8.2 Analysis of Organic Substances

Analytical chemistry

branch of chemistry which deals with the indentification and quantification of substances:

• Qualitative

• Quantitative

Qualitative analytic chemistry

what is present?

Quantitative analytical chemistry

how much is present?

Bromine water test

for unsaturation — used to identify the presence of double or triple bonds

electrophilic species (bromine water is an electrophilic reagent) will add across double or triple bonds.

distinct decolourisation of bromine water (from orange/brown to colourless) of the reagent when unsaturation is present

electrophilic species

electron-loving species

can add across double bonds

Identification of alcohols

have the hydroxyl (OH) group as their primary functional group

Primary and secondary can be oxidised using strong oxidant (e.g. acidified potassium dichromate)

oxidisation by strong oxidant

(potassium dichromate, acidified postassium permanganate)

tests for presence of primary or secondary alcohols

NOTE: aldehydes will also give a positive result

• primary alcohol → aldehyde → carboxylic acid

• secondary alcohol → ketone

orange dichromate ion (Cr₂ O₇^2-) will reduce to the green chromium (II) ion, Cr²⁺

purple permanganate ion (MnO₄ ^-) will reduce to the colourless manganese (II) ion, Mn²⁺

NOTE: tertiary alcohols cannot be oxidised and thus will not change colour to any oxidants

Lucas test

distinguishes between the three classes of alcohols based on the difference in their reactivity with hydrogen halides

• primary alcohols have no visible reaction after several minutes

• secondary alcohols form an oily layer after several minutes

• tertiary alcohols form an oily layer immediately

litmus tests, carbonate, metal tests

tests for carboxylic acids

• as they are acids, they turn blue litmus paper red

• produces CO₂ gas (fizzing) when a carbonate is added

• Hydrogen gas (fizzing) when reacted with sodium or magnesium metal

AgNO₃ (aq) tests on haloalkanes

• Fluoroalkanes do not react

• Chloroalkanes form a white precipitate (AgCl) after several minutes

• Bromoalkanes form a cream precipitate (AgBr) within a minute

• Iodoalkanes form a yellow precipitate (AgI) within seconds

Infrared (IR) spectroscopy

used as a quantitative test of organic compounds to determine the presence of functional groups

wet chemical tests

classical laboratory methods that use liquid solutions, reagents, and basic glassware to identify or measure the amount of a substance

• essentially limited to qualitative investigation

• nature of an organic compound can give misleading results if more than one functional group is present.

they can:

• identify each individual functional group present

• identify the molecular formula of a compound

• distinguish between structural isomers

• analyse mixtures of organic compounds

• quantify the amount of each organic compound present

mass spectrometer

an analytical instrument that identifies and quantifies molecules by converting them into ions and measuring their mass-to-charge ratio

ionisation chamber

here, the sample is bombarded by a high energy electron beam, which knocks a valence electron of the molecule to form a molecular ion (M⁺)

mass analyser

molecular ions are accelerated by an electric field which is curved, then deflected by a magnetic field. ions with different m/z (mass to charge) ratios are deflected to different extents

results in ‘sorting’ of ions according to their mass to charge ratio

detector

detects and quantifies the separated ions, producing a mass spectrum

base peak

most common fragmentation peak (mass spec) - highest relative intensity

parent peak

the signal that represents the complete, unfragmented molecule with a single electron removed (mass spec)

mass spectrometer

plot of relative intensity vs m/z

relative intensity assigns the molecular fragment with the highest abundance - a value of 100

NMR (nuclear magnetic resonance spectroscopy)

technique used to determine a compound’s unique structure

it identifies the carbon-hydrogen framework of an organic compound

• HNMR spectroscopy or proton NMR - analyses the hydrogen-1 nucleus

• CNMR spectroscopy - examines carbon-13 nucleus

magnetic moment

measure of a magnet's overall strength and the direction it tries to point inside an external magnetic field

resonance

the exact moment when an atomic nucleus in a magnetic field absorbs radio-frequency energy and flips its spin state

superconducting magnets

produce a very strong magnetic field

radio frequent (RF) generator

excites the nucleir causing a flip-spin

chemical environment

____ of a specific atom within a molecule includes the atom and its surrounding electrons

electron shielding

the way surrounding electron clouds protect a nucleus from an external magnetic field - reduces the experienced field. since the magnetic field experienced at the nucleus defines the energy between different spin states, it also defines what the chemical shift will be for the nucleus

signal splitting

many single peaks on a low-resolution spectrum will consist of multiple peaks on a high resolution spectrum (HNMR)

the division of a single NMR signal into multiple peaks caused by the magnetic influence of neighboring, non-equivalent nuclei

indicates with the n+1 rule how many hydrogens are on the neighbouring carbon for each environment

n+1 rule

predicts the number of peaks (multiplicity) a signal will split into based on the number of neighboring equivalent protons

e.g. if there are three peaks → the environment has two hydrogens on the neighbouring carbon (2 adjacent hydrogens)

relative peak area

HNMR - tells you how many hydrogens are in the environment that cluster of peaks represents - NOTE: THIS IS A RATIO (between the different environments)