Reactions of Benzene

Electrophilic substitution

  – electrophilic substitution by nitration

• The electrophlie is attracted to an electron rich ring, and the electrophile accepts a pair of the pi-electrons from the delocalised ring to form a covalent bond.
• The H is substituted by an electrophile, and the delocalised pi-electron cloud has been disrupted and the intermediate is less stable.
• The unstable intermediate repidly loses the hygrogen as a H⁺

Hydrogenation of benzene

Alkylation of benzene

AlCl₃ acts as catalyst as it causes the Cl₂ to split through heterolytic fission. Once the H⁺ is substituted, it takes the extra Cl molecule forming HCl.

Acylation of benzene

Sulfonation of benzene

A2 Chemistry – All about Benzene ring!

Discovery of Benzene C₆H₆

                                 

Properties of Benzene
Benzene is a clear, colourless, aromatic hydrocarbon which has a characteristic sickly, sweet odor. It is both volatile and flammable.

Benzene is highly stable, but it does react with electrophiles in substitution reaction.

• What is special about Benzene?

1. Benzene has bond angle of 120°, because it has exactly same bond length (whether it is double bond or not). And it has planar shape because that is the only way that the p orbitals can overlap sideways to give the delocalised pi system.
2. The electrons are delocalised, so Benzene is more stable than normal cycloalkenes.
3. Benzene is wrtien like this –

because the double bonds keep on swaping.

Kekule Structure
August Kekule was a German scientist who suggested the first sensible structure for benzene. The reason why it has snakes around the structure in the diagram above is because Kekule came up with the ‘ring’ idea after dreaming about snakes going around the elements.

• Problems with Kekule struture:

1. Benzene has three double bonds, so the reaction it undergoes should be similar to ethene. But instead of addition reaction, benzene undergoes substitution reaction.
2. The bond lengths are same in Benzene – but the length of C-C is 0.154nm and C=C is 0.134.
3. The theoratical enthalpy change of hydrogenating is 360kJ mol^-1, but the actual enthalpy change of hydrogenating of Benzene is 208 kJ mol^-1. This is less than the enthalpy change of hydrogenating of cyclohex-di-ene.

The Boltzmann Distribution

For reaction to take place, 3 things must happen –

1. A collision must occur.
2. Both particles must have enough energy.
3. The particles must have the correct orientation.

The rate of reaction can be increased by –

1. increasing the concentration/pressure, temperature and surface area as the number of collision increases.
2. increasing the temperature as more particles have enough activation energy.
3. adding a catalyst as more particles have the correct orientation/the activation energy reduces.

The Boltzmann distribution is the distribution of energies of molecules at a particular temperature which can be often shown as graph.

By increasing the temperature, the number of molecules that have activation energy increases –

And by adding the catalyst, the activation energy reduces, therefore the number of particles that can react increases more greatly.

C2 3.1 – 2 Definitions

Enthalpy means thr energy content that is stored in a chemical system.

Exothermic refers to a reaction in which the enthalpy of the products is smaller than the enthalpy of the reactants, resulting in heat loss to the surroundings.

Endothermic refers to a reaction in which the enthalpy of the products is bigger than the enthalpy of the reactants, resulting in heat being taken from the surroundings.

(Enthalpy profile diagram of exothermic)

Standard conditions are a pressure of 100kPa (1 atmosphere), a stated temperature, usually 298K (25°C) and a concentration of 1.0mol (for aq solutions).

Standard state is the physucal state of a substance under the standard conditions of 100 kPa and 298K.

Enthalpy change means the exchange of energy between a reaction mixture (system) and its surrounding when the reaction takes place at constant pressure.

Enthalpy change of combustion is the enthalpy change when one mole of a compound burns completely in oxygen. The compound and the products of burning must be in their normal stable states.

Enthalpy change of formation is the enthalpy change when one mole of compound forms from the elements in their normal stable states.

Enthalpy change of reaction is the enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in their standard states.

C2 – Alcohol

There are three different types of Alcohol molecules –
1. Primary Alcohol; is when the -OH is at the end of the chain.

2. Secondary Alcohol; is when the -OH group is attached to a carbon atom bonded to two alkyl groups.

3. Tertiary Alcohol; is when the -OH group is attached to a carbon atom bonded to three alkyl groups.

Combustion and Oxidation of alcohols

In a good supply of oxygen, alcohols burn completely to form carbon dioxide and water –
C₂H₅OH(l) + 3O₂ – 2CO₂(g) + 3H₂O(l)

Oxidation of alcohols

• Primary alcohol 

 

Figure 1 shows how alcohol forms aldehyde under distillation*.

Figure 2 shows how alcohol froms carboxylic acid under reflux**.

**Reflux is the continual boiling and condensing of a reaction mixture to ensure that the reaction takes place without the contents of the flask boiling dry.

*Distillation apparatus

 

**Reflux apparatus

 

• Secondary alcohol

 

Secondary alcohols are oxidised by acidified dichromate ions to produce ketones.

• Tertiary alcohol 

Tertiary alcohols are resistant to oxidation as the carbon atom is already bonded to three other alkyl groups.

Reactions of alkenes

Alkenes typically take part in electrophilic addition reactions.

1.

In this reaction shown above – there are delta charges in Bromine molecule, and delta positive bromine atom is attracted to the double bond of Carbons (as there are 4 electrons in the double bond). So the Bromine molecule is added across the double bond.

2.

The negatively charged Bromine molecule is then attracted by the positively charged Carbon, so the Bromine molecule is added to form dibromoethane.

Substitution reaction in Alkane

Subsritution reaction has three strps –

1. Initiation

In the reaction shown above, bromine is initiated under UV rays forming Bromine radicals.

2. Propagation

There are two steps in propagation;
-Firstly, Ethane reacts with Bromine radical. A single C-H bond is broken by homolytic fission, forming a ethyl radical and HBr.
-Secondly, the ethyl radical reacts with a bromine molecule. The organic product, bromoethane is formed together with a further Bromine radical.

3. Termination

In termination stage, two radicals combine together to form a molecule, and there are three possibilities as shown above.

Organic reagents and their reactions

• Organic reactions – in a chemical reaction, bonds can be broken in two different ways:

 1. Homolytic fission : one of the bonded atom takes one of the shaired pair of electrons forming a radical.

2. Heterolytic fission : one of the bonded atoms takes both of the shared pair of electrons forming ions.

• Nucleophile is an atom  that is attracted to an electron-deficient centre or atom, where it donates a pair of electrons to form a new covalent bond.

• Electrophile is an atom that is attracted to an electron-rich centre or atom, where it accepts a pair of electrons to form a new covalent bond.

 Examples: Br₂, HBr, NO₂

• Addition reactions : two reactants combine together to make one product.

 

• Substitution reactions : an atom is replaced with a different atom.

 

• Elimination reactions : one reactant reacts to form two products.

 

 

Functional Groups

Functional Groups

Name	Displayed formulae	Suffix	General Reaction
Alkane		-ane	Combustion, Cracking
Alkene		-ene	Addition, Polymerisation
Alcohol		-x-ol	Mild oxidation, Elimination
Aldehyde		-al	Oxidation/Mild reduction
Carboxylic Acid		-oic acid	Typical acid reaction
Ketone		x-one	Good Solvent
Halogenoalkane	etc.	Flouro- Chloro- Bromo- Iodo-

Naming organic compounds

Naming alkanes

Rules
1.pick the longest Carbon chain – it doesnt matter if it is not necessarily in one straight line.

2.Number from the end, so the sum of the substituent group is smallest – for exmple,

In 2-methylbutane, you could count from left to right because it will make the number smallest.

3. Substituents should be arranged alohabetically – here is another example –

This is 3-ethyl-2,3,2-trimethylheptane. There are ethyl and methyl, and because it should be arranged alphabetically, ethyl goes first and trimethyl follows.

Naming alkenes
Alkene is an unsaturated hydrocarbon with at least one carbon-carbon double bond, therefore has another level od isomerism as the double bond cannot rotate.

Alkenes are named in very similar way as alkanes are named, but the main differences are –
* The suffix is -ene.
* The position of the double bond has to be stated, and again, the sum of the numbers has to be as small as possible.
* The type of isomers has to be stated – either E isomer or Z isomer.

Example:

– There are four carbons in the longest chain, so but-.
– There is a double bond between carbons 1- and 2.
– The compounds is an alkene, so the suffix is -ene.
– The isomer is E (opposite).
– Therefore the name of this compound is e-but-1-ene.