Case study (4 Marks)

Question 14 Marks

The molecular orbital theory is based on the principle of a linear combination of atomic orbitals. According to this approach when atomic orbitals of the atoms come closer, they undergo constructive interference as well as destructive interference giving molecular orbitals, i.e., two atomic orbitals overlap to form two molecular orbitals, one of which lies at a lower energy level (bonding molecular orbital). Each molecular orbital can hold one or two electrons in accordance with Pauli's exclusion principle and Hund's rule of maximum multiplicity. For molecules up to $N _2$, the order of filling of orbitals is:

Bond order $=\frac{1}{2}$ [bonding electrons - antibonding electrons]
Bond order gives the following information:
i. If bond order is greater than zero, the molecule/ion exists otherwise not.
ii. Higher the bond order, higher is the bond dissociation energy.
iii. Higher the bond order, greater is the bond stability.
iv. Higher the bond order, shorter is the bond length.

1. Arrange the following negative stabilities of $CN , CN ^{+}$and $CN ^{-}$in increasing order of bond.
2. The molecular orbital theory is preferred over valence bond theory. Why?
3. Ethyne is acidic in nature in comparison to ethene and ethane. Why is it so?
OR
Bonding molecular orbital is lowered by a greater amount of energy than the amount by which antibonding molecular orbital is raised. Is this statement correct?

Answer

1. The increasing order of negative stabilities of $CN , CN ^{+}$and $CN ^{-}$is $CN ^{+}> CN > CN ^{-}$.
2. The molecular orbital theory is preferred over valence bond theory because molecular orbital theory explains the magnetic nature of the molecule.
3. In ethyne, hydrogen atoms are connected to sp hybridized carbon atoms, but in ethene, they are attached to $sp ^2$ hybridized carbon atoms and in ethane, they are attached to $sp ^3$ hybridized carbons.
OR
The given statement is not correct because the bonding molecular orbital is lowered by a lesser amount of energy than the amount by which antibonding molecular orbital is raised.

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Question 24 Marks

The existing large number of organic compounds and their ever-increasing numbers has made it necessary to classify them on the basis of their structures. Organic compounds are broadly classified as open-chain compounds which are also called aliphatic compounds. Aliphatic compounds further classified as homocyclic and heterocyclic compounds. Aromatic compounds are special types of compounds. Alicyclic compounds, aromatic compounds may also have heteroatom in the ring. Such compounds are called heterocyclic aromatic compounds. Organic compounds can also be classified on the basis of functional groups, into families or homologous series. The members of a homologous series can be represented by general molecular formula and the successive members differ from each other in a molecular formula by a $- CH _2$ unit.

1. The successive members of a homologous series differ by which mass of amu?
OR
Is tetrahydrofuran is aromatic compounds?
2. Does Pyridine, pyrrole, thiophene are all heteroaromatic compounds
3. Difference between heterocyclic and homocyclic compound.

Answer

1. The successive members of a homologous series are differ by a $- CH _2$ group. The molecular mass of a - $CH _2$ group is 14 amu . Hence, each successive homologue of a homologous series differ by a mass of 14 amu .
OR
Tetrahydrofuran is non-aromatic, due to absence of conjugation in $\pi$ electrons, and it does not follow Huckel's rule.
2. Heterocyclic compounds are a major class of organic compounds characterized by the fact that some or all of the atoms in their molecules are joined in rings containing at least one atom of an element other than carbon and follow Huckels rule, the most common heterocycles are those having five or six-membered rings and containing hetero members of Nitrogen, oxygen, sulphur. Pyridine, pyrrole, thiophene are all heteroaromatic compounds
3. A cyclic compound in which the ring includes at least one atom of an element different from the rest is called heterocyclic compound. A homocyclic compound is a cyclic compound in which all the ring atoms are the same.

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Chemistry Case study (4 Marks)

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