ATAR Notes: Forum
VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: hadia.united on March 21, 2013, 05:12:58 pm
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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
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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