Spectroscopy

Mass spectro tells you

the mass of the molecule

• via parent ion/molecular ion peak

Likely groups within a molecule

• via the mass of the fragments within the molecule OR the mass difference between the peaks

How mass spectro works

ionisation step removes an electron (it also can fragment the molecule and remove more than one e)

• becomes positively charged - mass spec can only detect positively charged substances (artificially generated ion)

Mass/charge signal (m/z) of COOH

45

CH3 m/z

15

OH m/z

17

What mass spectra looks like

base peak

fragment with the highest percentage abundance

Mass spectra: common mistakes

forgetting positive on fragment

Mass spectro isotopes effect

mass spec separates and detects m/z, different isotopes will impact on the position of peaks

When small tiny peak next to the molecular ion peak…

could be C-13 isotope - which is one unit apart from C-12 (longer peak)

35-Cl and 37-Cl

with relative ratio of 35-Cl: 37-Cl being 3:1 and two units apart

How mass spectra with Cl isotopes can look

IR absorptions - where to find

Data book only has wave numbers

How is the electromagnetic spectrum related to spectroscopy?

IR Radiation

• Causes vibrations in covalent bonds and makes the bonds bend and stretch

• It is used to detect bonds that are part of functional groups in organic molecules

Ground state

Bond vibrations

Different bonds absorb different wavelengths of IR light, which causes it to stretch, bend or twitch like a spring

IR: Strength of bonds

Stronger bonds require more energy to vibrate

IR: mass of atoms

Heavier masses vibrate at lower frequencies

Interpreting IR spectrum

High % transmission

Low % transmission

Describing absorption bands

Below 1500cm-1

fingerprint region - we don’t look at this

• unique for each organic molecule

Interpretation of IR - functional group region

OH acids absorbance band

Stronger absorbance and very broad band that masks the CH peak

Alcohol IR spectra

Carboxylic acid IR spectra

Ester IR spectra

Peak A: OH (acid)

Peak B: CH

Peak C: C double bond 0

Alcohol (Hexan-1-ol) IR

Carboxylic Acid (Butanoic Acid) IR

Aldehyde (Hexanal) IR

Ketone (Propanone) IR

Ester (Ethyl Ethanoate) IR

Amine (butanamine) IR

Primary Amide IR

Secondary amide

NMR (nuclear magnetic resonance)

Which nuclei are NMR active?

must have odd number of nucleons and spin is what is registered

• the orientation of spin

Carbon 13 NMR - why active

is active because there is an odd number of nucleons

TMS (Tetramethylsilone)

• reference peak

• has a chemical shift of zero ppm

• all other chemical shifts are compared relative to TMS

• this is because same chemical shifts are the same no matter what NMR you use (consistency)

What does number of peaks represent (C-NMR)

represents the number of carbon environments

• 2 peaks = 2 C environments

Chemical shift (ppm)

identifies the type of carbons and it’s surrounding atoms

• found in DB

Carbon environment

To be in the same carbon environment

1. carbons must be attached to the same number and type of atoms

2. neighbouring groups must be the same - look for symmetry

Trends in chemical shifts as per DB

Low Res H-NMR Number of peaks

represents the number of hydrogen environments

• 3 peaks = 3 H environments

Area of peak/Integration

Represents the ratio of hydrogens in the environment

eg. 3:2:3

Chemical shift H-NMR

identifies the type of hydrogens and surrounding atoms (chemical environment)

It also tells you about shielding

• in DB

Hydrogen environments

To be in same hydrogen environment…

1. Must contain the same number of hydrogens

2. The H’s must be attached to the same type of atom

3. The neighbouring groups must be the same - look for symmetry

Shielding

• The nucleus does not feel the full effect of the radio frequency

• Electrons spin and produce very small magnetic field

• Nucleus is partly shielded by the spinning electrons around it

• Elements with high electroneg have a greater share of e- in covalent bond

High Res H-NMR

chemical shift, peak area and number of signals still give same info as low res BUT splitting signals in high res also tells us about the number of protons on neighbouring carbons

Difference bw high res and low res H-NMR

low res = no splitting

high res = splitting

Splitting

1. H environment has hydrogen neighbours that are attached to a carbon

2. n + 1 rule used to determine how many bands it’s split into

Tricky points with H-NMR splitting

1. Hydrogens bonded to anything but carbon do not split and do not cause splitting

2. Hydrogens in same environment and next to each other are not split (i.e only one H-envir present = 1 peak only)

Example of Hydrogens not bonded to carbon

Combined spectra - what you can get out of each technique

Analytical technique

separating mixtures (organic substances) into components

Qualitative analytical technique purpose

get an idea of components in a mixture by comparison to pure sample

Quantitative analytical technique

tells you how much of a substance is present

Components of HPLC

form of column chromotography, involves…

• liquid mobile phase (solvent)

• solid/viscous liquid stationary phase

Mobile phase

moves through the stationary phase and carries the components of the mixture

• in HPLC, pumped under high pressure bc stationary phase has large surface area which causes resistance to flow of mobile phase

stationary phase

• stays still

• solid (or viscous liquid) attached to tiny beads

• substances in a mixture will have different affinity for stationary phase

In the column (process) + explain pink and yellow affinities

samples loaded at start of column (stationary phase is beads in pic)

Adsorbs

attaches onto column (SP)

Desorbed

moving WITH mobile phase (lower affinity to stationary phase)

Pentane vs pentan-1-ol example in non polar SP

Chromatograph x and y axes

x axis = retention time (mins)

y axis = area

retention time

how quickly the components move through the stationary phase

Area under curve on chromatograph

the concentration - higher area = higher conc

If not told polarity of SP or all molecules have similar polarity…(CG)

eg. all alcohols

larger molecules will have a higher affinity towards the stationary phase (higher Rt) since dispersion forces will be larger SINCE GREATER NO. OF E-

Chromatograph from pentan-1-ol and pentane

HPLC qualitative analysis

• analysis that tells you what your molecule is

• retention time is used

• if same conditions used (same SP, MP, temp, pressure that MP pumped through) the Rt will always be the same

• run the sample through + compare Rt times with DB times

ethanol qualitative analysis

HPLC Quantitative Analysis

Enables you to determine how MUCH of the component is present in the mixture

HPLC Quantitative analysis step 1

1. Identify the component by looking at the Rt and running a pure sample of the component to determine the component present

2. Make standard solutions of ethanol with different concentrations and run the samples through HPLC to get absorbance reading

HPLC Quantitative analysis step 2

HPLC Quantitative analysis step 3

unit conversions

Calculating concentration in gL-1

Dilutions formula

C1V1 = C2V2

Testing for presence of C=C (alkenyl)

bromine test : brown → colourless (DB)

iodine test : brown → colourless (DB)

Testing for presence of OH (primary of secondary)

MNO4- : purple to very pale pink

Cr2O72- : orange to green

Detecting presence of alkenyl (C=C)

Degree of unsaturation

tells us the number of C=C in a molecule

How many C=C in C16H28

Molecular formula and degree of unsat

Testing for presence of OH

precision glassware for titration

pipette, burette, volumetric flask

• same volume every time as long as used correctly

Volumetric flask (rinsing + use)

pipette (rinsing + use)

burette (rinsing + use)

conical flask (rinsing and use)

simple titration steps

the solution of unknown conc goes into the pipette and then into the conical flask

concordant results

titrations have to be repeated until concordant results are obtained. This increases PRECISION. Concordant results in titrations need to be 0.1mL apart from each other - all results must be within in 0.1mL range

Titres

the actual mL from burette used (final - initial)

Colour changes

redox reaction colour changes depends on reactants and products - look at DB

aliquot goes in…

conical flask

Steps with question like - “20mL aliquot of methanol is reacted with 1.28M MnO4-/H+ with 9.65mL, 9.60mL and 9.70mL - determine concentration of methanol”

Endpoint

a colour change (permanent) will occur

• seen in experiment