For alkenes A, B, and C: (a) Rank A, B, and C in order of...

For alkenes $A, B,$ and $C$: (a) Rank $A, B,$ and $C$ in order of increasing heat of hydrogenation; (b) rank $A, B,$ and $C$ in order of increasing rate of reaction with H$_2$, Pd-C; (c) draw the products formed when each alkene is treated with ozone, followed by Zn, H$_2$O.

Key Concepts

Catalytic Hydrogenation (H2, Pd-C Reaction)

Catalytic hydrogenation involves the addition of hydrogen to an alkene in the presence of a palladium catalyst supported on carbon (Pd-C). In this reaction, less substituted (and consequently less stable) alkenes typically react faster because they are more reactive, whereas more substituted alkenes, although generally more stable, may have steric hindrances that slow the reaction.

Ozonolysis Reaction

Ozonolysis is a method used to cleave carbon-carbon double bonds in alkenes by treating them with ozone to form unstable ozonide intermediates. Subsequent reductive workup, often with zinc in water, converts these intermediates into carbonyl compounds such as aldehydes and ketones, with the specific products being determined by the substitution pattern of the original alkene.

Heat of Hydrogenation

Heat of hydrogenation is the energy released when an alkene is converted to an alkane through the addition of hydrogen. In organic chemistry, it serves as a measure of alkene stability; more substituted alkenes typically have lower heats of hydrogenation because they are intrinsically more stable, while less substituted alkenes release more energy upon hydrogenation due to their relative instability.

Alkene Substitution Effects

The degree of alkene substitution, meaning the number of alkyl groups attached to the double-bonded carbons, greatly influences both the stability and reactivity of the alkene. More substituted alkenes benefit from hyperconjugation and inductive effects, making them more stable, which in turn affects their heat of hydrogenation and their behavior in catalytic reactions.

Transcript

00:03 This is the answer to chapter 12 problem number 36 from the smith organic chemistry textbook.

00:12 And this problem is giving us three alkenes and then asking us a series of questions about these alkenes.

00:20 And so i've drawn them here, a, b, and c.

00:30 And so first we're asked to rank them from lowest heat of hydrogenation to highest heat of hydrogenation.

00:38 And so remember, this question is really asking us about the stability of these alkenes.

00:44 And so in order to determine that, we look at the substitution pattern.

00:49 We see that a is mono substituted, b is tetra substituted, and c is tri -substituted.

00:58 And so based on that, we can make a pretty easy judgment.

01:04 B is going to be the most stable because it's tetras substituted, and therefore it's going to have the lowest heat of hydrogenation.

01:11 C, tri -substituted, is going to be slightly less stable than b.

01:15 So its heat of hydrogenation will be slightly higher.

01:19 And then a, mono -substituted is going to be significantly less stable than b or c, and so it will have the highest heat of hydrogenation.

01:28 So for part b of this question, we're asked to rank the same three molecules by their rate of reaction with h2 and palladium on carbon.

01:41 So how quickly they would be hydrogenated.

01:47 And so really, this is just another way of phrasing the same question we were asking part a.

01:53 So the least stable molecule, the molecule that gives off the most energy when it is hydrogenated, is also going to be the most eager to be hydrogenated...

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