6.1.1 Aromatic compounds need to be added

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

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