25) Aromatic chemistry Flashcards Preview

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Flashcards in 25) Aromatic chemistry Deck (27)
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1
Q

What is the molecular formula of benzene?

A

C6H6

2
Q

Give 3 reasons for the rejection of Kekulé’s model of benzene.

A

doesn’t undergo electrophilic addition reactions or decolourise Br2 under normal conditions (contains no C=C)
x-ray diffraction has shown benzene to contain bonds of equal length
enthalpy for hydrogenation of benzene was less exothermic than predicted

3
Q

Describe the delocalised model of benzene.

A

the sideways overlap of p-orbitals above and below the plane form a delocalised ring of electron density above and below the plane of the benzene ring (system of pi bonds)

4
Q

When does benzene become a substituent and the prefix ‘phenyl’ used?

A

when a benzene ring is attached to an alkyl chain with a functional group / with 7 or more carbon atoms

5
Q

In the electrophilic substitution mechanism for benzene, where are the 2 curly arrows?

A

from the benzene ring to the electrophile

from the H bond to the positive centre

6
Q

State the reagents and required temperature for the nitration of benzene

A

conc. HNO3 and conc. H2SO4

50 degrees C

7
Q

What acts as the nucleophile in the nitration of benzene?

A

NO2 +

8
Q

Give 2 examples of the halogen carriers that can be used for the halogenation of benzene

A

AlCl3 and FeBr3

9
Q

Describe the alkylation of benzene

A

substitution of a hydrogen atom in the benzene ring by an alkyl group (carried out with a haloalkane in the presence of AlCl3) which increases the number of carbon atoms by forming C-C bonds

10
Q

Describe the acylation of benzene

A

when benzene reacts with an acyl chloride in the presence of AlCl3 forming an aromatic ketone e.g. phenylethanone

11
Q

What are phenols?

A

a type of aromatic compound that contains a hydroxyl -OH functional group directly bonded to an aromatic ring e.g. C6H5OH

12
Q

Phenol is a weak acid. Does phenol react with Na2CO3, a weak base?

A

No - unlike carboxylic acids, it will only react with strong bases such as NaOH

13
Q

What organic product is formed when phenol undergoes a neutralisation reaction by NaOH?

A

sodium phenoxide, C6H5O-Na+

14
Q

Explain why phenol is more reactive / can react under milder conditions than benzene.

A

One of the lone pairs of electrons from the oxygen p-orbital of the -OH group (which overlaps with the delocalised pi system) donates electron density to the ring, making it more susceptible to attack from electrophiles

15
Q

What mixture of organic products will be formed when phenol undergoes nitration with dilute HNO3?

A

2-nitrophenol and 4-nitrophenol

16
Q

Why is no halogen catalyst required for the bromination of phenol?

A

the electron density in the phenol ring structure is sufficient to polarise Br2

17
Q

State the organic product and observations of the bromination of phenol

A

2,4,6-tribromophenol, white ppt

orange to colourless solution

18
Q

Define disubstitution

A

a second substitution

19
Q

What can different group on the benzene ring have on any second subtituent?

A

a directing effect

20
Q

What are activating groups?

A

electron donating groups which activate the benzene ring so the ring reacts more readily with electrophiles; 2- and 4- directing groups
e.g. -NH2, -NHR, -OH, -OR, -R, -F, -Cl, -Br, -I

21
Q

What are deactivating groups?

A

electron withdrawing groups which deactivate the benzene ring, so the ring reacts less readily with electrophiles; 3- directing groups
e.g. RCOR, -COOR, -SO3H, -CHO, -COOH, -CN, -NO2, -NR3 +

22
Q

Which type of group (activating or deactivating) will cause a mixture of products to be formed?

A

activating groups

23
Q

If 2 groups are on carbons 1 and 3, would the first substitution have been to put on an activating or deactivating group?

A

a deactivating group

24
Q

Why could you expect twice as many products with groups on the second carbon as the fourth carbon?

A

because positions 2 and 6 are equivalent

25
Q

Name 4 key stages to help with planning a two-step organic synthesis

A

1) identify the type of distribution
2) identify the directing effect of the two substituents
3) write out the steps with the reagents needed for the synthesis
4) note if there is a mixture of organic products to separate

26
Q

What is the empirical formula of benzene?

A

CH

27
Q

What are arenes?

A

aromatic hydrocarbons that contain one or more benzene rings