chapter 23- aromatic compounds

3 characteristics of benzene

• colourless

• sweet smelling

• highly flammable

what is each carbon bonded to in bezene

Each carbon in benzene is bonded to one hydrogen atom and two other carbon atoms,.

what did kekule suggest about benzene model

he thought it was a 6 membered carbon with alternating single and double bonds

what are the 3 reasons that kekules model for bezene was incorrect

• had a lack of reactivity

• the lengths of the carbon-carbon bonds

• hydorgenation

how did the lack of reactivity disprove kekules model

if benzene contained the c=c it would be able to undergo electrophilic addition and decolourise in bromine water however this did not happen meaning that benzene cant have double bonds

how did the lengths of the carbon bonds disprove kekules model

Kathleen lonsdale found that single bonds were longer than double bonds, however benzene is a symmetrical molecule

how did hydrogenation enthalpies disprove kekules model

the enthalpy change for benzene following kekules model shouldve been 3x that of cyclohexene however its bond enthalpy was found to be much lower, indicating greater stability of benzene than predicted.

what is the enthalpy of hydrogenation in benzene

-208kjmol-1, therefore 152kjmol-1 than expected from kekules model

draw kekules model for benzene

why does carbon only bond 3 times in benzene

one electron is in the p-orbital at right angles to the plane of the bonded carbon and hydrogen atoms

describe the interaction of p-orbitals in benzene

adjacent p orbitals overlap sideways above and below the plan to form a ring of electron density. This overlapping creates a system of pi bonds

where are the deloclaised electrons in bezenes

the 6 electrons occupying the system of pi bonds

how to name a benzene with one alkyl group

the alkyl group is named first and then followed by the word "benzene."

how to name a benzene attached to an alkyl group with a functional group 

you use phenyl benzene

which is the most common benzene reaction

electrophilic substitution

conditions for nitration of benzene

sulfuric acid and 50 degrees c

why is the nitration of benzene done at 50 c

to maintain a good rate of reaction (done with a water bath)

what happens if the nitration of benzene is carried out at a temp over 50c

further subsistution can occur resulting in the production of dinitrobenzene

use of nitrobenzene

dyes and pesticides

what is the electrophile in the nitration of benzene

NO₂⁺

how is the electrophile in nitration of benzene made

a reaction between conc nitric acid and conc sulfuric acid

steps of electrophilic substitution

• electrophile accepts an e- from benzene ring to form a dative covalent bond

• The dative covalent bond then rearranges to restore the aromaticity of the benzene ring, resulting in substitution of a hydrogen atom with the electrophile.

equation for the creation of an electrophile in the nitration of benzene

HNO₃ + H₂SO₄ —> NO₂⁺ + HSO₄^- + H₂O

final step of nitration of benzene

H⁺ + HSO₄^- —.> H₂SO₄

what is a halogen carrier

A halogen carrier is a substance that facilitates the generation of electrophiles by forming a complex with halogens

conditions for the bromination of benzene

room temperature and pressure and in the presence of a halogen carrier

what is the electrophile for the bromination of benzene

Br⁺

why is a Br⁺ needed for the bromination of benzene

benzene is too stable to react with a non polar bromine molecule and requires the electrophile Br⁺ to initiate the reaction.

halogen carriers for bromine

FeBr₃ or AlBr₃

steps for the bromination of benzene

• the Br+ ion accepts a pair of electrons from the benzene ring and forms a dative covalent bond

• the unstable organic intermediate breaks down to form the organic product and a H+ ion

step 1 in the bromination of benzene

Br₂ + FeBr₃ —> Br+ + FeBr₄^-

last step for bromination of benzene

FeBr₄^- + H⁺ —>FeBr₃ + HBr

what alkylation of benzene

the substitition of hydrogen for a alkyl group

what is needed for alkylation

benzene + haloalkane and a halogen carrier (AlCl₃)

what is the role of a halogen carrier

catalyst that generates an electrophile

why is alkylation useful

increases the length of the carbon chain

what is the non-organic product in alkylation

HCl

what is an acylation reaction

the reaction of benzene with an acyl chloride in teh present of AlCl₃

what forms from acylation

an aromatic ketone 

are arenes or alkenes more reactive

alkenes

why are alkenes more reactive than arenes

their pi bonds are localised making an area of high electron density in one spot which can easily attract electrophiles

why are arenes less reactive than alkenes

arenes have a stable deloclaised pi bond as electrons are spread evenly

why would a reaction with methyl benzene be faster than just benzene

methyl increases electron density on the benzene ring

this attracts an electrophile more

why would a reaction with propanone and propanal differ

the alkyl groups of propanone in a ketone would hinder the attack of an electrophile

what is a phenol

a benzene with an OH group

what is more soluble phenol or alcohol

alohols because phenol have a non-polar benzene ring

what type of acid is phenol

weak

how can you show a weak acid in an equation

double arrow 

what forms when a phenol dissolves in water

it partially dissociates forming a phenoxide ion and a proton

phenol + NaOH

sodium phenoxide + H2O

bromination of phenol

forms 2,4,6 tribromophenol and HBr

how can the bromination of phenol be used as a test for phenol

• there will be deolouirstaion

• the 2,4,6tribromophenol forms  white ppt 

what is the bromine test for phenol

react with bromine to form a white ppt

what is special about the bromination of phenol

needs no catalyst

nitration of phenol

the reaction of phenol and dilute nitric acid to form a mixture of 2-nitrophenol and 4-nitrophenol

what is an example of comparing reactviity between benzene and phenol

the nitration of benzene requires concentrated nitric acid whereas the nitration of phenol only require dilute nitric acid

what is the main test for phenols

• use a pH probe (low score for a weak acid)

• will not react with carbonates 

phenol + metal

forms a phenoxide salt and 1/2H2

why is phenol a better acid than a normal alcohol

it has a ring which stabilises the phenoxide ion

one example of evidence that could show that a molecule has kekules structure rather than the deloclaised structure

the bond lengths are different

what does NaOH hydrolyse

esters, phenols, an alcohol on a carboxylic acid

why is a phenylamine more reactive than benzene

• a lone pair of electrons on the N is partially delocalised

• this increases electron density (more than benzene)

• making the molecule more susceptible to electrophillic attack

why are phenols more reactive than benzene

they have a lone pair of electrons on the oxygen which is partially delocalised

this increases electron density

making it more susceptible to electrophilic attack

where does the -NH2 direct

2 or 4

where does -NO2 direct

3

Explain the differences between Kekulé’s model and the delocalised model of benzene

• P-orbitals overlap to form pi bonds

• Pi bonds are delocalised in the delocalised model, the delocalised nature makes it more stable

• In Kekulé’s model the pi bonds are localised between 2 carbons

phenol with a carboxylic acid + carbonate oragnic synthesis

only the carboxylci part reacts not the alcohol

phenol with a carboxylic acid + hydroxide oragnic synthesis

both the alcohol and the carboxylic acid react