Benzene and aromatic compounds

Kekule’s model of benzene:

• 6 carbon atoms in hexagonal ring, alternating single and double C-C bonds

What does Kekule’s model therefore suggest:

• It should react like an unsaturated alkene

What is the acc structure of benzene?

• Each carbon in ring forms 3 σ bonds using sp2 orbitals

• remaining p obriballs overlap laterally with p orbitals of neighbouring carbons forming a pie system

• side-ways overlap of p orbitals means electrons r delpcalised and move to freely spread over entire ring vausing pie system made of two ring shaped clouds of electron density (below + above)

What is the name and bond angle of benzene and other aromatic compounds?

regular and planar

120

what do the delocalised electrons therefore mean for C-C bonds?

bonds identical and have both single and double bond character

What is evidence for the delocalised ring structure of benzene?

same length

What other evidence is there to disprove kekule’s model?

enthalpy changes of hydrogenation

Enthalpy for cyclohexene vs. benzene?

Cylohexene- each molecule has one C=C double bond. Enthalpy is -120 kJ mol-1

Benzene- Kekules model has 3 double C=C bonds therefore its ecpected that the enthalpy change would be X3 the enthalpy change for the one C=C bond in cylohexane e.g. -360 kJ mol-1

But it is acc far less eco -280 kJ mol-1

Aromatic meaning

molecules with 1 or more benzene rings

How does benzene’s resistance to halogenation reject Kekule’s structure and why?

alkenes undergo bromination easily

in benzene there are no localised areas of high electron density, preventing it from being able to polarise bromine molecule

What are reactions of benzene and electrophile called?

electrophile substitution

General steps of electrophilic substitution mechanism for benzene:

Overall reaction of Benzene Nitration

Nitration of benzene mechanism:

once electrophile is formed NO2+, HNO3 and H2SO4 is refluzed with arene (aromatic compound) at 25-60

What do aromatic compounds need to react halogens and why?

Metal halide carrier

e.g. Irom(III) bromide

Aluminium chloride

Acts as catalyst and creates electrophile e.g. Cl+

Equation of metal halide breaking down

AlCl₃ + Cl₂ → AlCl₄^- + Cl⁺

FeBr₃ + Br₂ → FeBr₄^- + Br⁺

Overall equation of halogenation

C₆H₆ + X₂ → C₆H₅X + HX

chlorination of benzene

Friedel-Crafts Acylation

• Acyl group (alkyl group containing C=O group) is sub into benzene ring

• Metal catalyst needed to generate alkyl electrophile

• Benzene ring is reacted with acyl chloride in presence of AlCl3 cat

• Complex reacts with benzene ring

Phenol is a ___

very weak acid

typical pH __

5-6

How phenols break down?

Hydrogen ion can break from -OH group and transfer to a base

C₆H₅OH + H₂O ⇌ C₆H₅O^- + H₃O⁺

Phenol reaction with NaOH

Phenols dissolve in alkaline solutions and undergo acid-base reactions with bases to form a soluble salt and water

How phenols react with metals??

E.g. sodium

Forming soluble salt and hydrogen gas

How does benzene compare to phenol

• Phenols react readily with electrophiles compared to benzene

• cuz one lone pair on oxygen in OH overlaps with π bonding system

• increasing electron density ∴ more susceptible to electrophilic attack

Bromination of phenol

• At room temp with bromine water

• decolourises orange —> white ppt

• forms 2,4,6-tribromophenol

• Electrophilic substitution.

Nitration of phenols

What are arenes?

aromatic hydrocarbons that contain 1 or more benzene rings

What was used to identify the length of benzenes bonds?

X-ray Crystallography

Double bonds are __ than single bonds

shorter

This ∴ means Kekules structure would be ______

asymmetrical

X-ray showed the structure of benzene was ____

perfect hexagon, all internal bond angle were 120° and all bonds were of equal length

delocalised model of benzene:

• Each carbon forms bonds with 2 other carbon atoms and a hydrogen atom

• Remaining electron ound in p-orbitak at right angles to plane of bonded carbon + hydrogen atoms

• overlap of p-orbitals above and below ring creating pi bonds spread over 6 carbons

• 6 electrons in p-orbitals are delocalised

benzene is ___ (structure)

planar

the delocalised electrons make benzene ___

extra stable

∴ addition reactions would need to ____

disrupt the delocalised electron system, leading to lower stability so ∴ is resistent to electrophilic addition

benzene names

presence of other groups is denoted with use of prefix

exceptions to bezene naming

COOH-

CHO-

NH2-

COOH- benzoic acid

CHO- benzaldehyde

NH2- phenylamine

Benzene undergoes hydrogenation when ____ forming ___

heated under + Ni

cyclohexane

Nitration of benzene

Forms nitrobenzene

With nitric acid —> slow form

Rate increase- sulphuric acid as cat and 50°

Nitrobenzene uses:

• Dyes

• Pesticides

• Pharmaceuticals e.g. paracetamol

If reaction of nitration of benzene is carried out above 50° what happens?

further nitration of benzene forming 1,3-dinitrobenzene

Alkylation:

benzene is reacted with haloalkane in presence of halogen carrier e.g. AlCl3. Sub of hydrogen atom from benzene ring with alkyl group

acylation:

when benzene reacts with acyl chloride CH3COCH2Cl in presence of AlCl3

benzenes are ____ soluble than alc in water

less

phenol is ___ acidic than alc

and ____ acidic than carboxylic acids

more

less

How phenol reacts with sodium hydroxide:

neutralisation

What can be used to distinguish between phenols and carboxylic acids?

reaction with metal carbonates

Phenol not acidic enough to react

the ___ in phenols means it has different chemical reactions to benzene

hydroxyl group

what causes increased electron density in ring in phenol?

lone pair on oxygen of hydroxyl group donated to pie system

what does the increased electron density therefore mean and for reactivity of phenol compared to benzene?

can attract electrophiles more strongly than benzene

more reactive and easily undergo electrophilic sub

what forms when phenol reacts with bromine water + conditions?

form white ppt decloloursing bromine water

room temp

the increased electron density can polarise the bromine molecule meaning no halogen carrier is needed

what else does phenol readily react with a room temp?

dilute nitric acid

what forms when phenol reacts with dilute nitric acid?

2-nitrophenol and 4-nitrophenol and water

activating groups definition?

attached to aromatic ring that cause compound to be more susceptible to electrophilic sub

groups that cause it to be less susceptible are called ____

deactivating

NH2 ___

activates to 2/4 position so reacts more readily with bromine

forms t

2,4,6-tribromo

NO2 ___

deactivates to 3 position so is less susceptible

forms 3-bromonitrobenzene

-OH is

2-4 directing and activating

2,4- directing are activating except ___

halogens

Similarities between Kekulé model and the delocalised model of benzene?

• Overlap of p-orbitals

• π bond/system/ring above and below

Differences between Kekulé model and the delocalised model of benzene?

Kekule has: alternating π bonds OR 3 π bonds / localised (π electrons) / overlap in one direction / 2 electrons in π bond

Delocalised has: π ring (system) / all p orbitals overlap OR (π electrons) spread around ring / overlap in both directions / 6 electrons in π bond

Why is benzoic acid unreactive?

–COOH group on benzoic acid is an electron withdrawing group

arenes vs aromatic

"arene" specifically refers to only the hydrocarbon version of an aromatic compound

"aromatic" can encompass broader structures including rings with heteroatoms like nitrogen or oxygen, which are not considered "arenes.