Aromatic Chemistry Flashcards
the legendary, superior molecule that OWNS this chapter: BENZENE!! (18 cards)
What is BENZENE, and its properties…?
- Hexagonal, aromatic hrydrocarbon = arene molecule
- Properties = Colourless, aromatic, highly flammable
- carconegenic molecule! (on solus…)
- Many sweet-smelling molecules contain benzene rings, hence the name…
Differences + similarities between Kekule’s and Delocalised benzene molecules…
- Both = p-orbital overlapps, pi-bond structures presen
- K = alternating σ and π bonds, localised electrons per C atom
- D = all shared mobile e-, 6 e- in P orbitals, high e- density above and below plant of ring!
Describe in detail Kekule’s and Delocalised benzene models…
- K = 6 carbons atoms bonded in alternating C-C and C=C bonds
- D = Planar, cyclic with 6 C’s, 1 mobile e- per carbon in p-orbital, ADJACENT p orbitals will overlap, giving donut-ring of e- density above and below ring plane on all 6 carbon atoms!
Evidence to DISPROVE Kekule’s benzene model…?
- Minimal Reactivity»_space; won’t do electrophilic addition + won’t decolourise bromine water = no actual C=C bond
- C-C lengths»_space; via X-Ray Diffrcation, C-C bond of 0.153nm and C=C bond of 0.139nm, benzene has only bonds with intermediate lengths = no C bonding, rather high e- density above/below ring plane
- Hydration Enthalpies»_space; cyclohexene (-180 kjmol-1), meaning benzene should be 3 times, but (-208 kjmol-1) = less exothermic, MORE STABLE than Kekule idea
How to name benzene molecules…
Benxene molecule a “subsitutent” or “parent” group
1. Benzene = “parent” chain in alkyl groups <7 in C’s, functional groups being “subsituted”»_space; EXCEPTIONS = Carboxylic acid, amines, aldehydes and phenols…..
2. B = “subsituent” when otherwise…
3. alphabetical orders, small numbers still..
4. alkyl, halogens and nitro groups = prefixes!
Why can’t benzene undergo NORMALL electrophilic addition?
- Due to lack of real C=C bond, rather high e- density above and below, via π-orbital ring!
Describe mechanism of electrophilic SUBSITUTION!
- Electrophile attracts to ring of high e- density, donating e- pair
- H atom undergoes heterolyctic fission, donating e- pair to ringed structure, leaving final product + H+ ion
- C6H6 + X+»_space; C6H5X + H+
Describe + explain mechanism of NITRATION!
- reagents = conc HNO3 [providing NO2+], with conc H2SO4 [catalyst]
- temp = 50-55 centigrade = MULTIPLE SUBS may OCCUR, not needed..
- same mechanism, but nitronium ion acts as ELECTROPHILE, forming dative covalent bond, h+ ion as side product
-
FORMING NITRONIUM ION: H2SO4+HNO3»_space; HSO4- + H2NO3+
H2NO3+»_space; H2O + NO2+ [cancel rule..]
Useful NITRATION products?
- TNT producede via 2,4,6-trinitromethylbenzene
- aromatic amines!
- essential starting product to synthesise Paracetamol!
- all from nitrobenzene
Describe + explain mechanism of HALOGENATION!
- reagents = halogen carrier catalyst [AlCl3, FeCl3, FeBr3..] reacts with halogen [reactant] to form ELECTROPHILE
- temp = RTP
- same mechanism, but halogonium ion will sub for H atom, HaloH ion as side product
- regenerating CATALYST ION: H+ + AlBr4-»_space; AlBr3 + HBr
AL and Fe interchangable…!!!
Describe + explain mechanism of ALKYLATION!
- reagents = halogen carrier [catalyst] reacts with haloalkane [reactant] to form ELECTROPHILE!
- temp = RTP
- same mechanism, but alkyl group as ELECTROPHILE, subbinh H atom for alkyl chain!, h+ ion as side product
Describe + explain mechanism of ACYLATION!
- reagents = acyl chloride (COOCL) reacts with benzene reactant, with AlCl3 [catalyst]
- temp = RTP
- same mechanism, but Benzene + Acyl Chloide»_space; Benzene Ketone + h+ ion as side product
-
generating ELECTROPHILE
Acyl Chloide + AlCl3»_space; Acylium+ + AlCl4-
Catalyst Regeneration:
AlCl4- + H+»_space; AlCl3 + HCl
Distingiushing PHENOL molecules…
- -OH groups directly attached to benzene ONLY (not just part of the molecule), taking position 1 ALWAYS
- If not phenol, will be aromatic alcohol……
Unique e- density feature of PHENOL?
- Oxygen atom bonded to benzene contains 2 lone e- pairs!
- P orbitals from lone pair and benzene wil OVERLAP! > INCREASING e- DENSITY
- Electrophilic substiution occurs more readily, due to greater electrophile attraction…
3 main Important PHENOL properties to explain….
- Less SOLUABLE in water, than other alcohols
- Partially dissociates in H2O, forming Phenoxide + H+ ion [C6H5OH <> C6H5O- + H+]
- As acting as a WEAK ACID, can only react with a strong base!, forming ionic group [O-Na+] on C6H6
- Sodium Carbonate can be used to determine difference with CARBOXYLIC ACID and PHENOL (carboxylic can , phenol can’t react to produce CO2…)
how to undergo BROMINATION fo Phenol?
- Are able to react with Br2 water at RTP, and simply NO CATALYST! > due to increased e- density, more readily able to attract electrophiles..
- Causes decolourastion, leaving a WHITE PRECIPITATE as product
- will form 2,4,6-tribromophenol ALWAYS [triple-sub!]
how to undergo NITRATION fo Phenol?
- Are able to react with **dilute nitric acid at RTP, and simply NO halogen carrier CATALYST or conc H2SO4 **! > due to increased e- density, more readily able to attract electrophiles..
- Due to 2-4 directing effect, will form 2 and 4-nitrophenol ALWAYS [2 diff isomers]
Compare Bromination reaction with ALKENES, BENZENE and PHENOL
- A = pi-bond gives region of high e- density, above and below molecule, able to INDUCE a dipole in Br2
- B = pi-e- are spread cross entire ring, lower e- density at 1 point, compared to C=C; unlikey to induce dipole
- P = p-orbital e- lone pairs of oxygen will increase e- density than benzene; ABLE to induce dipole in Br2 like alkene, WITHOUT halogen-carrier catalyst!
