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1. Bonding of the carbon atom.
Electronic structure of the carbon atom is 1s2 2s2 2px1 2py1.
The outer electrons are located in orbitals (volumes of electron probability) having the following shapes:
Carbon is unable to take part in ionic bonding since it is not energetically possible to form either C4+ or C4-. It must therefore bond covalently by sharing electrons.
To achieve its maximum valency of 4. The 2s electrons must become uncoupled to give the electronic structure: 2s1 2px1 2py1
During bonding the 2s and 2p orbitals blend together to form four identical orbitals. This process is known as hybridisation.
2. Reactions of covalent bonds
For any organic reaction to take place, covalent bonds must be broken. The factors influencing bond breaking are:
a) Kinetic factors
For bond breaking to occur, the molecules must possess a certain amount of energy (activation energy). Sufficient energy may be obtained through collisions with other molecules and transfer of kinetic energy to the bond.
Increasing the temperature of the reaction increases the number of molecules with energies in excess of the activation energy, a rise of about 10oC doubles the rate of reaction.
b) Equilibrium of reactions
The reaction must have a favourable equilibrium constant.
3. Mechanism of bond breaking and making
A covalent bond can be broken in two ways:
a) Homolytic fission
- A:B → Ao + Bo
Produces free radicals which have an unpaired electron and are very reactive.
b) Heterolytic fission
- A:B → A+ + B-
- A:B → A- + B+
4. Bond polarity
Many organic molecules possess a dipole moment, due to an unequal distribution of electrons in the molecule. This occurs because some atoms tend to attractor repel electrons.
Consider the molecule, CH3Cl.
Chlorine has a greater share of the electrons due to the electronegativity it possesses:
This kind of bond polarisation is known as the inductive effect.
The inductive effect is the power of an atom or group of atoms to attract electrons compared to the power of a hydrogen atom.
NO2 > Cl > I > NH3 > C-H < CH3 C2H5 < (CH3)2CH
Groups which are attracting electrons have a negative inductive (-I) effect. They give rise to electron deficient atoms Cδ+.
Groups which are electron repelling have a positive inductive (+I) effect. They give rise to electron rich carbon atoms Cδ-.
Organic reagents can be classified as either:
1. Nucleophiles: Attack centres of low electron density (nucleus loving). They possess a lone pair of electrons and are usually negatively charged.
Examples include: H2O, ROH, OH-, RO-, Br-, NH3, RNH2, CN-.
2. Electrophiles: Attack centres of high electron density (electron loving) The are capable of accepting a lone pair of electrons and are usually positively charged.
Examples include: H+, Br+, R-N=N, CN-.
The properties of an organic molecule are predominately determined by the properties of the functional group in that compound. Functional groups are atoms or combinations of atoms such as OH-, -COOH.
Once the properties of the functional groups are known then the properties of any molecule containing a functional group maybe predicted.
The common functional groups are listed below:
Homologus series: This is a series of compounds in which all the members are similar in constitution (i.e. they contain the same functional group, if any) and therefore in chemical properties.
Each homologous series has a general formula (e.g. CnH2n+2 for the alkanes) and each member of the series differs from the next by CH2 unit in all cases.
There is a regular change in physical properties as one ascends the series for example: methane - gas, octane - liquid, and higher alkanes - solid.