Organic Chemistry II Final Exam Material

Chapter 21

Chapter 22

Chapter 23

Introduction to Aromaticity

Benzene = C6H6

Benzene has a low heat of hydrogenation due to stability and lack of static, isolated C=C bonds

In benzene, C-H and C-C bonds are shorter but C-C are all the same despite this; this is as a result of an average of the two resonance structures

a molecule is AROMATIC if it has 4N+2 pi bonds and is planar

molecules with 2,6,10, 14, 18 pi electrons etc. (even number but NOT a multiple of 4)

a molecule is ANTIAROMATIC if it has 4n pi electrons

multiples of 4 are ANTIAROMATIC

Aromaticity in Radicals, Cation, and Anions

heterocyclic molecules are aromatic molecules containing non-carbon atoms such as N,O, P, or S

the lone pair electrons on heterocyclic molecules can either be part of the pi system or not

in atoms with two sets of lone pair electrons (O,S), only ONE pair of them is part of the ring pi system

General Info:
AROMATIC = 4N+2; ANTIAROMATIC= 4N (multiple of 4);
NON-AROMATIC: not a multiple of 4 but not aromatic

Atoms that appear to be sp3 hybridized may actually behave as sp2 hybridized atoms

atoms with lone pairs next to pi bonds, only the pi bond counts as part of the ring pi system

General Info: one pi bond has 2 pi electrons; one lone pair ADDS two 2 pi electrons (3 pi bonds and 2 sets of lone pairs = 10 pi electrons = aromatic; cations do not change the number of pi electrons

Benzene and its family members can undergo electrophilic substitution (vid 1)

all the C=C reform

Friedel Crafts ALKYLATION includes multiple addition to the ring, rearrangment of cation, and NO reaction with deactivated rings

Friedel Crafts ACYLATION DOES NOT suffer from multiple additions, or rearrangement, but also does NO reaction with deactivated rings; involves use of acid chloride

Halogenation of benzene

Cl and Br need lewis acid catalyst. Including: AlCl3, FeBr3, BF3, HF, and TiCl4

I and F CANNOT be added through halogenation; must use amines, see chapter 23

Cl2/Br2 and a lewis acid are used to create the electrophile for the reaction; ex: Cl-Cl + FCl3 (LA) yields Cl+FeCl4-, where Cl+ is the ELECTROPHILE and FeCl4- acts as a BASE to remove the H to generate the final product

Sulfonation of benzene (conc. H2SO4, reflux)

only reversible reaction in chapter

using SO3 and H2SO4 allows the rate to go faster

Activating groups (vid 3)

activating groups generally direct electrophiles to the ORTHO and PARA positions

activating groups push electron density into the ring by induction and resonance, mainly at these two positions

the family of activating groups include ALL alkyl groups and groups where the atom ATTACHED to the ring has lone pair electrons (O,N) (these groups include delocalization of the lone pair through resonance or conjugation allowing them to donate density to ortho and para)

on an energy diagram (See back), ortho and para have the lowest Ea and tertiary allylic cations

see activating groups with ranking on back

Deactivating groups (vid 3)

direct the electrophile to the META position; deactivating groups withdraw electron density from the ring via resonance and induction, primarily at this location

REMEMBER: halogens are DEACTIVATING groups but they direct groups to the ortho/para position; also only deactivating group you can do Friedel Crafts with; energy diagram where benzene has lowest Ea, then ortho para, and then meta has the highest Ea

this family has atoms attached to the ring that are either partial positively charged or have a positive charge

halogens are an exception to this, see above

energy diagram (see back), meta has a lower Ea than ortho and para

see deactivating group rankings on back

Synthesis with activating and deactivating groups (vid 4)

depending on whether the group attached to the ring is an activator or a deactivator, there are several possible outcomes

1) the two groups can direct to the same spot on the ring and are reinforcing

2) the two groups oppose each other and in this case, the stronger deactivator will dictate the position of the electrophile on the ring

3) the two groups oppose each other and are either both activators or both deactivators and in this case, a mixture will occur

if the two groups are activating, generally the stronger of the two predominates

if one group is activating and one is deactivating, generally the activating group predominates

General Info: Ortho = 1,2 substitution; Meta = 1,3 substitution; Para = 1,4 substitution

amines are basic due to the lone pair of electrons on N

more basic than alcohols, ethers, water

General info: smaller pKb = stronger base; smaller pKa = stronger acid

conjugation leads to higher pKb

alkyl substituted amines are more basic than NH3 b/c they donate electron density by induction

amides are non basic, poor nucleophiles

Preperation methods (overview)

reduction of imides to amines (ch 16)

reduction of nitriles to primary amines (ch 18)

reduction of amides by LiAlH4 (ch 18)

reduction of -NO2 groups (ch 22)

nucleophilic ring opening by ammonia and amines (new, ch 11)

reduction of alkyl azides by LiALH4 (new)

alkylation of ammonia and amines (new)

aldehyde or ketone + NH2OH + reduction by LiALH4 = oxime

aldehyde or ketone + RNH2 + reduction = imine

aldehyde or ketone + R2NH + reduction = enamine

can use primary, secondary, or tertiary amines

amides are formed from the reaction b/w acid Cl and an amine

NH3 -> primary amine

primary amine -> secondary amine

secondary amine -> tertiary amine

CN- is a good nucleophile for SN2 reactions; will react with primary and secondary alkyl halides, which can then be reduced to primary amines

REMEMBER: this includes the ADDITION of ONE carbon!

use of azides (NaN3) does NOT add a carbon

Reduction of nitro compounds

everything that will reduce C=O to CH2 will do the same to NH2 EXCEPT Wolff Kishner

aromatic and aliphatic nitro compounds can be reduced to amines

[H2/Ni, Pt, or Pd] OR [H+/Fe, Sn, or Zn] can be used

Nucleophilic ring opening

can occur twice

amine acts as a nucleophile opening the ring in an SN2 fashion

Alkylation of NH3 and Amines

problematic because N will continue to attack the electrophile until there are no lone pairs left; SN2 reaction where amine is the nucleophile

Reaction with Amines

common thread is that due to the lone pair electron on N, amines are good nucleophiles

nitrous acid/diazonium salt

Hoffmann elimation

AR-N+ triple bond N + ......

H+ -> ArOH

CuCl (Br) -> ArCl (Br)

CuCN -> ArCN

HBF4 (KI) -> ArF (I)

H3PO2 -> ArH

ArH -> Ar-N=N-Ar

mechanism to get I and F on ring!

Sandmeyer reaction

replacement of -Cl, -Br, or -CN

involves the cleavage of C-N bonds

R4N+OH- is the starting material

product is an alkene and ammonium salt is a leaving group

Zaitsev = more substituted; Hoffmann = less substituted

There may be a possibility for external or internal double bonds, but the E2 elimination favors anti position (Newman) and for internal double bonds, in the anti position, there is too much steric hinderance. Therefore, an external double bond is formed because it can create the anti position product without steric hinderance