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Arenes Flashcards

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1
Q

Arenes / Aromatics

A

HYDROCARBONS with one or more BENZENE rings.

Aromatic is a traditional name as scientists thought benzene rings gave rise to FRAGRANCES (not in practice)

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2
Q

SOURCES of benzene

A
  • Fractional distillation of CRUDE OIL ~ condenses at its boiling point (80.1C)
  • COAL
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3
Q

Aromatic compounds ~ ONE substituent group on the benzene ring

A
  • Alkyl groups ~CH3, CH2CH3, CH2CH2CH3 (ethylbenzene)
  • Halogens ~ F, Cl, Br, I ( chlorobenzene)
  • Nitro groups (nitrobenzene)
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4
Q

Aromatic compounds ~ MORE THAN ONE substituent group on the benzene ring

A
  • The ring is NUMBERED like a carbon chain, starting at one of the substituent groups
  • Listed in ALPHABETICAL order
  • Using the LOWEST number possible
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5
Q

Identification of an arene

A
  • The number of carbons and hydrogens are ALMOST EQUAL
  • There are MORE carbons than hydrogens
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6
Q

When is ‘phenyl’ used as a prefix instead of ‘benzene’

A

Benzene ring attached to either:

  • An alkyl chain with a functional group
  • An alkyl chain with seven or more carbons
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7
Q

When AQUEOUS BROMINE is added to benzene …

A

It is NOT DECOLOURISED.

This indicates benzene does not:

  • Contain carbon-carbon double bonds , i.e
    cyclohexa-1,3,5-triene
  • Undergo electrophilic addition reactions, unless very strong reaction conditions are provided
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8
Q

Kekule’s Equilibrium Model

A
  • The double bonds move RAPIDLY
  • The two forms of benzene ( RESONANCE HYBRIDS) are in EQUILIBRIUM with each other.
  • BROMINE cannot be attracted to a C=C double bond in the time which they exist
  • An electrophilic addition addition reaction CANNOT take place
  • due to their being NOT TRUE C=C double
    bond
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9
Q

Which has a larger bond length, C=C or C-C?

A

C-C bond length ~ 0.154 nm

C=C bond length ~ 0.134 nm

( 1nm = 1x10^-9m)

C=C are shorter:
- Side-on overlap of p-orbitals
- Each holding one electron
- Pulls C atoms closer together

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10
Q

Kekule’s model of Benzene ~ Bond lengths

A
  • Two different Carbon-carbon lengths
  • THREE bonds should be of one length and the other three bonds should be of a different length
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11
Q

X-ray Crystallography of C6Cl6 ~ Lonsdale 1922

          The Process
A
  • Beam of x-rays strikes a CRYSTAL and SCATTERS in many different directions
  • The ANGLES and INTENSITIES of these scattered beams gives a 3-D picture of the DENSTY of electrons within the crystal

The ELECTRON DENSITY gives the:

  • The mean positions of the atoms
  • The chemical bonds of the atoms
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12
Q

What did x-ray crystallography conclude about benzene?

A
  • All C-C-C bonds angles 120 degrees
  • All C-C bond lengths ~ 0.139nm
  • These are INTERMEDIATE between C=C &
    C-C
  • Kekule’s structure of ALTERNATE double and single bonds DISPROVED
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13
Q

Hydrogenation enthalpies for Cyclohexene Vs Kekule’s benzene

A

Cyclohexene (one double bond)

C6H10(l) + H2(g) —– C6H12
Enthalpy change = -120 KJmol-1

kekule’s benzene/ cyclohexa-1,3,5-triene (Three double bonds)

C6H6 + 3H2 ——– C6H12

EXPECTED enthalpy change = -360 KJ mol-1 (3 x 120)

ACTUAL enthalpy change = -208 KJ mol-1

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14
Q

Evidence for STABILITY of Benzene ~ Results & what it showed

A
  • Actual benzene releases only 208 KJmol-1 when reduced, it is LOWER in energy
  • It is 152KJmol-1 MORE STABLE than expected
  • 152 KJ mol-1 = RESONANCE ENERGY / delocalised energy
  • Benzene is MORE STABLE and LESS REACTIVE than alkenes
  • The pi-electrons are DELOCALISED around the ring
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15
Q

How was the STABILITY of benzene measured?

A
  • Unsaturated hydrocarbons are reduced to the corresponding saturated compound
  • ENERGY is released
  • The amount of HEAT liberated PER MOLE (enthalpy of hydrogenation) can be measured
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15
Q

Evidence for the delocalised structure of benzene ~ SUMMARY

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A

BROMINE :
- Bromine water is not decolourised when added to benzene
- Indicates C=C double bonds are absent

BOND LENGTHS:
- All Carbon-carbon bond lengths are the same in benzene
- Between the length of a C-C & C=C

ENTHALPY CHANGE OF HYDROGENATION:
- Value is less exothermic than the theoretical value expected from Kekule’s structure
- Indicates the molecule is lower in energy and is more stable due to the delocalised electrons

16
Q

Benzene STRUCTURE

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A
  • CYCLIC hydrocarbon of 6 carbon atoms & 6 hydrogen atoms
  • Carbon atoms arranged in a HEXAGONAL PLANAR RING
  • Each carbon atom bonded to TWO CARBONS and ONE HYDROGEN
  • The shape around each carbon is TRIGONAL PLANAR , 120 degrees
17
Q

Benzene BONDING

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A
  • Each carbon atom has its FOURTH outer shell electron in a P-ORBITAL
  • These p-orbitals OVERLAP on BOTH SIDES to produce a ring of 6 pi delocalised electrons above & below the hexagonal ring of carbon atoms
  • The C-C BOND LENGTH & ENTHALPY in benzene is INTERMEDIATE between a single C-C bond & a C=C double bond
18
Q

Why does benzene not undergo ELECTROPHILIC ADDITION reactions?

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A

Electrophilic addition reactions require:

  • A LOCALISED pi-bond
  • Which has a HIGH enough electron density
  • To INDUCE a DIPOLE in a halogen

Benzene :

  • The 6 pi-electrons are DELOCALISED above and below the ring of carbon atoms
  • The ELECTRON DENSITY is NOT high enough to induce a dipole in a non-polar molecule
  • The 6 delocalised pi-electrons CONFER STABILITY to benzene, making it LOWER in energy
  • Addition reactions are UNFAVOURABLE ~ result in the LOSS of the delocalised ring of electrons
19
Q

Benzene REACTIVITY

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A
  • REPELS nucleophiles
  • REACTS with powerful electrophiles ~ NO2+, Cl+, Br+, CH3+
  • These are made during the reaction ~ IN SITU
  • Non-polar molecules DO NOT react with benzene
  • DELOCALISATION ENERGY means that benzene wants to RETAIN the ring of delocalised electrons
  • Undergoes ELECTROPHILIC SUBSTITUTION reactions
20
Q

Nitration

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A
  • Where a HYDROGEN ATOM in benzene is REPLACED with a NITRO GROUP , NO2

Overall equation:

C6H6 + HNO3 ————– C6H5NO2 + H2O

Uses of NITROBENZENE :
- dyes
- pesticides
- pharmaceuticals

REAGENTS & CONDITIONS:
- c.HNO3 (in equation)
- c.H2SO4
- 50 degrees

21
Q

Nitration ~ MECHANISM 1

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A

1:
- Nitrating mixture made by mixing c.HNO3 with c.H2SO4
- NO2+ electrophile, NITRYL CATION/NITRONIUM ION, is produced

HNO3 + H2SO4 ———- NO2+ + HSO4- + H2O

22
Q

Nitration ~ MECHANISM 2

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A

2:
The NO2+ electrophile reacts with benzene:
- NO2+ accepts pair of pi-electrons from delocalised ring of electrons
- A DATIVE covalent bond is formed

  • Unstable intermediate with delocalised benzene ring of electrons BROKEN
  • HORSESHOE represents FOR pi-electrons over FIVE carbon atoms
  • H+ lost to REGENERATE stable benzene ring where C-H electron pair moves into the ring
  • ELECTROPHILIC SUBSTITUTION takes place
  • Maintains STABLE benzene ring
23
Q

Nitration ~ MECHANISM 3

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A
  • The H+ released from the intermediate reacts with HSO4-
  • This REFORMS H2SO4
  • SULFURIC ACID behaves as a CATALYST

H+ + HSO4- —- H2SO4

24
Making NITROBENZENE in the LABORATORY
50 degrees temperature: - Prevents MULTIPLE SUBSTITUTIONS - Gives a GOOD RATE - If temperature reaches 70 degrees ~ 1,3- dinitrobenzene is produced - Today, only the nitration of BENZENE DERIVATIVES takes place as benzene is CARCINOGENIC
25
Isolation and Purification of a Solid product
RECRYSTALLISATION: - Dissolve solid in MINIMUM volume of HOT solvent - Allow solution to COOL & solid RECRYSTALLISES - Use VACUUM FILTRATION - Wash solid with small amount of COLD solvent MELTING POINT: PURER product ~ SHARP melting point IMPURE product ~ LOWER melting point and melts over a WIDER RANGE
26
Why do impurities LOWER the melting point?
- Disrupt the crystal lattice formed by a pure compound - Cause the crystal lattice to become LESS CROWDED & melts at a lower temperature
27
A HIGHER than expected melting point is not possible, but can occur when:
THE SAMPLE IS NOT DRY: - heat is being supplied to vaporise the water THE SAMPLE IS NOT TIGHTLY PACKED: - presence of impurities in the crystal lattice - Air pockets form which are poor conductors of heat
28
Halogenation of Benzene Reagents & Conditions
- When a HYDROGEN atom in benzene is REPLACED with a HALOGEN Reagents & Conditions: - Halogen carrier - Halogen - Heat under reflux - Anhydrous conditions
29
Halogenation of Benzene ~ The need for a HALOGEN CARRIER
- Benzene has a relatively low electron density ~ cannot induce a large enough dipole in X2 Halogen carriers (FeBr3 or AlBr3): - More powerful electrophile X+ is produced - Electrophile is able to attract 2 pi-electrons from benzenes ring of delocalised electrons Metallic Fe & Br2: - Can be added to the flask - A halogen carrier is made IN SITU 2Fe + 3Br2 -------- 2FeBr3
30
Halogenation of Benzene ~ Making the electrophile , X+
X-X ( lone pair of electrons) donates the electron pair to the halogen carrier and forms a DATIVE COVALENT bond - The X-X bond is POLARISED to the extent that HETEROLYTIC fission takes place
31
Mechanism for Halogenation
- The HALOGEN reacts with the HALOGEN CARRIER to make an ELECTROPHILE - The HALOGEN ION, X+, reacts with BENZENE - The H+ ion reacts with the AlX4-/FeX4- to REGENERATE AlX3/FeX3 and makes HBr - Halogen carriers act as CATALYSTS as they are REGENERATED at the end of the reaction
32
Friedal-Crafts ALKYLATION Reagents & Conditions
- A HYDROGEN atom in BENZENE is REPLACED with an ALKYL group - The number of CARBON atoms in the product is INCREASED as a result of the reaction Conditions: - Halogen carrier ~ ALBr3/FeBr3 - Haloalkane - Heat under reflux - Anhydrous conditions
33
Mechanism for ALKYLATION
- The HALOALKANE reacts with a HALOGEN CARRIER to make the ELECTROPHILE which is a CARBOCATION - CARBOCATION reacts with BENZENE - H+ ion reacts with AlX4- to REGENERATE AlX3/FeX3 and makes HX
34
Acyl Chlorides
Carboxylic acid group ~ RCOOH - OH is replaced with Cl (RCOCl)
35
Friedal-Crafts Acylation Reagents & Conditions
- When a HYDROGEN atom in BENZENE is REPLACED with an ACYL group Product ~ PHENYLKETONE - KETONE functional group attached to a carbon in the benzene ring - The total number of carbons in the product INCREASES Conditions: - Halogen carrier ~ FeCl3/AlCl3 - Acyl chloride RCOCl - Heat under reflux - Anhydrous conditions
36
Mechanism for ACYLATION
- The ACYL CHLORIDE reacts with a HALOGEN CARRIER to make the ELECTROPHILE which is an ACYL CATION - The ACYL CATION reacts with BENZENE - The H+ ion reacts with FeCl4- to REGENERATE FeCl3 and makes HCl
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