Organic Chemistry - FYUP First Semester NEHU Examination 2024

Farlando Diengdoh 22 Mar 2025

Q5 (a) Draw the molecular orbital picture of the following molecules, mentioning the type of hybridization, shape and bond angles: (Marks 1.5 x 2 = 3)

Solution: 

Here both Carbon and Oxygen atoms are sp² hybridised, thus they would form sp²-sp² σ bond, the other sp² orbitals of carbon form σ bonds with hydrogen atoms, while in oxygen they are occupied by lone pairs of electrons. The unhybridized p_z orbitals then form side-siwise overlap to form a π bond. The bond H-C-O bond angle is 120° and the shape of the molecule is triangular planar.

Here one Carbon and Oxygen atoms are sp² hybridised while the carbon atom in the middle is sp hybridised, thus they would form sp²-sp σ bond between C-C and C-O, the other sp² orbitals of the terminal carbon form σ bonds with hydrogen atoms, while in oxygen they are occupied by lone pairs of electrons. The sp hybridised carbon atom has two unhybridised orbitals - the p_z and p_y. Thus the unhybridized p_z orbitals then form side-siwise overlap with carbon form a π bond and the unhybridized p_y orbital would form π bond with unhybridised p orbital of oxygen. The bond H-C-C bond angle is 120° while the C-C-O bond angle is 180° and the shape of the molecule is triangular planar.

(b) Give the IUPAC names of the following compounds : (Marks 0.5 x 2 = 1)

Solution:

(i) The proper numbering is as follows as doublebond has higher priority than triple bonds in IUPAC.

The IUPAC name is hex-2-en-4-yne.

(ii) The parent molecule here is phenol, and following IUPAC rules for numbering as follows:

The IUPAC name is 3-bromo-5-chlorophenol

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