ochem examkracker

[premedstudent55]

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Does anyone have the examkracker 1001 book for ochem? I have a question for #137. I don't get why is CHO group has priority 2 instead of 3. I thought that the higher atomic number will prevail the lower atomic number/??? so the priority for this problem would be Br> OH > CHO tell me why I am wrong??

 

[premedstudent55]

also.. question # 138... isnt this a geometric isomer?? and diasterometers is not considered as geometric.... im confused now

 

[tttgo]

cho beats oh.
http://www.sparknotes.com/chemistry/organic3/enantiomersanddiastereomers/section1.rhtml

 

[michaelfranklin]

cho beats oh.
http://www.sparknotes.com/chemistry/organic3/enantiomersanddiastereomers/section1.rhtml

According to the Cahn-Ingold Priority Rules: -CHO should not beat -OH. We should "assign priorities based on atomic number of the four atoms" attached to the chiral center. Take for instance the assignment of priority to R-Lactic Acid. The -OH is assigned a priority of 1, while the carboxyl has a priority of 2. Therefore, -OH has priority over -COOH (which should have priority over -CHO).

http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/sterism3.htm

I apologize for introducing another molecule, but I didn't have the EK question in front of me. I didn't want to make assumptions about the substituents about the molecule in question.

 

[premedstudent55]

that's exactly what I thought too until this problem (137) from 1001 EK ochem says otherwise.. -__ im getting like mixed ideas... whether CHO > OH or vice versa......

 

[premedstudent55]

#137/138 / 223??

#137: priority CHO > OH??
#138: geometric isomer is diastereomers??? huh?
#223: why isnt carbon 1 being the most reactive since it is the most substituted..?

 

[AnastomoSteve]

#137/138 / 223??

#137: priority CHO > OH??
#138: geometric isomer is diastereomers??? huh?
#223: why isnt carbon 1 being the most reactive since it is the most substituted..?

#137 - Br > OH > CHO - EK seems to be wrong (seems to be a lot)
#138 - Cis/trans isomers are diasteromers
#223 - C1 is the most substituted, however there MUST be a hydrogen present for that carbon to go through a radical reaction (it must lose a H to become a radical). A C-C bond will not be broken to form a radical

 

[premedstudent55]

thanks! it makes a lot of sense to me now