11... Flashcards
Index of Hydrogen Deficiency (IHD)
add up all the rings + pi bonds (1 per double bond, 2 per triple bond)
OR
use formula:
CxHy - IHD: (2x + 2-y)/2
- ignore O and S
- count halogens as hydrogens
- add one C and One H for every nitrogen in the formula
mass spectrometry
talk to tutor then change the card
What is Nuclear Magnetic Resonance (NMR) and how does it work
- used for analysing organic compounds
- only atoms with odd mass numbers show signals on NMR spectra and have to property of nuclear spin
- In HNMR, the magnetic field strengths of protons in organic compunds are measured and recorded on a spectrum
- samples areirradiated with radio frequesncy energy while subjected to a strong magnetic field
- Protons on different parts of the molecule (in different molecular environments) absorb and emit(resonate) different radio frequencies
- all samples are measured against TMS
- TMS shows a sharp peak at 0
- ## sample peaks are then plotted as ‘shifts’ - chemical shifts are measured in parts per million (ppm)
What do the peaks in an HNMR show?
- the area under each peak is proportional to the number of protons in a particular environment
- shows the intensity against their chemical shift
- height shows the intensity/absorption from protons
different chemical encironment containign H
-Hydrogen atoms of an organic compound are said to reside in different chemical environments
e.g.
- methanol - CH3OH
- environments: CH3 and OH
- 2 different chemical shidt
- shifts are found in section 27 of formula booklet
how to interpret an HNMR spectrum
protons in same environmetn are cheically equivalent
- each peak relates to same proton environment
- heigh of peak is the number of hydrogens in environment
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How does IR spectroscopy work
covalent bonds are like springs
- frequency of vibration occurs in the IR spectrum
- if an organic molecule is irradiated with infrareed energy that matches the natural vibration frequency of its bonds, it absorbs some of that energy and the amplitude of vibration increases - this is called resonance
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What is IR spectroscopy
- technique used to identify compounds based on changes in vibration of atoms when they absorb IR
- spechtrophotometer irridates the sample with IR radiation and then detects the intensity of IR radiation absorbed by the molecule
- Ir energy is only absorbed if a molecule has permanent dipole
- O2 and H2 are symmetrical and are IR inactive
- ## bonds have a specific wavenumber at which ther show up on the spectrum
uncertainty of analogue instruments
half the smallest division on the scale
uncertainty of digital instruments
smallest division on the scale
random errors
when reading an instrument there is an equal chance that you may read it too high or too low
- will pull a result away from an accepted value in either direction - hogh or low
systematic error
occur as a result of a faulty or poorly designed experiemtnela procedure
- will always pull the result away from the accepted value in the SAME direction
Percentage Uncertainties
absolute uncertainty / measure value x100%
uncertainty when adding or subtracting measurments
add absolute measurement uncertainties
e.g. initial and final reading
uncertainty when multiply or divide measurements
add the percentage uncertainties
then calculate absolute uncertainty
precision vs accuracy
- accuracy is how close you are to an accepted value
- precision is a measure of how many decimal places you can express your results to
