CHEM545 Organic Chemistry Exam 2

What is a nucleophile?

Electron-pair donor; attacks an electron-poor site.

What is an electrophile?

Electron-pair acceptor; attacked by a nucleophile.

What is a base?

A proton (H+) acceptor.

What is an acid?

A proton (H+) donor.

What is a leaving group?

A group that departs with the bonding electron pair.

What is a substituent?

An atom or group attached to the main carbon framework.

What is an alkene?

A compound containing a C=C double bond.

What is an alkyl halide?

A hydrocarbon with one or more hydrogens replaced with a halogen.

What is an alcohol?

An OH group bonded to carbon.

What is an epoxide?

A three-membered ring with one oxygen.

What is a carbonyl?

A carbon double-bonded to oxygen (C=O).

What is a 1° carbon?

A carbon bonded directly to one other carbon.

What is a 2° carbon?

A carbon bonded directly to two other carbons.

What is a 3° carbon?

A carbon bonded directly to three other carbons.

How do you classify an alcohol?

Count carbon neighbors of the carbon bearing OH.

How do you classify a carbocation?

Count carbon neighbors of the positively charged carbon.

Rank of carbocation stability: 3°, 2°, 1°?

3° > 2° > 1° (without special resonance effects)

Which carbon is easiest for SN2 attack?

Usually methyl (CH3) > 1° > 2°; 3° is essentially blocked

Does more substituted mean more stable for every carbon?

No. Stability depends on the species and reaction.

What is steric hindrance?

Crowding that makes approach to a reacting atom harder.

What is resonance?

Different electron arrangements with the same atom connectivity.

What moves during resonance?

Electrons, not atoms or sigma-bonded hydrogens.

What is a carbocation?

A positively charged, electron-deficient carbon.

What is regioselectivity?

Preference for reaction at one position over another.

What is stereochemistry?

The three-dimensional arrangement of atoms.

What is Markovnikov addition?

For HX addition, H generally goes to the less substituted alkene carbon.

What is anti-Markovnikov hydration?

OH ends on the less substituted alkene carbon.

Where must a curved arrow start?

At an electron pair: a lone pair or bond

Where does a curved arrow point?

To the atom or bond receiving that electron pair.

What does an arrow from a lone pair to C mean?

A new bond forms to that carbon.

What does an arrow from a bond to an atom mean?

The bond breaks; that atom takes both electrons.

What happens when O has 1 bond and 3 lone pairs?

It usually has formal charge -1.

What happens when O has 2 bonds and 2 lone pairs?

It has formal charge 0.

What happens when O has 3 bonds and 1 lone pair?

What is a proton transfer?

A base bonds to H; the old H bond electrons move away.

What is nucleophilic attack?

An electron pair forms a bond to an electrophile.

What is leaving-group loss?

A bond breaks; leaving group takes the electron pair.

What is a rearrangement?

A neighboring bond/group shifts to stabilize an intermediate.

After alkene attacks H+ what may form?

A carbocation at the other former double-bond carbon.

What do you check before drawing the next arrow?

Available electrons, electrophile, octets, and leaving groups.

What does substitution do?

Replaces a leaving group with another group.

What does elimination do?

Removes H and a leaving group to form a double bond.

SN2: how many steps?

One concerted step; attack and leaving-group loss together.

SN1: what is the first step?

Leaving group departs, forming a carbocation.

E2: what happens in one step?

Base removes beta H as C=C forms and leaving group leaves.

E1: what is the first step?

Leaving group departs, forming a carbocation.

Which mechanisms form a carbocation?

SN1 and E1; some alkene additions also do.

Which mechanisms have no carbocation intermediate?

SN2 and E2.

What makes SN2 stereochemically distinctive?

What geometry does E2 usually require?

SN1 or SN2: methyl alkyl halide + strong nucleophile?

SN2 (no stable methyl carbocation).

SN1 or SN2: 3° halide + weak nucleophile in polar protic solvent?

Often SN1; E1 may compete.

E2 clue: 3° halide + strong base?

Usually E2 favored over SN2.

Strong bulky base + alkyl halide suggests?

Can NaOH or NaOMe indicate only one mechanism?

No. SN2/E2 depend on substrate, solvent, and conditions.

HCl or HBr + alkene?

Hydrohalogenation: H and halogen add across C=C.

HCl + alkene: key intermediate?

Usually a carbocation; rearrangement can occur.

H3O+ / H2O + alkene?

Acid-catalyzed hydration: Markovnikov alcohol.

BH3·THF; then H2O2/NaOH + alkene?

Hydroboration-oxidation: anti-Markovnikov alcohol, syn addition.

Br2 + alkene?

Halogenation: vicinal dibromide via bromonium ion.

Br2/H2O + alkene?

Halohydrin: Br and OH add; OH at more substituted carbon, anti.

mCPBA + alkene?

Epoxidation: double bond becomes a three-membered C-O-C ring.

NaOMe + epoxide; then H3O+?

Basic ring opening: attack less substituted C, then protonate O-.

Acid + epoxide + H2O?

Acidic ring opening: attack usually more substituted C; anti opening.

PCC + 1° alcohol?

Oxidation to aldehyde (under usual dry conditions).

PCC + 2° alcohol?

Oxidation to ketone.

Jones/chromic acid + 1° alcohol?

Usually oxidation to carboxylic acid.

Jones/chromic acid + 2° alcohol?

Oxidation to ketone.

Typical oxidation of 3° alcohol?

No simple oxidation without breaking a C-C bond.

NaBH4 + aldehyde?

Reduction to primary alcohol.

NaBH4 + ketone?

Reduction to secondary alcohol.

What does a Grignard reagent look like?

R-MgX; carbon behaves as a strong nucleophile/base and makes bonds

RMgX + formaldehyde, then H3O+ may give?

Primary alcohol with a new C-C bond.

RMgX + aldehyde, then H3O+ may give?

Secondary alcohol with a new C-C bond.

RMgX + ketone, then H3O+ may give?

Tertiary alcohol with a new C-C bond.

Strong acid + alcohol + heat may give?

Dehydration: alkene formation

What does 1) and 2) over the arrow mean?

Use reagents sequentially, in the numbered order.

What does a 3-membered O ring + nucleophile suggest?

Epoxide ring-opening reaction.

What is a carbocation rearrangement?

The movement of a carbocation to a more stable environment (tertiary > secondary> primary)

What are good leaving groups?

I-, Br-, Cl-, H20, TsO-

What are bad leaving groups?

OH-, RH-, NH2-, H-, carbanions

Nucleophiles commonly have…..

Lone pairs, double bonds and negative charges (strongest nucleophiles)

Electrophiles commonly have…..

Leaving groups, partially positive charges, positive formal charge