Q 14 P - Organic Chemistry (Mcmurry)

Question

Question: Propose structures for compounds that fit the following mass-spectral data:

(a) A hydrocarbon with ${\mathbf{M}}^{\mathbf{+}}\mathbf{=}\mathbf{132}$ 

(b) A hydrocarbon with ${\mathbf{M}}^{\mathbf{+}}\mathbf{=}\mathbf{166}$

(c) A hydrocarbon with  ${\mathbf{M}}^{\mathbf{+}}\mathbf{=}\mathbf{84}$

Step-by-Step Solution

Answer

Possible structures of molecule:

a)

                                               ${\mathrm{C}}_{10}{\mathrm{H}}_{12}$

b)

                                                    ${\mathrm{C}}_{12}{\mathrm{H}}_{22}$

                                                     

c)

                                                            ${\mathrm{C}}_6{\mathrm{H}}_{12}$

Step 1: Molecular mass

Molecular mass of a substance is the sum of atomic masses of all its atoms in a molecule.

Step 2: Calculation of molecular formula

• These molecules have only hydrogen and carbon. 

• Molecular mass of carbon is 12 and molecular mass of hydrogen is 1.

• Sum of molecular mass of C and H is $12+1=13$

• Now divide the given molecular mass by 13.

• The quotient indicates the number of carbon and number of hydrogen is equal to the quotient plus the remainder. 

Step 3: Molecular formula of given molecules

(a) A hydrocarbon with molecular weight 132: 

• Given by:  $\frac{132}{13}=10\frac{2}{13}$

• This indicates that there are 10 carbon and 12 hydrogens.

• So, the formula is ${\mathrm{C}}_{10}{\mathrm{H}}_{12}$

  
  

  
  

  
  

(b) A hydrocarbon with molecular weight 166: 

• Given by:  $\frac{166}{13}=12\frac{10}{13}$

• This indicates that there are 12 carbon and 22 hydrogen.

• So, the formula is  ${\mathrm{C}}_{12}{\mathrm{H}}_{22}$

  
  

  
  

  
  

Here one more structure is possible with formula ${\mathrm{C}}_{13}{\mathrm{H}}_{10}$.

(c) A hydrocarbon with molecular weight 84: 

• Given by: $\frac{84}{13}=6\frac{6}{13}$

• This indicates that there are 10 carbon and 12 hydrogen.

• So, the formula is ${\mathrm{C}}_6{\mathrm{H}}_{12}$