Topic 17.4 Benzene

Question 1

Arenes

Answer 1

Ring compounds in which there are delocalised electrons (based on benzene)

Question 2

Aromatic

Answer 2

The bonding in a compound which has delocalised electrons forming pi bonding in a hydrocarbon ring

Question 3

Aliphatic

Answer 3

Molecules have chains atoms and with rings that are not aromatic
(openchain)

Question 4

Reactions of benzene: as a hydrocarbon

Answer 4

-Hydrogenation
Heat under pressure with a nickel catalyst
C6H6 +3H2 –> C6H12

Question 5

Reactions of benzene: combustion

Answer 5

-Benzene burns with a smokey flame
C6H6 + 7.5O2 –> 6CO2 + 3H2O

Question 6

Reactions of benzene: bromination

Answer 6

Heated under reflux with a catalyst (a halogen carrier)

Question 7

Reactions of benzene: nitration

Answer 7

Reagents- concentrated nitric acid + sulfuric acid
Conditions- warm (not about 50°C)

Question 8

Reactions of benzene: Freidel-Crafts alkylation

Answer 8

Organic product is methylbenzene (touline)
Adding alkyl group

Question 9

Reactions of benzene: Freidel-crafts acylation

Answer 9

-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)

Question 10

Electrophile

Answer 10

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.

Question 11

Nucleophile

Answer 11

An electron-rich species that can donate a pair of electrons

Question 12

Substitution

Answer 12

A reaction in which a functional group in a compound is replaced by (or is substituted by) another.

Question 13

Addition

Answer 13

Chemical reaction wherein two or more reactants come together to form a larger single product.

Question 14

Four main things that led to Kekule’s structure being disproven: One

Answer 14

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

Question 15

Four main things that led to Kekule’s structure being disproven: Two

Answer 15

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.

Question 16

Four main things that led to Kekule’s structure being disproven: Three

Answer 16

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.

Question 17

Four main things that led to Kekule’s structure being disproven: Four

Answer 17

Should be 4 isomers however only 3 are known to exist

Question 18

Mechanisms: Bromination of benzene

Answer 18

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

Question 19

Mechanisms: Niration of benzene

Answer 19

-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

Question 20

Mechanisms: Friedel-Crafts alklyation of benzene with chloromethane

Answer 20

-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:

Question 21

Mechanisms: Friedel-Crafts with acetyl chloride

Question 22

Why does benzene undergo substitution reactions rather than addition reactions?

Answer 22

Because that would involve breaking the delocalisation and losing that stability.

Question 23

Show the general reaction and reagents for bromination of phenol

Answer 23

In the absence of a catalyst (cold conditions)
reagents: Aqueous bromine (?)

Question 24

Explain why bromination happens more readily with phenol than benzene

Answer 24

The increased reactivity compared to benzene is due to the lone pair of p-orbital electrons on the oxygen atom in the phenol group.

Question 25

Benzene

Answer 25

-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

Question 26

The (wrong) predicted benzene structure (Kekule)

Answer 26

-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)

Question 27

Electrophilic substitution

Answer 27

-Delocalised electrons
-High electron density
-Attract electrophiles; susceptible to attack
-delocalised ring is partially destroyed
(can be restored)