Chapter 25 - Aromatic Compounds Flashcards
What is benzene
aromatic hydrocarbon C6H6
Aromatic
Compound containing benzene ring(s)
Aliphatic
Chain of compounds without benzene
Alicyclic
Organic compound in any ring structure without benzene
Properties of benzene
- colourless with a sweet smell
- found naturally in crude oil
- carcinogenic
Disproofs of Kekule’s model
- Benzene does not undergo electrophilic addition (does not decolourise bromine water)
- X ray diffraction shows that all the C-C bond lengths in benzene were the same (length of double bond is longer than that of single bond)
- Hydrogenation enthalpy of benzene is less exothermic than expected, benzene is more stable than expected
Describe the structure of benzene (the delocalised model of benzene)
- The p-orbitals are above and below the plane of the benzene ring
- Sideways overlap of p-orbitals creates a delocalised ring of electron density above and below the plane of the benzene ring
Benzoic acid
phenylamine
benzaldehyde
Why does benzene not undergo electrophilic addition (decolourise bromine water)?
- An alkene can polarise the Br - Br molecule to make Br (delta plus) - Br (delta minus)
- Benzene cannot polarise Br - Br as the electrons are delocalised and spread across the ring
What are the conditions for the nitration of benzene?
Sulfuric acid catalyst and heat to 50 degrees Celsius with a water bath (if temp goes above 50 further substitution occurs)
Mechanism for nitration of benzene and equation for formation of electrophile
conditions for halogenation of benzene
halogens react with benzene in the presence of a halogen carrier catalyst (FeBr3 or AlCl3 or AlBr3 ect..)
Mechanism for halogenation of benzene and equation for formation of electrophile
conditions for the alkylation of benzene
react benzene with haloalkane in the presence of a halogen carrier catalyst (AlCl3)
equations for the alkylation of benzene
Conditions for the Acylation of benzene
React benzene with an acyl chloride in the presence of a halogen carrier catalyst (AlCl3)
equations and mechanisms for acylation of benzene
What is a phenol
an OH group directly bonded to a benzene ring (if Oh group is bonded to a side chain it will be an alcohol)
Why is phenol a weak acid
It partially disassociates with its H+ ion to form a phenoxide ion
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solubility of phenol
less soluble than alcohol as benzene ring is non polar so OH group has less significance
order of acidity
alcohols < phenols < carboxylic acids
formation of phenol salt (neutralisation reaction)
Phenol is a weak acid so reacts with a strong base in a neutralisation reaction
conditions for the electrophilic substitution of phenol with bromine
room temp with bromine water and NO halogen carrier catalyst
formula for the reaction of phenol with bromine
conditions for the nitration of phenol
room temperature and dilute nitric acid
formula for the nitration of phenol
Why does phenol react more readily than benzene
- Lone pair of electrons from the oxygen donated to the delocalised pi system
- electron density of benzene ring in phenol is increased so polarises electrophiles more strongly than benzene
Reasoning behind directing groups
substituted benzene rings distort the electron density of the ring and affect the reactivity of each carbon atom
electron withdrawing groups
- electronegative groups (like NO2) withdraw electron density from carbon 2,4,6
- electrophiles are more likely to attack carbons 3 and 5
electron donating groups
- donating groups (like OH and NH2) donate electrons to carbons 2,4,6
- so electrophiles are more likely to attack carbons 2,4,6
What is filtration under reduced pressure used for?
A technique for separating a solid product from a solvent or liquid reaction mixture
How to set up filtration under reduced pressure?
- Connect the büchner flask to the tap and to the büchner funnel ensuring the tubing is sealed
- Place filter paper in the funnel and dampen with the same solvent used when preparing the solid
- rinse the crystals with the solvent and leave them under suction for a few minutes so they dry
Why is recrystallisation used?
The solid product obtained after filtering still contains impurities, which can be removed by recrystallisation
How is recrystallisation carried out?
- Pour the chosen solvent into a conical flask. heat with water bath to warm the solvent
- tip the impure sample into a second conical flask or beaker
- slowly add the warm solvent to the sample until it dissolves, you should add the minimum amount of solvent needed to dissolve the sample to make a saturated sollution
- allow the sollution to cool, crystals should form.
- filter under reduced pressure
Why is melting point determination used?
To determine whether a solid compound is pure. A pure organic substance has a berm sharp melting range of one or two degrees.
- Impurities cause the compound to have a large melting range, they also lower the melting point
What does thin layer chromatography show?
Shows if the samples are different compounds
How to do thin layer chromatography?
- Draw thin pencil line at bottom of TLC plate
- spot the samples on the line
- place in a beaker with 10cm3 solvent and cover with lid to prevent solvent evaporating
- when solvent is about 1cm from top of plate remove the plate and mark the solvent front with pencil
-place plate in beaker with iodine crystals and wait for spots to form then calculate Rf values.
