A-level Chemistry: 3.3.6 Organic Analysis

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

1. Add 10 drops of alcohol to 2 cm3 of acidified potassium dichromate solution in test tube

2. Warm mixture gently in hot water bath

3. Watch for colour change
   

   1. Primary: Orange solution turns green as aldehyde forms

   2. Secondary: Orange solution turns green as ketone forms

   3. Tertiary: Nothing happens

1. Add excess alcohol to 2 cm3 of acidified potassium dichromate solution in round bottomed flask

2. Set up flask as part of distillation apparatus

3. 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)

1. Fehling’s solution

2. Benedict’s solution

3. Tollens’ reagent

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

1. Put 2 cm3 of 0.1 mol dm-3 silver nitrate solution in test tube

2. Add few drops of dilute sodium hydroxide to solution
   

   • Light brown precipitate should form

3. Add drops of dilute ammonia solution until brown precipitate dissolves completely

4. Place test tube in a hot water bath and add 10 drops of aldehyde or ketone

5. Aldehyde: silver mirror forms on walls of test tubes

6. Ketone: Nothing

Aldehyde reduced Ag+ ions to silver atoms

1. Add 2 cm3 of solution you want to test to test tube

2. Add 2 cm3 of bromine water to test tube

3. Shake test tube

4. Alkene: Solution will go from orange to colourless

1. Add 2 cm3 of solution you want to test to test tube

2. Add 1 small spatula of solid sodium carbonate

3. If solution begins to fizz, bubble gas that it produces through some limewater in 2nd test tube

4. Carboxylic Acid: Solution will fizz & CO2 gas produced will turn limewater cloudy

1. Add halogenoalkane (R—X) and sodium hydroxide to test tube

2. Warm it by placing it in hot water bath

3. Add dilute nitric acid and silver nitrate solution to test tube to acidify its contents

4. 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

1. Beam of IR radiation passed through sample of chemical

2. IR radiation absorbed by covalent bonds in molecules = ↑ vibrational energy
   

   1. Bonds between different atoms absorb different frequencies of IR radiation

   2. 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

extra peaks in fingerprint region

1. Some electromagnetic radiation emitted by Sun reaches Earth and is absorbed

   • Earth re-emits some of its infrared radiation (heat)

2. Molecules of greenhouse gases (CO2, CH4, water vapour) in atmosphere absorb IR radiation (it's bonds of these molcules that absorb it)

   • & re-emit some of it back towards Earth = keeps us warm

   • Called greenhouse effect

3. Human activities, burning fossil fuels & leaving rubbish to rot in landfill sites, caused rise in greenhouse gas concentrations

4. Means more heat being trapped and Earth getting warmer = global warming

• Board peak at 2500 – 3000 cm-1 shows O-H bond (in carboxylic acid)

• Sharp peak at 1680 – 1750 cm-1 shows C=O

(carboxylic acid = 2 peaks)

Alcohol

| (Acid's peak is boarder than alcohol)

Aldehyde