aromatic compounds Flashcards
what are the characteristics of aromatic compounds?
- resonance
- high thermochemical stability
- under substitution instead of addition reactions
- nmr ring currents
who discovered the actual bond length of benzene and when?
kathleen londsdale in 1929
what is the theoretical energy increase of hydrogenating cyclohexane 3 times and what is the reality? why is it?
theoretically 360 but actually only 208 due to the stability offered by conjugation/resonance structures
what type of catalyst does benzene need to form monosubsituted product with bromine (Br2)
lewis acid catalyst (eg. FeBr3)
What are the requirements for a compound to be considered aromatic?
- conjugation
- cyclic, planar structure
- every carbon must be sp2 hybridized with 1 p orbital to overlap with neighbours
- follows huckels rule
.
why does a molecular have to planar to be aromatic?
so that the p orbitals are aligned and can effectively overlap (adjacent to each other and are parallel)
what is huckels rule and what is the equation?
4n+2 = no. of pi electrons if n is an integer than the planar ring is aromatic
what is the difference between aromatic and antiaromatic compound?
anti-aromatics abides by all the rules except for huckels rule (abides by 4n instead)
what is a frost musulin diagram
MO diagram to determine aromaticity of a cyclic structure
(the bottom is always only 1, vertex pointing down)
What is the mechanism for SEAr? What conditions are required?
Reacting benzene ring with electrophile and then deprotonation (elimination) to reform the aromatic ring
required a lewis acid catalyst
how many steps is an SEAr reaction?
2 steps, forms cationic intermediate
what is the driving force between carbocation intermediate and aromatic compound
re-establishing aromatic stabilization
what stabilizes the cationic intermediate?
delocalization of the positive charge
how can the transition state of SEAr reactions be studied?
kinetic isotope effect: replacing hydrogen with deuterium atoms and NMR the components
how does replacing H with D effect the transition states in SEAr?
no change for TS1 as that is only effected by rate of electrophile (that determines RDS) attack and TS2 is not observed thus is not the rate determining step
how was the carbocation species proven?
using 1:1 ratio of antimony pentaflouride and flouro sulphuric acid with NMR
draw out the mechanism for how SEAr carbocation can be isolated
check lecture 11 slide 5
what are the conditions for nitration and draw out the mechanism
catalysts H2SO4 but HNO3 is required by the catalyst and mechanism is on L11 S8
what is a sulfonation reaction and what is the mechanism
sulfur trioxide and sulfuric acid
L11 S9, protonated sulfur trioxide as electrophile
is sulfonation reversible and if so under what conditions?
it is and above 80 degrees C
draw out the mechanism behind chlorination and bromination and reagents required? why is that specific catalyst used?
L11 P10
FeH3 catalyst where H is the corresponding halogen to avoid halogen scrambling
to polarise the X-X bond
why cant halogen molecules react with the benzene ring on its own
no dipole, needs to be polarized (by lewis acid catalyst)
but if the aromatic ring is substituted then a catalyst isnt required (due to activating groups)
what is friedel crafts alkylation?
adding a alkyl group to an aromatic compound using an alkyl halide and an aluminium halide catalyst
what is the problem with freidel craft alkylation?
- carbocations can rearrange via wagner-meerwin 1-2 hydride of alkyl shifts thus final product formed is different
- polyalkylation due to the donating effect of alkyl groups in general
what is friedel crafts acylation? what is an issue?
addition of acetyl group to benzene ring with lewis acid catalyst BUT the oxygen can coordinate with the lewis acid causing a loss in product or loss of catalyst
what is a further reaction after acetylation?
reduction of ketonic species with zinc/mercury with HCl to form alkyl chain (without the overalkylation because carbonyl is electron withdrawing)
draw benzene and label the relative positions to the substituent group
google it LOL
what are the 2 types of directing and 2 types of substituents on the ring and describe what they are
activating group: donate electron density into the ring
deactivating groups: withdraw electron density from the ring
ortho/para directing and meta directing
what are the 2 ways of electron donating/withdrawing?
inductive effects (sigma donating/accepting)
conjugation/resonance effects (lone pairs, pi donating/accepting)
*electron donation is a lot more significant than electronegative in the case of oxygen and nitrogen etc. CHECK CASE BY CASE
does inductive effect or pi donation have a greater effect with halogens
inductive effect is greater
what determines whether a reaction is meta or otho/para directing?
carbocation stability (resonance forms and positive charge distribution)
what groups are meta and what groups are ortho/para directing
electron widthdrawing is meta directing
electron donating is ortho/para directing
what are the axis labels to compare ortho/para and meta reactions
gibbs free y
reaction co-ordinate x
are halogens ortho/para or meta
deactivating but ortho/para directors
why are deactivating groups/EWG meta directing
inductive effect: conjugation makes ortho/para positions less nucleophilic (can have positive charge) and EWG deactivates the ring -> meta preference
inductive effect after electrophile is added also destabilizes ortho/para cationic intermediates
what is the ratio between ortho and para products and why?
1:1 instead of 2:1 because para position is less hindered and more accessible for an electrophile and quantum mechanically more activating than ortho position
what is the chaperoning effect? + one more thing that effects ortho/para position
substituent can coordinate with electrophile and direct it to ortho position rather than para
solvent (but complicated to predict)
what is a separate way to direct ortho group + what reagents are used?
add a really bulky substituent to the para position to make 100% ortho then remove the bulky substituent in the end (normally sulfunation is used because the process is reversible)
what are the 3 rules regarding disubstituted substrates?
- the most activating group will win as the directing group (conjugation/resonance is more activating than induction)
- among positions that are similarly favoured, the site with the fewest/smallest substituents are more likely to be the site of attack (less sterically hindered)
- if there is a deactivating and an activating then activating, if 2 deactivating then least deactivating
what is the 3 criteria that needs to be fulfilled for there to be an SNAr reaction?
- ring must contain a powerful electron-withdrawing group
- ring must contain a leaving group
- leaving group must be ortho/para to EWG
what intermediate is formed in SNAr and what main difference of that and a cationic intermediate?
meisenheimer intermediate, difference is that the intermediate is negatively charged instead of positively charged
does the rate of SNAr increase down group 17 or decrease? and why?
decrease due to decreasing electronegativity
higher electronegativity lowers the energy of the rate-determining addition step
why does the EWG have to be ortho or para to the leaving group?
so that the negative charge can be pushed around the EWG and onto the leaving group so that it can leave
are SNAr and SEAr under kinetic control and what does that mean?
yes and under kinetic control means that the reaction proceeds along the pathway that has the lowest activation energy, leading to the formation of the most rapidly formed product.
How can aniline become a good leaving group? why is this process done
adding NOX to turn the aniline into a diazonium ion (N2) and under heat that makes a good leaving group (turns into nitrogen gas)
Is the nitrosation reaction SN1 or SN2
SN1
What are the reagents required for nitrosonium salt fomation? what is formed
sodium nitrite, mineral acid (HCl, H2SO4 or HBF4), 0 degrees
forms NO+ that is highly reactive and an anion which is whatever is left over of the mineral acid used
(combined is also known as nitrosonium salt)
what is the mechanism of diazotisation?
- nitrosonium ion (NO+) is added to the aniline which is highly nucleophilic
- series of proton transfers from the aniline to the NO+ group
- water is then a good leaving group leaving the diazonium salt (cyclic compound + the anion)
what is an SRN reaction?
sandmeyer reaction involves diazonium salts and copper with the nucleophile
- free radical based mechanism
what is an SN1Ar reaction?
diazonium salt + any compound that does not involve a copper salt
eg. KX, BX4 (balz-schiemann), sulfuric acid + heat with water, KN3 for triazide
how to choose between an SN1Ar and an SRN reaction
which one does the reaction under chaper conditions, which one has higher yield, faster reaction profile etc.
what is the RDS for thee SN1Ar and SRN reaction?
the departure of the diazonium ion into nitrogen gas
what is the s character of the sp2 orbital in the aryl cation?
high s character -> higher energy carbocation making it more reactive thus faster reaction with the nucleophile
what is the mechanism of the sandmeyer reaction?
formation of a c-centered radical which is highly reactive and the nucleophile then reactive with it
draw out the mechanism for the SRN reaction
check L15 S5
how do hard and soft nucleophiles effect the site of nucleophilic attack or diazonium salt? what is the reaction called?
soft attacks the terminal nitrogen via diazo-coupling
hard attacks the carbon at the ipso position via ipso substitution
what is formed between a phenol and diazonium salt (draw out the mechanism)
L15 S6
how to determine which ring will have the diazo salt in it first?
the less activating group will have the diazo salt in it
what is the name of the position where you label the ipso position, leave it empty and attach a substituent to the carbon next to it?
cine
why are carbons in a triple bond more electron deficient?
the electrons in a triple bond are more strained and do not delocalize as well, thus that region of the compound becomes slightly electron deficient, causing it to be prone to nucleophile attack (di-radical like structure)
what are the 2 types of products that can be formed with a benzyne intermediate?
ipso and cine substitution
draw out the mechanism for a reaction that goes via elimination-addition with benzyne intermediate
L15 S7
is benzyne aromatic? when is it used?
yes because the no. of electrons is still huckel compliant
when you have a base/nuc but you cannot do SNAr
what are the conditions required for desulfonation?
dilute sulfuric acid and high temperatures
why is sulfonate used?
due to its ability to be removed, it can act as a temporary meta directing group then be removed later on
since it is a bulky substituent it is added para to any substituent that is already on the ring
which position of substitution does SiMe3 and HSO3 direct towards?
ipso position
