aromatic (benzenes) Flashcards

1
Q

aromaticity refers to

A

planar
stable due to delocalisation of p electrons
intermediate bond angles

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2
Q

describe the structure of benzenes

A

bond lengths are longer than normal cc and shorter than c=c

each c forms 3 bonds so 1 lone electron which gets delocalised as the p orbitals overlap to form a pi bond

e cloud above and below the ring plane

molecule is stable due to electron delocalisation

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3
Q

3 reasons why it’s benzene and not the hypothetical cyclo135triene

A
  1. bond lengths
    - C-C and C=C are different lengths. so this hypothetical molecule would be?? Impossible, less stable?
  2. Doesn’t undergo addition reactions
    - doesn’t decolourise bromine water, doesnt readily undergo addition reactions because it disrupts the stability of the e ring by ruining it basc.
  3. Enthalpies of hydrogenation
    - refers to the enthalpy change measures when different cyclic alkanes are hydrovenated to get to cyclohexene. All db get hydrogen added across them
    - they increase for 1ene to diene from triene, indicating that it’s more exothermic to become cyclohexane
    - but the enthalpy of hydrogenation for benzene is less than triene, indicating it is more stable
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4
Q

What mechanism does benzene undergo and why

A

Electrophilic substitution

Electron density makes it attractive to electrophiles

Addition would disrupt the delocalisation and therefore reduce stability but substitution only causes temporary disruption and preserves the delocalised system

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5
Q

What are the two electrophilic substitution reactions benzene undergoes

A

Nitration with nitronium ions to form nitro benzenes

Friedek crafts acylarion with acid anhydrides or acyl chlorides to form phenyl ketones (depends on r group)

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6
Q

Conditions of nitration

Formation of electrophile

A

Conc nitric acid
Conc sulfuric acid
50 degrees

Hno3 + h2so4 - > h2no3+ + hso4-
H2no3+ - > no2+ + h2o

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7
Q

Mechanism of nitration

A

Nitronium ion attacks the ring

Ring is disrupted and a C forms bond with N ion

Horseshoe

H on the C that is in that C-NO2 donates bond to ring

forms nitrobenzene and proton

could have successive reactions

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8
Q

Why are nitrobenzenes useful

A

Used as explosives

Important step in organic synthesis of other molecules eg amines. Produces primary amines that are used in the manufacture of dyes

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9
Q

Conditions of Fc acylation

A

Anhydrous
AlCl3

Both reagents have same group that is attached (molecule left after leaving group (different) goes):
O
II
R-C+

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10
Q

Formation of electrophile in Fc acylation

Regeneration of catalyst after the mechanism

A

ACYL CHLORIDES
AlCl3 + RCClO -> RCO+ + AlCl4-

ACID ANHYDRIDES
RCOOCOR + AlCl3 - > Cl3Al-OOCR] -
((the leaving group divide is RCO(|) OCOR))

[Cl3Al-OOCR] - + H+ - > AlCl3 + RCOOH

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11
Q

What two ways can nitrobenzenes - - > phenylamines

A

REDUCTION REACTIONS
- Sn & HCl, heating, followed by NaOH
- nitrobenzene + 6[H] - > phenylamine + 2water

this phenylamine becomes - NH3+ Anion- because it acts as a base, then forms a salt. follow by NaOH to react to give the insoluble phenylamine

CATALYTIC HYDROGENATION
- nickel catalyst
- nitrobenzene + 3H2 - > phenylamine + 2water

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12
Q

Why is Fc acylation good

A

products used as chemical feedstocks and synthetic intermediates. so they are necessary to make other useful chemicals

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13
Q

what’s interesting about AlCl3

A

Is more covalent than ionic

Al shows covalent character because small radius, high charge so high positive charge density

So high electronegativity, or polarising effect. It attracts electrons towards itself, from the anion for ex

Which distorts the E cloud introducing cov character to the bonds

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