What are the masses of the charged fragments produced in the...

Key Concepts

McLafferty Rearrangement

The McLafferty rearrangement is an intramolecular process observed in the mass spectrometry of carbonyl-containing compounds. It involves the transfer of a hydrogen from a gamma position relative to the carbonyl group to the oxygen, leading to a specific fragmentation pattern that provides vital structural insights about the molecule.

Dehydration Reaction in Mass Spectrometry

The dehydration reaction in mass spectrometry involves the loss of a water molecule from a precursor ion, a common pathway for alcohol-containing compounds. This reaction results in a characteristic mass loss, typically 18 amu, and can help confirm the presence and position of hydroxyl groups in a molecule.

Alpha Cleavage

Alpha cleavage is a fragmentation process where the bond adjacent to a functional group, such as a carbonyl or a heteroatom, is broken. This results in the formation of a charged fragment and a neutral species, and is particularly useful for deducing the structure around reactive centers in molecules.

Fragmentation Mechanisms

Fragmentation mechanisms in mass spectrometry describe how molecular ions break apart into smaller charged fragments under energetic conditions. Understanding these processes is crucial for interpreting mass spectra, as different functional groups and molecular structures tend to fragment in characteristic ways.

Mass Spectrometry

Mass spectrometry is an analytical technique used to identify compounds by ionizing chemical species and sorting the resulting charged fragments based on their mass-to-charge (m/z) ratios. It provides both molecular weight and structural information about a substance by analyzing the pattern and masses of the fragments formed.

Transcript

00:01 This is the answer to chapter 12, problem number three from the mcmurray organic chemistry textbook.

00:08 This problem asks us, what are the masses of the charged fragments produced in the following cleavage pathways? so we're given four cleavage pathways, and we're asked to identify the masses of those fragments.

00:25 Okay, so for a, we have alpha cleavage of two pentanone.

00:29 So i've drawn two pentanone here.

00:32 And the thing to remember is that alpha cleavage can happen between the carbonyl carbon and either of the adjacent carbons.

00:42 So we have one pathway that's going to be a cleavage at that bond that i've drawn in red.

00:48 And then a second pathway would be a cleavage of this bond that i've drawn a line through in green.

00:55 So the red pathway is going to produce a methyl radical.

01:12 So the product is going to be ch3 radical.

01:23 And then it's also going to produce the charged species, which would be the species that we detect in the mass spectrometer.

01:32 And so that is going to look like this.

01:51 And the charge here is going to be a positive charge.

01:54 This is a cation.

01:58 And the m to z here is going to be 71.

02:05 Okay.

02:06 So that's one pathway here.

02:10 The other pathway is going to be cleavage of the green bond.

02:16 And so in that case, what we're going to get is going to be this species.

02:36 And then this will be the one that has the positive charge.

02:40 So that's the one that we will actually detect in the mass spectrometer.

02:47 And then the other species will be this one.

02:58 And this will be the radical.

03:01 And the m to z here, and again, it's the first fragment that we're going to be detecting.

03:06 And the m to z here is going to be 43.

03:10 And so in reality, theoretically, either of these could take place.

03:15 In reality, we're going to get fragmentation at the bond with more substituted carbon, because that's going to make for a more stable species.

03:27 And so the only one that we're actually going to see is the m to z 43.

03:32 But theoretically, either one could happen.

03:34 And so that's the appropriate answer here.

03:37 So then for part b, we are asked dehydration of cyclohexanol.

03:43 And so remember, dehydration is going to be basically the water gets kicked out or takes off and leaves...

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