Question

(This is a long exercise suitable for group work.) Relative intenities for the molecular ion region of several compounds are listed in arts (a)-(d) and shown in the figure. Suggest a composition for each nolecule and calculate the expected isotopic peak intensities.

a) $\mathbf{m}\mathbf{/}\mathbf{z}\mathbf{}\mathbf{\left(}\mathbf{intensity}\mathbf{\right)}\mathbf{:}\mathbf{}\mathbf{94}\mathbf{}\mathbf{\left(}\mathbf{999}\mathbf{\right)}\mathbf{,}\mathbf{95}\mathbf{}\mathbf{\left(}\mathbf{68}\mathbf{\right)}\mathbf{,}\mathbf{96}\mathbf{}\mathbf{\left(}\mathbf{3}\mathbf{\right)}$

b)$\mathbf{m}\mathbf{/}\mathbf{z}\mathbf{}\mathbf{\left(}\mathbf{intensity}\mathbf{\right)}\mathbf{:}\mathbf{}\mathbf{156}\mathbf{}\mathbf{\left(}\mathbf{566}\mathbf{\right)}\mathbf{,}\mathbf{157}\mathbf{}\mathbf{\left(}\mathbf{46}\mathbf{\right)}\mathbf{,}\mathbf{158}\mathbf{}\mathbf{\left(}\mathbf{520}\mathbf{\right)}\mathbf{,}\mathbf{159}\mathbf{}\mathbf{\left(}\mathbf{35}\mathbf{\right)}$

c) m/z $\mathbf{\left(}\mathbf{intensity}\mathbf{\right)}\mathbf{:}\mathbf{}\mathbf{224}\mathbf{}\mathbf{\left(}\mathbf{791}\mathbf{\right)}\mathbf{,}\mathbf{225}\mathbf{}\mathbf{\left(}\mathbf{63}\mathbf{\right)}\mathbf{,}\mathbf{226}\mathbf{}\mathbf{\left(}\mathbf{754}\mathbf{\right)}\mathbf{,}\mathbf{227}\mathbf{}\mathbf{\left(}\mathbf{60}\mathbf{\right)}\mathbf{,}\mathbf{28}\mathbf{\left(}\mathbf{264}\mathbf{\right)}\mathbf{,}\mathbf{229}\mathbf{\left(}\mathbf{19}\mathbf{\right)}\mathbf{,}\mathbf{230}\mathbf{\left(}\mathbf{29}\mathbf{\right)}$

d)$\mathit{m}\mathbf{}\mathbf{/}\mathbf{}\mathit{z}\mathbf{}\mathbf{\left(}\mathit{i}\mathit{n}\mathit{t}\mathit{e}\mathit{n}\mathit{s}\mathit{i}\mathit{t}\mathit{y}\mathbf{\right)}\mathbf{:}\mathbf{}\mathbf{154}\mathbf{\left(}\mathbf{122}\mathbf{\right)}\mathbf{,}\mathbf{155}\mathbf{}\mathbf{\left(}\mathbf{9}\mathbf{\right)}\mathbf{,}\mathbf{156}\mathbf{}\mathbf{\left(}\mathbf{12}\mathbf{\right)}\mathbf{}\mathbf{}\mathbf{(}\mathit{H}\mathit{i}\mathit{n}\mathit{t}\mathbf{:}\mathbf{}\mathit{C}\mathit{o}\mathit{n}\mathit{t}\mathit{a}\mathit{i}\mathit{n}\mathit{s}\mathbf{}\mathit{s}\mathit{u}\mathit{l}\mathit{f}\mathit{u}\mathit{r}\mathbf{.}\mathbf{)}$

Short answer

(a) Expected intensity of $\mathrm{M}+2=0.0058\left(6\right)\left(5\right)+0.205\left(1\right)=0.38\%$

Observed intensity of $\mathrm{M}+2=0.3\%$

(b) The predicted intensity of $({\mathrm{C}}_6{{\mathrm{H}}_5}^{81}\mathrm{Br})$at M+2 is $0.0654(97.3)=6.4\%$of ${\mathrm{M}}^{+}$

Observed intensity of M+3 is $35/566=6.2\%.$

(c) Expected intensity of M+4 from${\mathrm{C}}_7{\mathrm{H}}_3{{\mathrm{O}}_2}^{35}{{\mathrm{Cl}}_2}^{37}\mathrm{Cl}\mathrm{is}$$5.11\left(3\right)\left(2\right)=30.7\%$ of${\mathrm{M}}^{+}$

Observed intensity of $=29/791=3.7\%.$

(d) Expected intensity of $\mathrm{M}+2=0.0058\left(4\right)\left(3\right)+0.205\left(2\right)+4.52\left(2\right)=9.52\%$

Observed intensity of$M+2=12/122=9.8\%.$

Step-by-step solution

Concept used

Step 2: m/z (intensity): 94 (999),95 (68),96 (3)

(a)

Form the given intensity we can identified a compound for${\mathbf{M}}^{\mathbf{+}}\mathbf{}\mathbf{=}\mathbf{94}$is phenol.

The molecular formula for phenol is${\mathrm{C}}_6{\mathrm{H}}_6\mathrm{O}$structure of phenol is

Now,

$\mathrm{Rings}+\mathrm{double}\mathrm{bonds}=\mathrm{c}-\mathrm{h}/2+\mathrm{n}/2+1\mathrm{n}=6-6/2+0/2=4$

The expected intensity for M+1 from the given table 21-2 :

$1.08\left(6\right)+0.012\left(6\right)+0.038\left(1\right)=6.59\%\mathrm{C}\mathrm{H}\mathrm{O}$Observed intensity of$\mathrm{M}+1=68/999=6.8\%$

Expected intensity of $\mathrm{M}+2=0.0058\left(6\right)\left(5\right)+0.205\left(1\right)=0.38\%$

Observed intensity of $M+2=0.3\%$

Step 3: m/z  (intensity): 156 (566),157 (46),158 (520),159 (35) 

(b)

Form the given intensity we can identified a compound for $M^{+-}=156$is bromophenol.

The molecular formula for bromophenol is${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{Br}$structure of phenol is

Now,

Here, h includes H+Br

The expected intensity for $\mathrm{M}+1=1.08\left(6\right)+0.012\left(5\right)=6.54\%\mathrm{C}\hspace{0.33em}\hspace{0.33em}\mathrm{H}$

Observed intensity of $\mathrm{M}+1=45/566=8.1\%$

Expected intensity of $\mathrm{M}+2=0.0058\left(6\right)\left(5\right)+97.3\left(1\right)=97.5\%$

Observed intensity of $\mathrm{M}+2=520/566=91.9\%$

The isotopic partner of M+2 is $\mathrm{M}+3({\mathrm{C}}_6{{\mathrm{H}}_5}^{81}\mathrm{Br})\mathrm{and}\mathrm{which}\mathrm{has}{}^{81}\mathrm{Br}{\mathrm{pluseitherone}}^{13}{\mathrm{Corone}}^1\mathrm{H}.$

Hence, the expected intensity of M+3 is $1.08\left(6\right)+0.012\left(5\right)=6.54\%$

The predicted intensity of $({\mathrm{C}}_6{{\mathrm{H}}_5}^{81}\mathrm{Br})$at M+2 is $0.0654(97.3)=6.4\%$of $M^{+}$

Observed intensity of M+3 is 35 / 566 =6.2 %

Step 4:  m/z (intensity): 224 (791),225 (63),226 (754),227 (60),28(264),229(19),230(29)

(c)

From the given data${\mathrm{M}}^{+}=224$we can assign these value for the following compound and which has the correct structure shown below. There is no some other way to assign the isomeric structure from the given data.

The molecular formula is$C_7{H}_3{O}_{2}C{l}_3$

The expected intensity for

$$\begin{gathered} \mathrm{M}+\mathrm{I}=1.08\left(7\right)+0.012\left(3\right)+0.038\left(2\right)=7.67\% \\ \mathrm{C}\mathrm{H}\mathrm{O} \end{gathered}$$

$$

Observed intensity of$\mathrm{M}+1=63/791=8.0\%$

Expected intensity of$\mathrm{M}+2=0.0058\left(7\right)\left(6\right)+0.205\left(2\right)+32.0\left(3\right)=96.7\%$

Observed intensity of$M+2=754/791=95.4\%$

The M+3 peak is the isotopic partner of $C_7{H}_3{O_2}^{35}C{l_2}^{37}ClatM+2.M+2ha{s}^{37}Clpluseitheron{e}^{13}Coron{e}^{1}Ho{r}^{17}O.$

Expected intensity of M+3 is$1.08\left(7\right)+0.012\left(3\right)+0.038\left(2\right)=7.67\%$

Predictedintensity of $(C_7{H}_3{O_2}^{35}C{l_2}^{37}Cl)$at $\mathrm{M}+2\mathrm{is}32.0\left(3\right)=96.0\%$of ${\mathrm{M}}^{+}$

Expected intensity of M+3=7.67% of 96.0%=7.4% of${\mathrm{M}}^{+}$

Observed intensity =60 /791 =7.6 /%

For M+4 which is completely composed of$C_7{H}_3{O_2}^{35}C{l}^{37}C{l}_2$along with little amount of$C_7{H}_3{O_2}^{35}C{l_2}^{37}C{l}^2.$

Other formulas like ${}^{12}\mathrm{C}_6^{13}{\mathrm{CH}}_6{}^{16}\mathrm{O}{}^{17}\mathrm{O}{}^{35}\mathrm{CI}_2{}^{37}\mathrm{CI}$also add up to M+4, which has two minor isotopes of ${}^{13}C$and ${}^{17}O$but there less likely to occur.

Expected intensity of M+4 from ${\mathrm{C}}_7{\mathrm{H}}_3{\mathrm{O}}_2{}^{35}\mathrm{CI}_2{}^{37}\mathrm{CI}$5.11(3)(2) =30.7% of${\mathrm{M}}^{+}$

The contribution from ${\mathrm{C}}_7{\mathrm{H}}_3{{\mathrm{O}}_2}^{35}{{\mathrm{Cl}}_2}^{37}\mathrm{Cl}$on basis of the predicted intensity of ${\mathrm{C}}_7{\mathrm{H}}_3{{\mathrm{O}}_2}^{35}{{\mathrm{Cl}}_2}^{37}\mathrm{Cl}\mathrm{at}$M+2

The predicted intensity of ${\mathrm{C}}_7{\mathrm{H}}_3{{\mathrm{O}}_2}^{35}{{\mathrm{Cl}}_2}^{37}\mathrm{Cl}\mathrm{is}$32.0(3) =96.0% of ${\mathrm{M}}^{+}$

The predicted intensity from ${\mathrm{C}}_7{{\mathrm{CH}}_3}^{16}{\mathrm{O}}^{18}{\mathrm{O}}^{35}{{\mathrm{Cl}}_2}^{37}\mathrm{Cl}\mathrm{at}$M+4 is 0.205(2)=0.410% of 96.0%=0.4.

The total expected intensity of M+4 is 30.7%+0.4%=31.1% of ${\mathrm{M}}^{+}$

Observed intensity =264 /791=33.4 /%

Expected intensity of M+5 from ${}^{12}\mathrm{C}_6{}^{13}\mathrm{CH}_3{\mathrm{O}}_2{}^{35}\mathrm{Cl}{}^{37}\mathrm{Cl}_2$and ${}^{12}\mathrm{C}_7{\mathrm{H}}_2{}^2\mathrm{HO}_2{}^{35}\mathrm{Cl}^{37}{\mathrm{Cl}}_2$ and $C_7{H_3}^{16}{O}^{17}{O}^{35}C{l}^{37}C{l}_2$which is based on the predicted intensity of $C_7{H}_3{O_2}^{35}C{l}^{37}C{l}_2$at m+4.m+5 should have 1.08(7)+0.012(3)+0.038(2)=7.7% of $C_7{H_3}^{16}{O}^{17}{O}^{35}C{l}^{3}7C{l}_2$at m+4=7.7% of 30.7%=2.4%

Observed intensity =29 / 791=3.7%

Step 5: m / z (intensity): 154(122),155 (9),156 (12)  (Hint: Contains sulfur.)

(d)

From the given data${\mathrm{M}}^{+}=154$we can assign these value for the following compound and which has the correct structure shown below. The observed M+2 is 12/122=9.8% which indicates that the compound is having two sulphur atoms. The composition${\mathrm{C}}_4{\mathrm{H}}_{10}{\mathrm{O}}_2{\mathrm{S}}_2$is exact match for this compound with two sulphur atoms and having molecular mass of 154.

The structure for this compound can be shown as follows.

Now let's find the following things,

Rings + double bonds$=c-h/2+n/2+1=4-10/2+0/2$

=0+1

The expected intensity$forM+1:$

$1.08\left(4\right)+0.012\left(10\right)+0.038\left(2\right)+0.801\left(2\right)=6.12\%\mathrm{C}\mathrm{H}\mathrm{O}\mathrm{S}$

Observed intensity of$\mathrm{M}+1=9/122=7.4\%$

Expected intensity of$\mathrm{M}+2=0.0058\left(4\right)\left(3\right)+0.205\left(2\right)+4.52\left(2\right)=9.52\%$

Observed intensity of$\mathrm{M}+2=12/122=9.8\%.$