Week 10/11/12

Question 1

Cyclohexanes in E2

Answer 1

-ß-H and LG must both be axial for anti-periplanar conformation to exist

Question 2

E1 acid catalyzed dehydration

Answer 2

• Heat, strong acid, hydroxyl LG
• hydroxyl is protonated
• electrophile dehydrated
• conjugate base of strong acid (v. weak base) deprotonates using ß-H, e- form pi bond

Question 3

E1 stereoselectivity

Answer 3

• is stereoselective but NOT stereospecific
• the more stable orientation (ie. anti) is preferred, produces major product
• antiperiplanar NOT needed, ß-H just needs to be parallel to the empty p-orbital

Question 4

Electrophilic Aromatic Substitution

Answer 4

• nucleophile is a pi-bond
• pi e-s are higher energy than sigma e-s, so more reactive
• pi-bonds typically react in addition rxn when electrophile is present

Question 5

Addition Rxns

Answer 5

-b/c of pi e- delocalization, aromatic mlc are less reactive w electrophiles, addition does NOT occur due to high activation energy barrier

Question 6

EAS mechanism

Answer 6

• cleavage of pi bond and addition of E+ is very slow step

- deprotonation of benzylic carbocation and formation of new double bond very fast, irreversible

Question 7

EAS electron donating groups

Answer 7

• N-(R)2 or NH2 most reactive
• O-R or -OH
• NHC=OR or -OC=OR less reactive
• catalyst (like AlCl3) not needed
• reaction occurs at ortho and para positions (where e- density in increased), NO RXN AT META

Question 8

Nitronium Ion

Answer 8

• very strong electrophile (O=N+=O)
• formed through protonation of nitric acid, then dehydration of nitric acid
• when added, is stable due to resonance, does not need additional protonation

Question 9

Sulfonation of Benzene

Answer 9

• S(=O)3 protonated by acid, strong E+
• pi bond donates E- to sulfur (has positive dipole due to oxygens)
• base deprotonates, new pi bond formed

Question 10

EAS alkylation

Answer 10

• E+ (3˚ carbocation) generated from 3˚ alkyl halide through SN1-like mech with catalyst (AlCl3)
• H removed by Cl from AlCl4-, aromaticity restored

Question 11

EAS acylation

Answer 11

• acyl (R-C=OCl) binds to catalyst (AlCl3)
• Oxygen LP form triple bond with C, C-Cl sigma broken
• benzene pi-bond donates e- to C, e- from C-O triple bond move to O
• H removed from sp3 C (C with E+ attached) by Cl from AlCl4-

Question 12

Regioselectivity for EDG

Answer 12

-look for steric clash at ortho/para positions

Question 13

Alkyl EDG

Answer 13

• are EDG due to hyperconjugation
• weaker than substituents with LP
• the C-H sigma orbital is relatively parallel to the empty p-orbital, some delocalization of charge

Question 14

Electron withdrawing groups

Answer 14

-pi e- density is reduced by these groups
reduced by:
-Inductive effects (electronegative atoms pull e- density through sigma bonds
-Conjugation (if atom directly connected to ring is also involved in pi-bond)
-greatest reactivity is at META position (meta-directing), b/c most e- deficiency is at ortho/para positions

Question 15

Halogen substituent in EAS

Answer 15

• behave as EDG (lone pairs) and EWG (inductive effects)
• inductive e- withdrawal slows rxn, BUT lone pairs still direct to ortho/para
• inductive effect weakest at the para position for F, relatively similar at o/p for I
• F more electronegative than I, so inductive deactivation strongest for F
• BUT delocalization of F LP are better than I b/c of similar size to the C p-orbitals (more effective overlap)

Question 16

Induction vs. Conjugation

Answer 16

• Conjugation is MUCH stronger than induction

- ie. for NH2 substituent, rate is more than 10^14 faster than for Cl substituent