Chemistry 3.6- Organic Analysis

What test can be used to identify aldehydes and primary and secondary alcohols?

• Add acidified potassium dichromate

• Positive test- orange solution → green precipitate

What test can be used to identify alcohols?

• Add sodium metal

• Positive test- bubbles (this is hydrogen gas)

What two tests can be used to identify aldehydes?

• Warm with Fehling’s solution

• Positive test- blue solution → red precipitate

OR

• Warm with Tollen’s reagent

• Positive test- colourless liquid → silver mirror

What test can be used to identify alkenes?

• Shake with bromine water

• Positive test- orange solution to colourless (decolourises bromine)

What test can be used to identify carboxylic acids?

• Add sodium carbonate or sodium hydrogen carbonate solution

• Positive test- bubbles (this is CO2- you can bubble it through limewater to confirm (clear solution → cloudy))

What test can be used to identify haloalkanes?

• Add aqueous sodium hydroxide

• Acidify with nitric acid

• Add silver nitrate solution

• Positive tests:

  • Chloroalkane- colourless solution → white precipitate

  • Bromoalkane- colourless solution → cream precipitate

  • Iodoalkane- colourless solution → yellow precipitate

What would the mass of this compound be?

58- the molecular ion peak (tallest peak to the right)

What m/z values are significant in mass spectrometry and why?

15, 29, 43, 57, 71 and 85

• They are the masses of the alkyl chains which are commonly seen as fragment values in mass spectra

Deduce the molecule in this mass spectrum

• Two peaks at equal heights, 2 units of mass apart

• So it contains a bromine atom, as bromine exists as the isotopes ⁷⁹Br and ⁸¹Br in equal proportions

• 110-81 = 29 which is the remaining mass

• So this is the mass spectrum for C₂H₅Br

How does infrared spectroscopy work?

• All bonds vibrate at a unique frequency, within the infrared range

  • Strong bonds and bonds between light atoms vibrate at high frequencies

• When you shine a beam of infrared at a sample, the bonds will absorb their natural infrared frequencies

• This means that the infrared that emerges will be missing the frequencies of the bonds in the sample

• A graph of frequency vs the intensity detected is plotted by an IR spectrometer and can be compared to identify molecules

  • Frequency is expressed in the unit wavenumbers (cm⁻¹)

What absorption corresponds with the O-H bond in alcohols?

Broad absorption between 3230 and 3550

What absorption corresponds with the O-H bond in carboxylic acids?

Broad absorption between 2500 and 3000

What absorption corresponds with the C=O bond in aldehydes, ketones and carboxylic acids?

Sharp absorption between 1680 and 1750

What absorption corresponds with the C=C bond in alkenes?

1620-1680

What is the region in an IR spectrum below 1500 cm⁻¹?

• The fingerprint region

• Incredibly specific and unique to each molecule

• This region is compared to other known spectra in a database to identify the compound

How can IR spectroscopy be useful?

• Identifying individual molecules and functional groups by fingerprinting

• Identifying impurities in compounds by looking at unexpected peaks