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La Chimie H2 > Arenes > Flashcards

Arenes Flashcards

1
Q

What is the structure of benzene?

A
  • C6H6 molecular formula
  • ring represents delocalised π e- (BUT cnt conduct electricity)
  • planar molecule w hexagonal ring, 6 C atoms, each w H atom attached
  • Each C is sp2 hybridised
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2
Q

How similar are alkenes and benzene? Explain

A
  • Both are unsaturated in nature
    bcos C=C bond present
    BUT
  • benzene less susceptible to electrophilic addit n rxn
    bcos benzene ring is resonance stabilised by delocal n of π e- cloud. Add n rxn destroys ring of delocal π e- in benzene, not energetically feasible
    so, benzene wld undergo electrophilic sub n but not electrophilic add n to preserve stable aromatic benzene ring structure (π delocalised system)
  • benzene need +vely charged E+ electrophile, compared to alkene oni need partially charged δ+ electrophile for rxn
    delocal n (ie resonance stabilisat n) of π e- of benzene ring make π e- less susceptible/available to electrophilic attack vs π e- localised in C=C, increasing π e- density of C=C. So, stronger electrophile needed for benzene ring
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3
Q

What is reagent, conditions & type of reaction of nitration of benzene?

A

R: conc HNO3 & conc H2SO4
C: 55°C
type of rxn: electrophilic sub n

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4
Q

Describe electrophilic substitution mechanism for nitration of benzene

A

Step 1:
HNO3 + 2H2SO4 ⇌ NO2+ + 2HSO4- + H3O+
Step 2:
full arrow fr centre of benzene ring (show 1 H-C bond) to N in +NO2 —(slow)—> carbocation intermediate (+ve charge smile oni spread to 5 C atom, not the H NO2 cross formed)
Step 3:
carbocation intermediate (full arrow fr H in cross to centre of benzene ring)
..
+ HSO4- (full arrow fr lp to H in cross)
—> (fast)
benzene ring w NO2 + H2SO4

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5
Q

What is general eqn of halogenation of benzene?

A

benzene + X2 –(FeX3)–> halogenoarene (benzene w mono sub X) + HX

FeX3 or AlX3 (w X=Br, Cl) used as catalyst
Fe can oso b used to generate FeX3

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6
Q

What are reagent, conditions, type of rxn & observation of chlorination of benzene?

A

R & C: Cl2 w anhydrous FeCl3/AlCl3/Fe
Type of rxn: electrophilic sub n
observat n: greenish-yellow Cl2 is decolourised

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7
Q

What are reagent, conditions, type of rxn & observation of bromination of benzene?

A

R & C: Br2 w anhydrous FeBr3/AlBr3/Fe
Type of rxn: electrophilic sub n
observat n: reddish-brown Br2 is decolourised

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8
Q

Describe the electrophilic sub n mechanism for halogenation of benzene. What is the function of FeX3/AlX3?

A

Name: Electrophilic sub n
Step 1 (no need arrows to show e- transfer)
X-X: + FeX3/AlX3 —> X+ + FeX4-/AlX4-

Step 2: Full arrow from benzene ring to X+ (slow rxn), X bond to carbon 2 or 4 (oni this carbon sp3 hybridised w 4 sigma)

*benzene ring hv delocalised e- so sp2 hybridised since oni 3 sigma bond

Step 3: lone pair above X4 in FeX4- full arrow to H of sp3 C and full arrow from this H to centre

FeX3/AlX3
- acts as Lewis acid, e-pair acceptor to generate electrophile X+
- acts as catalyst (regenerated at end of rxn)

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9
Q

What are the reagent, conditions and type of reaction of Friedel-Crafts Alkylation of benzene?
How is the mechanism like? What acts as Lewis base?

A

R & C: CH3Cl w anhydrous FeCl3/AlCl3 (cnt use Fe in absence of X2)
Type of rxn: Electrophilic sub n

Mechanism is similar to that of halogenation of benzene

eg CH3Cl acts as lewis base, as catalyst

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10
Q

Methylbenzene undergoes what 2 general reactions?

A
  1. ring rxn
    - involve benzene ring
    - electrophilic sub n
  2. side-chain rxn
    - involve alkyl side-chain
    - FRS & oxidation
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11
Q

What are the reagent, condition and type of reaction of nitration of methylbenzene? Which acts as a bronsted acid catalyst?

A

R: concentrated HNO3 & concentrated H2SO4
C: 30°C
type of rxn: electrophilic sub n

H2SO4 is the bronsted acid catalyst

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12
Q

What is the general OVERALL eqn of halogenation of methyl benzene?

A

benzene w CH3 + X2 –(in the dark)–> benzene w CH3 at 1, X at 2 OR X at 4 + HX

FeX3/AlX3 (w X = Br, Cl) are used as catalyst
Fe can oso b used to generate FeX3

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13
Q

What are the reagent, condition, type of reaction and observation of chlorination of methylbenzene?

A

R & C: Cl2 w anhydrous FeCl3/AlCl3/Fe, in the dark (to prevent FRS)
type of rxn: electrophilic sub n
observation: greenish-yellow Cl2 decolourised

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14
Q

What are the reagent, condition, type of reaction and observation of bromination of methylbenzene?

A

R & C: Br2 w anhydrous FeBr3/AlBr3/Fe, in the dark (to prevent FRS)
type of rxn: electrophilic sub n
observation: reddish-brown Br2 decolourised

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15
Q

What are the reagent, condition, type of reaction and observation of chlorination of methylbenzene (side-chain)?

A

R: limited Cl2 (g)
C: UV light
type of rxn: FRS
observation: greenish-yellow Cl2 decolourised

(under UV light, limited Cl2 -> monosub. With excess Cl2, further sub n can take place to form disub and trisub)

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16
Q

What are the reagent, conditions, type of rxn and observation of side-chain oxidation of alkylbenzene (eg methyl benzene)?

Why can this be used as chemical test to determine btw most alkylbenzenes (eg methylbenzene) and benzene?

A

(there must be >= 1 H atom attached to C atom of alkyl side-chain)
R & C: acidified KMnO4, heat
type of rxn: oxidation
observat n: purple KMnO4 sol n decolourised AND white ppt of benzoic acid observed when cooled

bcos KMnO4 does ont react w benzene (& alkanes oso)

17
Q

Describe mechanism for FRS of methylbenzene

A

Initiation:
(half arrow fr bond to each Cl)
Cl-Cl –(UV light)–> 2Cl ·

Propagation: (let O be benzene)
CH3 ·CH2
| |
O + Cl · –> O + HCl

·CH2 CH2Cl
| |
O + Cl2 –> O + Cl ·

Termination:
2Cl · –> Cl2
·CH2 CH2Cl
| |
O + Cl · –> O

·CH2 ·CH2
| + | –> O - CH2-CH2 - O
O O

18
Q

Where to find order of activating ability of substituent groups on benzene ring (directing effects)? What to take note of?

A
  • In data booklet
    eg
    for Br2, Cl2, CH3 etc.
    2 OR 6,4

for COOH, NO2, etc.
3 OR 5

  • u MUST draw both products if applicable
  • points at which to substitute is based on already present func grps
19
Q

When stating rxn & condition, what to take note of?

A
  • If sub in eg NO2 (needs certain temp), state ‘heat’ (no specific no like 55 degrees C)
  • If sub into benzene with few substituent grp edi, the benzene is less reactive; also state ‘heat’ in conditions to kickstart rxn
20
Q

As a substituent, benzene is known as… ? Give eg

A

phenyl grp

eg
2-phenylethanol
phenylethene
2-phenyloctane

21
Q

What are physical properties of benzene? Explain

A
  • colourless liquid, characteristic ‘aromatic’ odour. Non-polar, carcinogenic in nature
  • Low mp, bp
    bcos low amt energy needed to overcome weak id-id imf attract n btw molecules
  • soluble in non-polar solvents but insoluble in polar solvents eg water
    bcos oni in polar solvents,
    energy released from form n weak id-id attract n btw benzene & non-polar solute molecules enough to overcome weak id-id attract n btw benzene molecules & btw non-polar solute respectively
22
Q

Describe general electrophilic sub n mechanism of benzene

A

full arrow fr full benzene ring (show one H) to electrophile (E+)
–(slow)–>
H, E+cross (carbocation intermediate, smile to oni 5 carbon w/o cross)) & full arrow fr H-C bond to ctr
–(fast)–> benzene with E + H+

  • delocalised e- cloud in aromatic benzene ring serves as source of e- for electrophiles (e- pair acceptor)
  • hydrogen atom attached to benzene ring is readily sub by electrophile

La Chimie H2 (25 decks)

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