Aromatic compounds Flashcards
Describe Kekule’s benzene
Ring structure made of six carbons with alternating single, double cabron bonds.
List the 3 reasons as evidence disproving kekule’s benzene
- Benzenes lack of reactivity.
- The lengths of the Carbon-Carbon bonds in benzene. (being the same).
- Hydrogenation enthalpies.
Describe how benzenes lack of reactivity disproves kekule’s benzene structure?
The lack of reactivity can be shown by benzenes lack to:
- decolorise bromine water
- undergo electrophilic addition
If benzene had double C=C bonds like kekule’s model predicted it should react.
How does benzene’s X ray diffraction disprove kekule’s benzene model?
Benzene’s X ray diffraction shows that all the carbon-carbon bonds are of the same length. Kekule’s benzene should have two different bond lengths. One smaller for the C=C bond and one larger for the C-C bonds.
How does Benzene’s hydrogenation values disprove Kekule’s benzene model?
Benzenes hydrogenation value is much lower than what would be expected for kekule’s model. Expected hydrogenation value = -360kjmol
actual value = -208 kjmol
Benzene is more stable than kekule thought.
Describe the delocalised model of benzene
- Benzene is a planar, cyclic, hexagonal hydrocarbon with 6 hydrogens and 6 carbons.
- Each carbon of benzene uses 3 of its 4 available electrons to sigma bond to 2 carbons and 1 hydrogen.
- Each carbon has 1 available electron in a p orbital, at right angles to the plane of bonded carbons and hydrogens.
- Adjacent p orbitals overlap sideways to form a ring of electron density, via pi bonding.
- The electrons in the system of pi bonds are said to be delocalised.
How is benzene named when it is bonded to Halogen, alkyl or nitro groups?
Benzene takes the sufix
e.g. nitrobenzene
How is benzene named when it is bonded to any functional groups, and what are the exceptions?
Benzene takes the prefix -phenyl.
Execptions:
halogens, akyl or nitro groups.
Benzoic acid
phenylamine
benzaldehyde
Name all the electrophilic substitutions of benzene
-Nitration
-Halogenation (chlorination, bromination)
-Alkylation
-Acylation
What conditions are needed to produce nitrobenzene
Sulfuric acid, nitric acid and the temperature must be 50 °c
What conditions are needed to produce dinitrobenzene
Sulfuric acid, nitric acid, heat must be above 50 °c.
Write the equation for the catalytic process in nitration of benzene (including reformation of catalyst)
Step 1: H₂SO₄ + HNO₃ → HSO₄⁻ + NO₂⁺ + H₂O
Step 2: HSO₄⁻ + H⁺ → H₂SO₄
What catalysts are used in the halogenation of benzene and what are they also refered as?
AlBr₃
AlCl₃
FeBr₃
FeCl₃
halogen carriers
Draw the mechanism for the nitration of benzene
Look it up
Draw the mechanism for the bromination of benzene
look it up
Why is a catalyst needed for the bromination of benzene
Benzene is too stable to react with the non polar benzene so a halogen carrier is used to generate a bromonium ion Br⁺.
Describe what is needed for friedel-crafts alkylation
AlCl3 as the halogen carrier
and a haloalkane (of what alkyl chain you want to add).
What is needed for acylation?
Bezene reacts with the halogen carries AlCl₃ and an acyl chloride.
Describe in detail what happens when bromine reacts with an alkene
- The alkene contains a double carbon C=C bond which contains two localised electrons form the pi bond.
- The localised electrons in the pi bond induce a dipole in the non polar Br-Br molecule, making one bromine slightly electro positive and the other negative.
- The slight positivity allows the bromine molecule to reacts as an electrophile.
Why does benzene not react with bromine compared to alkenes
- Benzene has a delocalised pi system of electrons, spreading the electrons above and below the hexagonal plane of carbons.
- Benzene therefore has a lower electron density compared to the C=C of alkenes.
- Due to the low electron density unlike alkenes benzene cannot induce a dipole/ polarise the bromine molecule, preventing bromine from acting as an electrophile and preventing reaction.
Why are phenols weak acids
Phenols are weak acids due to the non polar benzene ring which isnt very soluable in water (polar).
However phenol does partially dissociate to a phenoxide ion and proton making it a weak acid.
Why are alcohols less acidic than phenols?
Alcohols dont react (neutralise) with sodium hydroxide (a strong base) whilst phenols do.
Why are carboxylic acids more acidic than phenols?
Carboxylic acids react with sodium carbonate (weak base), whilst phenols do not.
What is produce when brominating a phenol
2,4,6-trinitrophenol
What is needed to brominate phenol
just aqueous bromine, no halogen carrier needed.
At room temp.
What is produced in the nitration of phenol?
2-nitrophenol
or
4-nitrophenol
What is needed for the nitration of phenol
just dilute nitric acid.
at room temp.
Why are phenols more reactive than benzene?
- The lone pair form the -OH of phenols is donated from the pi orbital of oxygen to the delocalise pi system of the benzene ring in phenol.
- The electron density of the benzene ring in phenol is therefore increased, allowing phenols to attract electrophiles more strongly than benzene on its own.
- Increasing the susceptability of the ring to attack an electrophile - what spec says.
Why do phenols not need a halogen carrier for bromination?
- Phenols have a higher electron density and therefore can induce a dipole on the bromine molecule to make it act as an electrophile whilst benzene cant.
Which groups are 2,4 directing (2 groups)
NH₂ and OH
Which group is 3 directing?
NO₂
Describe what electron donating groups do to the reactivity of benzene
Groups like NH₂ and OH are electron donating groups, they activate the benzene ring (increase electron density), increasing the reacivity of the aromatic ring with electrophiles.
Describe what electron withdrawing groups do to the reactivity of benzene
Electron withdrawing groups such as NO₂ deactivate the benzene ring decreasing the reactivity of the aromatic ring with electrophiles.
