Benzene and its compounds

Benzene

Benzene is a hexagonal and planar molecule with the formula C6H6.

It is found in medicines and organic hydrocarbon with benzene rings are called arenes/ aromatic compounds/Aryl

Characteristics

It is planar
The C=C bond is shorter than the C-C bond, thus the diagram above would be distorted. That is why the picture on the right is used.
Double bonds are known to react with bromine very fast in electrophilic addition but it is found that benzene requires harsher conditions to react with bromine during electrophilic addition
The circle inside the hexagon is made up of moving delocalized electron that produces an electron cloud on the top and bottom of the hexagon giving benzene a relatively stable structure
Each carbon atoms contribute one electron to a pi bond and delocalised over all the six carbon atoms in the hexagonal ring from the P orbital
Each carbon in benzene is SP2 hybridised
Each carbon forms 3 sigma bonds
Bond angle is 120 since it is a planar compound

Difference between the pie bond of benzene and hex-3-ene

In benzene, the six electrons in the pie bonding system are no longer associated with any carbon atoms in the molecule, while in hex-3-ene the two electrons in the pie bond in the centre of the molecule are only found above or below the two carbon atoms

Naming Aryl compounds

Reaction of arenes

The delocalized pi bonding electrons remain intact for most reactions
Initial attack is done by electrophile, which is attracted to the high density around the benzene ring

Electrophilic substitution

Reagent : Chlorine/bromine gas, Cl2/Br2

Condition : Anhydrous Aluminium chloride, AlCl3 or iron(III) chloride, FeBr3 ,heated

Product : Halobenzene

Benzene reacts with chlorine/bromine in the presence of halogen carriers such as aluminium chloride, AlCl3 to form chloro- or bromobenzene, respectively

The halogen carriers react with the catalyst to form nucleophile and electrophile

Benzene is the attracted towards the nucleophile and forms an intermediate where it will react with FeBr4- and form the catalyst again. The end products are the benzene compounds and HN. N=halogen

In excess condition,

When methylbenzene or other alkylarenes are halogenated, the halogens are substituted in position 2 or 4 since Br and Cl are electron releasing
These positions are activated by any electron donating-groups bonded directly to benzene ring. Other compounds are phenols and phenylamine

Bond length

The carbon-halogen bond in halogenoarenes are stronger than equivalent bond in halogenoalkanes
This is because one of the lone pairs on the halogen atom overlaps slightly with the pi bonding system of the benzene ring
Thus the carbon-halogen bond is given a partial double bond character

Free radical substitution

Boiling methylbenzene and chlorine gas under the presence of UV light since it has a partial double bond character
No substitutions into the benzene ring
If excess, all the three hydrogen atoms in the methyl will be substituted

Nitration

Reagent : Concentrated nitric acid, HNO3

Condition : Reflux at a temperature of 55 ºC and the presence of concentrated sulfuric acid, H2SO4 as catalyst

Product : Nitrobenzene

Nitration happens when one or more hydrogen atoms in benzene is replaced by a nitro group, NO2.
Benzene reacts with nitric acid in the presence of concentrated sulfuric acid to give nitrobenzene

The mechanism of this reaction is electrophilic substitution.The electrophile, nitronium ion, NO2⁺ is formed by the reaction of nitric acid and sulfuric acid.

As before, the electrophile is attracted to the benzene ring and forms a bond with it. The delocalised electron system is partially broken.

Hydrogen ion is expelled and it bonds with HSO4⁻ to regenerate the catalyst. The delocalised electron system is restored. Electron is taken in by NO2 since it is a electron withdrawing compound and thus an electron is taken from hydrogen and hydrogen ion is the expelled.

If the temperature exceeds 50 ºC, 1,3-dinitrobenzene will be formed as well. Notice that the second nitro group is added to the 3 position of the ring

Mechanism of nitration

Stage 1-the electrophile is attracted to the high density of the pi bonding of the benzene
A pair of electrons from benzene ring is donated and forms a covalent bond
The delocalised ring is distrupted
Now we have four pi bonding electrons and a positive charge spread over five carbon atoms
Stage 2-full delocalised ring is restored
C-H bond breaks heterolytically
Both electrons go into the pi system
Further nitration produces 1,3-dinitrobenzene and 1,3,5- trinitrobenzene
Because NO2 is electron withdrawing and activates 3 or 5 positions

Electron releasing and withdrawal compounds

Alkylation and acylation

Reagent : Halogenoalkane, CH3CH2Cl or acyl halide, CH3COCl

Condition : Presence of catalyst, AlCl3, rtp

Product : Alkylbenzene and ketone

The reaction involve attack on the benzene ring by an electrophile carrying a positive charge on a carbon atom, i.e. a carbocation
The electrophile can be formed by adding AlCl3 catalyst to a halogenoalkane

Oxidation

Reagent : Alkaline potassium manganate(VII), KMnO4 then dilute H2SO4

Condition : Heat or reflux

Product : Benzoic acid

When methylbenzene is heated under reflux with acidified potassium manganate(VII), side-chain oxidation occurs. Benzoic acid is produced.

Any carbon side-chain group is oxidised to -COOH group under these conditions.

Can be used to introduce acyl group (carbonyl C=O and alkyl)

• Friedel-Crafts reactions result in the introduction of side chain into the benzene ring (also known as alkylation or acylation reactions)

Result is ketone

Hydrogenation

Reagent : Hydrogen gas, H2

Condition : Heat in the presence of nickel, Ni catalyst at 140 °C and high pressure

Product : Cyclohexanes

In hydrogenation, hydrogen atoms are added to the benzene ring. The delocalised electron system is permanently broken.

Phenol

Phenols are benzene compounds which have an -OH group attached directly to it.
In a phenol molecule, one of the lone pairs on the oxygen overlaps with the delocalised electron system to give a structure like this:

This increases the electron density of the delocalised electron system. It makes phenols much more reactive than benzene itself. Also, it increases the acidity of phenol as well.

Physical properties

Phenol has a higher melting and boiling points than methylbenzene. This is because phenols can form hydrogen bonds between them in addition of van der Waals' forces and permanent dipole-dipole forces.
Phenol itself is more soluble in water than other phenols. This is because a small phenol molecule can form more effective hydrogen bonds with water molecules. However, most phenols are generally insoluble in water.

Acidity of phenols

In a phenol molecule, one of the lone pairs on the oxygen overlaps with the delocalised electron system.Therefore the H ion are not strongly attracted to the phenoxide ion making it more to form dissociated molecules Or we can say phenol ionizes to form a more stable negative ion,so the ionization of phenol is more likely .This is because phenol can donate a proton to form a phenoxide ion. The presence of hydroxonium ions makes it acidic.

Comparison with other organic acids

The strength of organic acids depends on:

The strength of the O-H bond which is to be broken
The stability of the anion formed.

The strengths of the acids are as follow. The lower the value of pKa, the stronger the acid is Ethanoic acid>Phenol>Water>ethanol
Ethanoic acid is the strongest because of the stability of ethanoate ion formed. In an ethanoate ion, the negative charge is spread throughout the -COO group. This delocalisation of electron and negative charge stabilises it to a greater extent.
Phenol is a weaker acid. This is because electron delocalisation of the phenoxide ion is not as great as in ethanoate ion. Although the delocalised ring electrons are involved, the electrons are still heavily distorted towards the one electronegative oxygen atom rather than two in the ethanoate ion case.
If an electron-withdrawing group is attached to phenol, its acidity increases. This is because the electron-withdrawing group can attract electrons away from the oxygen, stabilising the phenoxide ion formed.
If an electron-donating group is attached to phenol, its acidity decreases. This is because the electron-donating group increases the electron density in the benzene ring, intensifying the charge on oxygen atom.
Ethanol is weaker acid than water because the alkyl group (electron donating) is attached to ethoxide ion. This results in concentrating more negative charge on the oxygen atom. It accepts hydrogen ions more and favours undissociated ethanol molecules
On the other hand, water is a stronger acid than ethanol but weaker than phenol. This is because in ethoxide ion, the presence of an alkyl group intensifies the negative charge on the oxygen atom. In a hydroxide ion, no such thing happens.

Preparation of phenols

Reagent : Phenylamine and nitrous acis/HNO2

Condition : Sodium nitrate/NaNo2 and Hydrochloric acid/HCl for HNO2 in 5°C

Product : Phenol, nitrogen gas and dilute HCl

HNo2 is very unstable so we react NaNO2 with HCl to form it

We use 5°C to prevent decomposition to give N2

Heat with water

Diazotisation

Reagent : Diazonium ion

Condition : Alkaline solution of phenol,NaOH

Product : Orang dye

2nd step after Preparation of phenols, Coupling reaction

Diazonium ion acts as an electrophile

Reaction with sodium metal

Reagent : Sodium metal, Na

Condition : Room temperature

Product : Alkoxides and hydrogen gas

Like alcohols, phenol will react with a reactive metal such as sodium to give sodium phenoxide and hydrogen gas.

The observation is that the sodium sinks and bubbles of hydrogen gas is produced. This reaction is more vigorous than the one with alcohol because phenol is more acidic.

Reaction with sodium hydroxide,NaOH

Reagent : Sodium hydroxide, NaOH solution

Condition : Room temperature

Product : Alkoxides and water

Phenol is a strong enough acid to react with sodium hydroxide solution to give sodium phenoxide and water.

Since alcohols will not react with sodium hydroxide, this can be used as a test to distinguish alcohols from phenols. However, phenol will not react with sodium carbonate and sodium hydrogencarbonate because it's not acidic enough to react with these.

Halogenation

Reagent : Chlorine gas/bromine water

Condition : Room temperature

Product : 2,4,6-trihalophenol

Phenol will react with halogens even without the presence of halogen carriers. This proves that phenol is more reactive than benzene itself due to the increase in the electron density

The observations are:

The reddish-brown of bromine decolourises
A white precipitate is formed, this is 2,4,6-tribromophenol
Steamy fumes of hydrogen bromide is observed.

Nitration

Reagent : Dilute Nitric acid, HNO3

Condition : Room temperature

Product : Nitrophenols

Unlike benzenes, concentrated sulfuric acid is not needed for nitration to occur. This proves that phenol is more reactive than benzene itself.
With dilute nitric acid, mono-substituion occurs. 2-nitrophenol and 4-nitrophenol is produced.

With concentrated nitric acid, tri-substituion occurs. 2,4,6-trinitrophenol is produced.

Tri-iodomethane test for alcohols

This is a test used to identify the presence of CH3CH(OH)- group in an alcohol. The R can be a hydrogen or an alkyl group.
Iodine solution is added to a small amount of an alcohol, followed by just enough sodium hydroxide solution to remove the colour of the iodine. ii. If the alcohol contains the CH3CH(OH)- group, then a pale yellow precipitate of tri-iodomethane, CHI3 is produced.
Ethanol is the only primary alcohol to give the tri-iodomethane (iodoform) reaction.
If R is a hydrocarbon group, then you have a secondary alcohol. Lots of secondary alcohols give this reaction, but those that do all have a methyl group attached to the carbon with the -OH group.
No tertiary alcohols can contain this group because no tertiary alcohols can have a hydrogen atom attached to the carbon with the -OH group. No tertiary alcohols give the triiodomethane (iodoform) reaction.

Tri-iodinemethane test for alcohols

This is a test used to identify the presence of CH3CO- group in carbonyl compounds. The R can be a hydrogen or an alkyl group.
Iodine solution is added to a small amount of an alcohol, followed by just enough sodium hydroxide solution to remove the colour of the iodine.
If the alcohol contains the CH3CH(OH)- group, then a pale yellow precipitate of tri-iodomethane, CHI3 is produced.
Ethanal is the only aldehyde to give the triiodomethane(iodoform) reaction.
If R is a hydrocarbon group, then you have a ketone. Lots of ketones give this reaction, but those that do all have a methyl group on one side of the C=O bond.

Naphthol

Reacts with:

NaOH
Na
Br2
HNO3

Naphthol + NaOH

Naphthol + Na

1-Naphthol + Br2

2-Naphthol + Br2

1-Naphthol + HNO3

2-Naphthol + HNO3

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