Aromatic compounds and benzene Flashcards
describe the length and strength of the C-C bonds in benzene
all of them are the same length and strength
T or F: benzene is planar
true
describe the electron clouds in benzene
the benzene has ring shaped delocalized pi electron clouds above and below the planar molecule
is benzene aromatic?
yes
T or F: benzene is an annulene
true
what is an annulene
conjugated monocyclic hydrocarbon
how do you name annulenes
[n]annulene, where n=number of carbons in the ring
T or F: all annulenes are aromatic
false! not all of them are
name the benzene compound based on the substituent: methyl
toluene
name the benzene compound based on the substituent: COOH
benzoic acid
name the benzene compound based on the substituent: OH
phenol
name the benzene compound based on the substituent: COH
benzaldehyde
name the benzene compound based on the substituent: OCH3
anisole
name the benzene compound based on the substituent: COCH3
acetophenone
name the benzene compound based on the substituent: NH2
aniline
name the benzene compound based on the substituent: CN
benzonitrile
T or F: benzene can form polycyclics
true
name the polycyclic based on the number of rings/their arrangement: 2 in a line
napthalene
name the polycyclic based on the number of rings/their arrangement: 3 in a line (straight)
anthracene
name the polycyclic based on the number of rings/their arrangement: 3 in a line (bent)
phenanthrene
name the polycyclic based on the number of rings/their arrangement: 4 in a cluster
pyrene
describe the placement of the substituents in the ortho prefix
right beside each other (1 and 2)
describe the placement of the substituents in the meta prefix
two away from each other (1 and 3)
describe the placement of the substituents in the para prefix
at opposite sides to each other (1 and 4)
for disubstituted benzenes, which sub is labelled as carbon one?
the highest priority sub is on carbon one
T or F: benzene is unstable
false; it is stable
T or F: benzene is highly reactive
false; it is stable, so it doesn’t want to react
list the three reasons why benzene isn’t reactive
- all the bond lengths are the same
- all carbons are sp2 hybridized and each p orbital overlaps equally with its 2 neighbors
- it’s resonance stabilized
what are the criteria for a molecule to be aromatic (there’s 4)
- contain 4n+2 electrons (in pi and/or lone pairs), where n=0,1,2,3…
- delocalized electrons must be in one or more rings
- must be conjugated
- must be planar
T or F: conjugate base aromaticity increases acidity
true
why does conjugate base aromaticity increase acidity
when an acids conj. base is aromatic, it is stable. Stable conjugate bases=super strong acids=increased acidity
are anti-aromatic compounds stable or unstable
unstable
are anti-aromatic compounds reactive or unreactive
highly reactive
what is the rule for anti-aromatic compounds
contain 4n electrons. All the other criteria is the same as the criteria for aromatic compounds
what happens to the aromaticity of benzene if it reacts
benzene will lose aromaticity
when benzene reacts, are the products or reactants favored
reactants are favored because the product is uphill
how can we increase the difference in electron density between the reactions so the product is favorable (in benzene reactions)
use a lewis acid to accept the electrons and make a better electrophile
when we use lewis acids for benzene reactions, what type of product do we get (hint: reaction will give us addition)
we get a product that underwent mono addition (not anti-addition like we would expect)
what lewis acid do we use in halogenation
AlBr5 (where Al is neg, and one of the Br is pos)
describe the halogenation reaction
benzene reacts with the lewis acid, picking off it’s electrophilic Br. When this happens, we get an intermediate that is a resonance stabilized sigma complex. Then there is a proton transfer and the lewis acid is regenerated (catalyst), and the pi bond is reformed and we’ve done mono addition to the benzene
in a nitration reaction, what are we looking to add to the benzene
NO2
T or F: for nitration, using a traditional lewis acid isn’t great
true
explain why traditional lewis acids aren’t very good in nitration reactions
if we react benzene with NO2 and a lewis acid, we get a low yield, harsh conditions, and side products
in nitration, what do we use instead of a lewis acid + why
H2SO4, because nitric acid (HNO3) is acidified by it
describe the acidification of nitric acid by H2SO4
- basically combining nitric acid and sulfuric acid together
- nitric acid deprotonates sulfuric acid
- the product is a cation (NO2+), which benzene cannot resist
- therefore, right as NO2+ is made, it reacts with benzene
in nitrogenation, describe the reaction of benzene with NO2+
benzene attacks N+ of NO2+, we get a new sigma bond that came from pi electrons, we get the sigma complex, then we deprotonate using sulfuric acid and drive towards benzene. Another proton transfer, and the sulfuric acid is regenerated and we’re left with mono addition of NO2 on benzene
what are we trying to add to benzene in sulfonation
SO3H
describe the SO3 equilibrium (and which side the equilibrium lays)
H2SO4 = SO3 + H2O
reactants are favored
how can we drive the H2SO4 and SO3 equilibrium to the products to get more SO3
heat!
describe the sulfonation mechanism
benzene attacks SO3, sulfonic sigma complex, proton transfer to make SO3- into SO3H, and we’re done
how do we convert a benzene with SO3H on it back into regular benzene
react it with dilute H2SO4
what are we trying to make with friedel-crafts alkylation
a benzene with an alkyl group attached (mono addition)
what do we use to add an alkyl group onto a benzene
use a lewis acid and an alkyl halide as an electrophile
T or F: friedel-crafts alkylation may also be intramolecular
true; practice this
describe the limits to alkylation
if the alkyl group is more than 2 carbons, a mixture of products is formed, and the number of side products increases as the alkyl halide grows
how do we add a long hydrocarbon chain onto a benzene
use a lewis acid and an acyl halide as an electrophile (friedel-crafts acylation)
what is an acyl halide
carbonyl attached to R and X
what is the product of friedel-crafts acylation
a benzene with carbonyl attached to R on it
in friedel-crafts acylation, how do we get rid of the carbonyl on benzene
wittig or clemmensen to reduce the carbonyl. It leaves the chain and R attached
what happens if we use anhydrides in friedel-crafts acylation
the method is analogous to that using acyl halides, because both have good leaving groups. We’re left with a benzene with a carbonyl attached
