A. What alkane, with molecular formula C5 H12, forms only...

a. What alkane, with molecular formula $\mathrm{C}_{5} \mathrm{H}_{12}$, forms only one monochlorinated product when it is heated with $\mathrm{Cl}_{2}$ ? b. What alkane, with molecular formula $\mathrm{C}_{7} \mathrm{H}_{16}$, forms seven monochlorinated products (disregarding stereoisomers) when heated with $\mathrm{Cl}_{2}$ ?

a. What alkane, with molecular formula $\mathrm{C}_{5} \mathrm{H}_{12}$, forms only one monochlorinated product when it is heated with $\mathrm{Cl}_{2}$ ?
b. What alkane, with molecular formula $\mathrm{C}_{7} \mathrm{H}_{16}$, forms seven monochlorinated products (disregarding stereoisomers) when heated with $\mathrm{Cl}_{2}$ ?

Key Concepts

Free Radical Halogenation

Free radical halogenation is a reaction mechanism in which a halogen, such as chlorine, reacts with an alkane under conditions (like heat or light) that generate radicals. The reaction typically involves the abstraction of a hydrogen atom by a chlorine radical followed by the recombination of the alkyl radical with a chlorine atom. The number of distinct monochlorinated products obtained in such reactions depends on the different types of hydrogen atoms available in the alkane, which is influenced by both the alkane’s constitutional isomerism and molecular symmetry.

Hydrogen Atom Equivalence

This concept is tied closely to molecular symmetry and refers to whether hydrogen atoms in a molecule are in identical chemical environments. In alkanes, if different hydrogens are equivalent due to symmetry, then substitution at any one of those positions leads to the same product. Conversely, when hydrogen atoms are in distinct environments, as in a highly unsymmetrical alkane, chlorination can yield multiple constitutional isomers because each unique position gives rise to a different product.

Molecular Symmetry

Molecular symmetry examines how parts of a molecule are arranged in space in a balanced or mirror-image fashion. In organic compounds, symmetry often leads to sets of equivalent hydrogen atoms. When a molecule has a high degree of symmetry, as in the case of neopentane (2,2-dimethylpropane), all hydrogen atoms can be equivalent. This results in only one product upon monochlorination because substituting any hydrogen yields the same structure.

Constitutional Isomerism

This concept refers to the existence of different molecular structures (isomers) that share the same molecular formula. In the context of alkanes like C5H12 and C7H16, several isomers may exist that differ in the connectivity of their carbon atoms. These structural differences can dramatically affect the number of unique sites available for chemical reactions, such as chlorination, because they influence the chemical environment of the hydrogen atoms attached to the carbon skeleton.

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