generally favored over the Hofmann product, unless a sterically hindered base in used, in which case the Hofmann product will be favored

• Stereochemical outcome: the procuess is both stereoselective and stereospecific; when applicable, a trans disubstituted alkene will be favored over a cis disubstituted alkene