Aromaticity
Aromaticity
Extremely stable compounds (stability through conjugation of double bonds and resonance)
o Extremely hard to reduce benzene ( ) vs ( ) AROMATIC HUCKEL’s RULE o 4n+2 π-electrons (n = 0,1,2..) o The n value is determined by counting the number of π-electrons in the molecule and seeing which interval of the formula it matches to o Ex. Benzene has 6 π-electrons 4n+ 2 = 6 n=1 o If the number of electrons falls on one of the intervals (2, 6, 10…) then it is said to be AROMATIC
o
o MO Reason for stability in aromatic compounds paired electrons in bonding orbitals Anti aromatic have unpaired electrons unstable ANTIAROMATIC UNSTABLE o 4n π-electrons (n = 0,1,2..) o These are classified as ANTIAROMATIC NON If the number of π-electrons does not fit into either of these categories it is known as NON AROMATIC
CHARACTERISTICS
Only Lone Pairs that are in the same plane are counted in the π-electron system Electrons in p-orbitals orthogonal to the system do not contribute to the π-electron count. Ex.
o Benzene – Due to resonance between bonds, no bond of benzene is strictly single or double ( (
) but rather a midway point between them
) in terms of length. o Single 1.32A o Double 1.53A o Benzene – 1.398A (all) o All C’s are sp2 hybridized entire molecule is flat Disubstituted Nomenclature o Ortho (beside) o Meta (One removed) o Para (across from) Common Names o Circled red are most important, HOWEVER, since the rest are in the notes, they are also fair game. But know the ones in 100%
REACTIONS OF AROMATICS Benzene ONLY undergoes substitution, NEVER addition (addition would ruin aromaticity permanently which is a very strong thermodynamic property) Substitution: the aromaticity is destroyed for only a very very very short amount of time, coupled with a low activation energy to lose an H+, making substitution viable STEP 1 STEP 2 STEP 1
STEP 2
NAME
REACTANTS
Protonation
Benzene Hydronium Ion (H+)
H+
None
Benzene HNO3
+NO2
Strong Acid H2SO4
Nitration
Halogenation Benzene Br2
E+
Br+
CATALYST
PRODUCT
SPECIAL
Benzene Hydronium Ion (H+)
Could Use Deuterium Good for Tracer studies
+ H2O
FeBr3 + HBr
Benzene Cl2
Cl+
FeCl3 +HCl
Sulfonation
Benzene SO3
+HSO3
H2SO4
FriedelCrafts ALKYLATION
Benzene R-Cl
R3C+ (3°, 2°, 1°, Methyl) Alkene (Using H2SO4)
AlCl3
Can be reversed Just add Water
FriedelCrafts ACYLATION
Benzene
AlCl3 Acylium Ion
DISUBSTITUTION --- EDG vs EWG EDG – donate electron density into the ring either via induction or resonance Generally direct ortho/para due to stabilization of positive charge by group present when second substituent is introduced into the ring
When the + charge is located on the carbon the EDG substituent is attached to this is know as inductive stabilization When the + charge can be transferred to the substituent by resonance this is known as resonance stabilization
Meta results in no stabilization of the cationic species not very good (not favoured) Standard EDG o Nitrogen with lone pair o Singly bonded oxygen o Carbons not attached to highly electron withdrawing groups
EWG When the positive charge ends up on the carbon bearing the Electron Withdrawing Group a lot of electron density is pulled from the ring BAD (ortho/para) o Can be extra destabilized by EWG on ortho/para that can have an extra resonance structure that is also unstable
Meta is the only one that does not result in a positive charge on the EWG carbon in the ring preferred Standard EWG o Nitrogen with positive charge (4 bonds) o Carbons attached to highly electron withdrawing groups Halogens Deactivating (slow down the rate of reaction)
Ortho/Para Directing
Extremely stable compounds (stability through conjugation of double bonds and resonance)
o Extremely hard to reduce benzene ( ) vs ( ) AROMATIC HUCKEL’s RULE o 4n+2 π-electrons (n = 0,1,2..) o The n value is determined by counting the number of π-electrons in the molecule and seeing which interval of the formula it matches to o Ex. Benzene has 6 π-electrons 4n+ 2 = 6 n=1 o If the number of electrons falls on one of the intervals (2, 6, 10…) then it is said to be AROMATIC
o
o MO Reason for stability in aromatic compounds paired electrons in bonding orbitals Anti aromatic have unpaired electrons unstable ANTIAROMATIC UNSTABLE o 4n π-electrons (n = 0,1,2..) o These are classified as ANTIAROMATIC NON If the number of π-electrons does not fit into either of these categories it is known as NON AROMATIC
CHARACTERISTICS
Only Lone Pairs that are in the same plane are counted in the π-electron system Electrons in p-orbitals orthogonal to the system do not contribute to the π-electron count. Ex.
o Benzene – Due to resonance between bonds, no bond of benzene is strictly single or double ( (
) but rather a midway point between them
) in terms of length. o Single 1.32A o Double 1.53A o Benzene – 1.398A (all) o All C’s are sp2 hybridized entire molecule is flat Disubstituted Nomenclature o Ortho (beside) o Meta (One removed) o Para (across from) Common Names o Circled red are most important, HOWEVER, since the rest are in the notes, they are also fair game. But know the ones in 100%
REACTIONS OF AROMATICS Benzene ONLY undergoes substitution, NEVER addition (addition would ruin aromaticity permanently which is a very strong thermodynamic property) Substitution: the aromaticity is destroyed for only a very very very short amount of time, coupled with a low activation energy to lose an H+, making substitution viable STEP 1 STEP 2 STEP 1
STEP 2
NAME
REACTANTS
Protonation
Benzene Hydronium Ion (H+)
H+
None
Benzene HNO3
+NO2
Strong Acid H2SO4
Nitration
Halogenation Benzene Br2
E+
Br+
CATALYST
PRODUCT
SPECIAL
Benzene Hydronium Ion (H+)
Could Use Deuterium Good for Tracer studies
+ H2O
FeBr3 + HBr
Benzene Cl2
Cl+
FeCl3 +HCl
Sulfonation
Benzene SO3
+HSO3
H2SO4
FriedelCrafts ALKYLATION
Benzene R-Cl
R3C+ (3°, 2°, 1°, Methyl) Alkene (Using H2SO4)
AlCl3
Can be reversed Just add Water
FriedelCrafts ACYLATION
Benzene
AlCl3 Acylium Ion
DISUBSTITUTION --- EDG vs EWG EDG – donate electron density into the ring either via induction or resonance Generally direct ortho/para due to stabilization of positive charge by group present when second substituent is introduced into the ring
When the + charge is located on the carbon the EDG substituent is attached to this is know as inductive stabilization When the + charge can be transferred to the substituent by resonance this is known as resonance stabilization
Meta results in no stabilization of the cationic species not very good (not favoured) Standard EDG o Nitrogen with lone pair o Singly bonded oxygen o Carbons not attached to highly electron withdrawing groups
EWG When the positive charge ends up on the carbon bearing the Electron Withdrawing Group a lot of electron density is pulled from the ring BAD (ortho/para) o Can be extra destabilized by EWG on ortho/para that can have an extra resonance structure that is also unstable
Meta is the only one that does not result in a positive charge on the EWG carbon in the ring preferred Standard EWG o Nitrogen with positive charge (4 bonds) o Carbons attached to highly electron withdrawing groups Halogens Deactivating (slow down the rate of reaction)
Ortho/Para Directing
