Question 1

Which structural feature distinguishes a cis alkene from a trans alkene in fatty acids, and how does this affect the physical properties of edible fats?

Accepted Answer

Cis alkenes have hydrogen atoms on the same side of the double bond, causing kinks in the chain and resulting in lower melting points.

Question 2

During the catalytic hydrogenation of alkenes using a palladium catalyst, what is the stereochemical outcome of the addition of hydrogen atoms?

Accepted Answer

Hydrogen atoms add to the same face of the double bond, resulting in syn addition.

Question 3

Partial hydrogenation of vegetable oils can lead to the formation of which type of isomer, and why is this significant for human health?

Accepted Answer

Trans isomers, which are associated with increased cardiovascular risk.

Question 4

Which compound is responsible for the 'taste of rancid' in spoiled fats, and what is its chemical structure?

Accepted Answer

Pelargonic acid (nonanoic acid), $$\mathrm{CH_3$(CH_2$)_7COOH}$$

Question 5

In the mechanism of heterogeneous catalytic hydrogenation of alkenes, what role does the metal surface (e.g., Pd) play?

Accepted Answer

It provides a surface for the alkene and hydrogen to adsorb, facilitating syn addition of hydrogen.

Question 6

Which of the following best explains why trans fats are more solid at room temperature compared to cis fats?

Accepted Answer

Trans fats have straighter chains, allowing tighter packing and higher melting points.

Question 7

A triglyceride contains three ester linkages. Which reaction is responsible for breaking these linkages during digestion?

Accepted Answer

Hydrolysis

Question 8

Which statement correctly describes the stereochemical outcome when hydrogen is added to a cyclohexene ring using Pd/C and $$\mathrm{H_2$}$$?

Accepted Answer

Both hydrogens add to the same face, creating a pair of enantiomers if a new stereocenter is formed.

Question 9

Why does steric hindrance affect the rate of catalytic hydrogenation of alkenes?

Accepted Answer

Bulky groups near the double bond prevent effective adsorption onto the metal surface.

Question 10

Which of the following is true regarding the chirality of products formed from the hydrogenation of a substituted cyclohexene?

Accepted Answer

If a new stereocenter is created, a pair of enantiomers will form unless the product is meso.

Question 11

Which process is used industrially to convert unsaturated vegetable oils into solid fats, and what is a major unintended consequence?

Accepted Answer

Partial hydrogenation, which produces trans fats.

Question 12

What is the main reason that cis double bonds in fatty acids lead to lower melting points compared to saturated fatty acids?

Accepted Answer

Cis double bonds introduce kinks, preventing tight packing and lowering melting points.

Question 13

Which of the following best describes the role of 'landing gear' in the mechanism of alkene hydrogenation on a metal surface?

Accepted Answer

It refers to the overlap of $$\pi$$ orbitals of the alkene with the metal surface, allowing adsorption.

Question 14

A student hydrogenates a chiral alkene using Pd/C and $$\mathrm{H_2$} in $$ethanol. What is the expected stereochemical outcome?

Accepted Answer

A racemic mixture of enantiomers due to syn addition.

Question 15

Which of the following reactions is most likely to occur when atmospheric oxygen reacts with unsaturated fats?

Accepted Answer

Oxidation, leading to the formation of short-chain acids like pelargonic acid.

Question 16

Why are trans fats considered more harmful than cis fats in the human diet?

Accepted Answer

Trans fats increase LDL cholesterol and decrease HDL cholesterol, raising cardiovascular risk.

Question 17

Which of the following best describes the product of complete hydrogenation of a triglyceride?

Accepted Answer

A saturated fat with no double bonds.

Question 18

In the context of edible fats, what is the chemical reason for the 'rancid' taste in spoiled oils?

Accepted Answer

Formation of short-chain carboxylic acids such as pelargonic acid due to oxidation.

Question 19

Which of the following statements about the hydrogenation of alkenes is correct?

Accepted Answer

Hydrogenation of alkenes on a metal surface proceeds via syn addition.

Edible Fats, Alkene Stability, and Catalytic Hydrogenation: Organic Chemistry Study Notes

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