3.3.6 Organic Analysis

Adding sodium metal to pure alcohol = fizzing as it gives off H2 gas

• Add 10 drops of alcohol to 2 cm3 of acidified potassium dichromate solution in test tube • Warm mixture gently in hot water bath • Watch for colour change • Primary: Orange solution turns green as aldehyde forms • Secondary: Orange solution turns green as ketone forms • Tertiary: Nothing happens

• Add excess alcohol to 2 cm3 of acidified potassium dichromate solution in round bottomed flask • Set up flask as part of distillation apparatus • Gently heat flask • Alcohol will be oxidised & product will be distilled off immediately = so you can collect it (if product produced aldehyde, then solution was a primary alcohol)

• Fehling’s solution • Benedict’s solution • Tollens’ reagent

• Add 2 cm3 of Fehling’s (or Benedict’s) solution to test tube • Add 5 drops of aldehyde or ketone to test tube • Put test tube in hot water bath to warm it for few minutes • Aldehyde: Blue solution will give brick red precipitate • Ketone: Nothing

• Put 2 cm3 of 0.1 mol dm-3 silver nitrate solution in test tube • Add few drops of dilute sodium hydroxide to solution • Light brown precipitate should form • Add drops of dilute ammonia solution until brown precipitate dissolves completely • Place test tube in a hot water bath and add 10 drops of aldehyde or ketone • Aldehyde: silver mirror forms on walls of test tubes • Ketone: Nothing

Aldehyde reduced Ag+ ions to silver atoms

• Add 2 cm3 of solution you want to test to test tube • Add 2 cm3 of bromine water to test tube • Shake test tube • Alkene: Solution will go from orange to colourless

• Add 2 cm3 of solution you want to test to test tube • Add 1 small spatula of solid sodium carbonate • If solution begins to fizz, bubble gas that it produces through some limewater in 2nd test tube • Carboxylic Acid: Solution will fizz & CO2 gas produced will turn limewater cloudy

• Add halogenoalkane (R—X) and sodium hydroxide to test tube • Warm it by placing it in hot water bath • Add dilute nitric acid and silver nitrate solution to test tube to acidify its contents • Halogenoalkane: Precipitate of AgX

To find Mr of compound

a molecule loses an electron

molecular ion peak

molecular mass of compound

Measure atomic and molecular masses extremely accurately

Identifying compounds that have same Mr when rounded to nearest whole number

• Beam of IR radiation passed through sample of chemical • IR radiation absorbed by covalent bonds in molecules = ↑ vibrational energy • Bonds between different atoms absorb different frequencies of IR radiation • Bonds in different places in molecules absorb different frequencies too (e.g. O-H group in alcohol and O-H in carboxylic acid absorb different frequencies)

Infrared spectrometer produces graph that shows what frequencies of radiation molecules are absorbing

To identify functional groups in molecule

500 cm-1 and 1500cm-1

Unique

• Can use computer database to check this region of unknown compound’s IR spectrum against known compounds • If it matches one of them = know what molecule it is

how pure compound is & identify any impurities