Question

The heat of hydrogenation per mole is highest for (A) Pent-1-ene (B) Pent-2-ene (C) Penta-1,3-diene (D) Penta-1,4-diene

Short answer

The compound with the highest heat of hydrogenation per mole among Pent-1-ene, Pent-2-ene, Penta-1,3-diene, and Penta-1,4-diene is Penta-1,4-diene. This conclusion is reached by analyzing the stability of their double bonds, their positions, and the presence of conjugation. Penta-1,4-diene, being a non-conjugated diene, has the lowest stability and thus releases the most energy upon hydrogenation. Additionally, the heats of hydrogenation for dienes are higher compared to that of mono-alkenes. Hence, Penta-1,4-diene has the highest heat of hydrogenation among the given options.

Step-by-step solution

Understand heat of hydrogenation

Heat of hydrogenation is the amount of energy released when one mole of a compound undergoes hydrogenation, that is, when H2 is added to the double bonds, converting them into single bonds. The more unstable the double bond, the higher the heat of hydrogenation will be because the reaction will release more energy to reach stability.

Compare the double bond positions

Now let's compare the positions of the double bonds in the given compounds: (A) Pent-1-ene: The double bond is at the first carbon. (B) Pent-2-ene: The double bond is at the second carbon. (C) Penta-1,3-diene: Two double bonds are at the first and the third carbon. (D) Penta-1,4-diene: Two double bonds are at the first and the fourth carbon.

Analyze conjugation and stability

The stability of a double bond depends on conjugation. Conjugation happens when double bonds are separated by a single bond (a 1,3-diene). More conjugation means higher stability, and less energy released upon hydrogenation. The effect of conjugation is only significant if the double bonds are present next to each other. (C) Penta-1,3-diene: Conjugated diene, higher stability, and a lower heat of hydrogenation. (D) Penta-1,4-diene: Non-conjugated diene, lower stability, and a higher heat of hydrogenation. Now we need to compare the stabilities of mono-alkenes.

Evaluate mono-alkenes stability

Mono-alkenes' stabilities can be compared based on their positions. Terminal alkenes have more steric strain compared to internal alkenes, making them less stable. (A) Pent-1-ene: Terminal alkene, less stable (B) Pent-2-ene: Internal alkene, more stable Comparing the heats of hydrogenation between mono-alkenes and dienes, dienes will have a higher heat of hydrogenation.

Final comparison and answer

The final comparison between the compounds gives us: Penta-1,4-diene (D) > Penta-1,3-diene (C) > Pent-1-ene (A) > Pent-2-ene (B) in terms of their heats of hydrogenation. Hence, the compound with the highest heat of hydrogenation per mole among the options is Penta-1,4-diene (D).

Key concepts

Hydrogenation

Hydrogenation is a chemical reaction that involves the addition of hydrogen (H_2) to another compound. In the context of organic chemistry, hydrogenation typically refers to the process of adding hydrogen to unsaturated hydrocarbons, such as alkenes and alkynes, to convert them into saturated hydrocarbons like alkanes. This addition occurs at the site of double or triple bonds, which are characteristically more reactive than single bonds.

During hydrogenation, the alkene goes through a mechanism that often requires a catalyst, such as palladium or platinum, to facilitate the reaction. The significance of the hydrogenation process lies not only in its applicability to chemical synthesis and industrial applications but also as a conceptual tool for understanding alkene stability. When hydrogen is added to an alkene, there is a release of energy known as the heat of hydrogenation. This value is a vital indicator of the relative stability of the unsaturated precursor; the lower the heat of hydrogenation, the more stable the original alkene was.

Alkene Stability

The stability of alkene molecules is a central concept in organic chemistry. Various structural factors influence the stability of an alkene, including the location of the double bond, the presence of substituents, and the potential for hyperconjugation and inductive effects. An internal double bond, as seen in alkenes like pent-2-ene, experiences less steric strain and more hyperconjugation than a terminal double bond, as seen in pent-1-ene. Hyperconjugation is when the electrons in the sigma bonds adjacent to the double bond can delocalize slightly into the empty pi orbital, effectively spreading out the electron density and lowering overall energy.

Alkenes can be systematically compared by their heats of hydrogenation, where less stable alkenes will release more heat upon hydrogenation. This paradigm allows us to predict and explain why some alkenes, such as mono-alkenes at terminal positions, exhibit a higher heat of hydrogenation—indicating lower stability—than their internal counterparts.

Conjugation in Organic Chemistry

Conjugation in organic chemistry refers to the alternating pattern of single and multiple bonds within a hydrocarbon molecule. This arrangement allows the overlapping of p-orbitals across the alternating single and double bonds, leading to delocalization of the pi electrons. As a result, conjugated systems, like 1,3-butadiene, are energetically more stable due to this electron delocalization, which distributes the negative charge across a broader region of the molecule.

Conjugated dienes like penta-1,3-diene have a lower heat of hydrogenation compared to non-conjugated dienes such as penta-1,4-diene, reflecting their increased stability. In our analysis, the more stable an alkene or diene is, the lesser the heat released during hydrogenation. Thus, conjugation is a stabilizing factor in organic molecules. This principle helps explain the ordering of the given compounds in terms of their heat of hydrogenation; conjugated systems demonstrate lower values which correlate with their inherent stability.