Organic Analysis Past Paper PDF - AQA

AS CHEMISTRY 3.3.6 ORGANIC ANALYSIS CHEMICAL TESTS FOR FUNCTIONAL GROUPS FUNCTIONAL CHEMICAL TEST OBSERVATION GROUP...

AS CHEMISTRY 3.3.6 ORGANIC ANALYSIS CHEMICAL TESTS FOR FUNCTIONAL GROUPS FUNCTIONAL CHEMICAL TEST OBSERVATION GROUP Decolourises Alkene + Br2(aq) Orange to Colourless +KOH(aq) + Heat White Precipitate = chloroalkane Halogenoalkane + AgNO3(aq) to test for Cream Precipitate = bromoalkane X- ion Yellow Precipitate = iodoalkane + 1o alcohol = Orange to Green + H / Cr2O7 + Heat 2- Alcohol 2o alcohol = Orange to Green (oxidation) 3o alcohol = no reaction Aldehyde = Silver mirror +Tollen’s + Heat Ketone = no reaction Aldehyde & Ketone Aldehyde = Red precipitate + Fehling’s + Heat Ketone = no reaction Carboxylic Acid + Na2CO3(s) or (aq) CO2(g) produced / Fizzing How to Use Chemical tests to ID Functional Groups AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.3.6 ORGANIC ANALYSIS MASS SPECTROSCOPY The process of mass spectroscopy for organic molecules works in the same way as in Section 3.1.1. Mass spectroscopy can be used to find the Mr of an organic molecule. e.g. ethanol C2H5OH The large peak furthest to the right on the spectrum is known as the molecular ion peak (M+ peak). It is caused by the complete organic molecule. This tells us the Mr of the compound as m/z = Mr for a 1+ charged molecule. In this case (C2H5OH)+ m/z = 46, therefore Mr = 46 Any small peak to the right of this is likely caused by a small number if molecule that have a 13C isotope in, increasing the Mr by 1. M+ peak Relative Abundance 5 10 15 20 25 30 35 40 45 50 m/z (Mass:charge ratio) Any other peaks you see are caused by fragmentation. The molecule can break into smaller pieces when hit by the electron gun, causing fragments of the molecules to create peaks with lower m/z values (lower Mr). e.g. a (CH3)+ fragment will cause a peak at m/z = 15 You don’t have to worry about these. They won’t be references in the exam. It’s just better to clear this up so you know what they are! AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.3.6 ORGANIC ANALYSIS MOLECULES CONTAINING ISOTOPES Halogenoalkanes can produce multiple M+ peaks. This is because chlorine has two isotopes, 35Cl and 37Cl, that exist in a 3:1 ratio. e.g. chloromethane M+ at m/z 50: + M peaks CH335Cl 3 M+ at m/z 52: Relative CH337Cl Abundance 1 As the molecule only contains 1 Cl atom, it 10 20 30 40 50 60 70 80 90 can be a 35 or a 37 m/z (Mass:charge ratio) If a halogenoalkanes contains two Cl atoms, you get the classic 9:6:1 ratio (as seen in Topic 3.1.1), but this time, they are classed as M+ peaks. e.g. dichloromethane M+ peaks M+ at m/z 84: CH235Cl35Cl 9 M+ at m/z 86: 6 CH235Cl37Cl Relative Abundance M+ at m/z 88: 1 CH237Cl37Cl 10 20 30 40 50 60 70 80 90 m/z (Mass:charge ratio) AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.3.6 ORGANIC ANALYSIS INFRARED SPECETROSCOPY Infrared (I.R.) spectroscopy can identify specific covalent bonds in an organic molecule. Multiple wavelengths of I.R. radiation are fired at the molecule. This causes the covalent bonds to vibrate, bend and stretch. When they do, specific bonds absorb specific wavelengths of the I.R. radiation. e.g. H O H I.R. Radiation C C H H Some wavelengths Some wavelengths absorbed are transmitted Each bond has a specific wavenumber (cm-1) that they absorb. So, if that wavenumber has been absorbed, we know that a specific bond is present in the molecule. e.g. a C=C has a wavenumber between 1620-1680 cm-1. You can find a list of covalent bonds and their wavenumber in your data sheet. In exam questions you will be required to analyse an infrared spectrum and deduce which covalent bonds are present using you “look up” table in the data sheet. It’ll look like this… Wavenumber Bond /cm-1 N—H 3300 - 3500 (amines) O—H 3230 - 3550 (alcohols) C=O 1680 - 1750 C=C 1620 - 1680 AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.3.6 ORGANIC ANALYSIS I.R. SPECTRA - KEY AREAS “Peaks” of low transmittance This means that a covalent bond has absorbed that wavenumber, so it is present in the molecule. 100 80 % 60 Transmittance 40 20 0 4000 3000 2000 1500 1000 Wavenumber (cm-1) “Fingerprint” Region As the name suggests, this is unique to each How to Analyse I.R. molecule. Only refer two this if you can’t Spectra distinguish two molecules using the peaks. How to Use a Combination of data to ID a Molecule AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.3.6 ORGANIC ANALYSIS USES OF I.R SPECTRA Fingerprinting This allows the identification of a molecule by comparison of spectra. Essentially what you will be doing in each exam question. It also allows chemists to spot impurities in compounds e.g. When analysing a sample of a product of a reaction, it can tell you if some reactants remain in the sample. Breathalysers H H Police use these to detect a person’s alcohol levels. H C C O It analyses their breathe for ethanol. H H H It specifically looks for C-O and O-H bonds in the sample. GLOBAL WARMING Now you know that covalent bonds absorb I.R. radiation, this makes a bit more sense. 1. I.R radiation is reflected from the Earth 2. Covalent bonds in gases such as CO2, CH4 and NOx absorb wavelengths of I.R. radiation. 3. Molecules gain kinetic energy. 4. This causes increased collisions between the molecules which releases thermal energy. 5. This causes an increase in global temperature. AQA www.chemistrycoach.co.uk © scidekick ltd 2024