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Chemistry 2 > Chemistry Y3: Aromatic chemistry > Flashcards

Chemistry Y3: Aromatic chemistry Flashcards

1
Q

What are the properties of benzene?

A

C6H6
Benzene is planar
Very stable
Pi electrons are delocalised above and below the ring

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2
Q

What is aromatic stacking?

A

The e rich centre or 1 benzene and e poor edge of another benzene are held together by aromatic stacking
*This happens in DNA

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3
Q

Catatonic Pi stacking

A

a cation is stacked onto the negative e rich centre of a benzene molecule

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4
Q

What are the 4 valence orbitals of C

A

2S, 2PX, 2PY, 2PZ

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5
Q

RULES OF AROMATICITY

A
  1. Must be cyclic
  2. Must be planar
  3. Must be conjugated with p-orbitals at every junction
  4. 4n+2 Huckel’s rule
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6
Q

what if a compound followed all the rules of aromaticity except Huckels?

A

It would be anti-aromatic

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7
Q

What is the maximum number of carbons you can have in a ring before it starts to pucker

A

7 C ring = planar and flat
8 C ring = Puckers

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8
Q

What is the difference between a Nu and an E

A

E+ = e accepting
Nu-= e donating

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9
Q

Why is benzene losing aromaticity often the RDS

A

Because benzene is very stable it takes a very good E/Nu to make benzene loser aromaticity
This takes time

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10
Q

What is SeAr

A

Is is electrophilic aromatic substitution

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11
Q

Which 5 SeAR reactions do we need to know

A

Halogenation
Nitrations
Sulfonation
Freidel-crafts acylation
Freidel-crafts alkylation

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12
Q

What reagents and conditions are needed for SeAr Bromination?
Ar-H—-> Ar-X

A

AlBr3/AlCl3 (LA)
Br2/Cl2
Heat

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13
Q

What reagents and conditions are needed for SeAr Nitration?
Ar-H —–> Ar-NO₂

A

E⁺= NO₂⁺

HNO3
H2SO4
HEAT

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14
Q

What reagents and conditions are needed for SeAr Sulfonation?
Ar-H —> Ar-SO₃H⁺

A

E⁺= SO₃H⁺

conc. H2SO4 + HEAT

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15
Q

What reagents and conditions are needed for SeAr Freidel-Crafts Alkylation?
Ar-H —–> Ar-C(R ₃)

A

E⁺= R₃C⁺

R₃C-X
LA
HEAT

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16
Q

What reagents and conditions are needed for SeAr Freidel-Crafts Acylation?
Ar-H —-> Ar-c(=O)R

A

E⁺= (RCO)⁺

RC(=O)Cl
LA
HEAT

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17
Q

In terms of product formed which is better acylation or alkylation?

A

ACYLATION

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18
Q

What does substituting an arene do to its reactivity

A

*Changes its reactivity
*determines location of the next attack

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19
Q

What happens if an EDG is attached to benzene

A

*Electrons are pushed into the ring
*Increases RoR
*Ring has more density
*O/P directing

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20
Q

What happens if an EWG is attached to benzene?

A

Pulls e density out of ring
*Decreases RoR
*Ring has less e density
*Meta

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21
Q

what is the best activator

A

NR2

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22
Q

What is the best Deactivator

A

NO2

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23
Q

Does activated or deactivate benzene stabilise the intermediate?

A

Activated benzene

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24
Q

Which has the higher activation energy EWG or EDG?

A

EWG - DEACTIVATES at O/P- therefore is meta directing. Slows reaction

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25
Q

What is the induction mechanism?

A

substituents donate/withdraw e density inductively via a sigma bond

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26
Q

What is the resonance mechanism?

A

Substituents donate/withdraw e density via the Pi network
*in resonance mechanisms you should be able to push e with curly arrows (resonance structures)

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27
Q

What are halogens

A

They’re EWG but direct to O/P not Meta

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28
Q

Are halogens resonance or induction?

A

They’re both
*They deactivate the ring due to inductive withdrawal
*LP resonance increases e density in O/P
*so, O/P directing for SeAR

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29
Q

Which has more effect resonance or induction?

A

RESONANCE

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30
Q

What happens if a positive charge in a cation ends up next to a substituent?

A

The substituent could stabilise or destabilise the cation (+ve) charge depending on what it is

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31
Q

Give 2 examples of EDGs that use induction

A

CH3 and C6H5

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32
Q

Give 2 examples of EWGs that use induction? And explain what happens when these react

A

NR3 and CF3- Nothing happens

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33
Q

Is Phenol activating of deactivating

A

V activating
acts as acid
O/P
can push e into ring

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34
Q

What does phenoxide look like? is it activating or deactivating

A

Benzene ring with O- attached

vvv activating

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35
Q

What does aniline look like ? is it activating or deactivating

A

Benzene with NH2 attached
LP on N
VV activating

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36
Q

what happens when you put aniline in acidic conditions?

A

It forms anilinium cation = extremely deactivating - reaction wont happen

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37
Q

what happens to Phenol in basic conditions?

A

it is converted to phenoxide
*This is more reactive than phenol

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38
Q

What is the benefit of reacting with phenol instead of benzene

A

It is more reactive. Reactions occur more easily
Less extreme conditions are needed

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39
Q

What happens when you brominate a phenol?

A

Tribromination

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40
Q

How do we achieve mono-nitrate a phenol?

A

Use dilute HNO3 to give a low conc

41
Q

What reagents do you need for a FC reaction with Phenoxide?

A

A weak catalyst e.g HF
or even non-catalytic electrophiles e.g. CO2

42
Q

How can we achieve SeAR reactions with a phenol?

A

We can temporarily protect it by converting it to an ester via ACYLATION
*With Acid chlorides and acid anhydrides
*Esters can still push e density into the ring
*O/P directing still

BUT
*Some e density is puller towards the ester (=O) less e density made available that in phenol
*Some e density is stabilised through ester double bond

43
Q

If there is a big group attached to a benzene which position is a 2nd group likely to attach at?

A

PARA - more space
less steric hinderance

44
Q

When doing all SeAR reactions with aniline what do we need to consider?

A

They all take place in acidic solution so,
-Need so work round this e.g Basic conditions
-Or anilinium cation is made

45
Q

How do we moderate aniline reactivity (EAS)? And what does the reaction look like?

A

can temorarily protect aniline as an amide via acylation
*activating effect is reduced due to competing withdrawal of a N LP by the carbonyl group of the amide

The reaction:
*must be in basic conditions
*SeAR will go to PARA 1st
*Gives Mono-substitution mainly in PARA (OCCASIONALLY ORTHO)

46
Q

why is reacting with an amide preferable to reacting with aniline

A
  1. Moderates reactivity
  2. can use acidic conditions
    3.product will be mono-substituted
  3. Aniline acylation is reversible (Just add a base)
47
Q

How do we make aniline

A

Benzene —(Nitraiton)—> Nitrobenzene —-(reduction)—-> Aniline

48
Q

What conditions are needed to go from Aniline to Anilinium cation (and vice versa)?

A
  1. H+
  2. OH-

or can form with a LA attached in which case
1. LA

49
Q

what conditions are needed to go from aniline to nitrobenzene (and vice versa)?

A
  1. Sn/HCl then neutralise
  2. Pd/C , H2 reduction
50
Q

what conditions are needed to go from aniline to amide (and vice versa)?

A
  1. NaOAc (Basic conditions)
  2. H3O+
51
Q

what conditions are needed to go from aniline to Diazonium salt

A
  1. NaNO2 and HCl
52
Q

How do aromatics react with nucleophiles

A

*e density is pushed into the ring from Nu
*aromaticity is lost =RDS
*Both LG and NU are attached
*SP3 hybridised - resonance possible
*LG eliminated
*aromaticity regained

53
Q

Why is a good EWG needed for SnAR

A

*to pull some of the e density out of the ring
*to stabilise the anion
*EWG should be O/P to LG

54
Q

what 3 things do we need for SnAR

A

*EWG O/P to LG/Nu
*Good leaving group F- or Cl-
*Nu

55
Q

Stabilisation can either be …?

A

resonance or inductive
-This stabilises the ring’s -ve charge

56
Q

Inductive stabilisation?

A

Pushes e density out via sigma bonds

56
Q

What position should the EWG be in in SnAR

A

ORTHO/PARA

57
Q

Resonance stabilisation

A

Pushes e density out via Pi system e.g NO2

57
Q

What are common Nucleophiles?

A

O, N or CN

58
Q

what are good EWGs and what do they do?

A

NO2, Sulfur based groups (SO3R), CN, and carbonyl groups
*They activate the SnAR reaction
*They stabilise the anion intermediate

59
Q

Why is resonance substitution better than induction?

A

*Single resonance substitution
*Faster and easier

60
Q

If there are 2 EWGs that are resonance what does this mean compared with 1?

A

that even less extreme conditions are needed and reaction goes faster.

61
Q

Can resonance substitution for SnAR occur in the META position?

A

No not even in extreme conditions

62
Q

Why are Halides good leaving groups?

A

*the halide creates a dipole with the carbon
*The -ve Nu can attack the partially +ve carbon
*e density can be pushed down to be stabilised by EWG
* the e density can be pushed back up to lose the F- ion (fast step)

63
Q

Which is the best Nu:
Amines
Alcohols
Alkoxides

A

Alkoxides>Amines>Alcohols

64
Q

Why are diazonium salts useful?

A

*Have N(triple bond)N
*Good starting point
*N2 is a v good LG, readily eliminated a N2 (g)
*Leaves benzene as carbocation (V hard to do)
*So DS is a source of aromatic carbocations

65
Q

Why are aryl carbocations so unstable

A

Breaks SP2 hybridisation by sharing e with N
*Ring can no longer be stabilised by resonance
*can be used as an E+

66
Q

How do you form a DS from benzene?

A
  1. Nitration -> nitrobenzene with HNO3 and H2SO4
  2. Reduction -> Aniline with H2 and Pd/C
  3. -> DS with HCl and NaNO2, <5℃
67
Q

How do you get from a DS to a phenol?

A
  1. -> Aryl cation with H2SO4, H2O and HEAT (N2 given off)
  2. Add H2O -> intermediate
  3. -> phenol with HSO4-
68
Q

How to make an Aryl iodide from DS?

A
  1. <25℃ -> aryl cation
  2. KI (I- attacks), <25℃
  3. -> Aryl iodide
69
Q

Why is the Sandmeyer reaction useful?

A

Helps to introduce weaker Nu
*Its a radical mechanism

70
Q

what does the Sandmeyer reaction use?

A

Cu catalysis: Cu-X and H-X

X can be:
Cl, Br, CN

71
Q

What is the Balz-scheimann reaction used for?

A

*To make aromatic fluorines

72
Q

How do you make an aromatic fluorine from a DS?

A
  1. DS —(NaBF4 and NaCl)–>
  2. Intermediate
  3. gentle heating, N2 removed
  4. Fluorobenzene + BF3 made
73
Q

How do we create tribrominated benzene from nitrobenzene?

A
  1. Nitrobenzene -> analine with H2 and Pd/C
  2. Aniline gets tribrominated using Br2
  3. -> DS using HCl, NaNO2, <5℃
  4. -> tribrominated benzene with H3PO2, H2O, HEAT (N2 removed)
74
Q

Reagents for DS -> Benzene

A

H2PO2, H2O, HEAT
(N2 removed)

75
Q

How can DS act as electrophiles in SeAR

A
  1. Introduce DS as E+ to benzene
  2. Benzene must have an EDG
  3. Forms bonds in O/P
  4. C-N bond at O/P
76
Q

What are heterocycles?

A

atoms other than c are incorperated into rings
e.g. N, S or O

77
Q

If a LP is in the SP2 orbital is it counted in Huckel’s?

A

No as its not in a P-orbital
-Can’t contribute to the ring

78
Q

What is Huckels rule?

A

4n+2

79
Q

In 5 membered rings and heteroaromatics what does a -ve charge mean

A

an extra e

80
Q

What is Pyrrole?

A

Neural, Stable and aromatic
non-basic
forms 5-memebered heteroaromatic
has 2 LPs that contributes to Pi system

81
Q

why are Pyrrole and pyridines LPs treat differently?

A

Pyrrole- Has LP that are in P orbital - so they contribute to Pi system

Pyridine- Has LP that sity in SP2 orbital, can’t contribute to ring
-Making it basic

82
Q

If an anion has resonance, what does this mean?

A

The LP must be in the P orbital as its contributing to the ring

83
Q

How can we make an aromatic from cyclopentadiene (non-aromatic)

A
  1. remove 1H
  2. gives a cyclopentadienyl anion = aromatic
84
Q

Imidazole has 2 N in a 5 membered ring, which orbitals are their LPs in

A

1st N (LP must be in P orbital, SP2 orbital all full)
2nd N (LP must be in SP2 orbitals

85
Q

is imidazole aromatic?

A

6 Pi e, neutral, planar, cyclical
=YES

86
Q

Once you have a nitro group in O/P where will the next Nitro group direct?

A

Meta- NO2 is M directing

87
Q

Once you have a CH3 group where will the next group attack?

A

O/P

88
Q

If we have an EWG attached where will the next group attach?

A

Meta

89
Q

what has the bigger effect activating or deactivating?

A

activating

90
Q

If you want to keep Nitrating what do you do?

A

NO2 is Meta directing- not favourable but have to keep increasing temperature

91
Q

How are halogens inductive and resonance?

A
  1. Inductive withdrawal through sigma bond-
  2. resonance can push LP into ring increasing e dinsity at o/p- sabilising cation
92
Q

how does halogenic inductive withdrawal have different effects in SeAR and SnAR

A

SeAR= inductive withdrawal destabilising cation
SnAR = inductive withdrawal stabilising anion

93
Q

how do you get from alkylbenzenes to benzoic acid?

A

oxidation using hot KMNO4

94
Q

Acyl to acyl chloride?

A
  1. Acylbenzene -> Alkyl uising Zn(Hg) nad Hcl
  2. Alkylbenzene to Benzoic acid using hot KMNO4
  3. Benzoic acid -> Acyl chloride using SOCl2
95
Q

What is retrosynthesis?

A

That the order of the reaction can change the outcome of the product
-Relation to M or O/P directing

96
Q

Benzene to 1-fluoro-3-bromobenzene

A
  1. Nitration (HNO3 and H2SO4)
  2. Bromination at Meta (Br2 and AlBr3)
  3. Reduction - Aniline (H2 and Pd/C)
  4. diazotisation/ fluorination (NaNO2 and HCl) followed by (NaBF4 and HEAT)

Chemistry 2 (7 decks)

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