organic chem question?

[messenger]

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i am having the hardest time figuring out free radical halogenation, between chlorination and bromination. i dont even know how to explain, can anyone please give me an explanation about these processes in general. on some kaplan full length exams, Br and hv, attach Br to the tertiary C on the benzene ring. but chlorination is supposed to attach to primary?
but then on the test questions, the correct answer is Cl attaching itself to the teriatry, most stable radical, like DAT achiever Sciences test 3, #99 and 81. could some please help me with any of these or anything at all. thank you. 🙂
messenger.

 

[asckwan]

This isn't going to help much in explaining the chlorination on the most stable radical, but the reason that you get the most stable substitution with Br is because it is selective about where it goes where as Cl isn't. So Cl is just about as likely to attach to a tertiary than to a primary or secondary. Br will yield in a very high percentage with the most stable radical substitution. Hope that helped!!

 

[DonExodus]

He is correct, Br is simply more selective / reactive. Cl will still attach to a tertiary C as its major prodict, it just less specific. The Br may attach to the tert C 99.9% of the time, and Cl to a tert C probably 80%.

Keep in mind, the product formed is dictated by the stability of the radical. A tertiary radical is more stable than a primary, hence major haolgenation products will go there.

-DonExodus

 

[asckwan]

Oi, what is it about me that screams "HE!" and not "SHE!" 😕 hehe

 

[DonExodus]

"They" doesnt sound right, and he is better than it 😉

lol jk

 

[asckwan]

😛

 

[messenger]

explaining these problems gets me confused still but thanks!

 

[DAT Destroyer]

Hi... I have seen a lot of answers to this question.....let me settle it. For the DAT, remember that Br2 will always abstract a Tertiary Hydrogen over a Secondary, and a Secondary over a Primary. Clearly the answer is due to radical stability. Cl2 is a bit more tricky because the transition state for Chlorination is DIFFERENT than that of Br2. We have radical stability versus statistical competition between hydrogens. For example, if we chlorinated butane we would get 28% 1-chlorobutane and 72% 2-chlorobutane.....thus the radical argument wins. However, if we chlorinated 2-methylpropane, we see something suprising.....63% 1-chloro-2-methylpropane and 37% 2-chloro-2-methylpropane,,,the statistical argument wins !!!! Draw it out and have a look.... I think that if you know the Br2 reaction,and the NBS reaction, it will be a safe bet to hit the problem dead on. This is not the Chem GRE,,,but the DAT. My NYU and Columbia students have never reported any problem with the Orgo questions. If you would like to discuss the matter further email me at Orgoman.com

Regards,
Dr. Jim Romano New York University

 

[messenger]

thanks for the replies. i have another question if anyone can help...i just took the ada sample test and did pretty well on the ochem.
i only missed one, abd the question i have is about "which H is the most acidic?"
i put down the H on the carboxylic acid, which was the correct answer, but in the kaplan book, it explains that the H alpha to the carbony carbon is the most acidic? just wanted a more clear thought on this? thank you again! 🙂
messenger.

ps--how hard is the actual dat to the sample dat test fromt he website? 😳

 

[DonExodus]

Yea, that online sample test sucked...

 

[thehipster]

ADA test is easier

 

[messenger]

one more question for help....how come CH4 is nonpolar and H20 is polar?
how do you determine this? can someone please help me with this? 🙂
messenger.

 

[messenger]

or i meant CCl4? im kinda confused? 😳

 

[asckwan]

CCl4 is non polar because you have equal electronegativity pulling on all 4 sides of the carbon, therefore they all cancel each other out. In H2O it's polar because it's not linear. The O has lone pairs of electrons that make it not linear, therefore the O pulls one way more than the two H's pull the other therefore making it unbalanced. Does that help?

 

[messenger]

CCl4 is non polar because you have equal electronegativity pulling on all 4 sides of the carbon, therefore they all cancel each other out. In H2O it's polar because it's not linear. The O has lone pairs of electrons that make it not linear, therefore the O pulls one way more than the two H's pull the other therefore making it unbalanced. Does that help?

thank you. 🙂

 

[RozhonDDS]

thank you. 🙂

The easiest way to distinguish between something polar and non-polar is its geometrical set up. Basically if the compound is symmetrical (tertahedral) like CH4 it is NON-POLAR, all the charges (dipole moments) cancel each other out. H2O is Polar because it is not symmetrical, it is bent and has dipole moments. If the compound was linear that it would be non-polar and so on.