Question

For the graphs, people write the structure of this compound and explanation for analysis for each graphs.

Molecular Formula: C9H12

Problem 3 - IR spectrum 100 80- 60- 40- 20- 3500 3000 2000 2500 1500 1000 Wavenumber (cm-1) -1027 -1095 1337 1378 -1455 -1602 1747 1801 1873 1941 2865 -3027 3062 Transmittance

Answer 1

Problem 3 :

The unknown structure has a molecular formula C₉H₁₂. It does not contain any hetero atoms. So, this compound is a purely hydrocarbon. Now we will use all the above spectral details to identify the unknown compound.

IR spectra :

• At 3027 cm-1 there is a signal which corresponding to C-H sp2 stretch
• At 2900 cm-1 there is a C-H sp3 stretch signal.
• At 1602 cm-1 there is C=C stretch signal
• around 1500 to 1450 cm-1 there are two signals which corresponding to the aromatic C=C stretch
• Around 899 and 741 cm-1 there are two peaks which represents the aromatic group is mono-substituted

Therefore, the unknown compound has a aromatic group and its mono-substituted with a alkyl group.

¹H-NMR spectra :

• In the aromatic region, around 7.3 region there is a multiplet which is corresponding to the 5 protons present in the benzene ring of unknown compound.
• At 1 ppm there is a triplet of 3 protons which is corresponding to the -CH3 terminal group
• At 1.7 ppm there is a sextet of 2 protons which is corresponding to the -CH2- group
• Another -CH2- groups gives a triplet at 2.6 ppm region
• Therefore, there is a -CH2-CH2-CH3 (propyl group) group attached to the benzene ring.  

  

Ia CH2 CH3 12 CH2 propylbenzene Y H Hd Type of Number of Splitting Chemical shift proton protons pattern (ppm) triplet sextet 1.7 2.6 triplet multiplet 7.3

¹³C-NMR spectra :

• In the aromatic region, around 142, 128.4, 128.1 and 125 ppm region there are 4 signals  which is corresponding to the 6 carbon atoms ( in that 2 carbon atoms are equivalent to each other) present in the benzene ring of unknown compound.
• At 13.8 ppm there is a signal of -CH3 carbon atom which experience downfield.
• At 24 and 38 ppm there are two -CH2- groups.
• Totally it gives 7 peaks in 13 C-NMR spectra.

Mass spectrum :

• Here, the molecular mass of the compound is 120, thus it gives a signal at 120 which is called as molecular ion peak. (m/z= 120)
• It gives a base peak (longest peak) at m/z = 91 which corresponding to C6H6CH2⁺ or it forms tropylium ion which gives the base peak
• There are many fragment ions are present at m/z = 78 (benzene group), 65 , 51 etc

Thereore, if we examine all the above spectra, the given unknown compound is propylbenzene

propylbenzene