Question

Which will have smallest heat of hydrogenation per mole? (a) Cis-2-butene (b) Trans-2-butene (c) 1,3 -butadiene (d) 1-butene

Short answer

Trans-2-butene has the smallest heat of hydrogenation.

Step-by-step solution

Understanding Heat of Hydrogenation

The heat of hydrogenation refers to the amount of heat released when a compound adds hydrogen across its multiple bonds. In general, the less stable a compound is, the higher the heat of hydrogenation it will have.

Analyzing Stability of the Compounds

Stability is influenced by factors such as steric hindrance and conjugation. Trans-2-butene is more stable than next most stable, Cis-2-butene, due to less steric hindrance. 1,3-butadiene is stabilized through conjugation. 1-butene is less stable because it lacks additional stabilizing factors, like trans-geometry or conjugation.

Predicting Heat of Hydrogenation Order

More stable compounds have lower heats of hydrogenation. In this case, Trans-2-butene, being most stable, will have the smallest heat of hydrogenation. Cis-2-butene follows, then 1,3-butadiene due to conjugation, and lastly 1-butene being the least stable having highest heat output.

Key concepts

Stability of Alkenes

Stability is key when talking about alkenes. Alkenes are hydrocarbons containing at least one carbon-to-carbon double bond.
Their stability depends on several factors including the degree of substitution around the double bond. Simply put, the more carbon atoms attached to the carbons of the double bond, the more stable the alkene.
This is because a highly substituted double bond can disperse its energy over more carbon atoms. When we compare different alkenes, you'll often hear terms like "cis" and "trans". These terms describe how groups are arranged around the double bond, influencing the alkene's stability. In general:

• Trans alkenes are usually more stable than cis because the bulky groups are farther apart, reducing repulsive interactions.
• Additionally, more substituted alkenes (with more carbon atoms directly on the double bond) are often more stable because they stabilize the overall structure.

Understanding these principles helps in predicting which alkenes are more securely held together in chemical reactions.

Steric Hindrance

Steric hindrance is a crucial concept in chemistry. It refers to the resistance to reactions that arises when the size of groups within a molecule prevents chemical reactions from occurring as easily as they would in a less crowded environment.
Imagine trying to fit a large puzzle piece into a packed puzzle – that’s steric hindrance! In alkenes, steric hindrance affects stability. For instance, in cis-2-butene, both bulky groups are on the same side of the double bond, making them closer to each other.
This proximity increases steric hindrance, causing instability compared to trans-2-butene where bulky groups are on opposite sides, reducing crowding. Key points about steric hindrance:

• Higher steric hindrance means less stability; thus, usually a higher heat of hydrogenation.
• Trans-isomers are less influenced by steric hindrance than their cis counterparts.

Understanding steric hindrance gives insight into molecule interaction and stability, which is invaluable for predicting reactivity and behavior in chemical processes.

Conjugation in Dienes

Conjugation is like harmonious teamwork among molecules. In the world of hydrocarbons, conjugation occurs when p-orbitals overlap in a sequence of alternating single and double bonds. 1,3-butadiene, for example, features this pattern.
Conjugation allows for electron delocalization across all the connected p-orbitals, creating a system where electrons can roam over several atoms.
This delocalization adds stability to the molecule, lowering the energy and heat released during reactions. The magic of conjugation in dienes comes from:

• Providing greater stability due to electron delocalization.
• Enhancing resonance energy, making the molecule more chemically desirable.

For students, it's important to note that conjugation often makes compounds like 1,3-butadiene less reactive when compared to their non-conjugated brethren, translating to smaller heats of hydrogenation because of this extra stability factor.

Trans-2-butene

Trans-2-butene is a popular example when discussing stability among alkenes.
This compound features a double bond between carbons 2 and 3, with two groups of atoms on opposite sides of this bond—a hallmark of the "trans" configuration.
This opposing arrangement of atoms means less steric hindrance, which makes the molecule more stable. Regarding heat of hydrogenation, the stability of trans-2-butene becomes particularly evident. As the compound experiences less internal crowding compared to its cis-relative, it needs less energy to stabilize, resulting in a lower heat of hydrogenation. Key features of trans-2-butene include:

• Superiority in stability compared to cis-2-butene, owing to minimized steric hindrance.
• Lower heats of hydrogenation, meaning it's energetically favored and more stable.

Trans-2-butene acts as a prime example to illustrate how molecular geometry impacts chemical energy and reactivity, solidifying its role as an important subject for students keen on mastering organic chemistry.