Question

Assuming the heterocyclic ring above to be planar, give the number of electrons that are contributed to the pi system of the heterocyclic ring by:

atom 1 = ? atom 2 = ? atom 3 = NA

How many electrons are int he pi system of the heterocyclic ring? (In the case of fused rings add all the pi elections)

Is the heterocyclic ring with the numbered atoms aromatic?

3-Methyl-1,2,4-thiadiazole (atom 1 is the nitrogen bonded to the sulfur atom, atom 2 is the nitrogen off the methyl group connected to another carbon)

Answer 1

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In this question seems like there is a figure missing. Nevertheless the compound’s name that appears below gives a good example and may be use as an example.

For 3-Methyl-1,2,4-thiadiazole answers are:

Atom 1: 2 electrons

Atom 2: 1 electron

Atom 3: 1 electron

How many electrons are in the pi system of the heterocyclic ring? Answer: 6 electrons

Is the heterocyclic ring with the numbered atoms aromatic? Answer: yes

Reasoning:

Sulfur atom is hybridized sp². It has two pairs of electrons that are non-bonded, one of them lying in the p orbital that is perpendicular to the ring plane. These two electrons are in the p cloud.

Nitrogen: Nitrogen is also hybridized sp² but unlike the sulfur it is establishing a double bond with the next carbon so it uses the p orbital that is perpendicular to the plane ring with one electron to make the p bond with the carbon atom. The lone pair lays in a sp2 orbital that el IN THE PLANE of the ring, so they cannot be part the p cloud.

Carbon: Carbon is also hybridized sp2 and establishes a double bond with the nitrogen so it uses the p orbital that is perpendicular to the plane ring with one electron to make the p bond. Carbon does not have non bonded electrons.