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Transcript

Atom

Economy

Definition

Measure the amount of starting reactants that become useful products.

Formula

Atom economy of

different reaction types

Condensation reactions:

atom economy: <100%

Two molecules join together to create a larger molecule and a small molecule is eliminated.

rearrangement and addition reactions:

atom economy: always 100% (no molecules are lost in the reaction)

Substitution reactions:

atom economy: <100%

one functional group is 'swapped' for another, meaning there will always be a small, unwanted molecule left over.

Elimination reactions:

atom economy: <100%

Atoms are removed from a molecule - for example, dehydration of an alcohol.

Order of decreasing

atom economy:

rearrangement & addition > substitution & condensation > elimination

Infra-red spectroscopy

Infra-red light is shone through a material. Some wavelengths are absorbed in the material, and these show as troughs on a graph of absorption against wavenumber (the inverse of wavelength)

Only certain wavelengths of infra-red light are absorbed. This is because the wavelengths that are absorbed cause the bonds between certain atoms, such as carbon and hydrogen, to be raised quantized energy levels and vibrate more. This energy corresponds to the frequency of the light absorbed, so only some frequencies of light can be absorbed by a particular bond.

The scale usually reads from 4000 to 0. The region from around 1500-0 is called the fingerprint region, and unique for each molecule - this can tell you precisely what the molecule being tested is, by comparing the fingerprint region to a database of fingerprint regions for different molecules.

Mass spectrometry

A technique used by chemists to determine the structure of molecules.

Process

A sample is vapourized and enters the mass spectrometer via the sample inlet.

In the first chamber, it is ionized by a stream of electrons, so it becomes positively charged.

In the second chamber, the ions are accelerated to the negative plate at the other end of the chamber so they have the same kinetic energy.

In the third chamber, the ions race each other across a vacuumed space, whereupon they reach the detector. Smaller ions travel faster, so this enables the Mr of the ion to be calculated.

Question: Why is the space a vacuum?

Fragmentation

The molecules of the sample being tested are ionised.

The ion produced when a molecule has just lost one electron is called the molecular ion. This ion is often unstable and can break up in a process called fragmentation into other smaller fragment ions and uncharged fragments.

For example, butanone could form fragments of:

CH

CH CH CO

CH CO

CH CH

Interpretation

A typical mass spectrum will have:

the x-axis showing mass to charge ratio (m/z)

the y-axis shows intensity

The height of the peak corresponds to how much of that ion is in the sample. For example, with isotopes of chlorine (35-Cl and 37-Cl), the peaks are always in a 3:1 ratio.

every line in the spectrum represents a positively charged ion. The line with the highest m/z usually represents the molecular ion, whilst other corresond to fragments or isotope peaks.

1) Identify the molecular ion peak

This is usually the peak with the highest m/z (if not, it will have a higher intensity than other peaks with a higher m/z).

2) List the masses of the other peaks

The other major peaks will have been caused by fragment ions. Find the difference in mass between these and the molecular ion peak - this difference in mass will be the mass of the fragment (that will equal the Mr of the fragment).

3) Identify any isotope peaks.

When molecules in a sample contain more than 1 isotope of any element, additional fragment peaks occur in the mass spectrum.

These peaks may have a higher peak than the molecular ion, due to the heavier isotope in the molecule.

Separates mixtures of molecules.

Can also be used to identify various substances.

Method:

1) draw a thin pencil line about 2cm away from the bottom of the chromatography plate.

2) add a small drop of mixture to the baseline, numbering each one to avoid confusion.

3) place in a solvent, making sure the solvent doesn't reach the base line.

4) rest the beaker in a place of quiet and peace and calm, and wait until the solvent has climbed to almost the top of the paper.

5) remove the plate from the solvent and make a thin pencil line to show where the solvent has reached.

6) spray some identifier spray to see the separated mixture: mark where each one has travelled to.

7) you can calculate the Rf value and compare it to a database value to help identify the chemical.

The End.

Analytical techniques

Chromatography

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Now identify the peaks on the diagram of the mass spectrum of butanone! Yay!

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