ochem midterm 3 shabbir

What does the Hammond Postulate state?

The transition state resembles the molecule (reactants or products) that it is closest to in energy.

What kind of transition state does an exergonic reaction have?

Early transition state; resembles reactants.

What kind of transition state does an endergonic reaction have?

Late transition state; resembles products.

What is the order of alkene stability?

Ethene < monosubstituted < cis disubstituted < trans disubstituted < trisubstituted < tetrasubstituted

Why are more substituted alkenes more stable?

Hyperconjugation and electron-donating inductive effects from alkyl groups.

What does Zaitsev's Rule predict?

The major alkene product is the more substituted alkene.

What is the order of radical stability?

Methyl < 1° < 2° < 3° < allylic/benzylic

Why are tertiary radicals more stable?

More hyperconjugation from alkyl groups.

What is the order of carbocation stability?

Methyl < 1° < 2° < 3° < allylic/benzylic

Why are tertiary carbocations more stable?

Hyperconjugation and inductive donation from alkyl groups.

Nucleophilicity order in polar aprotic solvents

F⁻ > Cl⁻ > Br⁻ > I⁻

Nucleophilicity order in polar protic solvents

I⁻ > Br⁻ > Cl⁻ > F⁻

Examples of polar aprotic solvents

DMSO, DMF, acetone, acetonitrile

Examples of polar protic solvents

Water, alcohols, acetic acid, ammonia

Which solvent favors SN2?

Polar aprotic

Which solvent favors SN1/E1?

Polar protic

Order of leaving group ability

I⁻ > Br⁻ > Cl⁻ > H₂O > F⁻ > RCOO⁻ > OH⁻ > CH₃O⁻ > NH₂⁻

Best leaving group

I⁻

Worst leaving group

NH₂⁻

Which alcohol dehydrates the fastest?

Tertiary > Secondary > Primary

Why do tertiary alcohols dehydrate fastest?

They form the most stable carbocation.

Labels on a reaction coordinate diagram

Reactants, products, transition state, activation energy, reactive intermediate (if present).

SN2 favored by what substrate?

Primary (sometimes secondary).

SN2 stereochemistry

Inversion of configuration (backside attack).

SN1 favored by what substrate?

Tertiary (sometimes secondary).

SN1 stereochemistry

Racemization.

Can tertiary substrates undergo SN2?

No; steric hindrance prevents SN2.

Strong base + tertiary substrate gives what?

E2

Weak nucleophile + tertiary substrate gives what?

SN1/E1

Primary alcohol + HX mechanism

SN2

Secondary alcohol + HX mechanism

Usually SN1

Better reagents for converting secondary alcohols

PBr₃ or SOCl₂

Tertiary alcohol + HBr mechanism

SN1

Priority when numbering alcohols

OH > alkene > alkyne > substituents

In IUPAC names, which comes first: R/S or E/Z?

R/S comes before E/Z.

What is OCH₃ called as a substituent?

Methoxy

Reagents for radical bromination

Br₂, hv

Which hydrogen is brominated during radical bromination?

The one that forms the most stable radical (3° > 2° > 1°).

Three steps of radical bromination

Initiation, propagation, termination

Reagents for allylic bromination

NBS, hv

Where does allylic bromination occur?

On the carbon adjacent to a double bond.

Reagents for anti-Markovnikov HBr addition

HBr + H₂O₂

Which carbon gets Br in anti-Markovnikov addition?

The less substituted carbon.

Which acid shows anti-Markovnikov addition with peroxides?

HBr only.

Definition of substitution reaction

Nucleophile replaces the leaving group.

Definition of elimination reaction

Base removes H while leaving group leaves, forming an alkene.

SN2 mechanism summary

One-step backside attack; leaving group leaves simultaneously.

SN1 mechanism summary

Leaving group leaves → carbocation forms → nucleophile attacks.

E2 mechanism summary

Strong base removes β-H anti to leaving group while leaving group leaves simultaneously.

E1 mechanism summary

Leaving group leaves → carbocation forms → base removes β-H.

First step of alcohol dehydration

Protonation of OH to form H₂O.

Second step of alcohol dehydration

Water leaves to form carbocation.

Third step of alcohol dehydration

Base removes β-H to form alkene.

PCC oxidizes a primary alcohol into what?

Aldehyde

Jones reagent oxidizes a primary alcohol into what?

Carboxylic acid

Jones reagent oxidizes a secondary alcohol into what?

Ketone

Can tertiary alcohols be oxidized?

No

Shape of an epoxide

Three-membered ring containing oxygen

Reagents to make a halohydrin

Br₂/H₂O

In halohydrin formation, where does OH go?

More substituted carbon.

Reagent that converts a halohydrin to an epoxide

NaOH

Reagent that directly converts an alkene to an epoxide

mCPBA

Acid-catalyzed epoxide opening attacks which carbon?

More substituted carbon.

Base-catalyzed epoxide opening attacks which carbon?

Less substituted carbon.

Stereochemistry of epoxide opening

Anti addition

Reagents for nucleophilic epoxide opening with hydride

LiAlH₄ then H₃O⁺

Which carbon does LiAlH₄ attack in an epoxide?

Less substituted carbon.

Product of LiAlH₄ opening an epoxide

Alcohol with H added to the attacked carbon.

Reagent that converts primary/secondary alcohols to alkyl bromides by SN2

PBr₃

Reagent that converts primary/secondary alcohols to alkyl chlorides by SN2

SOCl₂

Reagent that causes dehydration of alcohols

H₂SO₄ (heat)

Reagent that oxidizes 1° alcohol to aldehyde

PCC

Reagent that oxidizes 1° alcohol to carboxylic acid

H₂CrO₄ (Jones reagent)

Reagent that oxidizes 2° alcohol to ketone

H₂CrO₄ (Jones reagent)

Reagent that forms epoxides from alkenes

mCPBA

Reagent that forms halohydrins

Br₂/H₂O

Reagent that opens epoxides under acidic conditions

H₃O⁺

Reagent that opens epoxides under basic conditions

OH⁻

Reagent that opens epoxides with hydride

LiAlH₄, then H₃O⁺

Decision rule for epoxide opening under acidic conditions

Nucleophile attacks the more substituted carbon.

Decision rule for epoxide opening under basic conditions? Nucleophile attacks the less substituted carbon.

How many steps is SN2? One

How many steps is SN1? Two

How many steps is E2?

One

How many steps is E1?

Two

Which mechanisms have a carbocation intermediate?

SN1 and E1

Which mechanisms have a transition state but no intermediate?

SN2 and E2

Which mechanisms can rearrange?

SN1 and E1

Which mechanisms cannot rearrange?

SN2 and E2

Is OH⁻ a strong base?

Yes

Is OH⁻ a strong nucleophile?

Yes

Is CH₃O⁻ a strong base?

Yes

Is CH₃O⁻ a strong nucleophile?

Yes

Is NH₂⁻ a strong base?

Very strong

Is I⁻ a strong nucleophile?

Yes

Is H₂O a strong nucleophile?

Weak

Is ROH a strong nucleophile?

Weak

Markovnikov rule

H goes to carbon with more H's; X/OH goes to more substituted carbon.

Anti-Markovnikov rule

H goes to more substituted carbon; Br goes to less substituted carbon.

Which reagent gives anti-Markovnikov HBr addition?

HBr + H₂O₂