Chapter 25 - Aromatic chemistry Flashcards
What is benzene?
Aromatic hydrocarbon compound - C6H6
(Aromatic - has one or more benzene rings)
Define arene
Aromatic hydrocarbon containing one or more benzene rings
What was kekulé’s model of benzene?
suggested that structure of benzene was a 6 carbon ring joined by alternate single and double bonds
What were the 3 pieces of evidence to disprove kekule’s model?
- ) Lack of reactivity of benzene (electrophillic addition)
- ) Lengths of carbon-carbon bonds in benzene
- ) Hydrogenation enthalpies
What was the lack of reactivity argument against Kekule’s model?
- Kekule’s structure has C=C bonds so it should decolourise bromine in an electrophile reaction
- However, this doesn’t happen and benzene doesn’t decolourise bromine under normal conditions. Therefore, benzene can’t have C=C bonds
What is the length of carbon-carbon bond argument against kekule’s model?
using X-ray diffraction all the bonds in benzene were found to be 0.139 nm in length - in between the length of a single bond (0.153nm) and a double bond (0.134nm)
What is the hydrogenation enthalpies argument against kekule’s model?
enthalpy change of hydrogenation should be 3x that of cyclohexene (-120kJmol-1)
Actual value found to be -208kJmol-1 instead of -360kJmol-1. 152kJmol-1 less energy is produced than expected, so benzene more stable than expected.
Describe the delocalised model of benzene proposed after kekule’s model?
- Each carbon has one electron in a p-orbital which overlap sideways forming a ring of π-bonds above and below the plane of carbon atoms. Electrons in system of π-bonds are said to be delocalised.
What type of reaction is nitration of benzene?
electrophillic substitution
What are the conditoins for nitration of benzene?
benzene + HNO3 with H2SO4 at 50ºC
What happens when the reaction temp rises above 50ºC?
further substitution may occur leading to the production of dinitrobenzene
What are some uses of nitrobenzene?
dyes, pharamceuticals and pesticides
Write the 3 steps in the mechanism for nitration of benzene?
step 1: HNO3 + H2SO4 → NO2+ + HSO4- + H2O
step 2:
step 3 : H+ + HSO4- → H2SO4
As halogens can’t react with benzene on their own, what is required? Give examples of this
halogen carrier
e.g. FeCl3, AlBr3, FeBr3, AlCl3
What are the conditions for bromination of benzene? (hydrogen substituted by bromine)
Room temperature and pressure in presence of halogen carrier.
What is formed as a by product of bromination?
Hx where x is a halogen
*Write the 3 steps of the mechanism for bromination of benzene? (would be the same for chlorination)
step 1: Br2 + FeBr3 → FeBr4- + Br+
step 2:
step 3: H+ + FeBr4- → FeBr3 + HBr
What happens in alkylation of benzene?
substitution of a hydrogen atom in the benzene ring by an alkyl group.
- benzene reacted with haloalkane in presence of AlCl3
- AKA friede-crafts alkylation
Write the equation for alkylation of benzene with C2H5Cl
What happens in acylation of benzene?
Benzene + acyl chloride in presence of AlCl3 forms aromatic ketone (phenyl ketone)
Write equation for acylation of benzene with ethanoyl chloride and AlCl3
Why doesn’t benzene react with bromine?
- Benzene has delocalised π electrons spread above and below the plane of carbon atoms in the ring structure. Therefore, electron density less than C=C bond in an alkene.
- insufficient electron density to polarise bromine molecules
Alkenes can react with bromine as π bond electrons are localised in alkenes
What’s a phenol?
Organic chemical containing hydroxyl -OH functional group directly bonded to an aromatic ring
Why is a phenol classed as a weak acid?
Due to ability to partially dissociate to produce protons and a phenoxide ion
Why are pehnols lesss soluble in water than alcohols?
presence of non-polar benzene ring
Are phenols more acidic than alcohols and carboxylic acids?
more acidic than alcohols but less acidic than carboxylic acids
What can be used to distinguish between a phenol and a carboxylic acid?
Reaction with sodium carbonate
- carboxylic acid produces CO2
- Phenols don’t react
What happens in reaction of phenol with sodium hydroxide? Write an equation
Phenol reacts with sodium hydroxide to form a salt, sodium phenoxide and water in a neutralisation reaction.
Write equation for phenol with bromine
Write equation for nitration of phenols
What is the increased reactivity of phenols compared to benzene caused by?
Lone pair of electrons from the oxygen p-orbital of the -OH group being donated into the π-system of phenol
electron density increased, therefore phenol more susceptable to attack from electrophiles. Sufficient to polarise molecules e.g. bromine
What does an activating group do?
Makes aromatic ring that the group joined to react more readily with electrophiles
What does a deactivating group do?
Makes aromatic ring that the group joined to react less readily with electrophiles
What is a directing effect?
Different groups have different directing effects on any second substituent on the benzene ring. Determines which carbons the second substitutent group adds to
What directing effect do activting groups have?
2- and 4- directing groups
what directing effect do deactivating groups have?
3-directing groups
with the expception of the halogens - which are 2.4 directing
Give some examples of 2- and 4- directing groups and 3-directing groups?
