Organic Chemistry Final Review

What is a nucleophile?

A species with a lone pair or π bond that donates electrons

What is an electrophile?

A species that accepts electrons (electron poor)

What is a leaving group?

A group that can leave with an electron pair to form a stable species

What makes a good leaving group?

Weak bases (I- > Br- > Cl-, sulfonate ions)

What is a reaction mechanism?

A step-by-step sequence showing electron flow with curved arrows

What is nucleophilic attack?

Step where a nucleophile donates electrons to an electrophilic carbon

What is loss of leaving group (dissociation)?

Breaking a bond between carbon and the leaving group

What is proton transfer?

Movement of a proton from one species to another

What is carbocation rearrangement?

Shift of H or CH3 to form a more stable carbocation (hydride/methyl shift)

Which mechanisms can include rearrangements?

SN1, E1, addition of HX/H2O/alcohols to alkenes

Which mechanisms never include rearrangements?

SN2, E2, halogen addition to alkenes (X2), oxymercuration

Order of carbocation stability?

3° > 2° > 1° > methyl

What determines mechanism choice?

Substrate, nucleophile/base strength, solvent, LG quality

What are constitutional isomers?

Molecules with same formula but different connectivity

What is an eclipsed conformation?

Highest torsional strain; substituents overlap in Newman projection

What is a staggered conformation?

Lowest torsional strain; substituents are 60° apart

What is a gauche interaction?

Staggered but bulky groups 60° apart → medium energy

What is an anti conformation?

Staggered with bulky groups 180° apart → lowest energy

Which chair position is higher energy?

Axial due to 1,3-diaxial interactions

Which chair position is most stable?

Equatorial

When drawing chairs

what is rule #1?, Put bulky groups equatorial

How do you identify axial vs equatorial?

Axial is straight up/down; equatorial is angled around ring

What is a geometric (cis/trans) isomer?

Stereoisomer due to restricted rotation (alkenes or rings)

How do you identify a chiral center?

Carbon with four different groups

What is an enantiomer?

Non-superimposable mirror images

What is a diastereomer?

Stereoisomers that are not mirror images

What is a meso compound?

Molecule with chiral centers but overall achiral due to symmetry

How do you assign R/S?

Priority → orient lowest → trace path 1→2→3

What is optical activity?

Ability to rotate plane-polarized light

What is the key identity of SN2?

One-step backside attack

Stereochemistry of SN2?

Inversion of configuration

Best substrates for SN2?

Methyl > 1° > 2° (never 3°)

What kind of nucleophile favors SN2?

Strong, negatively charged nucleophiles

What solvents favor SN2?

Polar aprotic (DMSO, acetone, DMF)

Rate law for SN2?

Rate = k[substrate][nucleophile]

SN2 mechanism steps?

Nucleophilic attack + loss of LG (one step)

What is the key identity of SN1?

Carbocation formation

Stereochemistry of SN1?

Racemization

Best substrates for SN1?

3° > 2° (never 1°)

What kind of nucleophile for SN1?

Weak nucleophile OK (H2O, ROH)

What solvents favor SN1?

Polar protic (H2O, alcohols)

Rate law for SN1?

Rate = k[substrate]

SN1 mechanism steps?

Loss of LG → carbocation → rearrangement → nucleophilic attack → proton transfer

Strong nucleophile + 1°?

SN2

Weak nucleophile + 3°?

SN1

Polar protic solvent?

SN1

Polar aprotic solvent?

SN2

What is E2 mechanism type?

One-step β-elimination

Requirements for E2?

Strong base, β-H anti-periplanar

Stereochemistry for E2?

Anti-periplanar H and LG

Base strength for E2?

Strong bulky bases (t-BuO-), strong small bases (OH-, OR-)

E2 rate law?

Rate = k[substrate][base]

What substrates work for E2?

3° > 2° > 1° (1° requires strong base)

What is E1 mechanism type?

Carbocation formation then deprotonation

Steps of E1?

Dissociation → carbocation → rearrangement → deprotonation

Base strength for E1?

Weak base OK (H2O, ROH)

Substrate preference for E1

3° > 2°

Stereochemistry for E1

Forms most substituted alkene (Zaitsev)

Strong base?

E2

Weak base + 3°?

E1

Rearrangements within E1 and E2

E1 only

Strong nucleophile reaction

SN2

Strong bulky base reaction

E2

Weak nucleophile + weak base + 3°?

SN1/E1 mix

High heat favors?

E1/E2

Markovnikov addition of HX?

H adds to less substituted, X to more substituted

Anti-Markovnikov addition?

Hydroboration-oxidation (BH3/THF then H2O2/OH-)

Which alkene additions rearrange?

HX, H2O/H+, ROH/H+

Which alkene additions never rearrange?

X2, oxymercuration, hydroboration

Mechanism for HX addition?

Protonation → carbocation → X- attack

Mechanism for acid-catalyzed hydration?

Protonation → carbocation → water attack → deprotonation

Mechanism for acid-catalyzed alcohol addition?

Protonation → carbocation → ROH attack → deprotonation

Mechanism for halogenation (X2)?

Anti-addition through halonium ion

Mechanism for halogenation in water?

Halonium ion → nucleophilic attack by water → anti-addition

Mechanism for oxymercuration?

Hg(OAc)+ adds → nucleophile attack → reduction replaces Hg with H

Mechanism for hydroboration-oxidation?

Syn addition of BH3 → oxidation replaces B with OH

Catalytic hydrogenation of alkene?

Syn addition of H2 across double bond

Addition of HX to alkynes?

Markovnikov → geminal dihalide

Addition of X2 to alkynes?

Anti addition → trans dihalides

Full hydrogenation with Pd/C?

Forms alkane

Lindlar's catalyst?

Cis alkene product

Dissolving metal reduction (Na/NH3)?

Trans alkene product

What is a protic solvent?

A solvent that has an O-H or N-H bond and can donate H+ (H-bond donor)

Examples of protic solvents?

Water, methanol, ethanol, isopropanol, acetic acid

Which mechanisms do protic solvents favor?

SN1 and E1

Why do protic solvents favor SN1?

They stabilize carbocations and leaving groups

What is an aprotic solvent?

A solvent without O-H/N-H bonds; cannot donate H+

Examples of polar aprotic solvents?

DMSO, DMF, acetone, acetonitrile, HMPA

Which mechanisms do polar aprotic solvents favor?

SN2

Why do aprotic solvents favor SN2?

They keep nucleophiles "free" and strong

What makes a good leaving group?

Weak base that can stabilize negative charge

Examples of good leaving groups?

I-, Br-, Cl-, TsO- (tosylate), MsO- (mesylate)

Rank halide leaving groups from best to worst?

I- > Br- > Cl- >> F-

Why is fluoride a poor leaving group?

It is a strong base and unstable alone

What makes a bad leaving group?

Strong base or unstable negative ion

Examples of bad leaving groups?

OH-, OR-, NH2-, H-, F-

How can you make OH- a good leaving group?

Protonate to form H2O or convert to tosylate

What is a strong acid?

Substance that fully dissociates to give H+ in solution

Examples of strong acids?

HCl, HBr, HI, H2SO4, HNO3, HClO4

Which strong acids are used in alkene addition?

HCl, HBr, HI (for HX addition)