Introduction to hydrocarbon benzene 

Back to: o level organic chemistry notes

In the last chapter, we considered unsaturated compounds-Alkenes and Alkynes. There is another important class of unsaturated compounds that needs to be considered separately because their properties are so different from those of alkenes and alkynes.

This class of compounds is called the Aromatic hydrocarbon and its simplest member is Benzene.
The name Aromatic was originally used because some derivatives of these hydrocarbons have pleasant smells. Its is now known that just many of them have unpleasant and in any case the aromatic vapours are toxic
Therefore the name Aromatic has been retained to indicate certain chemical characteristics rather than odorous properties.

The structure of Benzene

Since 1834, molecular mass determination generated the molecular formula of benzene as C6H6. The
exact structural formula, however posed a problem for many years.
With such a high ratio of C: H, benzene must be a highly unsaturated with a possible structure of hexatetraene e.g.

However, benzene does not undergo addition reactions across the double bond as alkenes therefore Kekule (1865) proposed a ring structure in which alternate carbon atoms were joined by double bond as:

Bond lengths
X-ray diffraction studies show that benzene is planar and all the C-C bonds are the same.
Some of the bond lengths are:
C-C bond length in all bonds in benzene, 0.139nm
C-C bond length in cyclohexane, 0.154nm
C-C bond length in cyclohexene, 0.133nm
Kekule model would suggest unequal carbon-carbon lengths, alternating between double and single
bond values. In fact the constant bond length is found between single and double bond.
Question: Explain the variation in the bond length between cyclohexane, cyclohexene and benzene.
Solution: From Molecular orbital terms, Benzene is a planar molecular with all the six hydrogen and six carbon atoms lying in the same plane. Each Carbon atom is sp2 hybridized and explains why structure 3 is preferred.
Also, the variation can be attributed to resonance energy which indicates than benzene is more stable than cyclohexene and all carbon –carbon bond lengths in benzene are equal.

Thermochemistry of Benzene

Benzene is more stable than expected and this is illustrated using the enthalpy of hydrogenation to
cyclohexane. i.e. The enthalpy of hydrogenation of cyclohexene is -119KJ/mol

Considering Benzene as Cyclohexa-1, 3, 5-triene which has three double bonds then, we expect its
enthalpy of hydrogenation to be = 3 (-119) = -357KJ/mol.
However, the experimental value observed was = -207KJ/mol.
The difference in the values (-357 + 207) = -150KJ/mol and it indicates that Benzene is more stable than
cyclohexa-1, 3, 5-triene. The difference in the values is known as Resonance or Delocalization energy

Sources of Benzene

Benzene is obtained from petroleum oil by fractional distillation.
Synthetic Preparation
Owing to the great abundance of Benzene from Industrial sources, it is rarely prepared but may be
prepared by the following methods.

1. Decarboxylation of Sodium Salts

2. Reduction of Phenol

Polymerization of Ethyne

Physical properties Of Benzene

Benzene is a colourless liquid with a characteristic Aromatic smell. It is immiscible with water but highly soluble in organic solvents as well as being used as solvent itself. Like all Aromatic compounds, it burns with a sooty flame owing to its relatively high carbon content

Reactions of benzene
The availability of the pie electrons in benzene serves as a source of electrons which are donated to electron deficient species (Electrophiles). The reactions therefore are characterized by Electrophilic substitution reactions which differ from Addition reactions of Alkenes.

1. Nitration reaction
   This is the reaction of Benzene with nitrating mixture containing equal molar quantities of concentrated Nitric acid and sulphuric acid. In this reaction, the hydrogen atom is substituted by a nitro group to generate Nitrobenzene.
   Concentrated Sulphuric acid is used to protonate weaker Nitric acid such that Nitroniumcation is
   produced.

An example of an azo-dye is 4-hydroxyazobenzene which is orange solid synthesized from Nitrobenzene as shown below.

Mechanism

Note. The overall reaction is reversible with the equilibrium tending to the left. BenzeneSulphonic acid is used in the manufacture of phenol.i.e.

3. Halogenation
   Substitution by chlorine or bromine takes place readily at room temperature in presence of halogen carriers’ e.g iron, aluminium chloride or iodine. Iodine does not easily substitute.
   The halogen carrier functions as a Lewis acid inducing a certain degree of polarity in the halogen
   molecule.

Mechanism

The Haloalkanes can be replaced by other compounds such as Alkenes and Alcohols e.g

Note. The alkyl group can be oxidized to carboxylic acid by Acidified KMnO4 or dilute nitric acid

5. Friedel-Crafts Acylation
   Acid Halides react with Benzene in presence of Aluminium Chloride to generate Aromatic Ketones.

Mechanism

Note. Acid anhydrides i.e. (RCO)2O are generally preferred because they are easily obtained in a state of high purity, is more easily handled and resulting Ketones is more easily separated by distillation Other Important Addition reactions of Benzene include:

1. with Hydrogen

2. with Chlorine

Like Alkanes, Benzene undergo Addition reaction with chlorine in presence of sunlight or Ultraviolet radiation to form 1, 2, 3, 4, 5, 6-hexacyclohexane.

Position of Substitution in Benzene Derivatives

When a second atom or group of atoms is attached to monosubstituted benzene, the position of substitution is determined by the nature of the substituent already attached.

In fact, any substituent group attached to a benzene ring affects the rate and position at which further substitution occurs.

The position of substitution of the second substituent is governed largely by whether or not the atom
or group already present is donating electrons to the ring or withdrawing electrons from it.

Electron-donating groups enhance the availability of the pie electrons hence it activates the ring generating products at position 2-(ortho) and 4-(Para).

Examples of electron-donating groups include:

Example of substitution reaction with electron-donating group includes.

Presence of halogens on the benzene ring deactivates the ring to products at positions 2- or 4- on substitution.
Electron- withdrawing groups reduce the availability of the pie-electrons hence deactivate the ring therefore on substitution position 3- preferred.

Examples of electron-withdrawing groups include:

Example includes:

Question
(a) Complete the following reactions and in each case write the acceptable mechanism;

(b) How would distinguish between the following members of the pairs of compounds. In each case,state what would be observed when the reagent is treated with each member of the series.

(c) Write equation(s) to show how the following compounds can be synthesized. In each case , indicate the condition(s)/reagent necessary.

HALOHYDROCARBONS

Organic halogen compounds are halogen derivatives of alkanes. Monohalo derivatives (Alkyl halides) have a general molecular formula CnH2n+1 X (X= Cl, Br and I). Fluorine is excluded because C-F bond is strong due to high electronegativity therefore C-F bond is unreactive. Consequently fluorocarbon compounds are extremely inert and non-flammable.

Alkyl halides are divided into three classes depending on the number of the alkyl groups attached to the carbon atom bonded to the halogen i.e.

Aryl halides are halogen compounds in which the halogen atom is directly attached to the aromatic ring (Benzene). They are generally represented as ArX e.g.

If two halogen atoms are attached to adjacent carbon atom, the alkyl dihalides is known as Vicinal Dihalides while if are attached to the same carbon then it is known as germinal dihalides.
Note. Vinyl halides are compounds in which halogen is attached to a doubly bonded carbon e.g chloroethene while allyl halides are unsaturated halogen compounds.

Nomenclature
Organic halogen compounds are named using the prefixes fluoro, chloro, bromo or iodo-. Numbers are used to indicate the position of the halogen atom in the carbon containing molecule.

Example
Name the following organic compounds

Solution:
(a) Bromoethane (b) 2-iodobutane (c) 2-chloro-2-methylpropane (d) Chloromethylbenzene
(e) 4-iodomethylbenzene

Synthetic Preparation

For Alkyl halides:

1. From Alkenes

(See Alkenes for mechanism and detailed position for addition product)

2.From Alcohols

From Alkanes

Substitution with halogens in presence of sunlight or ultraviolet light is not suitable method since it
generates a mixture of products.
For Aryl Halides

1. From Diazonium salt
   The salt is warmed with appropriate copper (I) halide e.g.

2. Direct Halogenation of Benzene

(See Benzene for Mechanism and other halogen and Halogen carriers)

Physical Properties

Because of the polarity of the carbon-halogen bond due to electronegativity difference which enhances dipole-dipole interaction in liquid phase, Haloalkanes have a higher boiling point than alkanes of comparable molecular mass.

Most Haloalkanes are liquids (except Iodomethane) and their boiling points depend on the alkyl group and halogen atom. Fluoroethane and chloroethane are gases.

For corresponding halogen atoms, boiling points increases with increasing molecular mass. Densities decreases in the same order due to close packing of smaller molecules in the liquid phase.

Haloalkanes and Aryl halides are both insoluble in water but soluble in organic solvent due to inability to form hydrogen bonds with water.

Reactions

The Haloalkanes are fairly reactive compounds due to polarity of the Carbon-halogen bonds therefore the electron deficient carbon atom is susceptible to attack by electron rich species hence their reactions are characterized by Nucleophilic substitution reactions.

Some of the Nucleophilic substitution reactions include:

1.Ether Formation

This reaction provides the suitable method for preparation of alkoaromatics and alkoalkanes.

Mechanism

The reaction cannot be used to generate phenyl esters.
Note. A solution of Silver ethanoate in ethanol can be used to distinguish Haloalkanes from Acryl halides.

Amine Formation

When concentrated Ammonia solution is heated when Haloalkane in a sealed tube, different classes of amines are generated.

To generate Primary amine, excess ammonia is used. The reaction is limited to haloalkanes although aryl halides react if electron-withdrawing groups are present in 2- or 4- positions.

4. Cyanide (Nitrile) Formation
   When heated, haloalkanes react with potassium cyanide in an alcohol to generate alkanenitrile.

Example

Mechanism

Alkanenitrile are usually used to generate carboxylic acid of higher carbon chain. i.e.

Example

5. Higher Alkyne formation
   Haloalkanes react with sodium salt of acidic alkyne to form higher non-terminal alkynes.

Both haloalkanes and Aryl halides react with Magnesium in presence of dry ether to generate. Grignard reagent.

The reagent is widely used to generate alkanes, alkenes, alkynes, alcohols, Aldehydes, Ketones and carboxylic acid.

For Alkanes:

For Alcohols:

Example:

Example
Write equations showing how Propene can be converted to Butan-2-ol.

Example

Note. Aqueous Ammonium salt is used in hydrolysis to generate 3oAlcohol because the acid may dehydration to form Alkene.
Qn: Write equations showing how 2-methylpropan-2-ol can be synthesized from propan-2-ol.

For Ketones

For Carboxylic Acid

Write equations showing how Benzoic acid can be prepared from Bromobenzene

7. Alcohol Formation
   On heating, Alkyl halides react with sodium hydroxide solution on heating to form an alcohol.
   For 3 degree Alkyl halide, the kinetic studies show that it is First order with respect to the alkyl halides consequently the rate determining step involves only the haloalkane therefore the mechanism and reaction is known as Substitution Nucleophilicunimolecular (Sn1)

Mechanism

Qn: Draw a well labelled energy profile diagram for the reaction and State rate law for the reaction
above.
For 1 degree haloalkane alkaline hydrolysis, the kinetic studies show that it is second order reaction therefore the dependent on the concentration of Haloalkane and hydroxide ions.
Consequently the rate determining step involves both species therefore the mechanism and reaction is known

Question: Draw a well labelled energy profile diagram for the reaction and State rate law for the reaction
above.
Secondary haloalkanes show a mixture of both second and first order kinetics with the former predominating.

Alkene Formation or Elimination reaction

The reaction involves removal of halide ion and hydrogen to form the alkene. The hydrogen atom
removed is the one bonded to the carbon atom adjacent to the one bonded to the halogen.

Unimolecular elimination (E1)

This is undergone by 3 degrees haloalkane to generate an alkene in presence of alcoholic potassium
hydroxide and heat.

Qn: Write the mechanism for the reaction

Bimolecular Elimination (E2)
This is undergone by 1ohaloalkane and in the rate determining involves both the haloalkane and base to generate the alkene.

Question: Write the mechanism for the reaction.

9. Reduction reaction
   This can be done by:
   (a) Using Lithium Aluminium hydride in dry ether.

(b)Using sodium metal in dry ether (Wurtz reaction)

ELECTROPHILIC SUBSTITUTION REACTION OF ARYL HALIDES

Aryl halides differ in reaction from haloalkanes in that they do not undergo substitution of the halogen.
This is because the lone pair of electrons on the halogen atom interacts with the delocalized electrons of the benzene ring. This strengthens the carbon-halogen bond or makes the carbon-halogen partly double bonded and difficult to break.

In the Alkyl halides, the carbon-halogen bond is polar because the halogen is a more electronegative atom therefore the carbon atom becomes partial positively charged atom. It is then attacked by electron rich species (Nucleophile) leading to substitution of the halogen atom.

Generally Aryl halides undergo Electrophilic substitution reaction to generate ortho and Para products.
Some of the reactions include:

Reagents used to distinguish Aryl halides from Alkyl halides include:
Hot silver ethanoate in ethanol or hot sodium hydroxide solution and silver nitrate solution
Sample Question:

1. Suggest Explanations for each of the following observations:
   a) Iodobenzene is more reactive than chlorobenzene towards but much less reactive than iodoethane.

b) Tetrachloromethane is much less reactive towards Nucleophiles than chloromethane
c) Iodo-and Bromo-compounds are much more useful as intermediates in synthesis than chlorocompounds.

2. Using equations indicating the required conditions to show how the following compounds can be
   synthesized:

3. (a) Methyl Benzene reacts with chlorine to form Chloromethylbenzene. State the conditions for the
   reaction
   (b) Under different conditions the product is phenyl chloromethane instead of Chloromethylbenzene.
   (i) State these conditions of the reaction
   (ii) Write a plausible mechanism for the reaction.

4. Grignard Reagent Synthesis
   See Reactions of Haloalkanes.

5. Reduction Of Carboxylic acids
   Carboxylic acids are reduced to Primary Alcohols using LiAlH4 in dry ether.

7. Cannizzaro Reaction
   Aromatic Aldehydes without @hydrogen atoms undergo self-reduction and oxidation in sodium hydroxide solution.

The main sources of the starch materials include maize, millet, cassava, sorghum, potatoes, banana, rice,
molasses, etc. The enzymes that participate in the fermentation process and their stages include:

Question:
a) Describe the processes involved in the production of ethanol from a named material domestically
b) State briefly how the purification of the ethanol produced can be done
c) State four uses of ethanol
d) State two effects of over consumption of ethanol on the human body.

Physical Properties of Alcohols

a) Solubility
Lower members of the series are soluble however, solubility decreases as the hydrocarbon portion increases. The miscibility of alcohols is due to ability of the formation of hydrogen bond with water molecule.

b) Boiling Points
Alcohols boil at higher temperatures expected because the molecules associate through hydrogen
bonding which requires an extra energy to break. Within the series, boiling points increase with molecular mass.

c) Density
Density increases with increasing molecular mass although branching reduces the factor. Aliphatic alcohols are less dense than water while Aromatic alcohols tend to be slightly denser than water.

REACTIONS OF ALCOHOLS

1. With Electropositive Metals
   Although Alcohols are neutral to neutralization indicators, the hydroxyl group can be replaced by metals of Group I and II of the periodic table. The products are hydrogen gas and metal Alkoxide.

2. Halogenation
   This can be done by:
   a) Using Hydrogen Halide
   They react with hydrogen halides to form alkyl halides. The order of reactivity for alcohols is 3>2>1 degrees
   while for hydrogen halides is HI>HBr>HCl.

1 degree Alcohols undergo bromination with hydrogen bromide which is generated from sodium bromide and
concentrated sulphuric acid.
In case of hydrogen chloride, the reaction is carried by using anhydrous Zinc chloride and concentrated hydrochloric acid. (Luca’s Reagent)
The reagent is used to distinguish the classes of monools i.e.
3 degrees Alcohol forms a cloudy solution immediately
2 degrees Alcohols forms cloudiness between 5-10minutes
1 degree Alcohol no observable change.

Mechanism

b) Using Phosphorous Halides