organic exam 2

topics included: -EAS/how to make electrophiles, special EAS cases, multistep EAS, diels alder, conjugated dienes

describe what happens during an EAS

an electrophile reacts with an aromatic ring and substitutes for 1 of the hydrogens

what things can a H on an aromatic ring be substituted by?

a halogen (-Cl, -Br, -I)

a nitro group (NO₂)

a sulfonic acid group (SO₃H)

a hydroxyl group (-OH)

an alkyl group (-R)

an acyl group (-COR)

what reagents are needed for an electrophilic aromatic substitution HALOGENATION

X₂

FeX₃

_{these can be done with Br, Cl}

how do we make the electrophile for halogenation EAS? (Br or Cl) start with reagents you would see on the arrow and convert to E⁺

one of the Br atoms in Br2 will use a lone pair to attack Fe and connect to it

the electrons from the Br-Br bond follow that Br

makes complex Br^+-FeBr4

what reagents are needed for an electrophilic aromatic substitution NITRATION

HNO₃ and H₂SO₄

how do we make the electrophile for NITRATION EAS? start with reagents you would see on the arrow and convert to E⁺

lone pair on the O of HNO3 that is bonded to an H attacks one of the H on H₂SO₄

next, other single bonded O of HNO3 gives electrons in to form the second double bond and the newly formed water group gets kicked out

now we have O=N⁺=O (nitro group)

what reagents are needed for an electrophilic aromatic substitution SULFONATION

SO₃

H₂SO

how do we make the electrophile for SULFONATION EAS? start with reagents you would see on the arrow and convert to E⁺

similar to how we made it for nitration but simlpler

negatively charged oxygen on SO₃attacks one of the H on H₂SO₄

thats literally it

we just neutralize the negative O on SO₃, and we have ⁺SO₃H

what reagents are needed for an electrophilic aromatic substitution FRIDEL CRAFTS ALKYLATION

R-X where X is Cl

AlCl₃

how do we make the electrophile for FRIDEL CRAFTS ALKYLATION EAS? start with reagents you would see on the arrow and convert to E⁺

chlorines lone pair attacks Al of AlCl₃ and electrons from bond between R-X go with

this leaves R+ and ^-AlCl₄

what reagents are needed for an electrophilic aromatic substitution FRIDEL CRAFTS ACYLATION

carbonyl group with Cl on it

AlCl₃

how do we make the electrophile for FRIDEL CRAFTS ACYLATION EAS? start with reagents you would see on the arrow and convert to E

similar to fried craft alkylation

one of chlorines lone pair attacks Al and electrons from bond go with it

you get an acylium ion (+ change where Cl left w electrons) this ion has resonance

you also get AlCl₄^- (but she’s irrelevant)

what reagents are needed for an electrophilic aromatic substitution HALOGENATION adding Iodine

I₂ and CuCl₂

what reagents are needed for an electrophilic aromatic substitution HALOGENATION adding fluorine

F-TEDA-BF₄

TfOH (strong acid)

add a primary alkyl halide to an aromatic

make sure to draw out how the electrophile is formed and then how it reacts

what is the major product

remember that the carbocation will do a 1,2 hydride shift so your carbocation will turn to secondary (this is major product)

how could you attach an alkyl chain to an aromatic

• do an FC acylation with a terminal aldehyde and then wolf kishner aldehyde off

• do FC alkylation Br and then add Mg to make Grignard reagent, then have it react with alkyl halide

when you add multiple groups to an aromatic what do you have to consider

whether the groups are activating or deactivating

this will dictate where the following groups will add

activating groups are typically

those that donate electrons in and increase electron density

typically have lone pair

exception: alkyls

deactivating groups are typically..

groups that will decrease electron density in ring

those with a double bond

and exception: halides

what way will activating groups prefer the next thing to add?

Ortho and Para

what way will deactivating groups prefer the next thing to add

meta

diels alder requires high boiling pt solvent give 3 examples

toluene xylene bezene

which reagent has its stereochemistry preserved in diels alder

dienophile

what is an endo product in diels alder

substituent on opposite side of molecule from smallest bridge

why is the exo product for diels alder

substituent on same side of smallest bridge

diels alder can only happen if your diene is..

cis

sequence of arrows when you are doing reverse diels alder

• dienohpiles leave arrow

• newly formed double bond moves

• break other connection to dienopgile and put towards next bond

how to tell what way a diels alder will react

the electron rich end of the diene will react with the more electrophilic end of the dienophile

do conjugated dienes have increased or decreased molecular stability and why

they have increased molecular stability because of electron delocalization

on what carbons do the H and halide add of the conjugated diene for thermodynamic product? does this product occur at high or low temp

1 and 4 of the conjugated diene

high

on what carbons do the H and halide add of the conjugated diene for kinetic product? does this product occur at high or low temp

carbons 1 and 2

low

what is true about the alkene stability of the kinetic product in comparison to the thermodynamic product

alkene is less stable

what is true about the alkene stability of the thermonodynamic product in comparison to the kinetic product

alkene is more stable

what is true about the transition state stability of the thermonodynamic product in comparison to the kinetic product

transition state less stable

what is true about the transition state stability of the kinetic product in comparison to the thermodynamic product

more stable transition state

a lower pKa indicates..

a stronger acid

what is Huckels rule regarding aromaticity?

a cyclic continues conjugated pi system exhibits aromaticity if it contains 4n +2 pi electrons where n is any number

so systems with 6,10, 14, 18 pi electrons are aromatic

how do we consider carbocations when looking at aromaticity

the carbocation represents an empty p orbital so it CAN be a part of the racetrack even if it doesn’t contribute to the electron count

if a system has 4n pi electrons where n is any number..

the system is ANTIaromatic

when is a system nonaromatic

when sp3 hybridized carbon or charge breaks the racetrack therefore breaking the conjugated system

how do we consider lone pairs when looking at aromatic systems?

if an atom has one lone pair then count those electrons but if they have two lone pairs you can only count one set of electrons!

if an atom has lone pairs but is already a part of a double bond, do not count those lone pair electrons

what is the conditionality rule regarding lone pairs on atoms?

if including the lone pairs will result in anti-aromaticity, that atom will not include the lone pairs and is sp3 hybridized making the system nonaromatic

if including them will result in aromaticity, then the lone pairs are included and the atom is sp2 hybridized

how can you tell which molecule has a lower pKa (is more acidic)

is more acidic (more likely to donate a proton) when losing that proton produces a more stable conjugate base

for ex if the conjugate base is aromatic, then the beginning acid is very acidic and low pKa

if the conjugate base is anti-aromtic, then the beginning acid is less acidic and has high pKa