Topics

What a structure determination has to do

What the work is

You are given spectra and asked to identify aspects of a structure, determine it, or justify it.
The scope is unusually tightly defined, which is good news — there is a finite list of things you can be asked.

Technique	What it measures	What it tells you
Mass spectrometry	mass-to-charge ratio of ions	molar mass; presence of N, Cl or Br; fragments
Infrared (IR)	absorption of infrared radiation by vibrating bonds	which functional groups are present
¹³C NMR	the environment of each carbon nucleus	how many different carbon environments, and what kind

There is no ¹H NMR here. The only NMR used is carbon-13, so ignore anything you meet about splitting, coupling or integration.

Eleven families, and no others:

alkanes, alkenes — the hydrocarbons
alcohols, haloalkanes, amines — one heteroatom, no carbonyl
aldehydes, ketones, carboxylic acids, esters, amides, acid chlorides — the carbonyl families
Being able to list these is genuinely useful, because it bounds the answer. If your data is consistent with something outside this list, re-check the data.

There are three, and only three:

Molar mass and molecular formula
Functional groups
The carbon framework, including structural isomers
Structural isomers is where most of the difficulty lives. Two isomers have the same molar mass and often the same functional group, so distinguishing them is what ¹³C NMR is for.

Identify discrete aspects. Each answer is a single fact read from a single spectrum. It is the floor: correct, and it says nothing about what the molecule is.
- "The molecular ion peak is at m/z 74, so the molar mass is 74 g mol⁻¹."
- "The broad absorption at 3300 cm⁻¹ indicates an O–H group."
- "There are three signals, so there are three carbon environments."
Determine the structure. You combine the facts into a structure and explain the reasoning at each step.
Justify it by integrating. You show the three techniques agree, and you explicitly eliminate the structures that would also have fitted some of the data.

Not "I used all three spectra".
It means showing that the evidence converges: each technique narrows the possibilities, and the surviving structure is consistent with all of them simultaneously.
The clearest signal that you are integrating is that you can say why the alternative isomer is ruled out, and by which piece of data.

You are given spectra and asked to identify aspects of a structure, determine it, or justify it.
The scope is unusually tightly defined, which is good news — there is a finite list of things you can be asked.

Technique	What it measures	What it tells you
Mass spectrometry	mass-to-charge ratio of ions	molar mass; presence of N, Cl or Br; fragments
Infrared (IR)	absorption of infrared radiation by vibrating bonds	which functional groups are present
¹³C NMR	the environment of each carbon nucleus	how many different carbon environments, and what kind

There is no ¹H NMR here. The only NMR used is carbon-13, so ignore anything you meet about splitting, coupling or integration.

Eleven families, and no others:

alkanes, alkenes — the hydrocarbons
alcohols, haloalkanes, amines — one heteroatom, no carbonyl
aldehydes, ketones, carboxylic acids, esters, amides, acid chlorides — the carbonyl families
Being able to list these is genuinely useful, because it bounds the answer. If your data is consistent with something outside this list, re-check the data.

There are three, and only three:

Molar mass and molecular formula
Functional groups
The carbon framework, including structural isomers
Structural isomers is where most of the difficulty lives. Two isomers have the same molar mass and often the same functional group, so distinguishing them is what ¹³C NMR is for.

Identify discrete aspects. Each answer is a single fact read from a single spectrum. It is the floor: correct, and it says nothing about what the molecule is.
- "The molecular ion peak is at m/z 74, so the molar mass is 74 g mol⁻¹."
- "The broad absorption at 3300 cm⁻¹ indicates an O–H group."
- "There are three signals, so there are three carbon environments."
Determine the structure. You combine the facts into a structure and explain the reasoning at each step.
Justify it by integrating. You show the three techniques agree, and you explicitly eliminate the structures that would also have fitted some of the data.

Not "I used all three spectra".
It means showing that the evidence converges: each technique narrows the possibilities, and the surviving structure is consistent with all of them simultaneously.
The clearest signal that you are integrating is that you can say why the alternative isomer is ruled out, and by which piece of data.