arenes

Question 1

what are the criterias for aromatic compounds?

Answer 1

1. cyclic or ring structure
2. planar
3. alternating single and double bonds resulting in overlapping p orbitals
4. (4n+2) delocalised pi electrons in the overlapping p orbitals where n is any integer.
5. aromatic compounds generally have an added stability due to the delocalisation of pi electrons around the ring structure.

Question 2

describe the structure of benzene ring.

Answer 2

1. straight line represents the sigma bonds betweeen the sp2 C atoms.
2. the circle represents the cloud of six delocalised electrons.
3. resonance hybrid structure in which the system of six pi electrons are delocalised over the carbon atoms in the ring. the delocalisation confers additional stability, giving benzene its aromatic character. Benzene is said to have resonance stability.

Question 3

what are the physical properties of benzene ring?

Answer 3

1. colourless liquid
2. less dense than water
3. non-conductor
4. immiscible with polar solvents, soluble in non-polar solvents
5. low boiling point of 80C, volatile

Question 4

describe the carbon-carbon bond length in benzene.

Answer 4

1. carbon-carbon bonds are equal in length, as each resonance sturcture contributes equally to form the resonance hybrid.
2. the carbon-carbon bond length (0.139nm) is in btw the length of the a c-c single bond(0.154nm) and that of a c=c double bond(0.133nm).

Question 6

when is benzene considered a substituent?

Answer 6

when it is bonded to:
1. an alkane consisting 7 or more carbon atoms
2. alkane with a fuctional group.

it is called a phenyl group..

Question 6

why does Benzene not undergo electrophilic addition?

Answer 6

electrophilic addition will disrupt the side-on overlap of p orbitals throughout the ring. this changes the hybridisation of the 2 carbon atoms in benzene from sp2 to sp3, leading to a loss in aromaticity and resonance stability.

Question 6

why does Benzene undergo electrophilic substitution?

Answer 6

1. it can act as a nucleophile due to electron-rich ring, donating a pair of pi electrons to react with electrophiles.
2. as the pi electrons in the benzene ring are delocalised, the resonance stability of benzene decreases its reactivity towards electrophiles, making it a weaker nucleophile.
3. Hence, stronger electrophiles are required to react with benzene.
4. Benzene readily undergoes electrophilic substitution.

Question 7

describe electrophilic substitution in words.

Answer 7

1. the electrophile is first generated in the reaction mixture.
2. the electron-rich benzene ring uses 2 of the pi electrons in the ring to form a sigma bond between 1 carbon atoms and the electrophile, E+.
3. A carbocation intermediate is formed, and its positive charge is delocalised over the remaining 4 overlapping p orbitals.

formation of carbocation is a slow step as it has the highest Ea to destroy the extra stability associated with the six pi electrons delocalised in the benzene ring.

4. in this fast step, the sp3 C-H bond in the carbocation intermediate will break heterolytically, forming a proton, H+.
5. the 2 electrons in sp3 C-H bond will help to reform the stable benzene ring with its system of six delocalsied pi electrons.
6. A stable, aromatic and planar substituted benzene is formed.

Question 8

what is a lewis acid and a lewis base?

Answer 8

1. a lewis acid is an electron-pair acceptor.
2. A lewis base is an electron-pair donor.

Question 9

what is required for the halogenation of Benzene?

Answer 9

1. presence of AlX3 (where X = Cl/ Br) which acts as a lewis acid catalyst.

Question 10

what does the lewis acid do in the halogenation of benzene?

Answer 10

1. the AlX3 polarises the halogen molecule(X2) and accepts a lone pair of electrons from it, A stronger electrophile, X+ is generated.
2. general eq:
   X2 + AlX3 = X+ + AlX4-

Question 12

describe the halogenation of benzene in words.

Answer 12

1. X2 reacts with AlX3 Lewis Acid catalyst to generate the electrophile, X+.
2. In this slow step, electron-rich benzene ring attacks X+ to form a carbocation intermediate. Hybridisation of one C atom changes from Sp2 to sp3 to from the C-X bond.
3. the AlX4- regenerates the AlX3 catalyst and forms HX(g) by removing the H atom bonded to the sp3 C atom. Hybridisation of the sp3 C atom changes back to sp2. A stable, aromatic, and planar benzene ring reforms.

Question 13

state the reagents and conditons and observations for the reaction of Benzene with chlorine.

Answer 13

1. Cl2, anhydrous AlCl3(room temperature).
2. greenish yellow Cl2 decolourises; white fumes of HCl produced.

Question 14

state the reagents and conditons and observations for the reaction of Benzene with bromine.

Answer 14

1. Br2, anhydrous AlBr3(room temperature)
2. reddish brown Br2 decolourised; white fumes of HBr produced.

catalyst must be anhydrous as the catalyst can react with water to form other products.

Question 15

what happens in a friedal-crafts alkylation?

Answer 15

1. benzene reacts with halogenoalkanes, RX, only in the presence of ALX3(X=Br/Cl) lewis acid catalyst.
2. Lewis acid catalyst, AlX3 polarises the halogenoalkane(RX) and accepts a lone pair of electrons from it. A stronger alkyl electrophile, R+ is formed.
   general eq: RX + AlX3 = R+ + AlX4-.
3. electrophilic substitution still occurs, but AlX3 regenerated at the end.

Question 16

what are the conditions, reagents and observation for friedal-crafts alkylation of CH3Cl?

Answer 16

1. CH3Cl, anhydrous AlCl3( at room temperature)
2. white fumes of HCl produced.

Question 17

what are Bronsted-Lowry acid and Bronsted-Lowry base?

Answer 17

A bronsted-lowry acid is a proton(H+) donor
A bronsted-lowry base is a proton(H+) acceptor.

Question 18

what are the requirements for nitration of benzene to occur?

Answer 18

benzene only reacts with concentrated HNO3 in the presence of concentrated H2SO4 which acts as a Bronsted-Lowry acid catalyst.

reagents and conditions: concentrated HNO3 and concentrated H2SO4 at 50C (any higher will result in further substitution).

without a catalyst, concentrated HNO3 is not electrophilic enough to disrupt the aromatic stability of benzene ring. Using concentrated H2SO4 will generate a stronger electrophile.

Question 19

what to include in mechanism drawing for nitration of benzene?

Answer 19

1. HNO3 + 2H2SO4 = N+O2 + 2HSO4- + H3O-.
2. electrophilic substitution, but rmb that H2SO4 is regenerated at the end.

Question 20

what are the traits of activating groups?

Answer 20

1. electron-donating substituents in substituted benzenes increase the electron density of the benzene ring making the ring a stronger nucleophile than benezene.
2. reactivity of benzene ring towards electrophilic substitution increases.
3. eletrophilic substitution occurs faster and requires a lower temperature.

Question 21

what are the traits of deactivating groups?

Answer 21

1. electron-withdrawing substituents in substitued benzenes decrease the electron density of the benzene ring, making the ring a weaker nucleophile than benzene.
2. reactivity of the benzene ring towards electrophilic substitution decreases.
3. electrophilic substitution occurs slower and requires a higher temperature.

Question 22

how do substituents bonded to the benzene ring donate/withdraw electrons?

Answer 22

inductive effect/delocalisation.

Question 23

describe inductive effect.

Answer 23

1. the effect of electron donation or withdrawal felt by the polarisation of sigma bonds in a molecule. 2. substituents inductively donate or withdraw electrons through the sigma bond linking the substituent to the benzene ring. Examples: electron-donation: -CH3 electron-withdrawal: -NH3+, -Cl, -Br, -I.

Question 24

describe delocalisation.

Answer 24

1. delocalisation of pi electrons occurs when 3 or more p orbitals overlap side-on continuously. 2. for substituted benzenes, the p orbital of a substituent overlaps side-on with the pi electron cloud of the benzene ring. 3. the electron density of the benzene ring increases or decreases as the electron are delocalised into/away from benzene ring.

Question 25

describe electron donation by delocalisation.

Answer 25

1. substituents have a lone pair of electrons on the atom bonded to the benzene ring. the lone pair of electrons is delocalised into the benzene ring. 2. the P orbital of the substituent atom overlaps side-on with the pi electron cloud of the adjacent benzene ring. 3. As the lone pair of electrons on the substituent atom is delocalised into the benzene ring, the electron density of the ring increases. 4. Hence, the benzene ring in phenol is a stronger nucleophile than benzene.

Question 26

describe electron donation by withdrawal.

Answer 26

1. substituents typically have a double/triple bond present. the more electronegative atom in the substituent is usually located at the end of the group. 2. the pi electron cloud of the c=o bond overlaps side-on with the pi electron cloud of the adjacent benzene ring. 3. As the pi electrons of the benzene ring are delocalised into the c=o bond, the electron density of the ring decreases. 4. Hence, the benzene ring in benzaldehyde is a weaker nucleophile than benzene.

Question 27

what are the exceptions when it comes to activating and deactivating groups?

Answer 27

1. some substituents may exhibit opposing electronic effects. The predominant effect can be deduced from the reactivity of the ring in the substituted benzene relative to benzene. examples: 1. Cl, Br, I are deactivating groups: **althoughh the halogen substituents have a lone pair of electrons that can be delocalised into the benzene ring, the electron-withdrawal from the benzene ring by inductive effect is more dominant**. 2. -OH, -OR are activating groups. **although the electronegative nature of the O atom may result in electron0wtihdrawal from the benzene ring, the delocalisation of its lone pair of electrons into the benzene ring is more dominant.

Question 28

define steric effect.

Answer 28

repulsion of electron clouds of atoms due to **proximity/arrangement in space**.

Question 29

how does steric effect apply to 2,4-directing substituents?

Answer 29

1. when the 2,4-directing substituent on the benzene ring/incoming electrophile is made up of a large atom or gorups of atoms, the steric effect is significant. 2. there is greater **electron repulsion between incoming electrophile and the substituent** when the electrophile approaches the ring at the 2-position than at the 4-position. 3. Hence, the 1,4-isomer is typically formed in a larger proportion than the 1,2-isomer.

Question 30

describe the physical properties of methylbenzene.

Answer 30

1. colourless liquid 2. less dense than water 3. non-electrical conductor. 4. immiscible with polar solvents, soluble in non-polar solvents, 5. volatile with low boiling point of 111*c*.

Question 31

can methylbenzene undergo electrophilic substitution and side-chain reactions?

Answer 31

yes, side-chain reactions include free radical substitution and oxidation.

Question 32

what is the electrophilic substitution reaction rate at the benzene ring?

Answer 32

1. faster than benzene. 2. methyl substituent is electron-donating group, thus increases electron density of the benzene ring 3. methylbenzene becomes more reactive towards electrophiles than benzene. 4. incoming electrophile is directed by methyl group to 2, 4 positons.

Question 33

what are the reagents, conditions and observation for reaction of methylbenzene with chlorine at **benzene ring**?

Answer 33

Cl2, anhydrous AlCl3(at room temperature) observations: greenish yellow Cl2 decolourises, white fumes of HCl produced. **2/4-chloromethylbenzene are major products**.

Question 34

what are the reagents, conditions and observation for reaction of methylbenzene with bromine at **benzene ring**?

Answer 34

1. Br2, anhydrous AlBr3 at room temperature. 2. observations: reddish brown Br2 decolourised; white fumes of HBr produced.

Question 35

what are the reagents, conditions and observation for reaction of methylbenzene with CH3Cl at **benzene ring**?

Answer 35

1. reagents and conditions: CH3Cl, anhydrous AlCl3. 2. observations: white fumes of HCl produced.

Question 36

what are the reagents, conditions and observation for nitration of methylbenzene at **benzene ring**?

Answer 36

reagents and conditons: concentrated HNO3, concentrated H2SO4, 30*C*. **the increased electron density at the benzene ring due to the electron-donating methyl group results in nitration to occur at a lower temperature**. observations: nothing.

Question 37

what happens if nitration of methylbenzene takes place at temperature greater than 30?

Answer 37

further substitution reaction occurs. As the electron-withdrawing -NO2 group reduces the electron density at the benzene ring, it becomes deactivated and subsequent substitutions will require more vigorous conditions. **1st subsequent nitration: concentrated HNO3 & concentrated H2SO4, 90C**. **2nd subsequent nitration: fuming HNO3 and fuming H2SO4, 100C**.

Question 38

describe the reagents, conditions, and observation for free radical substitution between methyl group of benzene ring and chlorine.

Answer 38

1. limited CL2, uv light at room temp. 2. greenish yellow Cl2 decolourised, white fumes of HCl produced.

Question 39

describe the reagents, conditions, and observation for free radical substitution between methyl group of benzene ring and bromine.

Answer 39

1. limited Br2, UV light at room temp. 2. reddish brown Br2 decolourised, white fumes of HBr produced.

Question 40

what happens if halogen is in excess for free radical substituion?

Answer 40

further substitution of side-alkyl group, forming disubstituted and trisubstituted products.

Question 41

describe the reagents, conditions, and observation for oxidation of methylbenzene

Answer 41

1. general eq: methybenzene + 3[O] = benzoic acid + H2O. 2. acidified KMnO4, heat using water bath/alkaline KmnO4, heat followed by dilute acid. 3. Observations: purple KMnO4 decolourised, white ppt of benzoic acid forms. ## Footnote 1. not acidified KMnO4 does not oxidise alkanes or benzene. 2. Benzoic acid has low solubility in cold water, its solubility is higher in hot water. 3. the use of hot alkaline KMnO4 will produce a benzoate salt as the benzoic acid formed is first neutralised by the alkali. **adding the dilute acid to benzoate salt forms back the benzoic acid**.

Question 42

what are the requirements for the oxidation of alkyl side-chains?

Answer 42

for alkylbenzenes, the oxidation requires the **presence of at least 1 H atom attached to the benzylic acid C atom(C atom directly attached to the benzene ring)**

Question 43

what happens, during oxidation, to alkyl side-chains that have more than one C atom?

Answer 43

the rest of the akyl groups in the chain will be destroyed, forming CO2 and H2O. The effervescence observed during oxidation can help distinguish allkylbenzenes from methylbenzenes.