aromatic compounds

Question 1

what is the formula of benzene?

Answer 1

C6H6

Question 2

what is the empirical formula of benzene?

Answer 2

CH

Question 3

what is the state of benzene at room temperature?

Answer 3

liquid

Question 4

what is the bond angle of benzene?

Answer 4

120

Question 5

what are the 3 features of benzene that doesn’t support Kekule’s model?

Answer 5

• benzene is resistant to addition reactions (benzene doesn’t decolourise bromine water)
• enthalpy change of hydrogenation of benzene is more stable than predicted
• all the carbon bonds are the same length

Question 6

what technique was used to find the bond lengths of benzene

Answer 6

x-ray diffraction

Question 7

what happens to the 4th electron in the p-orbital of each carbon atom in benzene

Answer 7

• it delocalises to form rings of electron density above and below the hexagon
• forming rings of delocalised electron density above/below the hexagon

Question 8

how do the rings of electron density affect the stability of benzene

Answer 8

• makes benzene very stable
• even though it is unsaturated (aromatic stability)

Question 9

why does benzene have a relatively high melting point

Answer 9

close packing of flat hexagonal molecules when solid

Question 10

is benzene soluble in water? explain why?

Answer 10

• no
• it is non-polar

Question 11

dangers of benzene - why is it not used in school?

Answer 11

it is a carcinogen

Question 12

how do you name compounds containing a benzene ring?

Answer 12

benzene or phenyl

Question 13

why is benzene attacked by electrophiles?

Answer 13

high electron density above/below ring due to delocalised electrons

Question 14

nitration of benzene is what type of reaction?

Answer 14

electrophilic substitution reaction

Question 15

which ion is used to nitrate benzene?

Answer 15

NO2+

Question 16

what is the catalyst in nitration of benzene

Answer 16

sulfuric acid
H2SO4

Question 17

how is the electrophile NO2+ ion generated during the nitrification of benzene (conditions+equations)

Answer 17

• concentrated H2SO4 and concentrated HNO3
• 50°C
• overall: H2SO4 + HNO3 → HSO4- + NO2+ + H2O

Question 18

how is the H2SO4 catalyst regenerated in the nitration of benzene

Answer 18

HSO4- + H+ → H2SO4

(H+ from benzene ring)

Question 19

what type of catalyst is used for a Friedel-Crafts reaction?

Answer 19

a halogen carrier

e.g. AlCl3

Question 20

why does benzene not react directly with halogens?

Answer 20

the aromatic ring is too stable

Question 21

what is happening when AlCl4- is formed in terms of electrons?

Answer 21

the lone pair of electrons on the chlorine atom is forming a coordinate bond to Al

Question 22

how is the AlCl3 catalyst formed?

Answer 22

AlCl4- + H+ → HCl + AlCl3

(H+ from benzene)

Question 23

how could you use a Friedel-Crafts mechanism to add a methyl group to a benzene ring?

Answer 23

• use a halogenalkane
• add AlCl3 (catalyst)
• to create an electrophile that can attack the benzene

Question 24

what reactions can you carry out to show the weak acidity of phenol?

Answer 24

• neutralisation reaction
• with NaOH
• but absent when you react phenol with carbonates

Question 25

what is the relative ease of electrophilic substitution of phenol compared to benzene and why?

Answer 25

• it is easier for electrophilic substitution to occur with phenol
• because oxygen lone pair of electrons from the -OH group are partially delocalised into ring
• this increases the electron density of the ring
• electrophiles are more attracted to phenol

Question 26

what is the directing effect of electron donating groups OH and NH2?

Answer 26

• group donates electrons onto ring, so electron density increases
• reactivity increases
• 2nd substitution at 2- or 4- positions

Question 27

what is the directing effect of electron withdrawing group NO2?

Answer 27

• group withdraws electrons from ring
• reactivity decreases
• 2nd substitution at 3- position