Benzene

Question 1

What are aromatics?

Answer 1

contain a benzene ring

Question 2

What are aromatics used in?

Answer 2

dyes, explosives, drugs

Question 3

What is the empirical formula and Mr of benzene?

Answer 3

CH, 78

Question 4

Compared to alkenes, how reactive is benzene?

Answer 4

fairly unreactive

Question 5

What reaction indicates that benzene is a cyclic compound that contains 3 C=C?

Answer 5

when benzene is hydrogenated, 3 moles of H2 gas react with each mole of benzene to form cyclohexane

Question 6

What did Kekulé suggest about benzene?

Answer 6

that is has a cyclic structure with alternating single and double C=C bonds

Question 7

What is the problem with Kekulé stuctures?

Answer 7

-if benzene is unsaturated, it should undergo electrophilic addition reactions readily but it does not
-the reaction of benzene with chlorine is expected to form 2 different products, but only 1 product is formed. this indicates that all H atoms in benzene are equivalent
-x-ray diffraction studies have shown that benzene is planar and a regular hexagon, so all C-C bonds are equal, in kekulé’s structure it would have different bond lengths for C-C and C=C
-the enthalpy of hydrogenation values

Question 8

What is the bond length for C-C, C=C, and bonding in benzene?

Answer 8

c-c — 0.154nm
c=c — 0..134nm
benzene — 0.139nm

Question 9

What do the enthalpies of hydrogenation show for benzene?

Answer 9

that benzene is much more stable than kekulé’s structure, as when cyclohexene is hydrogenated 120 kJ/mol-1 is released, so we can assume that the enthalpy of hydrogenation for kekulé’s structure is three times this (360kJ/mol-1)
-however, the observed value for benzene is only 208 kJ/mol-1

Question 10

What does the fact that benzenes enthalpy of hydrogenation is lower than kekule’s structure mean?

Answer 10

that it requires less energy to break the double bonds in benzene than 3 double bonds in an alkene

Question 11

Why is benzene more stable than expected?

Answer 11

due to delocalisation stabilisation- the pi electrons in the C=C are delocalised around the ring, and the delocalisation brings extra stability

Question 12

Explain the structure of benzene?

Answer 12

-6 carbon atoms are planar and arranged in a regular hexagon, (all bond angles are 120), so each carbon atom is bonded to 2 other carbons and 1 hydrogen, leaving it with 1 unused pi electron. the 6 of these produce electron density above and below the plane of the ring

Question 13

What is resonance/delocalisation energy?

Answer 13

the increase in stability associated with electron delocalisation

Question 14

What do you need to include in an electrophilic substitution reaction?

Answer 14

step 1- formation of electrophile
step 2- mechanism

Question 15

What is the equation for making nitronium ion?

Answer 15

HNO3 + 2H2SO4 –> +NO2 + H3O+ +2HSO4-

Question 16

What is the structure of a nitronium ion?

Answer 16

O=N+=O

Question 17

What are the conditions for the nitration of benzene and why?

Answer 17

conc H2SO4 AND conc HNO3, and 50C, as above this multiple substitutions can occur

Question 18

Why is nitration important?

Answer 18

nitrated benzene derivatives used as explosives- the methyl group is electron releasing so that increasing the attraction to electrophiles even more

Question 19

What are the two ways that nitro groups can be reduced, and are they specific or not?

Answer 19

-catalytic hydrogenation, H2/Ni (non-specific)
-metal /acid combos (eg Sn/conc HCl), specfic

Question 20

What are primary aromatic amines used to make?

Answer 20

azo dyes

Question 21

How do you name aromatics with only 1 substituent?

Answer 21

just add to front- eg bromobenzene

Question 22

How do you name aromatics with more than one substituent?

Answer 22

-lowest possible number
-listed alphabetically

Question 23

What happens to alkyl side chains?

Answer 23

given number 1

Question 24

How is polystyrene made?

Answer 24

addition polymerisation of the monomer phenylethene

Question 25

What happens in a Friedel-Crafts Acylation?

Answer 25

-an acyl group is joined onto the benzene ring to form a ketone, leads to C-C bond formation, and the catalyst is an acylium cation (R-C+=O)

Question 26

How are aromatic ketones made?

Answer 26

benzene reacts with acyl chlorides/acid anhydrides in the presence of an aluminium chloride catalyst (product HCl)

Question 27

What is the equation for making the aluminium chloride electrophile?

Answer 27

CH3COCl + AlCl3 –> CH3-C+=O + AlCl4-

Question 28

What does the AlCl3 catalyst act as?

Answer 28

a lewis acid (electron pair acceptors)

Question 29

Why does the acylation of benzene not occur by nucleophilic addition?

Answer 29

electron pairs in nucleophiles repelled by the delocalised electrons in benzene