Topic 17.4 Benzene Flashcards
Arenes
Ring compounds in which there are delocalised electrons (based on benzene)
Aromatic
The bonding in a compound which has delocalised electrons forming pi bonding in a hydrocarbon ring
Aliphatic
Molecules have chains atoms and with rings that are not aromatic
(openchain)
Reactions of benzene: as a hydrocarbon
-Hydrogenation
Heat under pressure with a nickel catalyst
C6H6 +3H2 –> C6H12
Reactions of benzene: combustion
-Benzene burns with a smokey flame
C6H6 + 7.5O2 –> 6CO2 + 3H2O
Reactions of benzene: bromination
Heated under reflux with a catalyst (a halogen carrier)
Reactions of benzene: nitration
Reagents- concentrated nitric acid + sulfuric acid
Conditions- warm (not about 50°C)
Reactions of benzene: Freidel-Crafts alkylation
Organic product is methylbenzene (touline)
Adding alkyl group
Reactions of benzene: Freidel-crafts acylation
-Delocalised electrons in ring act as nucleophile
-Attack acyl chlorides
-Reactive intermediate needed
-AlCl catalyst:
R-COCl + AlCl3 –> R-CO^+ + AlCl4^-
(Anhydrous conditions (dry) as water would react with the catalyst)
Electrophile
An electron pair acceptor – an atom or group of atoms that is attracted to a region of high electron density and can accept a pair of electrons.
Nucleophile
An electron-rich species that can donate a pair of electrons
Substitution
A reaction in which a functional group in a compound is replaced by (or is substituted by) another.
Addition
Chemical reaction wherein two or more reactants come together to form a larger single product.
Four main things that led to Kekule’s structure being disproven: One
Benzene isn’t very reactive, so if Kekulé’s model were to be correct, there would be 3 double bonds in benzene, and so it would react with bromine and decolourise it
–> Benzene doesn’t decolourise bromine
Four main things that led to Kekule’s structure being disproven: Two
The bond lengths in benzene are all the same length. If Kekulé’s model was correct, there would be 2 different bond lengths, corresponding to the double bonds and the single bonds in the ring.
Four main things that led to Kekule’s structure being disproven: Three
The hydrogenation enthalpy of benzene is expected to be three times that of cyclohexene, if it were Kekulé’s model. However, it is smaller, and less energy is produced than expected, meaning that the actual model of benzene is a lot more stable than Kekulé’s model.
Four main things that led to Kekule’s structure being disproven: Four
Should be 4 isomers however only 3 are known to exist
Mechanisms: Bromination of benzene
Step 1: Formation of the electrophile
Bromine reacts with the catalyst to form Br+:
AlCl3 + Br2 –> Br+ + AlCl3Br-
Step 2: Electrophilic attack
Step 3: Formation of the aromatic product
Step 4: Formation of the inorganic products
AlCl3Br- + H+ –> AlCl3 + HBr
Mechanisms: Niration of benzene
-NO2 + ion = electrophile
Step 1: formation of electrophile
(conc.) nitric acid and (conc.) sulphuric acid to form NO2^+: HNO3 + H2SO4 –> NO2^+ + HSO4^- + H2O
Step 2: Electrophilic attack
Step 3: Formation of aromatic product
Step 4: Formation of inorganic products
HSO4^- + H^+ –> H2SO4
Mechanisms: Friedel-Crafts alklyation of benzene with chloromethane
-Alkalyation means the substitution of one of the hydrogen atoms of benzene by an alkyl group.
-The reagent is a halogenoalkane, and the products are an alklybenzene (methylbenzene) and hydrogen chloride.
-The equation for the reaction with chloromethane:
Mechanisms: Friedel-Crafts with acetyl chloride
Why does benzene undergo substitution reactions rather than addition reactions?
Because that would involve breaking the delocalisation and losing that stability.
Show the general reaction and reagents for bromination of phenol
In the absence of a catalyst (cold conditions)
reagents: Aqueous bromine (?)
Explain why bromination happens more readily with phenol than benzene
The increased reactivity compared to benzene is due to the lone pair of p-orbital electrons on the oxygen atom in the phenol group.
Benzene
-A ring of 6 carbons and 6 hydrogens
-A ring of delocalised electrons in the middle
-Each bond has an intermediate length
-One of the outer electrons from each carbon is delocalised from p orbital
-Much more stable than similar molecules
The (wrong) predicted benzene structure (Kekule)
-Predicted benzene would be similar but were wrong
-Predicted enthalpy change: -360 KJ mol^-1
-Actual value: -208 KJ mol^-1
(lower in energy so more stable)
Electrophilic substitution
-Delocalised electrons
-High electron density
-Attract electrophiles; susceptible to attack
-delocalised ring is partially destroyed
(can be restored)