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The attachment of a carbon atom to an adjacent atom determines its classification.

• When a carbon atom is directly linked to only one adjacent carbon atom, it is classified as1⁰ Carbon.
• When a carbon atom is directly connected to two adjacent carbon atoms, it is classified as 2⁰ Carbon.
• When a carbon atom is directly connected to three adjacent carbon atoms, it is classified as 3⁰ Carbon.

The stability of the carbon centre is of the order: 3⁰>2⁰ >1⁰

a. There are total nine hydrogen atoms attached to three 1⁰ carbon centers, two hydrogen atoms to a 2⁰ carbon center, and only one to a single 3⁰ carbon atom. The 3⁰ C-H bond will break readily to give the most stable radical, first the 2⁰ and then the 1⁰ C-H bond.

Classification of atoms in compound a

b. There are three hydrogen atoms attached to 1⁰ carbon, six hydrogen atoms to 2⁰ carbon, and only one hydrogen atom attached to 3⁰. Hence, the 3⁰ C-H bond will break easier than the 2⁰ and then 1⁰ C-H bond.

Classification of atoms in compound b

c. There are total six hydrogen atoms attached to the two 1⁰ carbon centers and four hydrogens attached to the two 2⁰ carbon centers. The 2⁰ C-H bond will break easier than the 1⁰ C-H bond to form stable 2⁰ carbon radical.

Classification of atoms in compound c