Organic analysis

How do you test for alcohols? What is the result?

• Add acidified potassium dichromate (K2Cr2O7) and heat

• Primary and secondary alcohols change from orange to green

• No change for tertiary alcohols as they cant be oxidised

How could you distinguish between primary and secondary alcohols?

• Use fractional distillation to collect the products produced upon oxidation with K2Cr2O7

• Test the product to see if an aldehyde or ketone is formed

• Aldehyde - made from primary alcohol

• Ketone - made from secondary alcohol

How do you test for aldehydes and ketones?

2 ways:

• Add Fehlings solution and warm

• Presence of aldehydes - blue solution to brick red precipitate

• Presence of ketones - remains blue

• Add Tollens’ reagent and warm

• Presence of aldehydes - silver mirror forms

• Presence of ketones - no silver precipitate forms

How do you make Tollens’ reagent?

• Add a few drops of NaOH to silver nitrate solution - pale brown precipitate forms

• Add a few drops of dilute ammonia until precipitate dissolves

How do you test for alkenes? What is the result?

• Shake with bromine water

• If alkene is present - water goes from orange to colourless

How do you test for carboxylic acids? What is the result?

• Add sodium carbonate

• In presence of carboxylic acid - effervescence due to CO2 given off

• Could confirm by bubbling through limewater - would turn cloudy

How do you test for haloalkanes? What is the result?

• Add acidified AgNO3

• Result = preipitate of AgX (Cl-white, Br-cream, I-yellow)

• If unclear - add dilute ammonia solution

• Result = AgCl precipitate dissolves

• If still unclear - add concentrated ammonia solution

• Result = AgBr precipitate dissolves

What is mass spectrometry used for?

To find the Mr of a compound

What does the x axis on a mass spectrum show?

• m/z value

• Since most ions are 1+, this effectively shows Mr

Why are there multiple peaks when molecules are put into a mass spectrometer?

The molecular ion (which is the same as a the Mr) is the last peak but it fragments into smaller molecules, so these peaks are shown as well.

Different peaks may also be seen due to isotopes

What is high resolution mass spectrometry used for and what makes it useful?

• Used to identify different molecules with the same Mr rounded to the nearest whole number

• Useful as they measure the Mr to several decimal places

What is Infrared spectroscopy? How does it work?

• A beam of IR radiation is passed through a sample

• Some frequencies are absorbed by the sample (because they match the natural vibration frequencies of certain bonds)

• A detector measures which frequencies are not absorbed

• This allows us to tell which types of bonds are present

How can we confirm the identity of a compound using an IR spectrum?

Compare the fingerprint region with a known spectrum

What does the frequency of IR radiation absorbed by a covalent bond depend on?

• The atoms that are on either side of the bond eg their mass

• The position of the bond in the molecule

• The strength of the bond

What do the troughs on an infrared spectrum show?

The frequencies where radiation has been absorbed

What is the fingerprint region?

The region in an IR spectrum that lies between 500cm-1 and 1500cm-1

The size and position of the peaks in this region are unique to a particular molecule

You can compare it against a known library of spectra to identify the molecule

What do extra peaks in the fingerprint region indicate?

Impurities in the sample