Chemsitry Checkpoints Question - help needed for the answer.

[hadia.united]

a hydrocarbon has the formula C6H14. its carbon 13 nmr spectrum consists of 2 lines at 33.89ppm and 19.49ppm. which one of the structures below is consistent with the information:

(a) CH3-CH2-CH2-CH2-CH2-CH3
(b) CH3-CH2-CH2-CH(CH3)₂
(c) CH3-CH2-CH(CH3)-CH2-CH3
(d) (CH3)₂CH-CH(CH3)₂

[dzames]

Hey there!

The question says that there are only 2 lines on the NMR spectrum. So this means that there must only be 2 unique carbon environments in the molecule. Now we have more than 2 carbons in this molecule, so we're gonna have a few carbon environments that are the same. Let's look at the options:

a) this is symmetrical right down the middle. So cut it down the middle and look at one side, how many unqiue carbons are there? 3 - no good
b) The only carbon environments that are the same are the ones on the very end. Every other one is unique, thats 5 carbon environments - no good
c) This one is also symmetrical down the middle. Cut it down the middle, look at one side, and count the carbons that straddle both sides. 5 carbon environments - no good
d) this one is again symmetrical down the middle. So if we cut down that middle C-C bond and look just at one side we get (CH3)2CH. now both of those CH3 groups are identical. So that's 1 carbon environment. Add on the CH, thats 2. So this molecule only has 2 unique carbon environments - BINGO

Hope that helps!
James