S6 - Unit 2
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What is an empirical formula?
The simplest ratio of atoms for a compound.
How can an empirical formula be worked out in real life?
By burning a sample and measuring the mass of the product
What is mass spectrometry?
A way to determine the accurate GFM and structural features of an organic compound
How is mass spectometry carried out?
By the ionisation and mass separation of molecules
What does the ionisation of a mass spectrometry sample involve?
The sample being bombarded with electrons An electron is removed and forms a parent ion that is + charged.
What are the four stages to mass spec?
• Ionisation • Acceleration • Deflection • Detection
What happens in the last three stages of mass spectrometry?
Acceleration: The ions are accelerated so they all have the same Ek Deflection: The ions are deflected by a magnetic field according to their masses (the lighter they are the more they’re deflected. Detection: The beam of ions passing through the machine is detected electrically.
What does the level of deflection in the 3rd stage of mass spec. depend on?
Depend on how many electrons were knocked off in the first stage.
What is IR spectroscopy?
A way of determining the functional groups present in a molecule
How does IR spectroscopy determine the F.G’s present in a molecule?
The radiation is of a frequency which causes molecules to vibrate and heat up. Functional groups have characteristic vibrational frequencies which can be used to identify them. Specific bonds/functional groups absorb IR at IR at different ranges of wavelength and frequencies.
What is nuclear magnetic resonance spectroscopy (NMR) used for?
to investigate the different types of hydrogen atom environments in organic compounds and how many atoms there are in each of these environments.
What is chemical shift?
When the radiation absorbed/emitted by a hydrogen nucleus is dependant on the local environment of the nucleus within the molecule.
What is Hybridisation?
When outer orbitals of the bonding atoms overlap to allow the sharing of electrons.
What is a sigma bond?
Two half filled atomic orbitals that overlap end-on.
What is a pi bond?
Two half filled atomic orbitals that overlap side-on.
When does hybridisation occur?
When different types of atomic orbitals are mixed.
What is a HOMO?
The highest bonding molecular orbital containing electrons
What is a LUMO?
The lowest antibonding molecular orbital
What is an sp3 orbital?
The hybridisation of 1x s orbital and 3x p suborbitals.
What is an sp2 orbital?
The hybridisation of 1 x s orbital and 2 x p suborbitals.
What is the shape and bond angle in an sp3 orbital?
Tetrahedral 109.5 o
What can cause electrons to move from the HOMO to the LUMO?
Absorption of electromagnetic energy
Why do most organic molecules appear colourless?
Because the energy difference between the HOMO and LUMO is relatively large.
What is a chromophore?
A group of atoms within a molecule that is responsible for absorption of light in the visible region of the spectrum.