Q24E - Organic Chemistry (Mcmurry)

Question

Question: 2-Methylpentane (C₆H₁₄)has the mass spectrum shown. Which peak

represents? Which is the base peak? Propose structures for fragment

ions of m/z =71, 57, 43, and 29. Why does the base peak have the mass it does?

Step-by-Step Solution

Answer

Answer

 

The base peak is at m/z = 43. The molecular ion, at m/z = 86, is present in very low abundance. The base peak at m/z = 43, represents a stable secondary carbocation.

Step 1: Base peak

The most intense peak in the mass spectrum is called the base peak. It is assigned a relative intensity of 100%, while the intensities of all other peaks including parent ion peak are expressed as percentage of base peak.

Step 2: Determine the base peak

m/z = 43 is the base peak. The molecular ion, at m/z = 86, is present in very low abundance. The base peak at m/z = 43, represents a stable secondary carbocation. So, the base peak has the mass.

Step 3: Propose structures for fragment ions

The structures for fragment ions of m/z = 71, 57, 43, and 29 are shown below.

                                       Structure of fragmented ions

 

The base peak at m/z = 43, represents a stable secondary carbocation. So, the base peak has the mass.