Question

Propose a structure for the compound that has the following spectra: NMR: ? 1.28 (3H, t, J = 7 Hz); ? 3.91 (2H, q, J = 7 Hz); ? 5.0 (1H, d, J = 4 Hz); ? 6.49 (1H, d, J = 4 Hz) ppm IR: 3100, 1644 (strong), 1104, 1166, 694 cm -1 (strong) no IR absorptions in the range 700-1100 cm -1 or above 3100 cm-1 Mass spectrum: m/z = 152, 150 (equal intensity; double molecular ion)

Answer 1

Concepts and reason

Any chemical structure can be deduced using spectroscopic techniques. There are different types of spectroscopic techniques used. In Major three types of techniques are used they are NMR (Nuclear Magnetic resonance Spectroscopy), IR (Infra-red Spectroscopy) and Mass spectrometry.

Fundamentals

In nuclear magnetic resonance spectroscopy, the number of non-equivalent protons present in the structure can be identified with the help of proton NMR. With the help of the chemical shift and neighboring protons in proton NMR, the total number of hydrogens present in the structure can be determined.

Infrared spectroscopy identifies the structure and functional group of the compound. With the help of infrared absorption range, the functional group of the structure and the double bond and triple bond present in the structure can be determined.

Mass spectroscopy identifies the total mass of the compound present in the sample. With the help of mass spectrum, there should be some assumption about the empirical formula of the structure.

Example:

Bromine atom exists as Br – 81 and Br - 79 with equal abundance. Therefore, in mass spectrometry, there exist two peaks (Example: 145 and 147). According to this way, the bromine atom present in the structure can be determined.

Also, by dividing the molecular ion peak by 13 $\left( {{\rm{C}}\,\left( {{\rm{12}}} \right){\rm{ + }}\,{\rm{H}}\,\left( {\rm{1}} \right)\,{\rm{ = }}\,{\rm{13}}} \right)$ , the assumption of empirical formula can be determined.

$\begin{array}{l}\\{\rm{m/e}}\,\,{\rm{ = }}\,{\rm{150}}\,{\rm{(Molecular}}\,{\rm{weight)}}\\\\{\rm{Divide}}\,{\rm{the}}\,{\rm{molecular}}\,{\rm{mass}}\,{\rm{by}}\,{\rm{13}}{\rm{.}}\\\\\frac{{{\rm{150}}}}{{{\rm{13}}}}\,{\rm{ = }}\,{{\rm{C}}_{\rm{n}}}{{\rm{H}}_{{\rm{n + r}}}}\,{\rm{where}}\,{\rm{n}}\,\,{\rm{is}}\,\,{\rm{numerator}}\,{\rm{r}}\,\,{\rm{is}}\,{\rm{remainder}}\\\end{array}$

$\begin{array}{l}\\\frac{{{\rm{150}}}}{{{\rm{13}}}}\,{\rm{ = }}\,{{\rm{C}}_{{\rm{11}}}}{{\rm{H}}_{{\rm{11 + 7}}}}\\\\{{\rm{C}}_{{\rm{11}}}}{{\rm{H}}_{{\rm{18}}}}\,{\rm{ - }}\,\left[ {{\rm{Br(79)}}\,{\rm{(}}{{\rm{C}}_{\rm{6}}}{{\rm{H}}_{\rm{7}}}{\rm{ = }}\,{\rm{79)}}} \right]\\\\\,\,\,\,\,\,\,\,\,\,{\rm{ = }}\,\,{{\rm{C}}_{\rm{5}}}{{\rm{H}}_{{\rm{11}}}}{\rm{Br}}\\\end{array}$

Ans:

The structure is