Aromatic Compounds

Question 1

general fromula of benzene

Answer 1

C6H6

Question 2

properties of benzene

Answer 2

colourless liquid
boiling point of 80.1 degrees celsius
immiscible in water
burns with a smokey flame due to high percentage of C in formula
present in Coal Tar (byproduct of coal production)

Question 3

bonding in benzene

Answer 3

each C atom is sp2 hybridized ie. during the bonding each C atom has 3 sp2 hybrid orbitals and 1 unhybridized p orbital

Question 4

characteristics of the bonding in benzene

Answer 4

each C-H bond is a sigma bond formed by the overlapping of sp2 hybrid orbital in C and one s hybrid in hydrogen

each C-C bond is a sigma bond formed by the overlapping of sp2 hybrid orbitals of C

Question 5

How do the 6 electrons move in an area above and below the nuclei of the C atom to make a compound aromatic?

Answer 5

the unhybridized p orbitals of eachC atom overlaps above and below the plane of the nuclei
each p orbital overlaps with the 2 adjacent p orbitals

Question 6

what is resonance?

Answer 6

the movement of electrons from one bond to another

Question 7

features of the structure of benzene

Answer 7

1. each CCC bond angle is 120
2. the benzene molecule is planar
3. all C-C bonds are identical in length and in strength (does not have double bonds and single bonds)
4. the C-C bonds are intermediate in lengthand strength between the C-C single bond and C=C double bond
5. the enthalpy change of hydrogenation is -209kJ

Question 8

steps in the mechanism of electrophilic substitution

Answer 8

1. since all 6 C atoms are identical the electrophile can approach any of the C atoms
2. as the electrophile approaches, the C-H bond begins to get weaker and the pi electrons gravitate toards that C atom
3. an unstable intermediate is formed in which the electrophile and he H are partially bonded to the same C atom
4. the unstable intermediate quickly breaks down the H and leaves as H+ and the C electrophile bond is formed
   the symmetical movement of pi electrons is restored

Question 9

what is the nitrating mixture?

Answer 9

concentrated sulphuric acid and concentrated nitric acid

Question 10

nitration of benzene

Answer 10

when benzeneis reacted with nitrating mixture, the benzene is converted to a pale yellow liquid called nitrobenzene
the mixture is placed in a water bath at 5 degrees Celsius

Question 11

what is a halogen carrier?

Answer 11

carriers which induce polarity of the non-polar halogen molecules helping with the generation of electrophiles

Question 12

chlorination of benzene

Answer 12

when Cl gas is passed through benzene at room temperature in the presence of a halogen carrier, the Cl atom will substitute H and chlorobenzene is formed

Question 13

properties of methylbenzene

Answer 13

colourless liquid
boiling point 111 degrees Celsius
insoluble in water
obtained from Coal Tar
good solvent

Question 14

parts of methylbenzene

Answer 14

the side chain
the aromatic nucleus (ring)/ phenyl group

Question 15

what are the reactions of the side chain/ methyl group?

Answer 15

substitution
oxidation

Question 16

substitution reaction of the side chain

Answer 16

when Cl gas is passed through methylbenzene boiling under reflux in the presence of sunlight, Cl atoms will substitute the H of the CH2 group

follows free radical substitution mechanism

Question 17

oxidation of the methylbenzene side chain

Answer 17

when methylbenzene is heated under reflux with oxidizing agents (H2SO4 or acidified KMnO7), the side chain is converted to a carboxylic group and benzoic acid is formed

can occur with other alkyl groups but the final product is always benzoic acid

Question 18

what are the positions of the C atoms in methylbenzene?

Answer 18

the C atom with the CH3 group is 1
two number 2 C atoms on either side of the 1
two number 2 C atoms below the position 2 atoms
one number 4 C atom

Question 19

characteristics of methylbenzene involving reactions with the aromatic ring

Answer 19

the C atoms of the ring are not equivalent
the C atoms differ in their distances from the C atom with the CH3 group

Question 20

how do positions work with electrophiles?

Answer 20

when an electrophile is approaching the ring, it will be directed to a particular position
the position to which the electrophile is directed is determined by the nature of the group

Question 21

types of groups

Answer 21

activating
deactivating

Question 22

what is an activating group?

Answer 22

electron donating groups which push electrons into the ring and increasing the density at all the C atoms

Question 23

what happens when an activating group is present?

Answer 23

1. the electrophile substitution reactions take place more easily than with benzene
2. the incomin electrophile will go to the second and fourth C atoms

Question 24

what are some common activating groups?

Answer 24

CH3/ C2H5
OH
NH2

Question 25

what are deactivating groups?

Answer 25

electron withdrawing group which pull electrons away from the ring, reducing the electron density at all C atoms

Question 26

what happens when a deactivating group is present?

Answer 26

the third carbon atom is the most negative due to the electron density being reduced more at the 2nd and 4th C atom

1. the electrophilic substitution reaction takes place less readily than with benzene
2. incoming electrophiles are directed to the third position

Question 27

common deactivating groups

Answer 27

COOH
NO2
CHO

Question 28

chlorination of methylbenzene

Answer 28

when methylbenzene is reacted with Cl at rom temperature in the presence of a halogen carrier a mixture of 2-chloromethylbenzene and 4-chloromethylbenzene is formed

a mixture of isomers is formed by the isomer in which the electrophile goes to the fourth position and is always formed in greater quantity

Question 29

nitration of benzene

Answer 29

when methylbenzene is reacted with the nitration mixture at 30 degrees Celsius, a mixture of 2-nitromethylbenzene and 4-nitromethylbenzene is formed

if the conditions are adjused, further nitration will occur and 2,4,6-trinitromethylbenzene is formed

Question 30

what is 2,4,6-trinitromethylbenzene?

Answer 30

also called dynamite or TNT
it is a high explosive and when it is detonated, it decomposes into large volumes of gases