Biphenyl has the following structure.

Question

c. The heat of hydrogenation for biphenyl is about 418 kJ/mol (100 kcal/mol). Calculate the resonance energy of biphenyl.

d. Compare the resonance energy of biphenyl with that of naphthalene and with that of two benzene rings. Explain the difference in the resonance energies of naphthalene and biphenyl.

Accepted Answer

Hey everyone and welcome back shown below is the structure of para try Fennell. Number one. If the heat of hydrogenation of para try fennel is 630 kg per mole and the heat of hydrogenation for an isolated elkin is 120 kg joules per mole determine the residents energy of para tree fennel. And number two compared the Residents energy of para tree fennel with that of three benzene rings and with that of nafta lean explain the difference in the residence energies of para tree fennel and nafta lean. The residents energy of benzene and naphthalene is 150 kg per mole and kg per mole respectively. Let's begin solving the first problem and let's keep in mind that we are trying to identify the residents energy of para try fennel and to do that. We need to understand what which equation we are going to use here, we call that Residents energy is defined as the difference between two valleys. So that would be theoretical heat of hydrogenation and experimentally determined heat of hydrogenation. Okay, so we are going to use that equation. Let's understand which two values. We need first. We'll notice it says the heat of hydrogenation of para tribe finalists 630. So this is our experimental value and we need to identify the theoretical valley of the heat of hydrogenation. We know that the heat of hydrogenation of an isolated elkin or essentially one double bond is 120 kg joules per mole. So the theoretical heat of hydrogenation would be calculated by taking this value. And multiplying by the number of pi bonds we have, assuming that these bonds are isolated Elkins. So we have 123456789. Right? So there will be nine pi bonds. We are going to multiply nine by 120 kg joules per month. And that's our theoretical heat of hydrogenation. We get 1080 kg joules per mole. Okay. And that this valley is our theoretical value. So we have the two required values. We have 6:30 as our experimental and 101,080 kg promote for the theoretical one. If we find the difference between them as defined by our equation, we will get the residents energy. So we can say that residents energy For Perot tried Fennel would be the Theoretical Valley. 1080. That'd be 1080 kg joules per mole. And we're going to subtract the experimentally determined heat of hydrogenation which is 630 kg joules per mole. Now, if we do the math, that would be 450 killer jewels per more. We have successfully determined the resonance energy for power try fennel. And we are ready to label this as our partial answer to this problem. Now let's move on to the next question number two. In order to compare the resonance energy of para try fennel with the resonance energy of Benzene which is 150. We have to understand that para try fennel contains three rings. Right? So this energy corresponds to three rings. We have three isolated rings. So our goal to have a direct comparison is to calculate The resonance energy per one ring. So we can essentially say that if we take 450 kg joules per mole and divide by three rings, We get 150 kg joules per mole per ring. Right? And when we think about the resonance energy of Benzene, we already said that for Benzene that's 150 kg joules per mole for ring. So that would be our residence energy for para tried fennel and this is residence energy per ring Benzene. So we have to acquire that measure because we can only compare resonance energies for the same quantity. So we can essentially compare the resonance energies per ring only. That's why we have to divide by three rings because part try final has three rings. Benzene consists of one ring and we may notice that the two values are exactly same right? Essentially these two values are exactly the same. So we can say that they are same and they have exactly same residents energy per wing. Now if you think about the comparison with Nafta lean, we call that Nafta line has the following structure. We can essentially draw it That would be our six member ring fused with another ring. And that we may essentially add our double bonds because we have two rings in the structure, we have to take the value of 252 kg per mole. That's the residence energy for Nafta lean. And since there are two rings, we are going to divide by two rings and we will end up with 126 kg joules per mole per ring. Okay. And now we are going to compare this valley to the value that we got for poetry. Final notice that the valley of poetry fennel is greater. We have 150 kg joules per mole per ring versus 126 killed juice per per per ring for nafta line. Right? So this would be the value for Nafta line. And we can essentially conclude that while Benzene and para try fennel have exactly the same resonance energy spur wing the residents energy per wing of Nafta lean is lower right? Because we have 126 which is lower than 1 50. And you may recall that the lower the resonance energy, the less stable our structure is. So we can conclude that Nafta lean is less stable than para try fennel. Okay, so that will be our answer. We clearly see that the residence energy is lower and because it's lower, it indicates that Nafta lean is less stable than our try fennel based on our answers. We may conclude that the correct answer to the multiple choice question is B. However, if you have any questions, don't hesitate to leave a comment down below. Thank you for watching.

Key Concepts

Resonance Energy

Heat of Hydrogenation

Comparative Resonance in Polycyclic Aromatic Hydrocarbons

Watch next