Chapter 25 - Aromatic Compounds

Question 1

What is benzene

Answer 1

aromatic hydrocarbon C6H6

Question 2

Aromatic

Answer 2

Compound containing benzene ring(s)

Question 3

Aliphatic

Answer 3

Chain of compounds without benzene

Question 4

Alicyclic

Answer 4

Organic compound in any ring structure without benzene

Question 5

Properties of benzene

Answer 5

• colourless with a sweet smell
• found naturally in crude oil
• carcinogenic

Question 6

Disproofs of Kekule’s model

Answer 6

• 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

Question 7

Describe the structure of benzene (the delocalised model of benzene)

Answer 7

• 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

Question 8

Benzoic acid

Answer 8

Question 9

phenylamine

Answer 9

Question 10

benzaldehyde

Answer 10

Question 11

Why does benzene not undergo electrophilic addition (decolourise bromine water)?

Answer 11

• 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

Question 12

What are the conditions for the nitration of benzene?

Answer 12

Sulfuric acid catalyst and heat to 50 degrees Celsius with a water bath (if temp goes above 50 further substitution occurs)

Question 13

Mechanism for nitration of benzene and equation for formation of electrophile

Answer 13

Question 14

conditions for halogenation of benzene

Answer 14

halogens react with benzene in the presence of a halogen carrier catalyst (FeBr3 or AlCl3 or AlBr3 ect..)

Question 15

Mechanism for halogenation of benzene and equation for formation of electrophile

Answer 15

Question 16

conditions for the alkylation of benzene

Answer 16

react benzene with haloalkane in the presence of a halogen carrier catalyst (AlCl3)

Question 17

equations for the alkylation of benzene

Answer 17

Question 18

Conditions for the Acylation of benzene

Answer 18

React benzene with an acyl chloride in the presence of a halogen carrier catalyst (AlCl3)

Question 19

equations and mechanisms for acylation of benzene

Answer 19

Question 20

What is a phenol

Answer 20

an OH group directly bonded to a benzene ring (if Oh group is bonded to a side chain it will be an alcohol)

Question 21

Why is phenol a weak acid

Answer 21

It partially disassociates with its H+ ion to form a phenoxide ion
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Question 22

solubility of phenol

Answer 22

less soluble than alcohol as benzene ring is non polar so OH group has less significance

Question 23

order of acidity

Answer 23

alcohols < phenols < carboxylic acids

Question 24

formation of phenol salt (neutralisation reaction)

Answer 24

Phenol is a weak acid so reacts with a strong base in a neutralisation reaction

Question 25

conditions for the electrophilic substitution of phenol with bromine

Answer 25

room temp with bromine water and NO halogen carrier catalyst

Question 26

formula for the reaction of phenol with bromine

Answer 26

Question 27

conditions for the nitration of phenol

Answer 27

room temperature and dilute nitric acid

Question 28

formula for the nitration of phenol

Answer 28

Question 29

Why does phenol react more readily than benzene

Answer 29

- 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

Question 30

Reasoning behind directing groups

Answer 30

substituted benzene rings distort the electron density of the ring and affect the reactivity of each carbon atom

Question 31

electron withdrawing groups

Answer 31

- electronegative groups (like NO2) withdraw electron density from carbon 2,4,6 - electrophiles are more likely to attack carbons 3 and 5

Question 32

electron donating groups

Answer 32

- donating groups (like OH and NH2) donate electrons to carbons 2,4,6 - so electrophiles are more likely to attack carbons 2,4,6

Question 33

What is filtration under reduced pressure used for?

Answer 33

A technique for separating a solid product from a solvent or liquid reaction mixture

Question 34

How to set up filtration under reduced pressure?

Answer 34

- 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

Question 35

Why is recrystallisation used?

Answer 35

The solid product obtained after filtering still contains impurities, which can be removed by recrystallisation

Question 36

How is recrystallisation carried out?

Answer 36

- 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

Question 37

Why is melting point determination used?

Answer 37

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

Question 38

What does thin layer chromatography show?

Answer 38

Shows if the samples are different compounds

Question 39

How to do thin layer chromatography?

Answer 39

- 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.