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Reactivity At Carbon Centers > aromatic compounds > Flashcards

aromatic compounds Flashcards

1
Q

what are the characteristics of aromatic compounds?

A
  1. resonance
  2. high thermochemical stability
  3. under substitution instead of addition reactions
  4. nmr ring currents
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2
Q

who discovered the actual bond length of benzene and when?

A

kathleen londsdale in 1929

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3
Q

what is the theoretical energy increase of hydrogenating cyclohexane 3 times and what is the reality? why is it?

A

theoretically 360 but actually only 208 due to the stability offered by conjugation/resonance structures

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4
Q

what type of catalyst does benzene need to form monosubsituted product with bromine (Br2)

A

lewis acid catalyst (eg. FeBr3)

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5
Q

What are the requirements for a compound to be considered aromatic?

A
  1. conjugation
  2. cyclic, planar structure
  3. every carbon must be sp2 hybridized with 1 p orbital to overlap with neighbours
  4. follows huckels rule
    .
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6
Q

why does a molecular have to planar to be aromatic?

A

so that the p orbitals are aligned and can effectively overlap (adjacent to each other and are parallel)

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7
Q

what is huckels rule and what is the equation?

A

4n+2 = no. of pi electrons if n is an integer than the planar ring is aromatic

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8
Q

what is the difference between aromatic and antiaromatic compound?

A

anti-aromatics abides by all the rules except for huckels rule (abides by 4n instead)

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9
Q

what is a frost musulin diagram

A

MO diagram to determine aromaticity of a cyclic structure
(the bottom is always only 1, vertex pointing down)

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10
Q

What is the mechanism for SEAr? What conditions are required?

A

Reacting benzene ring with electrophile and then deprotonation (elimination) to reform the aromatic ring

required a lewis acid catalyst

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11
Q

how many steps is an SEAr reaction?

A

2 steps, forms cationic intermediate

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12
Q

what is the driving force between carbocation intermediate and aromatic compound

A

re-establishing aromatic stabilization

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13
Q

what stabilizes the cationic intermediate?

A

delocalization of the positive charge

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14
Q

how can the transition state of SEAr reactions be studied?

A

kinetic isotope effect: replacing hydrogen with deuterium atoms and NMR the components

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15
Q

how does replacing H with D effect the transition states in SEAr?

A

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

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16
Q

how was the carbocation species proven?

A

using 1:1 ratio of antimony pentaflouride and flouro sulphuric acid with NMR

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17
Q

draw out the mechanism for how SEAr carbocation can be isolated

A

check lecture 11 slide 5

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18
Q

what are the conditions for nitration and draw out the mechanism

A

catalysts H2SO4 but HNO3 is required by the catalyst and mechanism is on L11 S8

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19
Q

what is a sulfonation reaction and what is the mechanism

A

sulfur trioxide and sulfuric acid
L11 S9, protonated sulfur trioxide as electrophile

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20
Q

is sulfonation reversible and if so under what conditions?

A

it is and above 80 degrees C

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21
Q

draw out the mechanism behind chlorination and bromination and reagents required? why is that specific catalyst used?

A

L11 P10
FeH3 catalyst where H is the corresponding halogen to avoid halogen scrambling

to polarise the X-X bond

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22
Q

why cant halogen molecules react with the benzene ring on its own

A

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)

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23
Q

what is friedel crafts alkylation?

A

adding a alkyl group to an aromatic compound using an alkyl halide and an aluminium halide catalyst

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24
Q

what is the problem with freidel craft alkylation?

A
  1. carbocations can rearrange via wagner-meerwin 1-2 hydride of alkyl shifts thus final product formed is different
  2. polyalkylation due to the donating effect of alkyl groups in general
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25
Q

what is friedel crafts acylation? what is an issue?

A

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

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26
Q

what is a further reaction after acetylation?

A

reduction of ketonic species with zinc/mercury with HCl to form alkyl chain (without the overalkylation because carbonyl is electron withdrawing)

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27
Q

draw benzene and label the relative positions to the substituent group

A

google it LOL

28
Q

what are the 2 types of directing and 2 types of substituents on the ring and describe what they are

A

activating group: donate electron density into the ring

deactivating groups: withdraw electron density from the ring

ortho/para directing and meta directing

29
Q

what are the 2 ways of electron donating/withdrawing?

A

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

30
Q

does inductive effect or pi donation have a greater effect with halogens

A

inductive effect is greater

31
Q

what determines whether a reaction is meta or otho/para directing?

A

carbocation stability (resonance forms and positive charge distribution)

32
Q

what groups are meta and what groups are ortho/para directing

A

electron widthdrawing is meta directing

electron donating is ortho/para directing

33
Q

what are the axis labels to compare ortho/para and meta reactions

A

gibbs free y
reaction co-ordinate x

34
Q

are halogens ortho/para or meta

A

deactivating but ortho/para directors

35
Q

why are deactivating groups/EWG meta directing

A

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

36
Q

what is the ratio between ortho and para products and why?

A

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

37
Q

what is the chaperoning effect? + one more thing that effects ortho/para position

A

substituent can coordinate with electrophile and direct it to ortho position rather than para

solvent (but complicated to predict)

38
Q

what is a separate way to direct ortho group + what reagents are used?

A

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)

39
Q

what are the 3 rules regarding disubstituted substrates?

A
  1. the most activating group will win as the directing group (conjugation/resonance is more activating than induction)
  2. 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)
  3. if there is a deactivating and an activating then activating, if 2 deactivating then least deactivating
40
Q

what is the 3 criteria that needs to be fulfilled for there to be an SNAr reaction?

A
  1. ring must contain a powerful electron-withdrawing group
  2. ring must contain a leaving group
  3. leaving group must be ortho/para to EWG
41
Q

what intermediate is formed in SNAr and what main difference of that and a cationic intermediate?

A

meisenheimer intermediate, difference is that the intermediate is negatively charged instead of positively charged

42
Q

does the rate of SNAr increase down group 17 or decrease? and why?

A

decrease due to decreasing electronegativity

higher electronegativity lowers the energy of the rate-determining addition step

43
Q

why does the EWG have to be ortho or para to the leaving group?

A

so that the negative charge can be pushed around the EWG and onto the leaving group so that it can leave

44
Q

are SNAr and SEAr under kinetic control and what does that mean?

A

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.

45
Q

How can aniline become a good leaving group? why is this process done

A

adding NOX to turn the aniline into a diazonium ion (N2) and under heat that makes a good leaving group (turns into nitrogen gas)

46
Q

Is the nitrosation reaction SN1 or SN2

A

SN1

47
Q

What are the reagents required for nitrosonium salt fomation? what is formed

A

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)

48
Q

what is the mechanism of diazotisation?

A
  1. nitrosonium ion (NO+) is added to the aniline which is highly nucleophilic
  2. series of proton transfers from the aniline to the NO+ group
  3. water is then a good leaving group leaving the diazonium salt (cyclic compound + the anion)
49
Q

what is an SRN reaction?

A

sandmeyer reaction involves diazonium salts and copper with the nucleophile

  • free radical based mechanism
50
Q

what is an SN1Ar reaction?

A

diazonium salt + any compound that does not involve a copper salt

eg. KX, BX4 (balz-schiemann), sulfuric acid + heat with water, KN3 for triazide

51
Q

how to choose between an SN1Ar and an SRN reaction

A

which one does the reaction under chaper conditions, which one has higher yield, faster reaction profile etc.

52
Q

what is the RDS for thee SN1Ar and SRN reaction?

A

the departure of the diazonium ion into nitrogen gas

53
Q

what is the s character of the sp2 orbital in the aryl cation?

A

high s character -> higher energy carbocation making it more reactive thus faster reaction with the nucleophile

54
Q

what is the mechanism of the sandmeyer reaction?

A

formation of a c-centered radical which is highly reactive and the nucleophile then reactive with it

55
Q

draw out the mechanism for the SRN reaction

A

check L15 S5

56
Q

how do hard and soft nucleophiles effect the site of nucleophilic attack or diazonium salt? what is the reaction called?

A

soft attacks the terminal nitrogen via diazo-coupling

hard attacks the carbon at the ipso position via ipso substitution

57
Q

what is formed between a phenol and diazonium salt (draw out the mechanism)

A

L15 S6

58
Q

how to determine which ring will have the diazo salt in it first?

A

the less activating group will have the diazo salt in it

59
Q

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?

A

cine

60
Q

why are carbons in a triple bond more electron deficient?

A

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)

61
Q

what are the 2 types of products that can be formed with a benzyne intermediate?

A

ipso and cine substitution

62
Q

draw out the mechanism for a reaction that goes via elimination-addition with benzyne intermediate

A

L15 S7

63
Q

is benzyne aromatic? when is it used?

A

yes because the no. of electrons is still huckel compliant

when you have a base/nuc but you cannot do SNAr

64
Q

what are the conditions required for desulfonation?

A

dilute sulfuric acid and high temperatures

65
Q

why is sulfonate used?

A

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

66
Q

which position of substitution does SiMe3 and HSO3 direct towards?

A

ipso position

Reactivity At Carbon Centers (8 decks)

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