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CHEM2010 Synthesis + Spectroscopy > Synthesis Aromaticity idk > Flashcards

Synthesis Aromaticity idk Flashcards

1
Q

4 rules of aromaticity

A
  1. Cyclic. 2. Fully conjugated pi electron system (every carbon is sp2 and p-orbs overlap). 3. Atoms in ring planar otherwise p-orbs don’t overlap. 4. Huckel’s rule: 4n+2 pi electrons in the ring
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2
Q

Why do anilines have a lower basicity than expected and why is it only slighlty lower

A

Delocalisation of N lone pair into aromatic ring (lone pair doesnt want to react with another H). However! N LP is in sp3 orbital therefore not optimal orbital overlap as it has a different angle so only PARTIAL DELOCALISATION

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3
Q

What is inductive effects

A

Bond polarisation due to electronegativity differences

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4
Q

What is mesomeric effects

A

How a functional group extends the resonance/delocalisation of e- in a molecule

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5
Q

What does a lower pKaH mean

A

It means a stronger conjugate acid therefore a weaker base

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6
Q

In the para position, does EWG or EDG give a stronger aniline base?

A

Donating group gives a stronger base

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7
Q

Benzoic acid can’t be stabilised by ring conjugation, but despite this what can increase the acidity of benzoic acid?

A

The substituents. An electron withdrawing group stabilises it more aka more acidic

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8
Q

Will a phenol be more acidic with an ewg or edg as a substituent

A

EWG otherwise both the OH and egd will donate e- into the ring giving 2 negative charges = unfavourable!

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9
Q

What is a heteroaromatic compound

A

Benzene ring where a C is replaced with any other atom but mostly O, N, S (+ all the rules for aromaticity)

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10
Q

What is pyridine

A

Isoelectronic w benzene but i C is an N

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11
Q

What is pyrrole, furan, thiophene

A

Isoelectric w cyclopentadiene anion but 1 C swapped with NH, O, S respectively

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12
Q

Why does pyridine (with an N) have a fully conjugated system

A

N is sp2, so p-orbital parallel to the rest. Extra LP is orthogonal therefore is not involved in delocalisation

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13
Q

Is sp3 or p orbital higher in energy

A

P because no s character. S closer to +bve charge in nucleus which stabilises e-

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14
Q

Explain change in reactivity (eg resonance stabilisation) of pyridine vs benzene

A

Electronegative N causes dipole. It has lower HOMO/LUMO energies than benzene -> worse orbital overlap -> less efficient delocalisation -> less resonane stabilisation. Lower HOMO causes less reactivity with nucleophile

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15
Q

Why is pyridine a weaker base than piperidine (pyridine without double bonds basically)

A

LP is sp2 hybridised vs sp3 so has more s-character. Held closer to nucleus -> more stabilised -> LP less available for reaction with protons

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16
Q

Do EWG or EDG substituents on pyridine make it more basic

A

EDG becasue it causes higher e- density on nitrogen -> more nucleophilic

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17
Q

What happens to basicity of pyridine if more nitrogen atoms are added

A

e- density pulled away from nitrogen -> less nucelophilic -> less likely to react -> less basic!

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18
Q

How is hydroxypyridine more acidic than phenol

A

N is slightly electronwithdrawing so stabilises negative charge formed when H is taken off

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19
Q

Is 2 and 4 hydroxypyridine more or less acidic than 3/5?

A

Less acidic. Tautomerism reaction happens where H goes from O to N forming =O and NH. Less likely to get H taken off by base

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20
Q

Is pyridone (=O and NH) still aromatic

A

Yes becasue LP on N can still be delocalised into ring

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21
Q

Name of molecule that 2 or 4 hydroxypyridine goes into during tautomerism

A

Pyridone

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22
Q

What can pyridine be used for

A

Nucleophilic catalyst in ester production

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23
Q

Structure of pyrrole

A

Like cyclopentadiene anion. LP on nitrogen is in p-orbital, part of conjugated system - delocalised - not available to do reactions.

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24
Q

Is HOMO/LUMO higher or lower in pyrrole than benzene

A

Higher because very e- dense due to delocalised LP

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25
Q

Why does furan (O) have a low stabilisation energy

A

O is very electronegative meaning not efficient p-orbs -> not efficient delocalisation -> not as stabilised aka more reactive

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26
Q

What is imidazole

A

Has one pyridine-like N and one pyrrole-like N

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27
Q

How to make Br2 react with benzene

A

Add lewis acid FeBr3. Br makes dative bond to empty p-orbital of Fe making Br+ and Fe-. Makes Br even more electrophilic

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28
Q

What is Wheland intermediate

A

When a benzene has added one atom on to a carbon which made a positive charge on the carbon next to it and “only 2 double bonds”

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29
Q

What can go wrong during friedel crafts alkylation of benzene

A

Over-alkylation. If straight chain alkyl halide then it can become branched in benzene because unstable primary carbocation rearranges to a more stable secondary carbocation

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30
Q

How to add a primary carbocation onto benzene

A

Friedel-Crafts ACYLATION (not alkylation) and reduce ketone to alkane

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31
Q

Why does only one ketone group add on during friedel crafts acylation

A

Ketone is electron withdrawing making product less reactive

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32
Q

What type of substituents increase the rate of aromatic nucleophilic substitution

A

Electron donatic making benzene more e- rich -> more nucleophilic

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33
Q

Why is ortho and para more likely than meta with EDG. And why is para most likely

A

Ortho and para have more resonance structures because they can be stabilised by EDG bc form a double bond to it. Intermediate is more stable. Ortho less likely bc of steric hinderance

34
Q

How does EDG substituent effect selectivity on benzene

A

EDG makes aromatic ring more nucleophilic. It stabilises positively charged intermediate. Stabilisation most effective when e-phile adds ortho or para to EDG.

35
Q

How does EWG substituent effect selectivity on benzene

A

EWG makes ring less nucleophilic. It destabilises positively charged intermediate. Least effective destabilisation (giving most stable) when electrophile adds meta to the EWG

36
Q

Mesomeric and inductive effect of halogens on benzene

A

Inductively e- withdrawing bc electronegative = deactivating - reacts slower. Mesomerically e- donating: o/p-directing but it is weak bc bad overlap betw halogen and C p-orbs. No effect on rate. In total deactovating BUT ortho/para directing

37
Q

Why does electrophilic aromatic substitution of pyridines not happen

A

Thermodynamics: N in ring lowers HOMO compared to benzene - makes ring e- deficient + less nucleophilic. Kinetics: LP on N will attack instead and cationic intermediate is inert to further elecphilic attack

38
Q

If electrophilic aromatic substitution of pyridine is forced, what position will electrophile add on to

A

3 bc doesn’t become δ+ - resonance structures

39
Q

How can you activate pyridines so they react

A

Add electron donating group or make an N-oxide

40
Q

How is an N-oxide made + useful on pyridine

A

Oxidise pyridine - forms N(+)-O(-) = N-oxide = dipolar but is stable. Eletro arom susbtitution as N-oxide becomes EDG. Lone pair is in bond so no side reactions. Deprotection by reduction with PPh3, PCl3 or P(OMe)3 - all form strong O=P bond. Faster + para selectivity

41
Q

What does it mean for HOMO etc if there is a build up of negative charge

A

High HOMO so very nucleophilic

42
Q

Difference in conditions between nitration of 6 membered and pyrrole

A

Milder conditions bc pyrrole more reactive (e- rich). Acetic acid instead of conc sulphuric + only 5 degrees temp

43
Q

5-membered heteroaromatics reactivity betw O, N and S

A

N>O>S because higher electronegativity means more localised on atom - less delocalised in ring - less nucleophilic. Also, S has high resonance stabilisation energy! Much less likley to react

44
Q

What happens with pyrrole and a very strong electrophile

A

It adds on to every position so fully bromonated if Br2

45
Q

Reaction conditions to mono-formylate (H-C=O) pyrrole (electronrich)

A

Pyrrole + DMF (HC(=O)NMe2) —-(1. POCl3)—(2. Na2CO3, H2O)–> pyrrole + formylate

46
Q

How to made furan (O) do mono-formylation

A

Normal Friedel-Craft conditions because less reactive than pyrrole

47
Q

Where is electrophilic substitution preferred on indole (benzene+pyrrole(5-N))

A

On 3-position bc it doesnt break aromaticity of benzene

48
Q

Why is SN2 impossible for aromatics

A

Negative nucelophile has electrostatic repulstion with e- in p orbitals

49
Q

Why is SN1 very unfavourable for aromatics

A

Leaving group leaves positive charge/empty sp2 orbital. Orthogonal to pi-system so is not stabilised by it. Also more s-character so closer to nucleus = repulsion

50
Q

What is a Meisenheimer complex

A

A negatively charged intermediate formed by the attack of a nucleophile upon one of the aromatic-ring carbons during the course of a nucleophilic aromatic substitution reaction

51
Q

What is needed for a 2-step addition-elimination reaction for aromatics

A

EWG on benzene. Good leaving group BUT this means F: something small to Nu can sterically get past and electronegative to pull e- away from carbon that Nu- adds on to

52
Q

What is the RDS for 2-step addition-elimination reaction for aromatics

A

The addition of the nucleophile (not leaving of leaving group so it doesnt need to be mega good at leaving)

53
Q

What is a diazonium compound

A

R-N≡N+

54
Q

What is an aniline

A

Benzene-NH2

55
Q

How to make diazonium salt from anilines (make aniline first from benzene-R)

A

Benzene-R + HNO3 and H2SO4 -> R-Ph-NO2. Then (Sn, HCl) or (H2, Pd/C) -> R-Ph-NH2. Then (NaNO2 + HCl, < 5C) -> R-Ph-N≡N+

56
Q

What can diazonium salts be converted into (4 things)

A

1) + PO2H3 (weak acid) -> benzene (protonated). 2) +H2O + heat -> phenol. 3) + CuX -> Benzene w halogen ligand. 4) CuCN -> benzene with CN ligand

57
Q

First step in diazotisation

A

Forming nitrosonium ion from NaNO2

58
Q

What is a nitrosonium ion

A

N≡O+

59
Q

What is benzyne/aryne

A

Benzene with one triple bond

60
Q

What is needed for benzyne formation then addition of nucleophile

A

Very strong base + benzene good leaving group + Nu

61
Q

What happens during Diels-Alder reaction

A

3 pi bonds broken, 2 C-C single bond formed and 1 C=C pi bond formed

62
Q

3 ways of adding nucleophile on to aromatics

A
  1. Addition-elimination mechanism. (Nu adds by LG). 2. Via SN1 mechanism (N2+ is the LG). 3. Benzyne intermediate
63
Q

3 modes of reactivity of imines

A
  1. Susceptible to nucleophilic attack (-ve charge on N + loses double bond). 2. Halogen derivative readily displaced w Nu. 3. α-protons are acidic (similar to ketoenol)
64
Q

What is an imine

A

carbonyl but N instead of O and it has one more bond to something else

65
Q

Increasing N-atoms in pyridine + reactivity (nucleophilic attack)

A

Ring becomes more e- deficient -> more susceptible to nucleophilic attack

66
Q

Is H- the leaving group when going from pyridine -> pyridine - NH2

A

No, forms NaH not just H-

67
Q

How to make pyridines more reactive towards ucelophilic attack

A

2-chloropyridine where Nu just swaps out the Cl in addit elim reaction

68
Q

What to do if we don’t have a good leaving group on pyridine

A

React pyridine N-oxide with POCl3 to form pyridine-Cl. No acts as nucleophile in beginning, then becomes good LG

69
Q

Is pyrrole or pyridine better at nucleophilic attack

A

Pyridine because pyrrole is electron rich

70
Q

Where does heteroatom direct electrophile during lithiation of 5 membered

A

Ortho

71
Q

What is needed to make lithiation work

A

Strong base eg Li-nBu

72
Q

What type of directing group is needed for benzene to do lithiation + reaction

A

Some electronegative atom to coordinatie with base to pull it close -> deprotonation on C-H next to directing group

73
Q

Last step in all lithiation reactions

A

Work up with water, - protonate negative charge

74
Q

Which position is preferred for electrophilic arom susbtit on 5-memebered heteroaromatics

A

2-position as it has more resonance forms!

75
Q

What is the Vilsmeier-Haak reaction for

A

If you want monosubstitution on pyrrole

76
Q

What is a acid-free alternative to friedel-crafts reaction

A

Vilsmeier-Haak reaction

77
Q

Why is Paal Knorr formation of furan easier than formation of pyrrole

A

Don’t need an external nucleophile - just acid which does tautomerism straight on the carbonyl

78
Q

Reactants for Paal-Knorr mechanism to make thiophenes (from 1-4 dicarbonyls)

A

P4S10 or Lawesson’s reagent

79
Q

Why is hydroxylamine (NH2OH) used instead of NH3 in synthesis of pyredines by 1,5-dicarbonyl by conjugate addition

A

Hydroxylamine used as nucleophile to allow for the elimination of three water molecules and the installation of 3 double bonds. Not needed for 5 membered ring

80
Q

What happens when pyridine is made with NH3 instead (Hantzsch synthesis)

A

First 1,4-dihydropyridine is made and then it oxidises in air to give pyridine

81
Q

2 main ways of making heteroaromatics

A

1 Synthesis of heteroaromatics exploiting carbonyl chemistry:
(imine, enamine and oxime formation, Aldol, tautomerism…)
2. Synthesis of fused heteroaromatics using electrophilic aromatic substitution
approach

82
Q
A

CHEM2010 Synthesis + Spectroscopy (2 decks)

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