Question

Straight chain alkane obtained on hydrogenation of \(\mathrm{S}\) can also be obtained by the reduction of (A) Ribose using Red \(\mathrm{P} / \mathrm{HI}\) (C) Arbinose using \(\mathrm{LiAlH}_{4}\) (B) Ribose using \(\mathrm{H}_{2} / \mathrm{Catalyst}\) (D) Mannose using Red \(\mathrm{P} / \mathrm{HI}\)

Short answer

The correct option for obtaining a straight chain alkane is (A) Ribose with Red \(P / HI\) reduction, which results in the formation of a straight chain alkane with the formula \(C_5H_{12}\).

Step-by-step solution

Understanding the reduction process

Reduction is the process of gaining hydrogen or losing oxygen by a molecule. In this case, we need to find the straight chain alkane that would be obtained by reduction.

Analyze Option (A) Ribose with Red P/HI

Ribose is a monosaccharide with the molecular formula \(\mathrm{C}_5\mathrm{H}_{10}\mathrm{O}_5\). Reduction of ribose using Red \(\mathrm{P} / \mathrm{HI}\) will remove all the oxygen atoms and add hydrogens in their place, forming a straight-chain alkane with the formula \(\mathrm{C}_5\mathrm{H}_{12}\). This process is known as complete reduction.

Analyze Option (B) Ribose with H2/Catalyst

When we reduce ribose using \(\mathrm{H}_{2} / \mathrm{Catalyst}\), it undergoes hydrogenation and turns into alditols. In this case, we will get ribitol which is not a straight chain alkane but a sugar alcohol.

Analyze Option (C) Arabinose with LiAlH4

Arabinose is another monosaccharide with the molecular formula \(\mathrm{C}_5\mathrm{H}_{10}\mathrm{O}_5\). Reduction of arabinose using \(\mathrm{LiAlH}_{4}\) will turn it into arabitols. Arabitols are sugar alcohols and not straight chain alkane. Therefore, the answer is not option C.

Analyze Option (D) Mannose with Red P/HI

Mannose is another monosaccharide with the molecular formula \(\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6\). Reduction of mannose using Red \(\mathrm{P} / \mathrm{HI}\) will remove all the oxygen atoms and add hydrogens, forming a straight-chain alkane with the formula \(\mathrm{C}_6\mathrm{H}_{14}\). This process is known as complete reduction.

Conclusion

Based on the analysis of all the options, we can conclude that option (A) Ribose with Red \(\mathrm{P} / \mathrm{HI}\) reduction is the correct answer, as it forms a straight chain alkane (\(\mathrm{C}_5\mathrm{H}_{12}\)).

Key concepts

Hydrogenation of Saccharides

Let's demystify the process of hydrogenation in the context of saccharides. Hydrogenation is the addition of hydrogen to a molecule, often requiring a catalyst such as palladium, nickel, or platinum. In saccharides, or sugars, this typically converts an aldehyde or ketone group into an alcohol, transforming the sugar into a different compound called an 'alditol'.

For instance, when ribose, a common monosaccharide, undergoes hydrogenation, it becomes ribitol. However, it's important to note that hydrogenation doesn't always yield the same product; the results can vary based on the sugar involved and the specific conditions of the reaction. To boost student understanding, focusing on the type of catalyst used and the resulting stereochemistry can contribute greatly to their grasp of the hydrogenation process.

This reaction is highly relevant in the food industry, where it's used to convert glucose into sugar alcohols which are less caloric than regular sugars.

Complete Reduction of Monosaccharides

When discussing the complete reduction of monosaccharides, we're entering a territory where these complex sugars are simplified into their most basic form – alkanes. The complete reduction involves not just the addition of hydrogen atoms but also the removal of all oxygen atoms within the sugar molecule.

A prime example, as outlined in our textbook problem, is the reduction of ribose or mannose using reagents like red phosphorus and hydrogen iodide (Red P/HI). Through this method, all oxygen atoms are removed and replaced by hydrogen atoms, ending with a molecule that is a straight-chain alkane. This is quite transformative, as the initial functional groups of the sugar are completely lost, leaving a molecule with distinctly different properties - ones characteristic of alkanes.

Emphasizing these dramatic changes in functional groups and molecular structure can significantly aid students in visualizing and understanding the significant impact of complete reduction on monosaccharides. It underscores the versatility and transformative power of chemical reactions in organic chemistry.

Chemical Properties of Alkanes

Alkanes are the simplest form of hydrocarbons, consisting of carbon and hydrogen atoms with single bonds. They are known for being relatively unreactive due to the strong C–H and C–C single bonds. One of the fundamental properties of alkanes is their nonpolarity, which dictates their solubility - alkanes are soluble in nonpolar solvents but not in polar solvents.

The chemical inertness of alkanes also means they do not easily undergo addition reactions but can participate in substitution and combustion reactions. When discussing the context of organic chemistry, pointing out that alkanes generally have higher boiling points with increasing molecular weight, yet are less dense than water, adds a layer of practical understanding for students.

A key teaching point can be around the fact that multistep processes might be required to convert alkanes into more reactive types of hydrocarbons for further chemical synthesis. Such properties are pivotal in fields like fuel production and the synthesis of more complex organic molecules.