Answers
Revision Questions #5

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1.
 		This is simply an exercise in predicting the multiplicity observed using the formula: multiplicity = 2nI + 1.
i)
 The 1H NMR spectrum of CH4 will be a singlet (no multiplicity) since the molecule is tetrahedral in shape and all of the protons are equivalent. There is no coupling from carbon since almost all C is 12C and this is NMR silent.
ii)
 The 13C NMR spectrum of CH4 is a quintet. The C is coupled to 4 equivalent protons and the spin of 1H is ½.

(2nI + 1) = (2 x 4 x ½) + 1 = 5
iii)
 The 1H NMR spectrum of NH4+ will have 3 lines. The molecule is tetrahedral in shape and all of the protons are equivalent and these will be coupled to 14N (which has a spin I = 1).

(2nI + 1) = (2 x 1 x 1) + 1 = 3
iv)
 The 14N NMR spectrum of NH4+ is a quintet. The 14N is coupled to 4 equivalent protons and the spin of 1H is ½.

(2nI + 1) = (2 x 4 x ½) + 1 = 5
v)
 The 1H NMR spectrum of PH3 is a doublet. The molecule (like ammonia) is pyramidal and all 3 protons are equivalent. The protons (with spin I = ½) are coupled to one 31P nucleus.

(2nI + 1) = (2 x 1 x ½) + 1 = 2
vi)
 The 31P NMR spectrum of PH3 will have 4 lines (quartet). The molecule is pyramidal in shape and all of the protons are equivalent (with a spin I = ½). and these will be coupled to 31P.

(2nI + 1) = (2 x 3 x ½) + 1 = 4
vii)
 The 13C NMR spectrum of H2PCH2PH2 will be a triplet of triplets. The 13C will be coupled to 2 x 31P (giving a triplet splitting) and the 13C will be coupled to 2 x 1H (giving a triplet splitting).
viii)
 The 13C NMR spectrum of (H2P)3CH will be a doublet of quartets. The 13C will be coupled to 3 x 31P (giving a quartet splitting) and the 13C will be coupled to 1 x 1H (giving a doublet splitting).

2.
i)
 Thiophene

 AA’XX’
ii)
 CH3CH2OCH3

 A3M2X3
iii)
 CH2=CH2

 A4
iv)
 m-dibromobenzene

 AMX2
v)
 CH2BrCH2Br

 A4
vi)
 CH2BrCHBrI
 AMX
  Note that this molecule has a chiral centre so the protons of the CH2 group are prochiral and hence they are chemically non equivalent.
vii)
 o-dibromobenzene

 AA’XX’
viii)
 p-dibromobenzene

 A4
ix)
 p-bromotoluene

 AA’XX’M3
x)
 o-xylene

 AA’XX’M3M3’
xi)
 cis-1,2-dichlorocyclopropane

 A2MX
xii)
 trans-1,2-dichlorocyclopropane

 AA’XX’
Any spin system with chemically equivalent nuclei which are magnetically non equivalent cannot be analysed by first order rules.

3.
i)
 The label A3MXY indicates that:
(a) There are 6 nuclei in the spin system, and
(b) The 6 nuclei occur at 4 different shifts with one shift environment having 3 chemically equivalent nuclei and the others having 1 proton; (c) The fact that two of the spins are given the letters X and Y (which are close together in the alphabet) indicates that this part of the spin system has Dn/J <3.
ii)
 Because one part of the spin system has Dn/J <3, the spin system is not first order and could not be analysed using normal first order rules.

4.
 		The signals in the spectrum can be readily assigned by inspection:

  • d 9.5 is due to the aldehydic proton - it appears as a doublet due to coupling with the vinylic proton on the adjacent carbon.

  • d 6.0 is due to the vinyl proton at C2. It appears as a doublet of doublets with one doublet splitting due to the aldehyde proton and the other doublet splitting due to the vinylic proton at C3.

  • d 6.9 is due to the vinyl proton at C3. It appears as a doublet of triplets with the doublet splitting due to the vinylic proton at C2 and the triplet splitting due to coupling to the CH2 group at C4.

  • d 2.2 is due to the CH2 group at C4. It appears as a multiplet but must be a doublet of quartets with the doublet splitting due to coupling to the vinylic proton at C3 and the quartet splitting due to the CH3 group at C5.

  • d 1.0 is due to the CH3 group at C5. It appears as a triplet due to coupling to the CH2 group at C4 (d 2.2).

    The effect of irradiation at each of these frequencies is to decouple the nuclei which have signals at these frequencies. Decoupling effectively removes the nucleus from the spin system.

    		•
    i)
    		 With irradiation at d 9.5, the multiplicity of the vinylic CH resonance at d 6.0 is simplified from a doublet of doublets to a doublet. The remaining resonances in the spectrum are unchanged.
    ii)
    		 With irradiation at d 6.9, the multiplicity of the vinylic CH resonance at d 6.0 is simplified from a doublet of doublets to a doublet and the CH2 resonance at C4 (d 2.2) is simplified from a doublet of quartet to a simple quartet. The remaining resonances are unchanged.

    iii)
    		 With irradiation at d 6.0, the multiplicity of the vinylic CH resonance at d 6.9 is simplified from a doublet of triplets to a simple triplet and the resonance of the aldehyde proton is simplified from a doublet to a singlet.
    iv)
    		 With irradiation at d 2.2, the multiplicity of the vinylic CH resonance d 6.9 is simplified from a doublet of triplets to a simple doublet and the multiplicity of the CH3 resonance (d 1.0) is simplified from a triplet to a singlet.
    v)
    		 With irradiation at d 1.0, the multiplicity of the CH2 group at C4 (d 2.2) is simplified from a doublet of quartets to a simple doublet. The remaining resonances in the spectrum are unchanged.

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    University of Sydney