Question

Write the structural formula of a hydrocarbon of molecular formula (a) \(\mathrm{C}_{6} \mathrm{H}_{12}\) having only secondary hydrogens (b) \(\mathrm{C}_{5} \mathrm{H}_{12}\) having only primary hydrogens (c) \(\mathrm{C}_{6} \mathrm{H}_{14}\) having only primary and tertiary hydrogens

Short answer

Question: Draw a hydrocarbon structural formula that satisfies the following conditions: A structure for C6H12 having only secondary hydrogens, A structure for C5H12 having only primary hydrogens, and A structure for C6H14 having only primary and tertiary hydrogens. Answer: The hydrocarbon structures that satisfy the given conditions are as follows: 1. C6H12 with only secondary hydrogens: H H H | | | H-C-C-C-H | | | H H H 2. C5H12 with only primary hydrogens: H H H H H | | | | | H-C-C-C-C-H 3. C6H14 with only primary and tertiary hydrogens: H H H | | | H-C-C-C-H | | H-C-H

Step-by-step solution

Understanding primary, secondary, and tertiary hydrogens

Primary hydrogens are attached to a carbon atom that is bonded to only one other carbon atom. Secondary hydrogens are attached to a carbon atom that is bonded to two other carbon atoms. Tertiary hydrogens are attached to a carbon atom that is bonded to three other carbon atoms.

Drawing a structure for C6H12 having only secondary hydrogens

To satisfy the condition of having only secondary hydrogens, we can construct a 6-carbon cyclic structure. In a cyclic structure, every carbon is bonded to two other carbons. A hexagonal cyclic structure will have the formula C6H12, as shown below: ``` H H H | | | H-C-C-C-H | | | H H H ```

Drawing a structure for C5H12 having only primary hydrogens

To ensure that C5H12 has only primary hydrogens, we should have a straight-chain structure with no branching. In this structure, there are two end carbon atoms, each with two primary hydrogens; and three other carbon atoms in the middle with one primary hydrogen each. ``` H H H H H | | | | | H-C-C-C-C-H ```

Drawing a structure for C6H14 having only primary and tertiary hydrogens

To construct a structure for C6H14 with only primary and tertiary hydrogens, we need a core carbon atom with three other carbons attached to it. Three primary hydrogens are then attached to each of the three outer carbon atoms, and the central carbon is connected to a hydrogen. This gives us a total of 6 carbon atoms and 14 hydrogen atoms, meeting the molecular formula requirement: ``` H H H | | | H-C-C-C-H | | H-C-H

Key concepts

Secondary Hydrogen

In organic chemistry, understanding the types of hydrogen atoms connected to carbon atoms is key to drawing and interpreting structural formulas. A secondary hydrogen is one that is attached to a carbon atom bonded to two other carbon atoms. These secondary hydrogens are significant in identifying isomers and understanding reactions involving hydrocarbons.

• When a hydrogen is labeled 'secondary,' it implies that it is connected to a secondary carbon.
• A secondary carbon is one that forms bonds with exactly two other carbon atoms within the molecule.
• This connection is commonly found in cycloalkanes, where each carbon forms a continuous ring linking to its neighbors.

In the context of a cyclic structure like cyclohexane (\( \text{C}_6\text{H}_{12} \), each carbon atom bonds with two adjacent carbons, allowing all hydrogens in the ring to be secondary hydrogens. Understanding this helps in predicting the formation of products in cyclohexane's reactions, such as halogenation, where secondary hydrogens are more reactive.

Primary Hydrogen

Primary hydrogens have a simple yet pivotal role in classifying the structural aspects of hydrocarbons. A primary hydrogen is attached to a primary carbon, which is bonded to only one other carbon atom.

• These are most frequently found at the extremities of molecule chains, such as in alkanes.
• Primary hydrogens are considered more stable and less reactive than secondary and tertiary hydrogens.

For example, in the linear alkane pentane (\( \text{C}_5\text{H}_{12} \)), the terminal carbon atoms are primary. These terminal carbons host primary hydrogens, two on each end carbon in a straight-chain format. Primary hydrogens largely feature in substitution reactions, like halogenation, where they participate in the formation of haloalkanes.

Tertiary Hydrogen

Tertiary hydrogens are linked to tertiary carbon atoms, which bond with three other carbon atoms. Tertiary hydrogens are particularly notable for their reactivity compared to primary and secondary hydrogens.

• These hydrogens participate prominently in free radical reactions due to the stability of resulting tertiary radicals.
• Tertiary carbons, by hosting these hydrogens, significantly influence molecular branching and steric hindrance.

An example of a hydrocarbon with tertiary hydrogen is isobutane, where the central carbon bonds with three other carbon atoms, hosting a tertiary hydrogen. In hexane derivatives with a branched form, tertiary hydrogens introduce critical points for predicting reactivity in mechanisms such as the combustion or alkylation processes.