Analysis of Organic Compounds Flashcards
electromagnetic energy?
light
what is radiation made of?
waves of light, each with their own unique energy value, frequency (Hz) range and specific wavelength values
what can UV or visible light do?
excite outershell electrons. colour is emitted as they return to ground state
radio waves
- far right
- low energy
- long waves
- greater distance between each wave
X rays
- far left
- high energy
- short wavelengths
- less distance between waves
visible light
- energy we can see
- composed of whit light that contains different wavelengths of light that correspond to different coloured light
what can the electromagnetic spectrum be used for?
- functional groups (IR spec)
- specific environments the assist with structural arrangement (MNR spec)
- molar mass (mass spec)
Why is it significant that covalent bonds are like springs?
- when IR energy is applied the atoms engaged in the bonds can vibrate towards or away from eachother
- as they vibrate, the bond angle changes
- bond length and mass of atoms is unique and thus vibration involves different of IR
- absorbing different wavelengths of IR results in different frequencies of vibration. if increased, molecule can move to a higeher energy level
types of vibration
- bond stretching
- valence angle bending
- out-of-plane wagging
- torsion
- symmetric stretching
- asymmetric stretching
- scissoring
- rocking
what is the energy involved in vibration of covalent bonds dependent on?
bond- the stronger the bond, the higher the signal
mass of atoms involved- a greater amount is absorbed when mass is lower, while a larger mass absorbs less IR
diatomic molecules and IR
they do not absorb as there is no dipole
how is IR spec carried out?
- compound is dissolved in a solvent
- sample is placed in a cell and IR energy is passed through
how does IR work?
- IR energy has different energy frequencies
- if absorbed they bonds vibrate
- some goes through and is measured as percentage transmittance
base line of 100%
all of a particular frequency got through and it was not absorbed
high absorption
small percentage of transmittance
how is the absorption of energy seen?
a dip or inverted peak
finger print region
shows the whole molecule vibrating to IR energy (1000 or 1400)
dips (inverted peaks)
correspond to specific bonds within functional groups
broad peak
O-H alcohol
what is special about the fingerprint region?
unique to each molecule (if not same then the graph is not of the same molecule even if rest looks similar)
what does NMR spec stand for?
nuclear magnetic resonance
what does NMR determine?
the structure of molecules
what does it cause?
the particles within the nucleus of atoms to spin (such as protons). The spin of particles is detected when there are an odd number of nucleons (1H or 13C) and in this way the nucleus has an overall spin
what are the types of NMR?
- proton NMR spec (for 1H)
- Carbon NMR spec (for 13C)
what is proton or H-NMR used for?
analyses the hydrogen environment and thus reveals the arrangement of the hydrogen atoms within the molecule and thus gives a structure for the molecule.
what happens in H-NMR when hydrogen is with a carbon?
the electron pair is shared evenly which provides a sheild for the hydrogen from the rasdiowaves
what happens in H-NMR when hydrogen is with an electronegative atom?
electrons are not shared evenly and thus hydrogen is not shielded
what is the effect of shielding on H-MNR?
- impacts the amount of spinning
- the amount of energy absorbed because of different shielding can be detected by NMR once the nuclei relaxes to a lower spinning state and the radio wave energy absorbed is emitted
- this amount of energy is translated into a chemical shift value which is represented on the NMR spec
what are chemical shift values?
- found in data book
- link to type of hydrogen bond
- expressen in ppm
- all are referenced to tetramethylsilane (TMS) which is the standard reference
TMS
has four hydrogen environments, methyl groups that are all identical
TsM is unreactive so will not react with molecule being analysed
at 0ppm
what happens when all hydrogen environments are the same?
shows as one split value as is only detected once
downfield
to the left, more polar
upfield
to the right, closer to TMS, more non-polar
what does 2 ppm mean?
hydrogen atoms need two millionths less than TMS to spin because of less shielding (more polar)
types of NMR spec
low res
high res
low res NMR
- shows how many hydrogen environments as a single peak at certain chemical shift values
- number of peaks = number of environments experienced by H nuclei
- not a lot of info
high res NMR
- shows how the hydrogen environment is arranged and what they are bonded to (adjacent hydrogen environments)
- adjacent environments have a direct impact on how hydrogen nuclei spin
- peaks splitting patterns represent each environment
splits
n + 1
n = number of hydrogens in neighbouring environments
spin is affected by
- neighbouring environments
- whether that neighbour is electronegative
for an environment to be the same an another…
must have same neighbours
how do you determine carbon environments in C-NMR?
examine the atoms the carbon atoms are attached to
a carbon environment is near an electronegative atom
higher chemical shift value
what does mass spec determine?
the structural formula and hence the molecular mass
what is the key principle of mass spec?
positively charged particles pass through a magnetic field that deflects along a circular path. deflection is based on size and change of the particle, referred to as mass charge ration (m/e-)
how does mass spec work with atoms?
- sample enters where electrons bombard is, causing valence electrons to fall off, forming positively charged particles
- these particles are accelerated by a magnetic field whilst neutral ones and negative ones are not
- deflection is based on size and charge
- greater charge and smaller mass, the greater the deflection
mass spec equation
M + e- —> M+ + 2e- where M is parent particle and M+ is charged particle
how does mass spec work with compounds?
- molecule is bombarded with electrons which knock off an electron from an atom present. a parent molecular ion is formed which is not stable and thus fragments into smaller positive charged particles
- they are accelerated and then deflected based on charge and lightness which allows chemists to decipher molecular formula
what is formed when a parent molecular ion fragments?
charged ions (positive) and free radicals which are uncharged and dangerous- they are removed through the bottom as they are not charged (they cannot be detected)
how is a mass spectra analysed?
- peaks equate to the size of the fragment
- in order to determine the mass of the fragment, the molar masses of each atom in the fragment are added together
what can a mass spec tell you?
- the peak line that occurs at the largest mass/charge is the parent molecular ion and therefore a chemist can quickly recognise the molecular mass of the molecule
- the peak closest to 100% relative intensity is the base peak and indicates the mass of the fragment most likely to occur as it is most stable
if asked to find a fragment? is asked to find a lost fragment?
charge
no charge
the effect of isotopes of mass spec
- halogens are often in compounds and they have isotopes
- an additional peak
- most abundant isotope has highest peak line
