6.1.1 Aromatic compounds

arenes

aromatic compounds that contain a benzene ring as part of their structure

Benzene

simplest arene with a planar ring structure

ring of six carbon atoms each bonded to one hydrogen atom

molecular formula of benzene

C6H6

2 primary models for benzenes structure

Kekulé model 

delocalised model 

Kekulé model

• ring of C atoms with alternating single and double bonds betwwen them

• later adapted model to say benzene molecule was constantly flipping between 2 forms(isomers) by switching over the double and single bonds

delocalised model

ring of electrons that are delocalised

how is the delocailed model formed

1. each carbon atom uses 3 of its 4 electrons to bond with other 2 carbon atoms and a hydrogen atom

2. Each carbon contributes one electron from its 2p orbital to a π-bonding system.

3. The p-orbitals overlap side-by-side around the ring, forming a delocalised system of 6 π-electrons.

4. This creates an electron density above and below the plane of carbon atoms.

5. The electrons are not fixed between specific atom pairs, but rather delocalised over the whole ring.

This delocalisation leads to equal C-C bond lengths between the carbon atoms and enhanced stability of the aromatic ring.

how many π and sigma bonds in benzene

12 sigma bonds

3π bonds delocalised

phyical properties of benzene

• colourless liquid at room temperature

• Bp comparable with that of hexanes as its flat hexagonal molecules pack together very well in the solid state therefore harder to seperaye and melt

• non polar compound and dissolves with other hydrocarbons and non polar solvents

evidence of delocalised model

• equal c-c bond lengths

• enthalpy of hydrogenation was less negative than expectedmore stable

• resistant to electrophilic addition reactions - doesnt decoulirise bromine water

similarities and differences between bonding in kekule model and delocalised model of benzene

• Similarities: overlap of p orbitals, π bond above and below atoms

• Difference:kekule has localised π electrons but deocalised has π ring system

how to name substituted benzene

the names of the substituents precede the word "benzene". Examples include chlorobenzene, nitrobenzene, and methylbenzene.

how to name phenyl derivatives

These compounds are named as derivatives of the phenyl group (C₆H₅^-). Examples include phenol and phenylamine.

how to name when multiple substituents on benzene ring

• the numbering begins from the substituent that gives the molecule its suffix (for example, -OH in phenol).

• If all substituents are identical, numbering starts from any position and proceeds to give the lowest possible numbers.

difference in reactivity between alkenes and benzene

• Alkenes are known for their readiness to undergo addition reactions with electrophiles, such as bromine, by breaking the π-bond in the C=C double bond.

• Addition reactions in benzene are difficult due to the stability provided by its delocalised π-electron system. 

• Instead, benzene is more inclined to participate in substitution reactions, which preserve the aromatic ring's integrity.

what is the reason for reactivity difference between benzene and alkene

• In benzene, the delocalised π-system across the ring has insufficient electron density to polarise the Br-Br bond, making addition reactions difficult. Heat and a catalyst are required to initiate the substitution reaction.

• In ethene, the localised π-system around the C=C double bond has sufficient electron density to polarise the Br-Br bond, allowing addition reactions to occur readily at room temperature without the need for a catalyst

how to number benzene ring

• if more than one functional group attached the carbons need to be numberd

• if all functional groups are the same make it be the smallest number

• if the functional groups are different start from whichever functional group gives the molecule its suffix and continues counting round whichever way gives the smallest numbers

why does benzene not undergo electrophic addition

would involve breaking up stable delocalised ring of electrons

what mechanism does benzene undergo

electrophilic substitution

electrophilic substitution reaction of benzene

involves a hydrogen atom being replaced by an electrophile

2 stages of electrophilic substitution

addition of electrophile

loss of hydrogen

Halogenation of benzene

nitration of benzene conditions

50C

nitration of benzene mechanism

friedel crafts reactions 

are really useful for forming C–C bonds in organic synthesis. They are carried out by refluxing benzene with a halogen carrier and either a halogenoalkane or an acyl chloride.

2 types of friedel crafts reactions

1. Friedel-Crafts alkylation puts any alkyl group onto a benzene ring using a haloalkane and a halogen carrier.

2. Friedel-Crafts acylation substitutes an acyl group for an H atom on benzene using an acyl chloride and halogen carrier. This produces phenylketones or benzaldehyde

how can something become a stronger electrophile

using a catalyst caled a halogen carrier e.g. AlCl3

alkylation of benzene using chloromethane

acylation of benzene using ethanoyl chloride and AlCl₃

why is benzene resistant to bromination

delocalised electron density of the π system in benzene compared with the localised electron density of the πbond in alkenes

the delocalised model makes benzene relatively stable and the negative charge spread out

phenol

organic compound containing a benzene ring with an OH alcohol group

aromatic alcohol

type of acid phenol

weak

can weak acids react with weak alkalis

no

phenol formula

C6H5OH

phenol skeletal formula

how to name phenol

add suffix -phenol instead of  -ol carbon with oh is always carbon 1

phenol reaction with sodium hydroxide solution

neutralisation reaction to form sodium phenoxide and water

phenol reaction with sodium carbonate

doesnt react as sodium carbonate is not a strong enough base ans so cant remove the hydrogen ion from the oxygen atom

phenol strucute

8π electron

4π bonding regiosn

can phenol react with bromine water

yes

why is phenol more reacitve than benzene

the lone pair of electrons on the oxygen atom is partially delocalised into the π system making henol more susceptible to electrophilic attack

reaction of phenol with bromine water

• Phenol causes bromine water to decolourise as substitution occurs at the 2- and 4- positions, producing 2,4,6-tribromophenol as a white precipitate.

reaction of phenol with dilute nitric acid

• Direct nitration of phenol at room temerature yields two main isomers, 2-nitrophenol and 4-nitrophenol, at the 2- and 4- positions respectively.

directing effect

refers to the influence of a functional group on the position where a second substituent is added to a benzene ring during an electrophilic aromatic substitution reaction

electron donating groups

• contribute electrons into the delocalised π-system of the ring, increasing its electron density.

• they enhance the electron density at carbons 2-, 4-, and 6-, making these positions more likely to react with electrophiles.

• Thus, electron donating groups activate positions 2-, 4-, and 6- for electrophilic attack.

electron withdrawing groups

do not have orbitals that overlap with the ring's π-system. Instead, due to their electronegativity, they pull electron density away from the ring, particularly from positions 2-, 4-, and 6-.

This action directs electrophilic substitution towards the 3- and 5- positions, which retain relatively more electron density.

Thus the -NO₂ group activates positions 3- and 5- for electrophilic attack.

examles of electron withdrawing groups

NO2 Cl

examples of electron donating groups

OH NH2 CH3