NMR

Question 1

¹H NMR features

Answer 1

• one peak for each set of equivalent H atoms
  
  • equivalent H atoms are bonded to the same groups/atoms
• the intensity of the peak is proportional to the number of equivalent H atoms within the set
  
  • area of peak is proportional to the number of H atoms it represents - given as a number usually a decimal
  • the simplest whole number ratio of the relative areas tells you how many equivalent H atoms for each peak
• chemical shift tells you how far along the x-axis the peak will be found
  
  • protons in the same environment are shifted by the same amount
  • the size of chemical shift depends on the atoms/groups bonded to the H
  • the more electronegative the group , the greater the shift
    *

Question 2

samples are dissolved in solvent free of ¹H atoms, explain why and give examples of alternative solvents

Answer 2

to prevent signals in spectrum due to H atoms in solvent

CCl₄ and CDCl₃

Question 3

what is the standard used for NMR why is it used?

Answer 3

TMS (tetramethylsilane)

low electronegativity of Si means the signal produced is far away to the right from other organic molecules (chemical shift of 0ppm)

only produces one signal as all H and C atoms in the same environment

non-toxic

inert

volatile due to low BP so easily removed from sample

Question 4

how many signals would be produced for ethylpropanoate and what is the ratio of each signal

Answer 4

Question 5

what does splitting mean?

Answer 5

the number of smaller peaks a signal is split into gives us information about the H atoms on adjacent C atoms

n+1 rule = number of Hs on adjacent Carbon + 1

Question 6

here is the spectrum for a compound with molecular formula C₄H₈O₂ , deduce its structure

Answer 6

when you see a quartet and a triplet this usually indicates a ethyl group is present

Question 7

when does splitting not occur?

Answer 7

equivalent H atoms on the next atoms and OH groups do not cause splitting (so produce a singlet)

Question 8

deduce the structure of this compound

its molecular formula is C₆H₁₄O

Answer 8

the IR spectrum tell us the O is part of an OH group

integration ratio is 4:1:3:6

4 peaks so there are 4 different H environments

singlet with integration 1 must be due to H in OH alcohol group

singlet with integration 3 due to CH₃ with no adjacent H (attached to 3 R groups)

quartet + triplet indicates CH₃CH₂ group

integration 4 and 6 indicates there are 2 CH₃CH₂ groups

Question 9

deduce the structure of this compound

Answer 9

Question 10

¹³C NMR

Answer 10

no link between area under peak and number of equivalent C atoms

no splitting

spectrum contains a number of single peak that correspond to the number of sets of equivalent C atoms, the chemical shift determines how far along the x-axis the peak is

Question 11

determine the number of peak in the ¹³C spectrum for each compound and give the IUPAC name

Answer 11