4.2.4 Analytical Techniques: Module 4: Core Organic Chemistry: Chemistry OCR A A Level

What happens when a molecule absorbs infrared radiation?

It makes the covalent bond vibrate more in a stretching or bending motion.

What factors affect the amount of vibration of a bond?

● Bond strength

● Bond length

● Mass of each atom in the bond

What happens when a molecule absorbs infrared radiation?

It makes the covalent bond vibrate more in a stretching or bending motion.

What factors affect the amount of vibration of a bond?

● Bond strength

● Bond length

● Mass of each atom in the bond

How does infrared spectroscopy work?

Every bond has a unique vibration frequency in the infrared region of the EM spectrum.

Bonds absorb radiation that has the same frequency as their frequency of vibration.

Infrared radiation emerged from a sample is missing the frequencies that have been absorbed → this information can be used to identify the compound’s functional group.

What do the peaks on an infrared spectrum represent?

Absorbance of energy from the infrared radiation.

What are the 2 peaks that must be present to identify a substance as carboxylic acid?

● Very broad peak at 2,500 - 3,300 cm-1 → O-H group

● Sharp peak at 1680 - 1,750 cm-1 → C=O group

What happens inside a mass spectrometer?

● Organic compound is vaporised and passed through the spectrometer.

● Some molecules lose an electron and forms molecular ions.

● Excess energy from the ionisation makes the bonds vibrate more.

● Vibration causes bond to weaken.

● Molecular ion splits by fragmentation.

What is the symbol of molecular ion?

M+

Is fragmentation predictable? Why?

No, because it can happen anywhere in the molecule.

In a mass spectrometry how is a molecular ion represented?

It is the peak with the highest mass/charge ratio.

The molecular mass of the molecular ion is equal to what?

Relative molecular mass of the compound.

Will the molecular ion peaks of two isomers of the same compound be same or different?

Same.

What is the m/z value of OH- from alcohol?

17

What are the advantages of using mass spectrometry?

● Cheap.

● Small quantities of samples required.

What is the main disadvantage of using mass spectrometry?

The sample is completely destroyed.