Q12-4P - Organic Chemistry (Mcmurry)

Question

List the masses of the parent ion and of several fragments you might expect to find in the mass spectrum of the following molecule:

Step-by-Step Solution

Answer

The mass of the parent ion is 102 g/mol.

The possible mass of the charged fragments is 87, 84, and 59 g/mol.

Step 1: Molecular mass

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

Step 2: Fragmentation:

Fragmentation happens when high-energy cation radical flies apart by spontaneous cleavage of the bond. One fragment has the positive charge, carbocation, while the other fragment is a neutral radical. 

The positive charge often remains with a more stable carbocation that forms.

Step 3: Carbocation:

Carbocations are species in which a carbon atom has a positive charge. They are electron deficient species.

Step 4: Alpha-cleavage:

Alpha-cleavage ( $\mathrm{\alpha }$-cleavage) indicates breaking the carbon-carbon bond adjacent to the carbon bearing a specified functional group. Here, cleavage forms one carbocation and one radical.

Cleavage can occur on either side of the molecule but practically on the side, giving more stable carbocation. 

Step 5: Dehydration:

Dehydration results in the loss of water molecules and forms an alkene cation radical. 

Step 6: Methods to form fragments from parent ion:

• Dehydration results in the loss of water molecules and forms an alkene cation radical. 
• Alpha-cleavage ( $\mathrm{\alpha }$-cleavage)

Step 7: Carbocations formed by alpha-cleavage:

Given molecule is  $\mathbf{2}-\mathrm{Methyl}-2-\mathbf{pe}\mathrm{ntanol}$   and has a molecular weight of 102 g/mol. Here, alpha-cleavages can give two carbocations depending on the bond which is cleaved. The two carbocations are   ${[{\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{C}\left({\mathrm{CH}}_3\right)=\mathrm{OH}]}^{+}$and  ${[{\mathrm{CH}}_3\mathrm{C}\left({\mathrm{CH}}_3\right)=\mathrm{OH}]}^{+}$ .

${[{\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{C}\left({\mathrm{CH}}_3\right)=\mathrm{OH}]}^{+}$  is formed when methyl radical is cleaved and ${[{\mathrm{CH}}_3\mathrm{C}\left({\mathrm{CH}}_3\right)=\mathrm{OH}]}^{+}$  is formed when propyl radical is cleaved.

${[{\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{C}\left({\mathrm{CH}}_3\right)=\mathrm{OH}]}^{+}$  has a molecular mass of 87 g/mol and  ${[{\mathrm{CH}}_3\mathrm{C}\left({\mathrm{CH}}_3\right)=\mathrm{OH}]}^{+}$ has molecular mass of 59 g/mol.

Alpha-cleavage

Step 8: Carbocation formed by dehydration:

In dehydration, the water molecule is removed from  $\mathbf{2}-\mathrm{Methyl}-2-\mathbf{pe}\mathrm{ntanol}$, which forms ${[{\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{CH}=\mathrm{C}{\left({\mathrm{CH}}_3\right)}_2]}^{+.}$  cation radical. The molecular mass of this cation radical is 84 g/mol.

Dehydration