Aromatic compounds Module 6

Question 1

What is the Kekule model of Benzene?

Answer 1

C6H6, cyclic structure with there being 3 alternating double bonds, also that there were 2 isomers where the double bonds were in different positions, and it was constantly shifting between these 2 forms.

Question 2

Why is the Kekule model incorrect?

Answer 2

X-Ray diffraction studies show that all the bonds in Benzene are 140pm, when expected there should be 3 C-C bonds length 154pm and 3 C=C bonds with length 134pm

Question 3

Describe the delocalised model of Benzene?

Answer 3

All the p-orbitals overlap to create a π-system
The π-system is made up of 2 ring shaped clouds of electrons, one above, and one bellow the plane of the 6 carbons
All the bond lengths are the same as the they are the same bonds
The electrons in the rings are said to be delocalised because they don’t belong to a specific Carbon, so represented as a ring inside the Hexagon

Draw by having P orbitals up and down C atoms, then a whole ring forms above and bellow showing they overlap

Question 4

What is hydrogenation?

Answer 4

When you react an Alkene with Hydrogen gas, and 2 Hydrogen atoms are added across the double bond

Question 5

How to enthalpy changes of hydrogenation give more evidence of delocalisation in benzene?

Answer 5

In cyclohexene there is 1 double bond, and when its hydrogenated it’s enthalpy change is -120kjmole^-1
Therefore you would expect that for benzene would be (3x-120) = -360kjmol^-1, however experimental values show that for Benzene it’s actually only -208
It’s far less exothermic than expected, so more energy must of been put in to break the bonds in Benzene than in the kekule structure
Showing that Benzene is more stable than kekule strcuture, due to delocalised ring of electrons

Question 6

Another reason why delocalised model is correct?

Answer 6

Benzene is very resistant to addition reactions, needs to be hot and UV light present

Question 7

What’s chlorine bonded to benzene called?

Answer 7

Chlorobenzene

Question 8

What’s NO2 bonded to benzene called?

Answer 8

nitrobenzene

Question 9

What’s methyl group bonded to benzene called?

Answer 9

methylbenzene

Question 10

What’s OH bonded to benzene called?

Answer 10

Phenol

Question 11

What’s NH2 bonded to benzene called?

Answer 11

Phenylamine

Question 12

When naming benzene compounds which have chemicals which normally name benzene and phenyl what do you prioritise in naming and starting carbon

Answer 12

Phenyl chemical

Question 13

What type of organic molecule likes addition reactions?

Answer 13

Alkenes, react with bromine water at room temperature it’s an electrophillic addition reaction

Question 14

What’s different with addition reactions with benzene rather than alkenes?

Answer 14

For it to work with Benzene require hot Benzene and ultraviolet light, as Benzene is far more stable due to delocalised electrons

Question 15

Why do alkenes prefer electrophillic addition, and benzene requires different conditions for an electrophilic addition reaction?

Answer 15

In alkenes C=C bond is an area of high e- density which strongly attracts electrophiles
However in Benzene the attraction is reduced due to the negative charge being spread out

Question 16

Briefly describe electrophillic substiution in Benzene?

Answer 16

Benzene reacts with the electrophile, breaking the delocalised ring
So an unstable intermediate is formed, the delocalised ring is broken, the Electrophile and H+ are bonded making it positively charged
H+ is lost and the delocalised ring is reformed

Question 17

How do Halogen carriers help to make good electrophiles?

Answer 17

An electrohphile has to have a pretty strong positive charge to attack the stable benzene ring, most aren’t polarised enough but this can be changed using a halogen carrier

A halogen carrier accepts a lone pair of electrons from a halogen atom on an electrophile, as the lone pair is pulled away the polarisation of the molecule increases and sometimes a carbocation forms. This makes the electrophile stronger

Question 18

Examples of halogen carriers?

Answer 18

Aluminium Halides
Iron Halides
Iron

Question 19

Equation for Haloalkane with a halogen carrier?

Answer 19

R(delta +) - Cl(delta -) + AlCl3 = R+ (carbocation) + AlCl4 -

Question 20

How do Halogen carriers help Halogens substitute into the Benzene ring?

Answer 20

The Halogen carrier is a catalyst and polaries the diatomic halogen, allowing one of the halogen molecules to act as an electrophile
So during the reaction a halogen atom is substituted in place of a H atom = halogenation

Question 21

How would you draw Bromine reacting with Benzene with aid of AlCl3 catalyst?

Answer 21

Draw the bromine molecule with one end delta + and one delta -
Draw Benzene
Write AlCl3
Have a curly arrow going from Br-Br bond to AlCl3, and a curly arrow from the delocalised electrons from ring to delta + Br
This produces benzene ring where delocalised ring as hole in it, is positively charged and has Br and a H visibly bonded to it
Also produces AlCl3Br-
Finally forms Bromobenzene + HBr + AlCl3

Question 22

What do Friedel craft reactions form?

Answer 22

C-C bonds

Question 23

How do Fridel craft alkylation reactions happen?

Answer 23

Puts on any alkyl group onto a benzene ring using a haloalkane and a halogen carrier

Question 24

General formula for an alkylation Fridel craft reaction?

Answer 24

C6H6 + R-X =(AlCl3 catalyst + reflux) C6H5R + HX

R= alkane part of halogen
X= Halogen

Question 25

What do Friedel craft acylation reactions do?

Answer 25

Substitutes an acyl group for an H atom on Benzene, refluxing acly chloride with benzene forming either a ketone or an aldehyde if R only = H

Question 26

General formula for an acylation reaction?

Answer 26

C6H6 + RCOCl = ( AlCl3 catalsyt + Relfux) C6H5COR + HCl

COR = Carbon doubled bonded to Oxygen, then bonded to a Carbon Chain (R)

Question 27

How does Nitric Acid act as an Electrophile with a sulfuric acid catalsyt?

Answer 27

Nitronium ion is formed
HNO3 + H2SO4 = H2NO3+ + HSO4-
H2NO3+ = NO2+ +H2O

Question 28

How to draw Nitric acid reacting with Benzene with a sulfuric acid catalyst?

Answer 28

Benzene ring with curly Arrow going to NO2 + (nitronium ion)
Delocalised ring broken, Benzene also has + charge
NO2 and H bonded to ring
Nitrobenzene formed, H+ ion reacts with HSO4 - to reform the catalyst

Question 29

How do you only allow one NO2 group to be added (mononitration)?

Answer 29

Keep temperature bellow 55 degrees

Question 30

What’s a phenol?

Answer 30

Benzene bonded to an OH group

Question 31

Why is Phenol more reactive than Benzene?

Answer 31

-OH group means Phenol is more likely to undergo electrophillic substiution than Benzene
One of the lone pairs of electrons in the p-Orbital overlaps with the delocalised ring of electrons in the Benzene ring
So the lone pair of electrons from the Oxygen atom is partially delocalised into the π system
This increases the electron density of the ring making it more likely to be attacked by electrophiles

Question 32

How do functional groups affect the position of substitution?

Answer 32

In an unsubstituted ring all Carbons just as likely as e- density the same everywhere

Electron donating groups (OH and NH2), direct substiution to Carbons 2, 4 and 6, as orbitals overlap increasing the e- density specifically at these Carbons

Electron withdrawing groups (NO2) direct substiution to Carbons 3 and 5, has no overlapping orbitals and it’s electronegative so withdraws e- density from ring, most specifically on carbons 2, 4 and 6

Question 33

What do you get if you react phenol with Bromine water?

Answer 33

2,4,6-tribromophenol is formed aswell as 3HBr

Bromine water is decoulourised
Insoluble in water and precipitates out of the mixture, smells of antiseptic

Question 34

What’s produced when Phenol is Nitrated with Dilute Nitric acid?

Answer 34

2-nitrophenol or 4-nitrophenol and water

Question 35

Why does phenol react with bases and sodium to form salts?

Answer 35

It’s a weakly acid

Question 36

What happens if you react Phenol with NaOH?

Answer 36

Sodium phenoxide (benezene ring bonded to O- which is bonded to Na+) is formed and water

Question 37

What functional group do acyl’s contain?

Answer 37

C=O