Arenes Flashcards
Names of mono-substituted benzene derivatives - 2 types
Most: ___ -benzene
Eg methylbenzene, ethylbenzene, nitrobenzene (NO2 connected to benzene ring), chlorobenzene
Some: special names Eg benzaldehyde (CHO connected...), phenol (OH connected...), phenylamine (NH2 connected), benzoic acid(COOH connected)
Naming disubstituted benzene derivatives
If 2 sub grps are diff:
If one of the sub grp is the kind that gives a special name:
If both sub give derivates with special names:
If 2 sub. grp are diff, the names of the grps prefix benzene in alphabetical order
eg 1-bromo-2-methylbenzene
If one of the sub grp is the kind that gives a special name, then the compound is named as a derivative of that special compound.
eg 2-bromophenol
If both sub give derivates with special names, preference is given to the sub of highest nomenclature priority
eg 3-hydroxybenzoic acid
Naming polysubstituted benzene derivatives
Numbering should give LOWEST SUM, with preference given to grp with highest nomenclature priority
When is benzene ring treated as a substituent and what’s it called (____)?
& what’s the substituted ____ grp called?
Called a Phenol grp when
- substituent on the benzene ring has more than 6 carbon atoms
- highest priority functional grp is not a substituent on the benzene ring
eg if there’s an OH, then it’s 2-phenylethanol
Substituted phenyl grp: benzyl grp = benzene ring - CH2
Physical properties of benzene
- colour, state
- polarity? solubility? density?
- bp n mp
- flame when burnt
- Colourless liquid
- np , insoluble in water & less dense than water
- soluble in all org solvents, good solvent for organic compounds
- freezes at 5.5°C & boils at 80
- like all aromatic compounds, it burns with a smoky and luminous flame, owing to its relatively high carbon flame
Resonance struc of benzene
Each C atom has
3 sp2 hybrid orbitals that are arranged in a trigonal planar manner
- 2 sp2 hybrid orbitals are used to overlap head-on with the sp2 hybrid orbitals of 2 adjacent C atoms to form 2 C-C pi bonds
- 1 sp2 hybrid orbital is used to overlap head-on with the 1s orbital of the H atom to form the C-H pi bond
1 unhybridised p orbital
- each singly-filled p-orbital overlaps side-on with the adjacent p orbital on either side
- this continuous side-on overlap of the p-orbitals results in a delocalised pi e- cloud above and below the plane of the ring, i.e. resonance is present
Properties of benzene as a result of its aromatic character
(a) all C-C bonds in benzene are equal & intermediate in length b/w C-C & C=C (partial double bond character)
(b) Enthalpy change of hydrogenation of benzene is less exothermic than expected - benzene has a struc where 6 pi e- are delocalised - extra stability on benzene - extra stability is reflected by its less exo resonance energy value than expected
(c) Undergoes substitution rctns rather than addition reactions to retain its aromaticity - not destroy the extra stability associated with the delocalisation of the 6 pi e-
How to draw benzene ring
C6H6
regular hexagon containing a circle, a H atom is attached to each vertex of the hexagon
4 eg of electrophilic substitution rctns of benzene
- Nitration
- Halogenation
- Friedel-Crafts Alkylation
- Friedel-Crafts acylation
When they ask for type of reaction, just give electrophilic sub not ^^
R&C + catalyst for electrophilic sub reactions (4)
Nitration
Reagent: conc HNO3
Catalyst: conc H2SO4
Condition: 55 degree celcius
Halogenation
Reagent: eg Br2
Catalyst: AlCl3 or FeCl3 or Fe
Friedel-Crafts alkylation
Reagent: R-Cl
Catalyst: AlCl3
Friedel-Crafts acyclation
Reagent: R - C - Cl
||
O
Catalyst: AlCl3
General steps for electrophilic substitution rctn of benzene
- Generation of the electrophile, E+
Followed by 2-step electrophilic substitution rctn mechanism:
Step 1: Electrophilic attack by E+ on the benzene ring to form a carbocation
- This step is the slow step as it involved the destruction of the aromaticity of benzene. The extra stability associated with the delocalisation of the 6 pi e- is lost
- Electrophile attacks the e-rich benzene ring. Involves the movement of 2pi e- from the benzene ring to the electrophile, forming a pi bond to one carbon atom of the benzene ring.
- A carbocation is formed. It is resonance-stabilised but not aromatic
Step 2: Loss of proton from the carbocation to form the prod
- Fast step
- carbocation intermediate loses a proton from the C atom that bears the electrophile
- 2 e- that bonded this proton to carbon become a part of the delocalised pi-e- system
- aromatic character of the benzene ring is restored and the substituted prod is formed
Bronsted-Lowry acid and base - what type of rctn (rct here is a bronsted ___)
A Bronsted-Lowry acid is a proton (H+ donor). A Bronsted-Lowry base is a proton (H+ accepter)
Nitration - HNO3 is a bronsted… base
Nitration of benzene: (for 1 nitro grp)
1) Generation of the electrophile ___
2) ____
(for more than 1 nitro grp) - major? minor?
Also what’s the eqn? general prod?
1) Generation of the electrophile NO2+ - check notes
2) A 2-step electrophilic substitution rctn mechanism then follows:
Step 1: Electrophilic attack by NO2+ (rate determining/slow step)
COMMON DRAWING MISTAKES:
- Arrow never pt from the circle to the N
- the ‘U’ in the circle must pt towards C with E+
Step 2: Loss of a proton from carbocation (fast step)
Check pic/pg 11 notes
General prod: H2O
Lewis acid & base - for what type of rctn (the react in this rctn is a lewis___)
A Lewis acid is an e- pair acceptor. A Lewis base is an e- pair donor.
rctn with halogens - AlCl3 is a lewis acid
Halogenation
Conditions when reacting with Cl2 / Br2 (2)
Eg AlCl3 …
Benzene does not react with cl n br in the dark (unlike alkenes)
In the presence of a suitable Lew acid catalyst: AlCl3 or FeCl3 or Fe, Benzene undergoes eletrophilic substitution rctn with chlorine at rm temp
Why can Fe/iron filings be used?
it is converted into FeCl3 or FeBr3 in the rctn mixture
Eg AlCl3
AlCl3 accepts a lone pair of e- from the halogen molecule, generating the Cl+ electrophile. Hence, AlCl3 functions as a Lewis acid.
pg 12
~ AlCl3 catalyst reacts readily with water. Hence rctn can only proceed under anhydrous condition
Defn of friedel-crafts alkylational
Alkylation with a halogenoalkane (RX) and a trace amt of anhydrous AlCl3 as catalyst
Defn of friedel-crafts Acylation
SImilar to that of alkylation except that an acid halide (RCOX or ArCOX) instead of a halogenalkane (RX) is used. The acid halide providees the acyl grp needed for the rctn.
How friedel crafts alkylation works
Pic 3
How Friedel-crafts acylation works
Pic 4
How to find position of substitution
Table in D.B.
When an electrophile attacks an aromatic ring, the grp already attached to the ring determines
- how readily the attack occurs
- position of substitution
Relative reactivity/rate of electrophilic sub. rctn depends on ____
How do Activating grps & deactivating grps affect the relative reactivity…
Depends on the tendency of an existing substituent grp to release or withdraw e-
Activating grps are e- donating
- increases the e- density in the benzene ring and makes the ring more susceptible to electrophilic attack
- helps to disperse the +ve charge in the intermediate carbocation and stabilise the carbocation
-> A grp attached to a benzene ring is classified as activating if it makes the benzene ring more reactive than benzene towards electrophilic sub. rctns
Deactivating grps are e- withdrawing
- decreases the e- density in the benzene ring and makes the ring less susceptible to electrophilic attack
- tends to intensify the +ve charge in the intermediate carbocation and destabilise the carbocation
-> A grp attached to a benzene ring is classified as deactivating if it makes the benzene ring less reactive than benzene towards electrophilic sub. rctns
What makes a substituent on a benzene ring e- -donating or e- - withdrawing? (2)
Thro inductive and resonance effects
Inductive effect: donation or withdrawal of e- thro sigma bonds due to the electronegativity diff b/w atoms
Resonance effect: donation or withdrawal of e- thro pi bonds due to the continuous side-on p-orbital overlap of the substituent and the benzene ring. This results in delocalisation of e-, either towards or away from the benzene ring.
Both e- - donating and e- - withdrawing substituents can be inductive OR resonance effect
Position of electrophilic attack on the ring
A 2,4-directing grp causes an electrophile to attack chiefly at 2-position and 4-position relative to it
A 3-directing…
Combustion of benzene
benzene + 15/2 O2(g) -> 6CO2(g) + 3H2O(l)
Reduction of benzene
Benzene + H2 —–Ni catalyst high temp high pressure —> cycloehexane
Alkylbenzene (eg methylbenzene) alkyl side chain undergoes (2) reaction
Undergoes FRS sub
Oxidation
FRS of alkylbenzene - mechanism
just the alkyl chain
Oxidation of alkylbenzene
Criteria for oxi to happen
R&C (what works n what doesn’t)
eqns for methyl benzene vs longer alkyl chains
Alkylbenzenes without a H atom bonded to the C that is bonded to the benzene ring do not undergo oxidation when heated with acidified KMnO4
KMnO4 (aq), H2SO4(aq), heat
methylbenzene + 3[O} ————–> benzene-COOH + H2O
KMnO4(aq), NaOH(aq), heat
methylbenzene —–KMnO4(aq), NaOH(aq), heat—> benzene-COO- Na+ —dilute H2SO4–> benzene-COOH
* salt is formed cuz it cannot exist in a basic medium
K2Cr2O7(aq), H2SO4(aq), heat
NO RCTN
*** When the side chain is oxidised, only a carboxylic acid grp (-COOH) remains to indicate the position of the org side chain
*** For longer alkyl chains, the side chain will produce the same COOH n the others are converted to CO2 + H2O
