Heats of hydrogenation of three alkenes are as follows:...

Heats of hydrogenation of three alkenes are as follows: 2-methyl-1-butene $\left(-119 \mathrm{kJ} \mathrm{mol}^{-1}\right)$ 3-methyl-1-butene $\left(-127 \mathrm{kJ} \mathrm{mol}^{-1}\right)$ 2-methyl-2-butenc $\left(-113 \mathrm{kJ} \mathrm{mol}^{-1}\right)$ (a) Write the structure of each alkene and classify it as to whether its doubly bonded atoms are monosubstituted, disubstituted, trisubstituted, or tetrasubstituted. (b) Write the structure of the product formed when each alkene is hydrogenated. (c) Can heats of hydrogenation be used to relate the relative stabilities of these three alkenes? (d) If so, what is the predicted order of stability? If not, why not? (e) What other alkene isomers are possible for these alkenes? Write their structures. (f) What are the relative stabilities among just these isomers?

Heats of hydrogenation of three alkenes are as follows: 2-methyl-1-butene $\left(-119 \mathrm{kJ} \mathrm{mol}^{-1}\right)$ 3-methyl-1-butene $\left(-127 \mathrm{kJ} \mathrm{mol}^{-1}\right)$ 2-methyl-2-butenc $\left(-113 \mathrm{kJ} \mathrm{mol}^{-1}\right)$
(a) Write the structure of each alkene and classify it as to whether its doubly bonded atoms are monosubstituted, disubstituted, trisubstituted, or tetrasubstituted. (b) Write the structure of the product formed when each alkene is hydrogenated. (c) Can heats of hydrogenation be used to relate the relative stabilities of these three alkenes? (d) If so, what is the predicted order of stability? If not, why not? (e) What other alkene isomers are possible for these alkenes? Write their structures. (f) What are the relative stabilities among just these isomers?

Key Concepts

Isomerism in Alkenes

Alkene isomerism includes geometric (cis/trans or E/Z) isomerism as well as positional isomerism. Geometric isomers occur when the groups around the double bond can be arranged differently in space due to restricted rotation, leading to different physical and chemical properties. Positional isomers differ in the location of the double bond or substituents. Identifying and comparing these isomers is crucial for understanding the nuanced stability and reactivity of alkenes.

Hydrogenation Reaction

This is a chemical reaction that adds hydrogen across a carbon–carbon double bond to convert an alkene into an alkane. The reaction typically occurs in the presence of a catalyst. Understanding hydrogenation is important in organic chemistry as it not only provides the means of altering unsaturation but also serves as an indirect measure of alkene stability.

Alkene Substitution

This concept involves classifying alkenes on the basis of the number of alkyl groups attached directly to the carbon atoms involved in the double bond. A monosubstituted alkene has one alkyl group attached, disubstituted has two, trisubstituted has three, and tetrasubstituted has four. The degree of substitution affects the alkene's stability, with more substituted alkenes generally being more stable due to hyperconjugation and inductive effects.

Heat of Hydrogenation as a Measure of Stability

Heats of hydrogenation refer to the energy released when an alkene is converted to an alkane via addition of hydrogen. This exothermic process provides insight into alkene stability: a less exothermic reaction (less negative heat) indicates that the starting alkene was already at a lower energy level and, hence, more stable. Conversely, a more exothermic hydrogenation suggests a less stable alkene.

Transcript

00:03 The first structure we are going to look at is 2 -methyl 1 butene, which as you can see is illustrated by this structure as there's a methyl on carbon 2 and the alkene starts on carbon 1.

00:17 When this is halogenated by h2 and pd, the double bond on the alkyne disappears and it becomes an alkane, which will look like this.

00:36 And that is because a hydrogen is added here and here, which gets rid of that double bond.

00:49 To classify this alkene in the reactants, we first want to count how many substituents it has.

01:01 And so an alkyne can have a total of four.

01:04 And so, as you can see, this first carbon has zero, since both of them are hydrogen coming off of it.

01:12 And then the other alkene has two.

01:15 So this is a di -situent.

01:26 And i'm just going to put a little note so we don't forget.

01:29 And then it's also important that we look at this heat of it that the molecule will need to be heated up to in order for halogenation to occur.

01:42 Some isomers of 2 -metall -1 butene will be this structure.

02:00 Of the alkyne moving from here to here and then another structure for isomer would be here as since the bond has moved here it can then move to here and this will just involve hydrogens being removed and placed on other parts of the structure in order to figure out which ones are more stable we look at how many substantiate substituents the alkyne has and the more substituents it has the more stable it is.

02:42 So we've already characterized as our main isomer being die substituted.

02:51 The first isomer that we labeled we see has one, two, three.

02:58 So it'll be a little bit more stable.

03:01 This might be a little misleading.

03:05 And then we see here it has zero on here but one carbon or alcohol group off the other carbon in the alkene, and so it is mono substituted.

03:19 Therefore, if we're labeling this in terms of stability, the tri -substituted will be the most stable, then the dye substituted, and then lastly the one that only has one group that's not hydrogen.

03:40 In our next example, we have 3 -methal -1 butene...

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