Quiz 3 Flashcards
describe a benzene ring
six-membered ring with alternating double bonds
describe toluene
benzene ring with one methyl (CH3) attached
describe xylene
benzene ring with a two methyls either in the ortho or meta position
describe indene
one benzene ring with a 5 membered ring attached (the two rings share a double bond)
describe napthalene
two benzene rings attached to one another (the two rings share a double bond)
describe biphenyl
two benzene rings attached by a single bond in the center (looks like a dumbell)
describe anthracene
three benzene rings all attached to each other (one end ring and the center ring share a double bond, while the other end ring and the center ring just share a single bond) - ARE ALL IN A ROW
describe phenanthrene
three benzene rings all attached to each other (one end ring and the center ring share a double bond and the other end ring and the center ring share a double bond) - IN ALMOST A TRIANGLE SHAPE
what would be the name of a benzene ring with a bromine attached
bromobenzene
what is -CH2CH2CH3
propyl
what would be the name of a benzene ring with a -CH2CH2CH3 attached
propylbenzene
what is a benzene ring with an OH group attached called
phenol
what is a benzene ring with an NH2 group attached called
aniline
what is a benzene ring with an SO3H group attached called
benzenesulfonic acid
what is a benzene ring with an OCH3 group attached called
anisole
what is a benzene ring with a CH=CH2 group attached called
styrene
what is a benzene ring with an aldehyde group attached called
benzaldehyde
what is a benzene ring with a carbon double bonded to an oxygen and single bonded to an OH group attached called
benzoic acid
what is a benzene ring with a carbon double bonded to an oxygen and single bonded to a CH3 group attached called
acetophenone
phenyl vs. benzyl
phenyl: benzene ring
benzyl: benzene ring with a CH2 bond
what would be one benzene ring on the left bonded to an O bonded to another benzene ring on the right called
diphenyl ether
what is an ether
R-O-R
ortho
(1,2)
meta
(1,3)
para
(1,4)
naming disubstituted benzenes rules
- the substituents are listed in alphabetical order
- the first-mentioned substituent is assigned as position 1
- the ring is numbered to give the second substituent the lowest possible number
- some substituents can be incorporated into the common names
- an incorporated substituent is assigned as position 1 (but not necessarily numbered as one)
what is a benzene group with two methyls in the meta position called
meta-xylene
1,3-dimethylbenzene
can ortho / meta / para only be used for disubstituted benzenes
yes. once a benzene derivative has three or more substitutents attached to it, you can only number the substituents
what is the molecular formula of benzene
C6H6
how many degrees of unsaturation does benzene have
4 (a ring and 3 double bonds)
what does it mean when it says “benzene has delocalized bonding”
benzene is also a cyclic molecule in which all of the ring atoms are sp2-hybridized that allows the π electrons to be delocalized in molecular orbitals that extend all the way around the ring, above and below the plane of the ring. all the benzene bonds have the same length
what is the hybridization of the carbon atoms for a benzene ring? what is the structure of benzene? what is the bond angle of the bonds? what is the orbital for the hydrogen?
- carbon atoms = sp^2 hybridized
- planar structure
- 120 degrees
- hydrogen = s orbital
explain the electropotential landscape of benzene
- each orbital has one electron (6 pi electrons)
- all pi electrons are fully and equally shared across all siz carbons in the ring (fully delocalized pi system)
- the delocalizd pi orbitals make the center of a benzene ring electron rich.
- the outer edge of the ring is less electron dense
explain delocalization energy
the delocalization energy (resonance energy) is the extra stability that a compound has as a result of having delocalized electrons.
the resonance hybrid is more stable than any of its resonance contributors
what is the value of the delocalization energy of benzene
36 kcal/mol
(36 kcal/mol of aromatic stability)
why doesn’t cyclooctatetraene have delocalized electrons
for maximal p orbital overlap, the atoms that share the delocalized electrons must lie in the same plane (cyclooctatetraene is shaped like a boat due to the four double bonds). Hence, the molecule doesn’t have delocalized electrons
whats the criteria for a compound to be aromatic
- parallel p-orbitals
- uninterrupted cloud of pi electrons that are cyclic, planar, and every ring atom must have a p orbital.
- the pi cloud must have an odd number of pairs of pi electrons (Huckel’s Rule - 4n + 2)
Explain Huckel’s Rule
4n + 2 Rule for Aromaticity
- aromatic compounds have 2 (n=0), 6 (n=1), 10 (n=2), etc. pi electrons in their uninterrupted cyclic pi cloud
explain antiaromatic compounds and give an example
Example: cyclobutane (two double bonds, four pi electrons)
antiaromatic –> molecules have an uninterrupted cyclic cloud of 4n (n = 1, 2, 3, etc.) pi electrons. these molecules are very unstable
explain non-aromatic compounds and give an example
Example: cyclooctatetraene (four double bonds, eight pi electrons)
non-aromatic: it would be antiaromatic if it had a planar structure, but it’s not planar so it doesn’t have an uninterrupted cyclic cloud of pi electrons
can a compound be an aromatic ion if it is a five membered ring but contains one lone pair in one of the orbitals? (6 e- total)
yes
can a compound be an aromatic ion if it is a seven membered ring but contains one empty orbital? (6 e- total)
yes, empty p-orbital but still has the correct number of electrons so it can delocalize and be aromatic
non-aromatic
- no uninterrupted cyclic cloud of pi electrons because of the sp^3 carbon atom (CH2) at the top of the ring –> none of the benzene carbons can be sp^3
- 4n + 1
- UNSTABLE
antiaromatic
- 4n uninterrupted cyclic cloud of pi electrons
- UNSTABLE
aromatic
- 4n + 2
- uninterrupted cyclic cloyd of pi electrons
- STABLE
pyridine
benzene but instead of one of the carbons, a nitrogen is there with a lone pair (three double bonds total)
- lone pair is orthoganol to the pi system and do not participate in the aromatic pi system.
- a doubly bonded N only contributes ONE electron to the aromatic pi system (electron density around the sp^2 nitrogen)
- electron dense aroun the nitrogen, electron poor everywhere else
pyrrole
five membered ring but instead of one of the carbons, a nitrogen is there with a lone pair and a hydrogen attached (two double bonds total)
- lone pair is part of the aromatic pi system
- singly bonded nitrogen = contributes one electron, doubly bonded nitrogen = contributes two electrons.
- more electron rich than pyridine because the electron rich nitrogen can dump / donate lone pair into the backbone
furan
five membered ring but instead of one of the carbons, an oxygen is there with two lone pairs (two double bonds total)
- the first lone pair is part of the aromatic pi system
- the second lone pair is orthoganol to the pi system and does not participate in the aromatic pi system
- because furan is a five membered ring, on of the pi orbitals (the one above the oxygen) has a lone pair (2 e-s)
thiophene
five membered ring but instead of one of the carbons, a sulfur is there with two lone pairs (two double bonds total)
- lone pair is part of the aromatic pi system
what are polycyclic aromatic hydrocarbons
molecules with multiple aromatic rings fused together
naphthalene structure
two benzene rings
how many resonance forms of naphthalene are there
three (one resonance with no double bond in the middle)
delocalized naphthalene
the ten electrons are fully delocalized throughout both rings
quinoline
two benzene rings with one carbon replaced by a nitrogen with a lone pair
indole
benzene ring attached to a five membered ring with one carbon replaced by a nitrogen with a lone pair and a hydrogen
imidazole
five membered ring that has a nitrogen with one lone pair in place of one carbon, and another nitrogen with a lone pair and a hydrogen in place of another carbon. The two nitrogens are across from each other (meta)
purine
benzene attached to a 5 membered ring.
on the benzene: theres a nitrogen with a lone pair in place of one carbon and a nitrogen with a lone pair in place of another carbon (meta).
on the five membered ring: one nitrogen with a lone pair in place of a carbon and another nitrogen with a lone pair and a hydrogen in place of another carbon (meta)
pyrimidine
benzene with a nitrogen with a lone pair in place of one carbon and a nitrogen with a lone pair in place of another carbon
orbital structure of imidazole and pyrimidine
electron pair on NH is part of the aromatic pi system, whereas the lone pair on the doubly-bonded nitrogen atom participates in the aromatic pi system (it only contributes one pi electron)
is benzene aromatic? why or why not?
yes, because benzene is cyclic, planar, and has an uninterrupted cloud of pi electrons, which provides benzene with stability.
how does benzene react with electrophiles
aromatic compounds undergo electrophilic aromatic substitution reactions.
why do aromatic rings undergo substitution rxns? what would happen if it was an addition rxn?
substitution leaves an aromatic ring in the product. addition across one of the double bonds in the ring would destroy the aromatic ring, losing its 36 kcal/mol of resonance stabilization
what are the last two steps that many mechanisms have in common for benzene reactions with electrophiles
- a base (with lone pair) attacks and captures the electrophile (the H+)
- regain the aromaticity (H+ removed)
compare the reaction of an electrophile with an alkene and with benzene
benzene rxn is slower than the alkene rxn because of the stability of the aromatic ring. benzene does not want to lose its 36 kcal/mol of resonance. the delta G double dagger for benzene is much higher than that of alkene
is the benzene substitution exothermic or endothermic
exergonic because the product retains the stability of the aromatic ring
what is the complete reaction mechanism for the electrophilic aromatic substitution
- electrophile adds to the aromatic ring which breaks or disrupts aromaticity. This makes a higher energy intermediate
- higher energy cationic intermediate is resonance stabilized. the proton is removed from the carbon that has formed the bond with the electrophile.
- loss of proton regenerates the aromatic ring (product: ring with electrophile attached + HB+)
halogenation
benzene with a halogen (F, Br, Cl, I) attached
nitration
benzene with an NO2
sulfonation
benzene with an SO3H
alkylation
benzene with an R group
acylation
benzene with a ketone (c with O double bond then R group)
explain the bromination and chlorination of benzene
requires a lewis acid and catalyst (FeBr3 or FeCl3) because the aromaticity of the benzene causes the molecule to be less reactive than a normal alkene
how to generate the electrophile for bromination and chlorination
(Cl can be replaced with all Brs here)
TYPICALLY you have a ring + Br2 –FeBr3–>
1. lone pair on the Br from the Br2 is the leaving group that bonds to the FeBr3 to create Br-Br-FeBr3
2. double bond on ring attacks one of the Br2 Brs which creates a ring with one Br and one H on a carbon
then you have the rest of the process
how to generate the electrophile for iodination of benzene
you have a benzene ring + I2 —-H2O2—->
1. I2 + H2O2 –> 2 H-O-I
2. double bond from benzene ring attacks I+ to create ring with H and I bonded to a carbon
3. OH (remaining from I) helps the proton come off to regenerate the ring
bromination and chlorination solvent
FeBr3 / FeCl3
iodination solvent
H2O2
how to generate the electrophile for nitration
benzene ring + HNO3 –H2SO4–>
- oxygen with two lone pairs attached to the hydrogen of HNO3 attacks hydrogen on H2SO4 to produce H2O+-NO2
- now water is a good leaving group and it leaves to produce +NO2
- double bond on benzene ring attacks nitrogen on NO2
- base attacks hydrogen one the same carbon as NO2
nitration solvent
HNO3 and H2SO4
sulfonation solvent
fuming H2SO4
how to generate the electrophile for sulfonation
benzene + H2SO4 –SO3 and heat–>
- lone pair on oxygen on SO3 (all oxygens are double bonded) attacks the hydrogen of H2SO4 to produce SO3 with a hydrogen attached (that specific oxygen is only a single bond) + HSO4-
- sulfuric acid becomes sulfonium ion + water
- benzene ring attacks +SO3H (sulfonium ion)
- base attacks hydrogen which leaves just SO3H on the ring + HB+
which reaction is reversible
the sulfonation of benzene
what are the friedel-crafts rxns
two electrophilic substitution rxns:
- friedel crafts acylation installs and acyl group on a benzne
- friedel crafts alkylation installs an alkyl group on a benzene ring
what is a limitation of the friedel-crafts rxns
- cannot undergo rxns with the electrophile in the meta position (meta deactivates benzene)
- cannot undergo reactions that would leave -NH2, -NHR, or -NR2 in the electrophile position because these compounds are so basic that they react with the solvent instead
explain friedel-crafts acylation
benzene + acyl chloride (acyl bonded to Cl) –AlCl3–>
- the strong lewis acid AlCl3 is required to rip off the acyl group
what is an acyl group
C bonded to R and double bonded to O
(essentially it is half of a ketone or half of an aldehyde)
(R has to be an alkyl (methyl, ethyl, etc.) or aryl (ring) substituent)
friedel-crafts acylation solvents
AlCl3
for friedel-crafts acylation what is the lewis acid and what is the lewis base
acyl chloride = lewis base
AlCl3 = lewis acid
what is the mechanism for the friedel-crafts acylation
benzene + R-C=O (acylium ion)
- benzene attacks carbon
- H attached to C double bond O and bond to R is attacked by base
explain friedel-crafts alkylation
- any alkyl halide works
- aryl halides and vinyl halides do not react because their carbocations are too unstable
- carbocation is the electrophile so watch out for rearrangements
- requires a strong lewis acid (AlCl3)
friedel-crafts alkylation mechanism
benzene + RCl –AlCl3–>
- double bond on benzene attacks R+ which produces ring with R and H
- base attacks H bonded to the carbon with the R which produces benzene with R + HB+
friedel-crafts alkylation solvent
AlCl3
friedel-crafts alkylation what is the lewis acid and what is the lewis base
lewis base: alkyl halide (R-Cl)
lewis acid: AlCl3
what is the caveat for the alkylated benzene
- alkyl groups donate electrons so the product is more reactive than benzene, therefore a large excess of benzene is needed to avaoid multiple alkylations of the ring
class encore rule ab ortho / para / meta directors
a substituent is an electron donating group if the first atom is MORE electronegative than the second.
a substituent is an electron withdrawing group if the first atom is LESS electronegative than the second.
