Q12-2P - Organic Chemistry (Mcmurry)

Question

Two mass spectra are shown in Figure 12-8. One spectrum is that of 2-methyl-2-pentene; the other is of 2-hexene. Which is which? Explain.

a. 

b.

Step-by-Step Solution

Answer

1. Here, the m/z peak is present at 69 indicating loss of ${\mathrm{CH}}_3$ . So, this molecule is 2 - mthyl - 2 - pentene.
2. Here, the m/z peak is present at 55 indicating loss of ${\mathrm{CH}}_3{\mathrm{CH}}_2$. So, this molecule is 2 - hexene.

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. 

Step 3: Inductive effect:

When atoms of different electronegativity form a covalent bond, then the electron density is more toward a more electronegative atom in the bond; this is known as the inductive effect (I-Effect).

–I-Effect is when the substituent attached is electron-withdrawing. +I-Effect is when the substituent attached is electron-donating. The Halogen group is an electron-withdrawing group, so it shows the –I effect, while the alkyl group attached to halogen is electron donating, so it shows the +I effect.

Step 4: Carbocation:

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

Step 5: Stability of carbocation

There are two factors for the stability of carbocations:

• Inductive effect: More the alkyl groups, more will be the inductive effect. Hence, tertiary carbocation is most stable.
• Hyperconjugation effect: Hyperconjugation refers to the delocalization of electrons with sigma bonds. The stability of carbocation increases with the increase in hyperconjugation.
• Resonance in alkene.

Step 6: Molecular formula:

The molecular formula of $2-\mathrm{methyl}-2-\mathrm{pentene}$ and $2-\mathrm{hexene}$ is ${\mathrm{C}}_6\mathrm{H}{}_{12}$ .

Step 7: Carbocations formed:

As alkene in  $\mathbf{2}-\mathbf{methyl}-\mathbf{2}-\mathbf{pentene}$ will show resonance, we get ${\left({\mathrm{CH}}_3\right)}_2\mathrm{C}={\mathrm{CHCH}}_2^{+}$ as  carbocation, which has a molecular mass of 69 g/mol, and in $2-\mathrm{hexene}$ we get ${\mathrm{CH}}_3\mathrm{CH}={\mathrm{CHCH}}_2^{+}$ carbocation which has a molecular mass of 55 g/mol. It is also resonance stabilized.

${\mathrm{CH}}_3{\mathrm{CH}}_2$ is removed in $2-\mathrm{hexene}$ and ${\mathrm{CH}}_3$ in $2-\mathrm{methyl}-2-\mathrm{pentene}$ .

Step 8: m/z peak in part (a):

Here, the m/z peak is present at 69 indicating loss of ${\mathrm{CH}}_3$. So, this molecule is $\mathbf{2}-\mathbf{methyl}-\mathbf{2}-\mathbf{pentene}$ .

Step 9: m/z peak in part (b):

Here, the m/z peak is present at 55 indicating loss of ${\mathrm{CH}}_3{\mathrm{CH}}_2$ . So, this molecule is  $\mathbf{2}-\mathbf{hexene}$.