1
$\begingroup$

I'm having trouble understanding how to "estimate" the value of the BDE for the compound shown below (I believe it is a secondary benzylic C-Br value) shown below (the 58 kcal/mol). We are supposed to be able to estimate the value from the table values shown below. My teacher said to use the C-H values and C-Br values from the table below to estimate it, but I don't understand how they are able to indicate the value. Any helpful drawings would be nice since I'm new to organic chemistry and have trouble just understanding the names! Thank you.

enter image description here

enter image description here

enter image description here

Improve this question
$\endgroup$
4
  • $\begingroup$ Welcome to ChemSE. We would like to see how you arrived at your values. We like to help but loath doing homework. $\endgroup$ – user55119 Feb 14 at 22:47
  • $\begingroup$ @user55119 Thanks! Sorry if I wasn't clear, but the red is what I am trying to understand - the values are not mine; it is what the teacher said. Specifically, I am confused about how they got the 58 kcal/mol estimation. $\endgroup$ – Brian Feb 15 at 4:04
  • $\begingroup$ Adding a reference for the source of that table might help. Also, ask your teacher if possible, it does look off. $\endgroup$ – Buck Thorn Feb 15 at 7:11
  • $\begingroup$ @BuckThorn All the information I have is that it comes from Pearson Education : / . My teacher wants us to use this table to calculate our BDE values. She said the estimate can have a range of 55-60 $\endgroup$ – Brian Feb 15 at 17:37
2
$\begingroup$

What you have shown in red is the second propagation step in the benzylic free radical bromination of ethylbenzene. The bond dissociation energy (BDE) of bromine is +46 kcal/mol while the overall heat of reaction of the step is -12 kcal/mol. Thus, the formation of the C-Br bond must be -58 kcal/mol. The BDE for the C-Br bond in (1-bromoethyl)benzene is +58 kcal/mol.

However, the BDE for bromine is not listed in your tables. So how can one estimate the BDE of the C-Br bond in (1-bromoethyl)benzene using the data in the tables? A simple ratio and proportion will suffice. In equation A compare the primary C-H bond of ethane to that of toluene. This ratio is compared to the ratio of the BDE of the secondary C-H bond of propane and its counterpart in ethylbenzene, which is the unknown "x". An estimated value of 82 kcal/mol is obtained. In equation B, this ratio compares secondary C-H bonds with secondary C-Br bonds. The estimate for the BDE of the C-Br bond in (1-bromoethyl)benzene is the value of "y" which is equal to 59 kcal/mol. Pretty close, wouldn't you say?

Improve this answer
$\endgroup$
2
  • $\begingroup$ thank you so much for the response! this technique definitely works. Just a clarifying question, how did you know how to make the fractions? For instance, why did you choose to put CH3CH2H on top of PhCh2H? $\endgroup$ – Brian Feb 15 at 20:10
  • $\begingroup$ Doesn't matter as long as the two ratios are in the same order. 2:4::1:x. So, x=2. But for 4:2::x:1, the answer is still 2. $\endgroup$ – user55119 Feb 15 at 22:56

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.