3.4 - Organic Analysis Flashcards
How do you test for a primary, secondary or tertiary alcohol?
Add 10 drops of the alcohol to 2cm^3 of acidified potassium dichromate solution in a test tube. Warm the mixture gently in a hot water bath. If primary or secondary, the orange solution turns green. If tertiary, nothing happens. You can also test for alcohols using sodium metal. If you add sodium, it will fizz as it gives off H2 gas. The colour change of dichromate is when it’s reduced to Cr^3+ ion, which is green.
What is the set up to collect alcohols?
Add excess alcohol to 2cm^3 of acidified potassium dichromate solution in a round bottomed flask. Set up the flask as part of the distillation apparatus, with a fractionating column, thermometer, condenser, and container to collect liquid. Once you’ve collected the product, you need to test whether it’s an aldehyde or a ketone.
How do you test between an aldehyde or a ketone?
Use Fehling’s/Benedict’s solution. Fehlings solution is blue of copper(II) ions dissolved in sodium hydroxide. Add 2 cm^3 of either to a test tube (it is a clear blue solution). Add 5 drops of the collected liquid to the test tube. Put the test tube in hot water for a few minutes. If aldehyde, solution turns red because copper (II) ions are reduced to a brick-red precipitate of copper (I) oxide, or else stays blue. Use tollens’ reagent. Put 2 cm^3 of 0.10 mol dm^-3 silver nitrate in a test tube (colourless). Add a few drops of dilute sodium hydroxide solution, and a light brown precipitate should form. Add drops of dilute ammonia solution until the brown precipitate dissolves completely. This is tollens reagent. Place the test tube in a hot water bath and add 10 drops of the collected liquid. Wait for a few minutes. If aldehyde, silver mirror forms because the aldehyde reduces the Ag^+ ions to silver atoms. If not, solution still colourless.
How do you test for alkenes?
This test is for double bonds. Add 2cm^3 of test solution to a test tube. Add 2cm^3 of bromine water and shake. If alkene, solution will go from orange to colourless. If not, solution stays orange.
How do you test for carboxylic acids?
Use sodium carbonate. Carboxylic acids react with carbonates to form a salt, carbon dioxide, and water. This test will also give a positive result with any acid. Add 2 cm^3 of test solution to test tube. Add 1 small spatula of solid sodium carbons or 2cm^3 of sodium carbonate solution. If solution fizzes, bubble the gas produced through limewater in a second test tube. If carboxylic acid, there will be fizzing and limewater will turn cloudy.
Why is m/z value usually the same as the molecular mass of a compound?
Because the ion will usually have a 1+ charge.
What is high resolution mass spectrometry?
These mass spectrometers measure atomic and molecular masses extremely accurately. Useful for identifying compounds with the same Mr when they’re rounded to the nearest whole number.
What is infrared spectroscopy?
Helps to identify organic molecules. A beam of IR radiation is passed through a sample of a chemical. The IR radiation is absorbed by the covalent bonds in the molecules, increasing their vibrational energy. Bonds between different atoms absorb different frequencies of IR radiation. Bonds in different places inna molecule also absorb different frequencies. So OH in OH and COOH are different.
What is the fingerprint region?
The region between 500 cm^-1 and 1500 cm^-1. Its unique to a particular compound. You can use a computer database to check this region of an unknown compound’s IR spectrum against those of known compounds. Infrared spectroscopy can also be used to find out how pure a compound is, and identify any impurities. Impurities produce extra peaks in the fingerprint region.
How is infrared radiation absorption linked to global warming?
Some of the electromagnetic radiation emitted by the sun reaches the earth and is absorbed. The earth then re-emits some of it as infrared radiation.
Molecules of greenhouse gases, like carbon dioxide, methane, and water vapour, in the atmosphere absorb this infrared radiation. It’s the bonds that absorb the radiation. They then re-emit some of it back towards the earth, keeping us warm. This is called the greenhouse effect. Human activities causes a rise in greenhouse gas concentrations.
