Organic Analysis

Section 1 of 18Test for an alkene

Reagent

• Reagent bromine water, an orange-brown solution

Method

• Method shake the unknown compound with the bromine water

Positive result

• Positive result the orange-brown colour disappears because bromine adds across C=C

Alternative

• Alternative iodine also decolourises with a double bond

Section 2 of 18Test for a halogenoalkane

Test for a halogenoalkane

Warm the halogenoalkane with sodium hydroxide in ethanol and water, releasing the halide ion by nucleophilic substitution.

Acidify with dilute nitric acid to remove carbonate ions, which could give a false positive.

Add silver nitrate solution.

• Equation
  • Ag⁺(aq) + X^-(aq) → AgX(s)

Halide	Precipitate
Chloride	White AgCl
Bromide	Cream AgBr
Iodide	Yellow AgI

• Memory aids
  • the precipitates get more coloured down the group, or list bromide, chloride and iodide alphabetically as cream, white and yellow

Section 3 of 18Tests for an aldehyde

Principle

• Principle the aldehyde is oxidised to a carboxylic acid, whereas ketones cannot be oxidised and give a negative result

Tollens’ reagent

• Tollens’ reagent aqueous silver nitrate in excess ammonia
  • (ammoniacal silver nitrate)
• Warm with the aldehyde.
• Ag⁺ is reduced to Ag, forming a silver mirror on the inside of the tube.

Fehling’s solution

• Fehling’s solution a blue solution of Cu²⁺ ions
• Warm with the aldehyde.
• Cu²⁺ is reduced to Cu⁺, giving an orange or brick-red precipitate.
• A ketone gives no visible change.

Section 4 of 18Test for an alcohol

• Classification
  • primary alcohols oxidise to aldehydes and acids, secondary alcohols to ketones, and tertiary alcohols do not oxidise

1

Add 1 cm³ of the substance to a test tube with a pipette.

2

Add 1 cm³ of acidified potassium dichromate with a different pipette.

3

Stopper and shake.

4

Place in a hot water bath at about 60 °C for a few minutes.

Test for an alcohol

Primary or secondary alcohol

• Primary or secondary alcohol the solution changes from orange to green

Tertiary alcohol

• Tertiary alcohol the solution stays orange

Caution

• Caution an aldehyde also turns acidified dichromate from orange to green

Section 5 of 18Test for a carboxylic acid

pH

• pH carboxylic acids in solution have a pH of about 3, so an indicator or pH probe can be used

Carbonate test

• Carbonate test add 1–2 cm³ of sodium carbonate or sodium hydrogencarbonate solution, and bubbles of gas indicate an acid

Confirmation

• Confirmation bubble the gas into limewater, which turns milky if it is carbon dioxide

Section 6 of 18Required Practical 5: apparatus

Aim

• Aim to test for alcohol, aldehyde, alkene, carboxylic acid and halogenoalkane functional groups

Apparatus

• Apparatus ethanol, small pieces of sodium under petroleum ether, ethanal or propanal, Fehling’s A and B, cyclohexene, bromine water, solid sodium hydrogencarbonate, dilute ethanoic acid, 1-bromobutane, sodium hydroxide, dilute nitric acid, 0.05 mol dm^-3 silver nitrate, 250 cm³ beaker, anti-bumping granules, test tubes, boiling tubes and holder, thermometer and pipettes
  • (with a beaker of ethanol for disposing of excess sodium)

Format

• Format five separate experiments, with results in a suitable table

Section 7 of 18Required Practical 5: methods 1 to 3

Required Practical 5: methods 1 to 3

Part 1

• Part 1 alcohol
• Put 1 cm³ of ethanol in a dry test tube.
• Add a small piece of sodium and record the observations (fizzing).
• Dispose of excess sodium safely in the beaker of ethanol.

Part 2

• Part 2 aldehyde
• Put 1 cm³ of sodium carbonate solution in a tube and add anti-bumping granules.
• Mix equal volumes of Fehling’s A and B to give a clear dark blue solution.
• Add 5 drops of it to the carbonate tube, then 1 cm³ of ethanal or propanal.
• Warm gently in a water bath for about 5 minutes and stand for several minutes.
• Record the observations.

Part 3

• Part 3 alkene
• Put 2–3 drops of cyclohexene in a tube and add 1 cm³ of bromine water.
• Shake vigorously and record the observations.

Section 8 of 18Required Practical 5: methods 4 and 5, and evaluation

Required Practical 5: methods 4 and 5, and evaluation

Part 4

• Part 4 carboxylic acid
• Put one small spatula of solid sodium hydrogencarbonate in a boiling tube.
• Add 2 cm³ of dilute ethanoic acid and record the observations.

Part 5

• Part 5 halogenoalkane
• Put 1 cm³ of sodium hydroxide in a tube and add 5 drops of 1-bromobutane.
• Warm in a water bath at about 60 °C for a few minutes.
• Add 2 cm³ of dilute nitric acid, then 1 cm³ of silver nitrate, and record the observations.

Volumes

• Volumes they need not be precise because only general observations are needed

Evaluation

• Evaluation check that each functional group gives the expected result

Example identification

• Example identification decolourising bromine water gives an alkene, a yellow precipitate with silver nitrate gives an iodoalkane, fizzing with NaHCO₃ gives a carboxylic acid, blue to red precipitate with Fehling’s gives an aldehyde,
  • and fizzing with sodium gives an alcohol

Section 9 of 18Mass spectrometry: molecular ion

Purpose

• Purpose to identify unknown compounds

Ionisation

• Ionisation the vaporised sample is bombarded with electrons, forming molecular ions, M⁺•

Molecular ion peak

• Molecular ion peak the peak with the highest m/z is M⁺, and its m/z equals the relative molecular mass

M+1 peak

• M+1 peak a smaller peak caused by carbon-13, whose height increases with the number of carbon atoms, so hexane has a larger M+1 peak than ethane

Example

• Example pent-1-ene has M⁺ at m/z 70 and M+1 at 71

Section 10 of 18Fragmentation

Fragmentation

• Fragmentation the molecular ion breaks into new ions, molecules and radicals, and the same molecule gives several different fragments

Mass spectrum

• Mass spectrum the relative abundances of ions form a molecular fingerprint that a computer can match to a database

Fragment	m/z	Small molecule lost	m/z
CH3+	15	H2O	18
C2H5+	29	CO	28
C3H7+	43	CO2	44

• Example
  • butan-2-ol cannot give a peak at m/z 43, because it contains neither CH₃CH₂CH₂⁺ nor (CH₃)₂CH⁺

Section 11 of 18Low and high resolution mass spectrometry

Low resolution

• Low resolution masses to 2 decimal places using values such as H 1.00, C 12.00 and O 16.00
• It suits general analysis and simple compounds.
• It cannot distinguish molecules of similar molecular mass.

High resolution

• High resolution masses to 4 or 5 decimal places, such as H 1.00783, C 12.0000 and O 15.99491
• It gives a precise molecular mass and so the molecular formula.
• It distinguishes molecules of similar mass but not isomers, which have identical molecular mass.

Molecule	Low resolution	High resolution
Propane, C3H8	44.00	44.06264
Ethanal, CH3CHO	44.00	44.02623

• Example
  • CHCF (44.00623) and CO₂ (43.98982) both have relative molecular mass 44.00 at low resolution, but high resolution separates them

Section 12 of 18Infrared spectroscopy: principle

Bond vibration

• Bond vibration covalent bonds act like springs and vibrate in different ways

Method

• Method a spectrophotometer passes IR radiation through the sample, some energy is absorbed by the bonds and the rest is transmitted to a detector

Spectrum

• Spectrum the x-axis is wavenumber in cm^-1 and the y-axis is transmittance as a decimal or percentage

Peaks

• Peaks characteristic peaks identify functional groups

Section 13 of 18Infrared absorption data and shapes

Bond	Wavenumber / cm-1
N–H (amines)	3300–3500
O–H (alcohols)	3230–3550
C–H	2850–3300
O–H (acids)	2500–3000
C≡N	2220–2260
C=O	1680–1750
C=C	1620–1680
C–O	1000–1300
C–C	750–1100

O–H

• O–H alcohols and carboxylic acids give a broad scoop, and position and shape distinguish them

C=O

• C=O aldehydes and ketones give a sharp intense spike

Example

• Example propanone has a sharp peak near 1700 cm^-1, and propan-2-ol has a broad peak at 3200–3500 cm^-1

Section 14 of 18Using infrared spectroscopy

Limitation

• IR alone usually cannot identify an unknown compound

Uses

• Uses to identify functional groups, to confirm a known compound is present, and to detect impurities by comparing with database spectra

Combined with other techniques

• Elemental analysis gives the empirical formula.
• Mass spectrometry gives the molecular mass and fragments.
• NMR is not covered at AS.

Section 15 of 18The fingerprint region

Definition

• Definition the region below about 1500 cm^-1, which is unique to every molecule

Peaks

• Peaks many peaks come from complex vibrations between bonds and are hard to assign

Use

• Use matching it to a database gives an exact identification, for example of one member of a homologous series

Example

• Example propan-1-ol and propan-2-ol show the same bonds above 1500 cm^-1 but different fingerprint regions

Section 16 of 18Infrared radiation and global warming

Energy path

• Energy path short-wavelength UV from the Sun is absorbed by the Earth’s surface and re-emitted as longer-wavelength infrared

Greenhouse gases

• Greenhouse gases carbon dioxide, methane and water vapour trap much of this infrared, which maintains the Earth’s temperature
  • (the greenhouse effect)

Absorbing bonds

• CO₂ has two C=O bonds, methane has four C–H bonds and water has two O–H bonds, all of which absorb IR

Human activity

• Human activity it raises greenhouse gas levels, and more absorption of infrared causes global warming

Section 17 of 18Key terms

Term	Meaning
Tollens’ reagent	Ammoniacal silver nitrate used to identify aldehydes.
Fehling’s solution	An alkaline copper(II) reagent giving a brick-red precipitate with aldehydes.
Positive result	The observation that shows a functional group is present.
Decolourise	To lose colour, as bromine water does with an alkene.

Term	Meaning
Molecular ion	The ion formed when a molecule loses one electron, giving the highest m/z peak.
Wavenumber	The reciprocal of wavelength, in cm-1.
Fingerprint region	The region below about 1500 cm-1 unique to each molecule.
Fragmentation	The breakdown of the molecular ion into smaller ions.
Greenhouse effect	The trapping of infrared radiation by atmospheric gases.

Section 18 of 18Exam technique

1

Skipping the nitric acid step — carbonate ions would give a false precipitate.

2

Saying a ketone gives a silver mirror — only aldehydes do.

3

Thinking only alcohols turn dichromate green — aldehydes also do.

4

Using hydrochloric acid in the halide test — use nitric acid.

1

Saying the molecular ion is the base peak — it is the peak of highest m/z.

2

Saying IR alone identifies an unknown — it is used with other techniques.

3

Saying high resolution MS distinguishes isomers — isomers have the same molecular mass.

4

Saying the broad scoop peak is a C=O — C=O is a sharp spike.