29) Chromatography and spectroscopy Flashcards by Jasmine Mussell

What is the purpose of chromatography?

to separate individual components from a mixture of substances

How well did you know this?

Not at all

Perfectly

Define stationary phase

phase that does not move in chromatography

How well did you know this?

Not at all

Perfectly

Define mobile phase

phase that moves in chromatography

How well did you know this?

Not at all

Perfectly

Define thin layer chromatography (TLC)

quick and inexpensive analytical technique that indicated how many components are in a mixture

How well did you know this?

Not at all

Perfectly

What is the stationary phase in TLC?

TLC plate (plastic or glass) coated with a thin layer of solid adsorbent substance (e.g. silica)

How well did you know this?

Not at all

Perfectly

Define adsorption

the process that occurs when a gas or liquid or solute is held to the surface of a solid

How well did you know this?

Not at all

Perfectly

How is separation achieved in TLC?

by relative adsorptions of substances with the stationary phase (different components in the mixture have different affinities and bind with different strengths to the surface)

How well did you know this?

Not at all

Perfectly

Describe a 5 step method for TLC

1) base line drawn across the plate 1cm from the edge in pencil
2) small amount of sample solution spotted using a capillary tube onto the base line
3) prepare a chromatography tank with a small beaker and watch glass, solvent depth 5mm, then place the TLC plate inside ensuring the solvent doesn’t cover the spot
4) leave undisturbed until the solvent has risen to about 1cm below the top of the plate, remove + mark solvent front with pencil. allow to dry
5) circle any visible spots with pencil. You may need to use a UV lamp or spray with a chemical/locating agent e.g. iodine

How well did you know this?

Not at all

Perfectly

How can each component of TLC be identified?

by comparing its Rf value with known values recorded using the same solvent system and stationary phase
OR TLC of a sample could be run against pure/known values

How well did you know this?

Not at all

Perfectly

retention factor, Rf = ?

distance moved by component / distance moved by solvent front

How well did you know this?

Not at all

Perfectly

What is the purpose of gas chromatography?

separating and identifying volatile organic compounds in a mixture

How well did you know this?

Not at all

Perfectly

What is the stationary phase in gas chromatography?

high boiling liquid adsorbed onto an inert solid support

How well did you know this?

Not at all

Perfectly

What is the mobile phase in gas chromatography?

inert carrier gas e.g. helium or neon

How well did you know this?

Not at all

Perfectly

Describe a 3 step method for gas chromatography

1) small amount of volatile mixture is injected into the gas chromatograph + is carried by the mobile phase through the capillary column
2) components slow down as they interact with the stationary phase. the more soluble the component is in the stationary phase, the slower it moves through the column. therefore, compounds are separated depending on their solubility in the liquid stationary phase
3) components reach the detector at different times. the compound retained in the column for the shortest time has the lowest retention time and is detected first

How well did you know this?

Not at all

Perfectly

Define retention time

the time taken for each component to travel through the column (from the column inlet to the detector)

How well did you know this?

Not at all

Perfectly

Each component is detected as a _ on the gas chromatogram

peak

How well did you know this?

Not at all

Perfectly

How can compounds be identified in GC?

retention times can be compared to known components

How well did you know this?

Not at all

Perfectly

Define peak integrations

the areas under each peak

How well did you know this?

Not at all

Perfectly

How can you determine the concentrations of components in the sample, in GC? Give a 4 step method

compare peak integrations to values obtained from the standard solutions of each component:

1) prepare standard solutions of known concentrations of the compound being investigated
2) obtain gas chromatograms for each
3) plot a calibration curve of peak area against concentration = external calibration
4) obtain a gas chromatogram of the compound being investigated under the same conditions + use the calibration curve to measure the concentration of the compound

How well did you know this?

Not at all

Perfectly

Give the chemical test and observation for an alkene

+ bromine water dropwise

orange -> colourless

How well did you know this?

Not at all

Perfectly

Give the chemical test and observation for a haloalkane

\+ silver nitrate, ethanol
water bath at 50 degrees C
Cl -> white ppt
Br -> cream ppt
I -> yellow ppt

How well did you know this?

Not at all

Perfectly

Give the chemical test and observation for a carbonyl

+ 2,4-DNP

orange ppt

How well did you know this?

Not at all

Perfectly

Give the chemical test and observation for an aldehyde

+ Tollens’ reagent and warm

silver mirror

How well did you know this?

Not at all

Perfectly

Give the chemical test and observation for an alcohol (primary/secondary) or aldehyde

+ acidified potassium dichromate (VI) and warm in a water bath
orange -> green

How well did you know this?

Not at all

Perfectly

Give the chemical test and observation for a carboxylic acid

+ aqueous sodium carbonate | effervescence

Define nuclear magnetic resonance (NMR) spectroscopy

technique using a very strong magnetic field and radio frequency radiation so that the nuclei of some atoms absorb radiation. the energy for the absorption can be measured and recorded as a NMR spectrum

Name two types of NMR

proton NMR and carbon-13 NMR

When is nuclear spin significant?

when there is an odd number of nucleons (protons and neutrons)

Describe resonance and how it can occur

when the nucleus rapidly flips between two spin states the nucleus has 2 different spin states which have different energies. with the right combination of a strong magnetic field and radio frequency radiation the nucleus can absorb energy

What uses the same technology as an NMR

a MRI

What do the e- surrounding the nucleus do for NMR to take place?

shift the energy and radio frequency

What can frequency shift be measured on?

chemical shift δ, in ppm | a scale that compares the frequency of an NMR absorption with the frequency of the reference TMS at δ = 0ppm

What is tetramethylsilane (TMS), (CH3)4Si used as?

the standard reference chemical against which all chemical shifts are measured

What is the amount of chemical shift determined by and why?

the chemical environment, especially the presence of nearby electronegative atoms. depending on the chemical environment, NMR requires a different energy and frequency, producing absorption peaks at chemical shifts

Give a 5 step method for running a NMR spectrum

1) sample dissolved in solvent, placed inside a narrow NMR tube with a small amount of TMS and placed inside the NMR spectrometer 2) spun to even out any imperfections inside the NMR spectrometer 3) spectrometer zeroed against TMS standard + sample given a pulse of radiation containing a range of radio frequencies whilst maintaining a constant magnetic field 4) absorptions of energy resulting from resonance are detected and displayed on a computer screen 5) after analysis the sample can be recovered by evaporation of solvent

Why is a deuterated solvent often used in NMR?

common solvents contain C and H atoms which will produce a signal in the spectra in a deuterated solvent the 1H atoms are replaced by 2H atoms (deuterium) which produce no NMR signal in the frequencies used

What is done in response to deuterated trichloromethane (a common solvent) still producing a peak in carbon-13 spectra?

a computer will usually filter out the peak

In carbon-13 NMR spectroscopy, the number of peaks indicate...?

number of different carbon environments

In carbon-13 NMR spectroscopy, the chemical shift indicates...?

types of carbon environment present

What is the chemical environment of a carbon atom determined by?

the position of the atom within the molecule

Carbon atoms bonded to different atoms/groups have _ environments and will absorb at _ chemical shifts

different | different

If two carbon atoms are positioned symmetrically within a molecule, then they are _ and have _ chemical environment. What implication does this have?

equivalent same They will then absorb radiation at the same chemical shift and contribute to the same peak.

In proton NMR spectroscopy, the number of peaks indicate...?

number of different proton environments

In proton NMR spectroscopy, the chemical shift indicates...?

types of proton environments present

In proton NMR spectroscopy, the integration traces / ratio numbers of relative peak areas indicate...?

relative number of each type of proton

In proton NMR spectroscopy, the spin-spin splitting pattern indicates...?

number of non-equivalent protons adjacent to a given proton

What 3 factors may move a peak outside of the normal ranges?

solvent concentration substituents

If two or more protons are equivalent, they will absorb at _ chemical shift, _ the size of the peak

the same increasing (look for any plane of symmetry)

Protons of different types have _ chemical environments and are _ - they absorb at _ chemical shifts

different non-equivalent different (look for any plane of symmetry)

What does the NMR spectrometer measure?

the area under each peak as an integration trace

What is an integration trace shown as?

an extra line on the spectrum or as a printed number of the relative peak areas

A proton NMR peak can be split into sub-peaks or splitting patterns. How is this caused?

by the proton's spin interacting with the spin states of nearby protons that are in different environments

Define spin-spin coupling / spin-spin splitting

the splitting of a main peak into sub-peaks | the interaction between spin states of non-equivalent nuclei that results in the splitting of a signal

The number of sub-peaks is _ than the number of adjacent protons causing the splitting.

one greater

State the n+1 rule

for a proton with n protons attached to an adjacent carbon atom, the number of sub-peaks in a splitting pattern is n+1

``` If a proton has 0 protons attached to an adjacent carbon atom, state the following? n+1 splitting pattern relative peak areas within splitting structural feature ```

1 singlet 1 no H on adjacent atoms

``` If a proton has 1 proton attached to an adjacent carbon atom, state the following? n+1 splitting pattern relative peak areas within splitting structural feature ```

2 doublet 1:1 adjacent CH

``` If a proton has 2 protons attached to an adjacent carbon atom, state the following? n+1 splitting pattern relative peak areas within splitting structural feature ```

3 triplet 1:2:1 adjacent Ch2

``` If a proton has 3 protons attached to an adjacent carbon atom, state the following? n+1 splitting pattern relative peak areas within splitting structural feature ```

4 quarter 1:3:3:1 adjacent CH3

When you are analysing spin-spin splitting, what are you seeing?

the number of hydrogen atoms on the adjacent carbon atom

Describe a common splitting that results in a heptet (7) splitting pattern

CH(CH3)2 | where a CH proton has 6 protons on the adjacent carbon atoms

Sometimes, adjacent protons have different environments so resulting splitting would show as a...?

multiplet

Why does spin-spin coupling occur in pairs?

because each proton splits the signal of the other

Why is proton exchange used?

to identify OH and NH protons which are otherwise difficult to assign

How is proton exchange carried out?

by running a second NMR spectrum with a small amount of deuterium oxide, D2O, added to the mixture (shaken) before deuterium exchanges and replaces OH and NH protons in the sample and does not absorb in the chemical shift range so OH/NH peaks disappear

What can you find out with elemental analysis?

percentage composition by mass -> empirical formula

What can you find out with mass spectra?

M+ peak -> molecular mass | fragment ions -> determine parts of the molecule

What can you calculate with an empirical formula and molecular mass?

molecular formula

What can you find out with infrared spectra?

absorption peaks -> bonds present and functional groups

What can you find out with NMR spectra?

chemical shifts of peaks -> number and types of carbon and hydrogen atoms splitting patterns -> order of atoms within molecules

Explain why deuterated dimethylsulfoxide, (CD3)2SO, is used as a solvent in NMR rather than (CH3)2SO

no peaks from interfering protons

What information can be deduced about a compound from an integration trace in proton NMR?

area under graph measured | number of protons present

29) Chromatography and spectroscopy Flashcards

(72 cards)