Hydrogenation Problems PDF

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Hydrogenation Problems CHE -235 DUE DATE: 4/16/2020 INSTRUCTIONS: Please answer all questions and write legibly. Show all work if necessary. This will be graded out of 50 points and count as a post-lab questions.

1. List three catalysts typically used in catalytic hydrogenation. Explain why a catalyst is needed for hydrogenation reactions. (5 Points) 1. Nickel 2. Palladium 3. Platinum The hydrogenation reaction occurs above 480℃ in the absence of catalyst. In the presence of catalyst the hydrogenation takes place at lower temperature. Catalyst provide an alternative path with lower activation energy thus the substrate can easily convert to product in the presence of catalyst at lower temperature.

2. Explain why hydrogenation is stereospecific (10 Points) Hydrogenation is stereospecific because when hydrogen is added to the double bond then both the hydrogen atoms added to the same side of double bond and result in cis isomer or the addition of hydrogen to double bond is syn addition.

3. Describe the differences between catalytic hydrogenation and transfer hydrogenation.(10 Points) [Hint: Look at the sources of hydrogen]

Catalytic hydrogenation Transfer hydrogenation 1. In catalytic hydrogenation, the 1. In transfer hydrogenation,

the

source of hydrogen is hydrogen

source of hydrogen is hydrogen

gas.

donor molecules.

2. Metal catalyst like nickel, palladium or

No metal catalyst is used in transfer

platinum

hydrogenation.

are

used

in

catalytic

hydrogenation. 4. For each of the following unsaturated fatty acid, write and balance the reaction that will produce the corresponding saturated fatty acid via catalytic hydrogenation. (15Points) a. Oleic Acid b. Linoleic Acid c. Linolenic Acid

A. CH3(CH2)7CH=CH(CH2)7COOH + H2 Pd/C Oleic acid

CH3(CH2)16COOH

stearic acid

B. CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH + 2H2 Pd/C CH3(CH2)16COOH

Linoleic acid

stearic acid

C. CH3(CH2CH=CH)3(CH2)7COOH +3H2 Pd/C CH3(CH2)16COOH Linolenic acid

stearic acid

4. In the transfer hydrogenation of olive oil, what is the purpose of adding cyclohexene to the reaction mixture? What compound is cyclohexene ultimately converted to? (5Points) In transfer hydrogenation of olive oil, cyclohexene is used as a source of hydrogen. The cyclohexene ultimately converts to benzene after the lose of two pairs of hydrogen atoms. Cyclohexene first convert to cyclohexadiene and then cyclohexadiene convert to benzene.

6. What is the driving force for the transfer hydrogenation of olive oil when using cyclohexene? (5 Points)

In transfer hydrogenation, cyclohexene act as source of hydrogen. Cyclohexene convert to cyclohexadiene after the lose of one pair of hydrogen atoms. Cyclohexadiene convert to benzene after the lose of one pair of hydrogen atoms. Thus cyclohexene convert to benzene after the lose of two pairs of hydrogen atoms. Benzene is highly stable molecule due to delocalization of electrons. The delocalized electrons makes covers benzene ring and makes it stable. Thus the formation of benzene is the driving force for the transfer hydrogenation....