Organic Chemistry: Techniques & Spectra — IAL Edexcel Modular (Chemistry) Revision Notes & Topical Questions

Section 1 of 20Heating under reflux

• Why reflux
  • organic reactions are often slow at room temperature, so they need prolonged heating
  • Reactants and products are volatile, so a vertical condenser returns vapour to the flask.
  • Nothing escapes and the flask does not boil dry.

1

Place the reaction mixture and anti-bumping granules in a round-bottomed or pear-shaped flask.

2

Heat with a heating mantle or water bath, not a naked flame, because organic liquids are flammable.

3

Fit a vertical condenser with a water jacket, with water entering at the bottom and leaving at the top.

4

Heat until the mixture boils, keeping the top of the condenser open, then allow it to cool.

Heating under reflux

Examples

• Examples a carboxylic acid from a primary alcohol with acidified dichromate, and an ester from an alcohol and acid with an acid catalyst
• Ethyl ethanoate is prepared by refluxing for about 15 minutes.

Never heat a sealed system

• Never heat a sealed system the condenser must be open at the top

Section 2 of 20Distillation

• Purpose
  • separates a product from a mixture by boiling point, with the lowest boiling point distilling first

1

Place the mixture and anti-bumping granules in the flask and fit a still head and a sloping condenser leading to a receiver.

2

Heat with an electric heating mantle, because the temperature can be controlled and the chemicals are flammable.

3

Place the thermometer bulb at the side-arm opening to measure the vapour temperature.

4

Discard any distillate below the boiling point of the product, collect the fraction within about 2 °C of the boiling point,

• and stop if the temperature rises further.

• Aldehyde from a primary alcohol
  • the aldehyde has a lower boiling point because it cannot hydrogen bond, so it distils off at once
  • This prevents further oxidation to the acid.

Section 3 of 20Boiling point determination

Boiling point

• Boiling point a pure liquid boils at a fixed temperature at a given pressure
• It is found by gentle distillation, recording the steady thermometer reading.

Impurities

• Impurities raise the boiling point and cause boiling over a range of temperatures
• The value is compared with literature or database values.

Section 4 of 20Solvent extraction

1

Transfer the contents of the reaction vessel to a separating funnel and add the stopper.

2

Invert the funnel and open the stopcock to release pressure, repeating this 15–20 times.

3

Allow the two layers to separate, with the denser layer at the bottom.

4

Drain the aqueous layer through the stopcock and run the organic layer into a clean beaker.

Identifying the layers

• Identifying the layers add water and see which layer increases in volume

Washing with sodium carbonate solution

• Washing with sodium carbonate solution neutralises acidic impurities
• CO₂ is released, so the stopcock is opened slowly to avoid losing product.

Section 5 of 20Drying the organic liquid

• Drying agents
  • remove traces of water
  • Anhydrous calcium chloride dries hydrocarbons.
  • Anhydrous calcium sulfate or magnesium sulfate are general-purpose drying agents.

Drying the organic liquid

Add a spatula of drying agent to the organic liquid and swirl.

If the agent clumps, water is still present, so add more until some remains as a fine powder.

Decant or filter the dry liquid, which should appear clear.

• Low boiling point liquids
  • a stopper reduces evaporation

Section 6 of 20Preparing and purifying a liquid

1

Heat under reflux.

2

Distil off the crude product.

3

Wash in a separating funnel.

4

Dry with an anhydrous salt.

5

Redistil at the boiling point.

6

Check purity by boiling point and infrared spectrum.

Section 7 of 20Mass spectra in brief

Molecular ion, M+

• Molecular ion, M+ the peak at the highest m/z gives the relative molecular mass
• A small M+1 peak arises from carbon-13, about 1.1% of carbon.

Fragment peaks

• Fragment peaks the mass difference from M⁺ shows the lost neutral group
• A difference of 15 is CH₃.

Halogen isotopes

• Halogen isotopes chlorine compounds show M and M+2 peaks in a 3 : 1 ratio and bromine compounds show them in a 1 : 1 ratio

Section 8 of 20Infrared spectra in brief

Infrared spectra in brief

Infrared absorption

• Infrared absorption bonds absorb IR radiation at specific frequencies and vibrate by stretching and bending
• Wavenumber in cm^-1 is used.

Alcohol

• Alcohol broad O–H but no C=O

Carboxylic acid

• Carboxylic acid very broad O–H and C=O

Aldehyde, ketone or ester

• C=O only

Fingerprint region

• Fingerprint region below about 1500 cm^-1, unique to each compound and compared with a database

Section 9 of 20More from the specification

1Mass spectrometry

• Mass spectrometry sample is ionised, and the mass to charge ratio of ions is measured, where the molecular ion peak gives the relative molecular mass

2Fragmentation

• Fragmentation the molecular ion breaks into fragments, and the abundance of each is plotted, with relative abundance of the tallest peak taken as 100

3Infrared spectroscopy

• Infrared spectroscopy absorption corresponds to bond vibrations, such as the C=O and O-H stretches, identifying functional groups

4Techniques

• Techniques apparatus for reflux and distillation, drying agents added to remove water, and melting point checks for purity

5Fragmentation

• Breaking of the molecular ion into a smaller cation and a radical.

6Charge

• Q = It, measured in coulombs.
  What the symbols mean: Q = charge (C) · I = current (A) · t = time (s)

7Abundance

• The percentage or proportion of atoms of an element that are a particular isotope.

8Relative abundance

• Percentage of each isotope.

Section 10 of 20More on this topic

• Preparation and purification
  • reflux, distillation, solvent extraction and drying

Section 11 of 20Mass spectrometry and m/z

Mass spectrometer

• Mass spectrometer a sample is ionised, and the positive ions are accelerated, deflected by a magnetic field and detected
• High-energy electrons knock an electron off each molecule to form a molecular ion.

m/z

m/z mass ÷ charge

• Most ions have a charge of 1+, so m/z equals the mass.
• A 2+ ion of mass 16 appears at m/z 8.

Deflection

• Deflection ions with a smaller mass and larger positive charge (smaller m/z) are deflected more

Base peak

• Base peak the peak of the most abundant ion

Section 12 of 20The molecular ion and M+1 peak

Molecular ion, M+

• Molecular ion, M+ the whole molecule that has lost one electron
• It gives the relative molecular mass, so M⁺ at m/z 70 identifies pent-1-ene rather than but-1-ene at 56.
• It is the peak with the highest m/z, ignoring the small isotope peaks.

M+1 peak

• M+1 peak due to molecules containing one carbon-13 atom
• Carbon-13 is a little over 1% of carbon.
• The more carbon atoms in the molecule, the taller the M+1 peak.

Section 13 of 20Fragmentation

• Fragmentation
  • the molecular ion breaks into a cation and a radical, and only positively charged fragments are detected
  • Characteristic peaks identify structures, and a peak is a fragment of the whole molecule.

m/z	Fragment ion
15	CH3+
29	C2H5+ or CHO+
31	CH2OH+
43	C3H7+ or CH3CO+
45	COOH+

Loss of small molecules

• Loss of small molecules gives peaks at M − 18 (H₂O), M − 28 (CO) and M − 44 (CO₂)

Alkanes

• Alkanes fragment by breaking C–C bonds
• Successive peaks are 14 mass units apart because fragments differ by CH₂.

Alcohols

• Alcohols lose water, giving a peak 18 below M⁺, and show CH₂OH⁺ at 31
• Propan-1-ol shows peaks at 59, 42, 31 and 29.
• A peak at 31 points to a primary alcohol and a base peak at 45 (loss of CH₃) points to propan-2-ol.

Isomers

• Isomers propanal gives m/z 29, but propanone gives a strong peak at 43 (CH₃CO⁺) and 15

Section 14 of 20Halogens in mass spectra

Chlorine

• M and M+2 peaks in a 3 : 1 ratio
• ³⁵Cl and ³⁷Cl occur in a 3 : 1 ratio.

Bromine

• M and M+2 peaks of almost equal height
• CH₃Br has molecular ion peaks at m/z 94 (⁷⁹Br) and 96 (⁸¹Br), and a peak at 15 for CH₃⁺.

Section 15 of 20Infrared spectroscopy

Principle

• Principle the sample is irradiated with infrared radiation and the transmitted intensity is recorded against wavenumber
• Bonds absorb particular frequencies, causing vibrations such as stretching and bending.
• A dip in transmittance is an absorption, and the pattern identifies functional groups.

Wavenumber

• Wavenumber the reciprocal of wavelength, in cm^-1

Appearance

• Appearance hydrogen bonding makes O–H absorptions broad, and the C=O absorption is strong and sharp

Section 16 of 20Characteristic infrared absorptions

Bond	Wavenumber / cm-1	Appearance
O–H (alcohol)	3200–3550	Broad
O–H (carboxylic acid)	2500–3300	Very broad
N–H	3300–3500	Sharp
C–H	2850–3100	Sharp
C=O	1680–1750	Strong, sharp
C=C	1620–1680	Medium

Characteristic infrared absorptions

Alcohol

• O–H but no C=O

Ketone or aldehyde

• Ketone or aldehyde strong C=O near 1710 cm^-1 and no O–H

Overlap

• Overlap some absorptions overlap, so mass spectrometry is used alongside IR

Section 17 of 20Fingerprint region and uses of IR

Fingerprint region

• Fingerprint region below about 1500 cm^-1, unique to each compound and compared with a database

Breathalysers

• Breathalysers use C–H and C=O absorptions to detect ethanol

Greenhouse gases

• CO₂, CH₄ and H₂O absorb infrared radiation, which is why they cause the greenhouse effect

Section 18 of 20More from the specification

More from the specification

Reflux heating and condenser setup

• Organic synthesis frequently requires heating a reaction mixture under reflux with a water-cooled Liebig condenser mounted vertically to prevent volatile reactants and products from escaping.

Fractional and simple distillation

• Distillation separates a liquid reaction mixture based on boiling point differences;
  • the lower-boiling product vapour ascends, condenses in the condenser,
• and collects as pure distillate.

Liquid-liquid extraction and drying agents

• When isolating organic products with a separating funnel, the organic layer is separated from the aqueous layer and shaken with an anhydrous drying agent to remove residual water.
  • (e.g. anhydrous MgSO₄ or CaCl₂)

Identification via infrared spectroscopy

• In infrared (IR) spectroscopy, characteristic absorption bands correspond to vibrational excitations of specific functional groups.
  • (e.g., a sharp peak at 1710 cm^-1 corresponds to a carbonyl C=O bond)

Reflux heating and condenser setup

• Organic synthesis frequently requires heating a reaction mixture under reflux with a water-cooled Liebig condenser mounted vertically to prevent volatile reactants and products from escaping.

Fractional and simple distillation

• Distillation separates a liquid reaction mixture based on boiling point differences;
  • the lower-boiling product vapour ascends, condenses in the condenser,
• and collects as pure distillate.

Liquid-liquid extraction and drying agents

• When isolating organic products with a separating funnel, the organic layer is separated from the aqueous layer and shaken with an anhydrous drying agent to remove residual water.
  • (e.g. anhydrous MgSO₄ or CaCl₂)

Identification via infrared spectroscopy

• In infrared (IR) spectroscopy, characteristic absorption bands correspond to vibrational excitations of specific functional groups.
  • (e.g., a sharp peak at 1710 cm^-1 corresponds to a carbonyl C=O bond)

Fractional distillation

• Fractional distillation separates liquid mixture components based on differences in boiling temperatures using a fractionating column and water-cooled condenser.

Infrared carbonyl absorption

• Infrared carbonyl absorption a sharp, intense absorption band near 1710 cm⁻¹ corresponds characteristically to carbonyl C=O bond stretching.

Drying organic liquids

• Drying organic liquids adding anhydrous magnesium sulfate absorbs residual moisture from an immiscible organic layer before final distillation.

Separating funnel

• Apparatus for separating two immiscible liquids.

Drying agent

• An anhydrous salt that removes water from an organic liquid.

Boiling

• Liquid to gas throughout the liquid at the boiling point.

Distillation

• Separating a liquid from a solution by boiling and condensing the vapour.

Condenser

• Apparatus that cools gas back to liquid.

Mixture

• Two or more substances mixed but not chemically combined.

Reflux

• Heating with a condenser to stop volatile liquids escaping.

Temperature

• A measure of the average kinetic energy of the particles.

Section 19 of 20Key terms

Term	Meaning
Heating under reflux	Continuous heating with a vertical condenser so vapours return to the flask.
Distillation	Separation of a liquid from a mixture by boiling and condensing its vapour.
Anti-bumping granules	Small porous pieces added to a liquid to promote smooth boiling.
Separating funnel	Apparatus used to separate two immiscible liquid layers.
Drying agent	An anhydrous salt that removes water from an organic liquid.
Molecular ion peak	The peak of highest m/z in a mass spectrum, giving the relative molecular mass.
Fingerprint region	The infrared region below about 1500 cm⁻¹ that is unique to each compound.

Term	Meaning
m/z	The mass-to-charge ratio of an ion.
Molecular ion	The ion formed by loss of one electron from the whole molecule.
Fragment ion	A positive ion formed when the molecular ion breaks up.
Wavenumber	The reciprocal of wavelength, in cm⁻¹, used in IR spectra.
Fingerprint region	The region below about 1500 cm⁻¹ that is unique to each compound.
Base peak	The peak of the most abundant ion.

Section 20 of 20Exam technique

1

Heating a sealed flask — a reflux condenser must be open at the top

2

Adding water to the condenser from the top — water enters at the bottom and leaves at the top

3

Using a naked flame to heat flammable organic liquids — use a water bath or electric heater

4

Collecting distillate at any temperature — collect only near the boiling point of the product

5

Assuming drying is complete when the agent clumps — add more until some stays as a fine powder

1

Forgetting that only positive fragments are detected

2

Assuming the molecular ion is always the tallest peak

3

Confusing broad O–H with sharp N–H absorptions

4

Using the fingerprint region to find a particular functional group