Arenes

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)

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

Numbering should give LOWEST SUM, with preference given to grp with highest nomenclature priority

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

• 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

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

(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-

C6H6 regular hexagon containing a circle, a H atom is attached to each vertex of the hexagon

• Nitration • Halogenation • Friedel-Crafts Alkylation • Friedel-Crafts acylation When they ask for type of reaction, just give electrophilic sub not ^^

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

• 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

A Bronsted-Lowry acid is a proton (H+ donor). A Bronsted-Lowry base is a proton (H+ accepter) Nitration - HNO3 is a bronsted… base

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

A Lewis acid is an e- pair acceptor. A Lewis base is an e- pair donor. rctn with halogens - AlCl3 is a lewis acid

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

Alkylation with a halogenoalkane (RX) and a trace amt of anhydrous AlCl3 as catalyst

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.

Pic 3

Pic 4

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

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

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

A 2,4-directing grp causes an electrophile to attack chiefly at 2-position and 4-position relative to it A 3-directing…

benzene + 15/2 O2(g) -> 6CO2(g) + 3H2O(l)