2.12a: interpret fragment ion peaks in the mass spectra of simple organic compounds, eg the difference between propanal and propanone

Mass spectrometry can be used to:

• determine the RAM of an element by measuring the ratio of isotopes present
• determine the molecular mass (Mr) of a compound as this will be the largest mass detected - the molecular ion peak
• find the structure of an unknown compound by identifying fragments of the original molecule
• advantages:
• only requires tiny amounts of a sample
• very fast process

How does a mass spectrometer work?:

• the sample is vaporized by heating in a vacuum, then ionised by bombarding it with high energy electrons from an electron gun which knock an electron off each atom in the sample to generate a positive ion
• sometimes more than one electron is knocked off, affecting the mass/charge ratio
• the ions are highly unstable, so are very reactive
• the inside of the mass spectrometer is a vacuum, so there is nothing for them to react with, but they may fragment
• fragmentation patterns are useful as they give characteristic masses that can allow us to build up a “jigsaw” of the parent molecule
• molecules usually fragment along the weakest bond (eg. tertiary more stable than secondary, secondary more stable than primary -  due to the electron donating effect of the alkyl groups to stabilise the positive charge)

Some common fragment ions:

mass to charge ratio (m/z)	possible fragment ion
17	OH+
15	CH3+
29	CH2CH3+ or CHO+
31	CH2OH+ or OCH3+
43	CH3CO+

2.11g: demonstrate an understanding of how oxygen, O2, and ozone, O3, absorb UV radiation and explain the part played by emission of oxides of nitrogen, from aircraft, in the depletion of the ozone layer, including the free radical mechanism for the reaction and the fact that oxides act as catalysts

Formation of ozone and oxygen:

• O2 → 2O-, O- + O2 → O3
• O3 + O- → 2O2, O- + O- → O2
• all occur in the presence of UV light

How do nitrous oxides help to deplete the ozone layer?

• nitrous oxides come from jet and plane engines
• oxides catalyse the reaction
• NO + O3 → NO2 + O2, NO2 + O- → NO + O2
• free radical

2.11f: describe the mechanisms of the substitution reactions of halogenoalkanes and recall those in 1.7.2e and 1.7.3e

free radical substitution of chlorine:

• initiation
• Cl2 → Cl- + Cl-
• propagation
• C4H10 + Cl- → C4H9- + HCl
• C4H9- + Cl2 → C4H9Cl + Cl-
• termination
• Cl- + Cl- → Cl2
• C4H9- + Cl- → C4H9Cl
• C4H9- + C4H9- → C8H18

electrophilic addition of bromine and hydrogen bromide to ethene:

• INSERT DIAGRAM HERE

electrophilic addition of hydrogen bromide to propene:

• INSERT DIAGRAM HERE

2.11e: demonstrate an understanding of the link between bond polarity and the type of reaction mechanism a compound will undergo

A polar molecule will react differently than a non-polar molecule

• eg. C-H will react differently to C-C

Two non-polar molecules can induce a dipole

• eg. INSERT DIAGRAM HERE

2.11d: demonstrate an understanding of why it is helpful to classify reagents

2.11c: give definitions of the terms free radical, electrophile and nucleophile

free radical: an atom with an unpaired electron INSERT DIAGRAM HERE

electrophile: a reactant that accepts an electron pair

nucleophile: a reactant that donates an electron pair (must have a lone pair of electrons)

2.11b: demonstrate an understanding of the concept of a reaction mechanism and that bond breaking can be homolytic or heterolytic and that the resulting species are either free radicals, electrophiles or nucleophiles

Reaction mechanisms are step-by-step descriptions of what occurs on a molecular level in chemical reactions.

Bond-breaking

• homolytic: when the bonding electron pair is split evenly INSERT DIAGRAM HERE
• often produces free radicals
• heterolytic: when the bonding electron pair in unevenly split INSERT DIAGRAM HERE