Q.12-17E - Organic Chemistry (Mcmurry)

Question

Question: The nitrogen rule of mass spectrometry says that a compound containing an odd number of nitrogen’s has an odd-numbered molecular ion.

Conversely, a compound containing an even number of nitrogen’s has

an even-numbered M⁺peak. Explain.

Step-by-Step Solution

Answer

Answer

Carbon is tetravalent, and nitrogen is trivalent if a C-H unit (formula weight 13) is replaced by an N atom (formula weight 14), the molecular weight of the resulting compound increases by one unit.

Step 1: Nitrogen rule

According to nitrogen rule an organic compound having odd molar mass contains odd number of nitrogen atoms, and compound having even molar mass contains even number of nitrogen atoms.

Step 2: Relating nitrogen rule to molecular ion

The Carbon atom is tetravalent, and nitrogen is trivalent if a C-H unit (formula weight 13) is replaced by an N atom (formula weight 14), the molecular weight of the resulting compound increases by one unit. The general formula for hydrocarbon is given below.

                                 C_nH_2n+2(alkanes), C_nH_2n (alkenes), C_nH_2n-2 (alkynes)

Since all neutral hydrocarbon have even-numbered molecular weights, the resulting nitrogen containing compound have odd-numbered molecular weights. If two C-H units are replaced by two N-atoms, the molecular weight of resulting compound increases by two and remains an even number.