Ochem Problem!!

[MansonTiles]

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please HELP!

1. how many different NMR peaks wouls appear if the molecule being sythesized were:

a. ethane

b. propane

c. 1,3-dichloropropane


2. How many diffeent types of equivalent protons are there in 1-bromo-2-chloropropane?

CAN ANYONE HELP ME AND EXPLAIN WHY AND HOW YOU CAME UP WITH THEM? this is a question from KAPLAN NOTECARDS.....does anyone think they are very helpful?? 🙂 THANK YOU!!

 

[armorshell]

1. Should be A.1 B.2 C.2
2. There are 3 types of protons (3 peaks)

explanation:
1a. All of the protons on the molecule are exactly the same (no proton "sees" the molecule any different)
1b. Note that the protons on the 1 and 3 carbon "see" the molecule as exactly the same, but the protons on the 2 carbon do not, meaning there are two peaks.
1c. Becasue the same symmetry is present as in 2, there are only 2 peaks, but the chemical shifts for this molecule would likely be different.

2a. Becasue in this case there are substituents on the 1,2 carbons of the propane molecule, the protons on each carbon "see" the rest of the molecule differently, therefore there must be 3 peaks.

As you can tell, my method for determing the nimber of peaks is to visualize the molecule from the perspective of each hydrogen, hope this helps. Also ist's good to remeber that if the molecule is symmetrical, some of the hydrogens are going to be identical.

 

[MansonTiles]

wow, thank you for THAT!

on number 2, is having an n amount of peaks = the same number of equivalent protons?????

I am also confused on how you determine what TYPE of peak they are ( ie,. for the ethane, it is a SINGLET???? BUT WHY!!!! HOW?! propane a septet and triplet???? what?!?!

thanks for the explanations though! helps out!!! 🙂 please.

 

[armorshell]

wow, thank you for THAT!

on number 2, is having an n amount of peaks = the same number of equivalent protons?????

I am also confused on how you determine what TYPE of peak they are ( ie,. for the ethane, it is a SINGLET???? BUT WHY!!!! HOW?! propane a septet and triplet???? what?!?!

thanks for the explanations though! helps out!!! 🙂 please.

Not sure what you're asking about the equivalent protons. The protons on different carbons (C1 compared to C2, etc...) are different, but the protons which are attached to the same carbon are equivalent.

spin/spin splitting is determined by the number of different hydrogens on adjacent carbons.

Ethane is a singlet because equivalent protons don't split each other, but non, equivalent protons follow the n+1 rule, the peak is split into the number of ajacent equivalent protons +1 peaks.

So in propane, the C2 protons have 6 adjacent equivalent protons and is split into a septet, and the C1,3 protons have 2 adjacent equivalent protons, so they are split into triplets.

 

[RockstarDMD]

[ sure what you're asking about the equivalent protons. The protons on different carbons (C1 compared to C2, etc...) are different, but the protons which are attached to the same carbon are equivalent.

spin/spin splitting is determined by the number of different hydrogens on adjacent carbons.

Ethane is a singlet because equivalent protons don't split each other, but non, equivalent protons follow the n+1 rule, the peak is split into the number of ajacent equivalent protons +1 peaks.

So in propane, the C2 protons have 6 adjacent equivalent protons and is split into a septet, and the C1,3 protons have 2 adjacent equivalent protons, so they are split into triplets.[/QUOTE]

In determining NMR it is always helpful to draw the carbon skeleton, especially if you are having difficulties.

 

[armorshell]

In determining NMR it is always helpful to draw the carbon skeleton, especially if you are having difficulties.

Oh for sure! I was assuming he had already done that 😉

 

[doctor-wanabe]

wow, thank you for THAT!

on number 2, is having an n amount of peaks = the same number of equivalent protons?????

I am also confused on how you determine what TYPE of peak they are ( ie,. for the ethane, it is a SINGLET???? BUT WHY!!!! HOW?! propane a septet and triplet???? what?!?!

thanks for the explanations though! helps out!!! 🙂 please.

Always look for symetry, that gives you a hint on the number of peaks.

 

[MansonTiles]

man....i GUESS i understand this one.

I am just having a hard time figuring out number of peaks....STILL, even after your posts!!! For some reason, i cant seem to understand this section on peaks and such from the kaplan book notes!!!!!

thank you.

 

[DMD-2-B]

man....i GUESS i understand this one.

I am just having a hard time figuring out number of peaks....STILL, even after your posts!!! For some reason, i cant seem to understand this section on peaks and such from the kaplan book notes!!!!!

thank you.

The number of peaks is equal to n+1, with n being the number of chem equiv hydrogens on the adjacent atom.