Question

Primary amides give a strong peak at \(m / e 44\) in their mass spectra. Indicate the nature of this peak and suggest how it might be formed.

Short answer

The \(m/e = 44\) peak is from a stable carbamoyl ion \(NH_2CO^+\), formed from primary amide fragmentation.

Step-by-step solution

Understanding the Mass Spectra

In mass spectrometry, the peak at a particular mass-to-charge ratio \(m/e\) represents the ion's molecular mass. For a primary amide to give a strong peak at \(m/e = 44\), this peak likely corresponds to a fragment ion created during fragmentation.

Identify Relevant Structures

Common primary amides have the general structure \(R-CONH_2\). In the mass spectrum, fragmentation can occur where bonds are broken, and stable ions are formed. The peak at \(m/e = 44\) is significant and must correlate with a stable ion related to the structure of an amide.

Consider Fragmentation Patterns

Analyze potential fragmentation patterns of a primary amide. When the \(-CONH_2\) group loses parts of its structure but leaves a stable ion, it could potentially give rise to the \(m/e = 44\) ion. A common fragmentation involves the loss of small stable molecules or radicals.

Identify the Fragment Ion

The \(m/e = 44\) peak is often attributed to the formation of the \(NH_2CO^+\) ion. This ion forms by the cleavage of adjacent bonds to provide a stable carbamoyl ion. This corresponds to the theoretical mass of 44 amu.

Conclude the Mechanism

The \(NH_2CO^+\) ion stems from the primary amide by fragmentation. Specifically, the carbonyl group \(C=O\) remains intact with the \(NH_2\) group to form the stable carbamoyl ion, recognized at \(m/e = 44\).

Key concepts

Primary Amides

Primary amides are a fascinating class of compounds in organic chemistry. They are characterized by the presence of a carbonyl group \((C=O)\) directly bonded to an amine \((NH_2)\). This class of compounds is generally denoted as \(R-CONH_2\), where \(R\) represents an alkyl or aryl group.
Amides are important due to their occurrence in biological molecules and their utility in synthetic chemistry. Their unique structure, with both a carbonyl and amine group, makes them reactive in certain chemical transformations and prone to specific fragmentation in mass spectrometry. Understanding these reactions and fragmentation behaviors is crucial for interpreting mass spectra data and identifying amides.
In mass spectrometry, primary amides exhibit a distinct peak due to the fragmentation of their specific bond. This process involves breaking specific bonds in the molecule and includes the generation of ions by cleaving bonds adjacent to the amide group.

Fragmentation Patterns

Fragmentation patterns in mass spectrometry reveal much about the structure of a compound. When a molecule is ionized in a mass spectrometer, it often breaks into smaller pieces or fragment ions. The pattern follows underlying chemical principles that predict which bonds are likely to break.
For primary amides, the process usually involves breaking the bonds around the amide linkage \((CONH_2)\). The energy supplied by the mass spectrometer can breakdown the molecule at weak points, forming stable ions that appear as distinct peaks in a mass spectrum.
To understand fragmentation, it's essential to know:

• The likelihood of particular bonds breaking.
• The stability of potential ions that may form, with stable ions forming at predictable locations.
• The molecular mass-to-charge ratio \((m/e)\), as it indicates the mass of specific ions.

These factors help chemists to deduce the original structure of the molecule from the peaks produced during fragmentation.

Carbamoyl Ion

The carbamoyl ion \((NH_2CO^+)\) plays a crucial role in understanding the mass spectra of primary amides. Among the different ions formed during fragmentation, the carbamoyl ion is particularly stable in the gas phase.
First, consider the process where a primary amide \((R-CONH_2)\) undergoes fragmentation in a mass spectrometer. Bond cleavage adjacent to the amide bond happens, typically leading to the formation of the carbamoyl ion.
This ion is notable for producing a peak at \(m/e = 44\), representing its molecular weight in atomic mass units (amu). The formation of this ion involves retaining the carbonyl group \((C=O)\) bonded with the amine \((NH_2)\). The stability of the carbamoyl ion is key to its strong signal in mass spectrometry, providing a crucial clue in identifying the presence of primary amides within a sample.