Question

Like any catalyst, palladium, platinum, or nickel catalyzes both directions of a reaction: addition of hydrogen to (hydrogenation) and its elimination from (dehydrogenation) carbon double bonds. (a) Which variable determines whether an alkene will be hydrogenated or dehydrogenated? (b) Which reaction requires a higher temperature? (c) How can all-trans fats arise during hydrogenation of fats that contain some double bonds with a cis orientation?

Short answer

The variable is the partial pressure of hydrogen. Dehydrogenation requires a higher temperature. Cis fats can turn into trans fats during partial hydrogenation.

Step-by-step solution

- Determine the Variable for Reaction Direction

Identify the factor that dictates whether an alkene undergoes hydrogenation (adding hydrogen) or dehydrogenation (removal of hydrogen). This variable is the partial pressure of hydrogen gas in the reaction environment.

- Analyze the Temperature Requirements

Evaluate which of the two reactions, hydrogenation or dehydrogenation, requires a higher temperature. Dehydrogenation typically requires a higher temperature because breaking bonds (such as removing hydrogen) generally needs more energy than forming them.

- Understanding Trans Fat Formation

Explain how hydrogenation of fats with cis double bonds can lead to the formation of trans fats. During the partial hydrogenation process, the cis configuration of the double bonds in fatty acids can rearrange to a more stable trans form.

Key concepts

Hydrogenation

Hydrogenation is a process where hydrogen atoms are added to carbon-carbon double bonds. This reaction converts alkenes (which have double bonds) into alkanes (which have single bonds). The presence of a catalyst such as palladium, platinum, or nickel speeds up the reaction without being consumed by it. The critical variable that determines whether hydrogenation will occur is the partial pressure of hydrogen gas. When the partial pressure is high, the alkene molecule is more likely to interact with and absorb hydrogen, transitioning into a fully hydrogenated product. In everyday life, hydrogenation is a key process in the food industry. For example, it's used to turn liquid vegetable oils into semi-solid fats like margarine, which have a longer shelf life and are easier to spread. Understanding hydrogenation helps us comprehend not just culinary practices but many chemical manufacturing processes.

Dehydrogenation

Dehydrogenation is the reverse of hydrogenation; it involves the removal of hydrogen atoms from a molecule. This reaction is significant in organic chemistry and industrial applications because it transforms alkanes back into alkenes. The catalysts used for dehydrogenation are the same as those for hydrogenation—palladium, platinum, or nickel. However, dehydrogenation typically requires higher temperatures. This is because breaking the carbon-hydrogen bonds requires more energy than forming them. One notable application of dehydrogenation is in the production of high-octane gasoline, since alkenes have higher octane ratings than alkanes. So when we think about processes like these, we're actually seeing dehydrogenation in action. It's a key technique in the petrochemical industry and many other sectors where organic transformations are required.

Trans Fats Formation

Trans fats formation occurs during the partial hydrogenation of fats that contain double bonds in a cis orientation. When vegetable oils undergo partial hydrogenation, not all the double bonds get converted into single bonds. Some double bonds remain but their orientation can change from cis (where the hydrogen atoms are on the same side of the bond) to trans (where the hydrogen atoms are on opposite sides). This is because during hydrogenation, the fat molecule becomes flexible and can rearrange itself. Trans fats are associated with higher risks of heart disease, as they can increase bad LDL cholesterol and decrease good HDL cholesterol. Understanding this transformation is crucial for both chemists and consumers, as trans fats are often found in processed foods, baked goods, and fast foods. By knowing how trans fats are formed, we can make more informed dietary choices and advocate for better food processing practices.