Investigate The Relationship Between Structure And Properties Of Hydrocarbons - Session 2

Homolytic Cleavage is a type of bond cleavage in which a covalent bond between two atoms breaks symmetrically. Each atom retains one electron from the shared pair. This process results in the formation of two highly reactive species called radicals. For example, the Homolytic Cleavage of covalent bond in chlorine molecule results in the formation of two chlorine radicals. Homolytic cleavage process is represented by a single-headed-arrow.

The Homolytic Cleavage process typically requires energy input to break the bond and initiate the cleavage. This energy can be provided by various means, such as heat, light or the interaction with other reactive species. The energy input disrupts the equilibrium between the attractive forces holding the atoms together and the repulsive forces between the electrons. As a result, the bond is broken, and each atom retains one electron. Can you give any example of the chemical reaction that involves Homolytic Cleavage?.

Heterolytic Cleavage is a type of bond cleavage in which a covalent bond breaks asymmetrically. This results in the formation of two charged species called ions. In this process, one of the bonded atoms retains both electrons from the shared pair. It becomes a negatively charged specie called anion. The other atom becomes electron deficient. It becomes a positively charged specie called cation. For example, the bond in hydrogen chloride molecule breaks asymmetrically to produce Hydrogen ion and Chloride ion.

Catalytic Hydrogenation of Alkenes is a chemical reaction in which hydrogen gas reacts with an alkene in the presence of a catalyst to form alkane. This process involves the addition of hydrogen atoms across the double bond of the alkene. This results in the conversion of the double bond to a single bond. The catalyst commonly used for the hydrogenation of alkenes is typically a transition metal, such as platinum palladium, or nickel. For example, catalytic hydrogenation of ethene results in the formation of ethane.

Catalytic hydrogenation of alkynes is a chemical reaction in which hydrogen gas reacts with an alkyne in the presence of a catalyst to form an alkane. This process involves the addition of hydrogen atoms across the triple bond of the alkyne. This results in the conversion of the unsaturated bond to a single bond. The catalyst commonly used for the hydrogenation of alkynes is typically a metal catalyst, such as palladium, platinum or nickel. For example, catalytic hydrogenation of ethyne results in the formation of ethane.

The nature of bonding in benzene can be explained by considering the Kekule Structure and the concept of pi electron delocalization. The Kekule Structure is an early model proposed by Friedrich August Kekule to describe the structure of benzene. It suggests that benzene is a cyclic molecule with alternating single and double bonds between Carbon atoms.

In kekule structure of benzene, each carbon atom is connected to one hydrogen atom and two neighboring carbon atoms by a sigma bond. The remaining one pi bond is formed by the carbon atom with the neighboring carbon atom. This shows that each of the carbon atoms in benzene is sp² hybridized.

Kekule presented that benzene could exist in two forms or structures. He referred these structures as resonance structures. In one structure, the alternating single and double bonds are arranged in a hexagonal ring. In other structure, the positions of the single and double bonds are reversed. The idea of Kekule was that the actual structure of benzene is not described by either of these resonance structures alone. It is an intermediate or hybrid of the two structures.

According to the Kekule structure, the carbon carbon single bonds and double bonds in benzene are distinct. It says that in benzene, carbon carbon single bond length is 154pm. Bond length of double bonded carbon atoms is 133pm. However, experimental data reveals that all carbon carbon bond lengths in benzene are equal. All carbon carbon bond lengths in benzene are 138pm. This contradicts the alternating bond lengths presented by the Kekule Structure.

Now we shall compare the value of Standard Enthalpy of Hydrogenation of Kekule structure of benzene with observed value. As we know, 1-cyclohexatriene has one double bond. Standard enthalpy of hydrogenation of 1-cyclohexatriene is -120 kj/mol. Kekule Structure says that benzene possesses three double bonds. So the standard enthalpy of hydrogenation for three double bonds will be -360 kj/mol.

Meanwhile experimental results say that the standard enthalpy of hydrogenation of benzene is -208 kj/mol. This data shows that standard enthalpy of hydrogenation of benzene is less than standard enthalpy of hydrogenation of Kekule Structure of benzene. This shows that benzene is stable than its Kekule Structure by amount of one five two kilo joule per mole. This means benzene does not undergo addition reactions easily as compared to other alkenes.

We can conclude that kekule structure does not correctly describe the structure of benzene. This is because benzene is more stable than its Kekule Structure. The structure of benzene is correctly described by pi electron delocalization. As we already know that all of the carbon atoms in benzene are sp². This means that each carbon in benzene has unhybridized p orbital. So there are total of six unhybridized p orbitals in benzene. These unhybridized p orbitals are perpendicular to the carbon carbon sigma bonds in benzene.

Electrons in these unhybridized p orbitals are called pi electrons. The delocalized pi electrons in benzene are spread out over the entire ring. They are not localized between any specific carbon atoms. This delocalization gives benzene its unique stability and reactivity compared to other unsaturated hydrocarbons. The circle in the given structure of benzene represents the delocalization of pi electrons all over the benzene ring.

As we know, in alkynes there are triple bonds between carbon atoms. For example, ethyne has triple bond between two carbon atoms. These carbon atoms are s p hybridized. One of the triple bond is sigma bond. Other two are pi bonds. Due to presence of one sigma bond and two pi bonds, electron density is concentrated between two carbon atoms. As a result the hydrogen atom is loosely attached with the carbon atoms.

When alkynes are treated with a strong base, the hydrogen atom attached to s p hybridized carbon atom is displaced by the metal ion of the strong base. This shows that alkynes demonstrate acidic behavior because they can donate the hydrogen ion. For example, When ethyne is treated with sodium amide, the hydrogen atom of the ethyne is displaced by sodium metal.