Structual characterisation

Question 1

heteroatoms

Answer 1

any element that is not carbon or hydrogen

eg nitrogen,oxygen, sulphur

Question 2

empirical formula vs molecular formula

Answer 2

EF = simplest whole number ration of elements in the structure

MF = actual number

Question 3

Double bond equivalence DBE

Answer 3

also called index of hydrogen deficiency IHD

H reference is the number of hydrogens required to make CnH2n+2 - saturated alkane

H molecule = number of hydrogens actually present

DBE = H ref - H mol /2

DBE = 1 = one double bond or ring
DBE = two double bonds or rings or one triple bond

Question 4

molecules with heteroatoms

Answer 4

with oxygen - can be ignored as does not affect DBE so no change to H ref - so COH/COOH/COC

Subtract one H from H ref for every halogen present

Add one H to H ref for every nitrogen

Question 5

mass spectrometry

Answer 5

this gives us the mass of molecules and can therefore distinguish between empirical and molecular formulas

• the mass and charge affects the acceleration as will have different path lengths so will produce different peaks

Question 6

Base peak

Answer 6

largest peak to which the relative abundance of all other peaks are measured to

Question 7

parent ion or molecular ion peak

Answer 7

the peak at which is produced by the original molecule before fragmentation

Question 8

exact mass

Answer 8

measuring to 4 decimal places

• calculating the exact mass using the most common isotope. - this helps distinguish between compounds with similar RAM

Question 9

electromagnetic radiation

Answer 9

is a wave consisting of orthogonal electrical magnetic fields that can travel through space

as energy decreases, frequency decreases but wavelength increases

Question 10

defining waves

Answer 10

wavelength = lamdaba (metres)
frequency = v (s-1)
wavenumber -V (cm-1)
speed light = c = 2.998x108ms-1

c=vlamdaba

need to remember to convert these to m ->cm

Question 11

wave particle duality

Answer 11

electromagnetic radiation can be a wave and small packet of energy

energy of photon is related to its frequency

converting to wave number for this allows us to put the information onto a better scale

Question 12

wave particle duality equations

Answer 12

E = hv
h = 6.626x10-34Js = plancks constant

E=hc/lambada

E=hc-V

Question 13

quantisation of energy levels

Answer 13

electronic, vibrational and rotational energy levels in molecules can be compared

• energy gap between electorns in valence orbitals is quite large
• energy gap between vibrational energy levels smaller
• energy gap between rotational energy levels smallest

decreasing energy gap between different states

Question 14

UV/Visible spectroscopy

Answer 14

when photon in Uv or visible range is absorbed, an electron is promoted from a full bonding orbital to a empty anti bonding orbital

the electrons that do this are double bonded electrons, pi bonds - undergone a pi tranistion

Question 15

energy gap between pi and sigma orbitals

Answer 15

energy gap of sigma orbitals are not in the UV/visible region of EM spectrum as the gap is too big to be associated with this region

Question 16

transmittance and absorbance

Answer 16

T = IT/IO
io = intensity of incidient light - starting light
it = intensity of transmitted light - after been through solution

A = log 10 IO/IT
A = absorbance

Question 17

beer lambert law

Answer 17

A = ecl

Question 18

graphs with transmittance and absorbance

Answer 18

the absorbance is directly proportional to the concentration and transmittance has an exponential relationship

Question 19

infra red spectroscopy

Answer 19

photons in IR region do not have enough energy to promote electrons so instead they excite vibrations in a molecule

these molecular vibrations occur at fixed frequencies which depend upon how heavy the atoms are and how strong the bonds are in order to identify them

Question 20

vibrations

Answer 20

will be vibrating and stretching at a fixed frequency

if we excite a vibration the frequency does not change but the amplitude of the vibration does - stretching further so easily detected

Question 21

dipole moments

Answer 21

this occurs when there is a separation of charge

this is larger when
- the difference in electronegativity increases
- the separation of the atom decreases

to be IR active, the dipole moment must change during the vibration - if not then there is no peak

Question 22

strectches

Answer 22

is there is a symmetrical compound like co2

if there is a symmetric stretch then there is no dipole moment

if asymmetric then there is

Question 23

IR spectroscopy

Answer 23

the vibrational frequency depends on:
- how stiff the bond is - bond force constant
- the sizes of the atoms involved in the bond

v=1/2pi x square root k/u

u=m1xm2/m1+m2

Question 24

infrared spectra

Answer 24

always plotted as transmittance vs frequency

there is a fingerprint region which is 1500> and then the rest of the region which can be used to identify the compound

-OH in alcohol = whale
-OH in COOH = mountain

Question 25

IR amine

Answer 25

N-H stretch, n-h bend just before fingerprint

Question 26

NMR

Answer 26

used to obtain the information on the structure of the carbon hydrogen framework of the molecules and functional groups

Question 27

nuclear spin

Answer 27

have to have odd mass number in order to have a nuclear spin hence with H1 and carbon 13 are used

when there is no external magnetic field the nuclear spins are random but when applied, they align parallel or anti-parallel to the field

Question 28

change in energy levels and splitting to give different peaks

Answer 28

tis is dependent on the strength of the magentic field, the bigger the magnet, the stronger the magnet field so larger the energy level split

degree of shielding - different H environments have different degrees of shielding which causes the different peaks

magnetogyric ratio - specific to a particular type of nucleus

Question 29

magnetic field strength

Answer 29

the energy difference between spins is proportional to the magnetic field of the spectrometre

Question 30

magnetogyric ratio

Answer 30

the energy difference between spins is also proportional to the magnetogyric ratio

this depends on the magnetic moment of that nucelus and the spin where the energy separation achieved for protons is quoted as the strength

v=yB/2pi

Question 31

shielding

Answer 31

The magnetic field experienced will be different for H atoms in different environments and therefore absorb a different frequency

Question 32

degree of shielding

Answer 32

fluorine is more electronegative that chlorine

the electron density will be pulled away from the hydrogen atoms

these atoms are less shielded than chlorine counterparts

so the magnetic field experience on the hydrogen atoms in fluorine will be bigger so will absorb at a higher frequency

Question 33

TMS

Answer 33

the reference signal for 13C and H NMR - tetramethylsilane
4 ch3 groups around Silicon

Question 34

chemical shift and resonance frequency

Answer 34

the difference in resonance frequency is quoted by the chemical shift (the natural frequency at which a medium vibrates at the highest amplitidue ).

chemical shift = shift in frequency from reference / operating freq of spectrometre

Question 35

splitting in proton NMR

Answer 35

the signal intensity is proportional to the number of protons

when there are H atoms on adjacent carbon atoms, then the magnetic field is affected

Question 36

splitting

Answer 36

singlet - no hydrogens in neighbouring environment

double - one hydrogen in neighbouring environment

triplet - 2

quartet - 3

have to be in different environments