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OCR A-Level Chemistry: Alkenes, Polymers, and NMR Spectroscopy, Exams of Chemistry

College of The AlbemarleChemistry

This comprehensive guide covers key concepts in organic chemistry, including alkenes, polymers, and nuclear magnetic resonance (nmr) spectroscopy. it provides detailed explanations, examples, and practice questions to help students master these topics. Particularly useful for a-level chemistry students preparing for exams, offering a structured approach to understanding complex chemical structures and reactions.

Typology: Exams

2024/2025

Available from 05/02/2025

SERENAWILLIAMS
SERENAWILLIAMS 🇺🇸

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OCR A-Level Chemistry Paper 2 2022
ALL ANSWERS 100% CORRECT
GUARANTEED GRADE A+
What is an alkene and its general formula?
It is an unsaturated hydrocarbon with a least one C=C bond. General
formula is CnH2n
What is a sigma bond? σ
single covalent bond formed when two s orbitals overlap. High
electron density
What is a pi bond?
sideways overlap of p orbitals that occur in a double or triple bond.
Much weaker than sigma, low bond enthalpy and lower electrostatic
attraction
Is an alkane or an alkene more reactive?
Alkene. contains both sigma and pi bonds with high electron density
and low bond enthalpy.
Define steroisomers
have the same structural formulae but have a different arrangement of
atoms in a space
What is a cis isomer? (Z)
the two Xs are on the same side of the double bond
What is a trans isomer? (E)
the two x's are on opposite sides of the double bond
What is an electrophile?
An electron pair acceptor
What is a nucleophile?
An electron pair donor
How do you produce alkanes from an alkene?
React with hydrogen gas in addition reaction.
requires nickle catalyst and 150℃
How do you produce dihaloalkanes from alkenes?
Halogens react with alkenes in electrophillic addition.
Bromine water turns orange to colourless as it is added across the
double bond to form a colour dibromoalkane
How do you produce alcohols from alkenes?
Alkenes need to be hydrated by steam at 300℃ and pressure 60 - 70
atm.
Needs solid phosphoric (V) acid catalyst
How do you form haloalkanes from alkenes?
Alkenes undergo addition with hydrogen halides
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Download OCR A-Level Chemistry: Alkenes, Polymers, and NMR Spectroscopy and more Exams Chemistry in PDF only on Docsity!

OCR A-Level Chemistry Paper 2 2022

ALL ANSWERS 100% CORRECT

GUARANTEED GRADE A+

What is an alkene and its general formula? It is an unsaturated hydrocarbon with a least one C=C bond. General formula is CnH2n What is a sigma bond? σ single covalent bond formed when two s orbitals overlap. High electron density What is a pi bond? sideways overlap of p orbitals that occur in a double or triple bond. Much weaker than sigma, low bond enthalpy and lower electrostatic attraction Is an alkane or an alkene more reactive? Alkene. contains both sigma and pi bonds with high electron density and low bond enthalpy. Define steroisomers have the same structural formulae but have a different arrangement of atoms in a space What is a cis isomer? (Z) the two Xs are on the same side of the double bond What is a trans isomer? (E) the two x's are on opposite sides of the double bond What is an electrophile? An electron pair acceptor What is a nucleophile? An electron pair donor How do you produce alkanes from an alkene? React with hydrogen gas in addition reaction. requires nickle catalyst and 150 ℃ How do you produce dihaloalkanes from alkenes? Halogens react with alkenes in electrophillic addition. Bromine water turns orange to colourless as it is added across the double bond to form a colour dibromoalkane How do you produce alcohols from alkenes? Alkenes need to be hydrated by steam at 300 ℃ and pressure 60 - 70 atm. Needs solid phosphoric (V) acid catalyst How do you form haloalkanes from alkenes? Alkenes undergo addition with hydrogen halides

What is Markownikoffs rule? The major product from addition of a hydrogen halide to an unsymmetrical alkene is the one where hydrogen adds to the carbon with the most hydrogens already attached How do you find the monomer of a polymer? Take the repeat unit and add a double bond State an advantage of polymers Very unreactive so food doesn't react with the coating on pans, plastic windows don't rot, etc. State a disadvantage of polymers Most polymers aren't biodegradable and therefore difficult to dispose of. This leads to more environmental damage. State the ways of dealing with waste plastics Burning as a fuel Burying in landfill Remoulding for new objects Cracking in monomers to become feedstock for new plastics or other chemicals State two types of polymers that aren't wasteful (Ecofriendly) Biodegradable (eaten by other microorganisms) Photodegradable (Broken down by sunlight) What is nuclear magnetic resonance (NMR)? Analytical technique used to work out the structure of an organic molecule What are the basics of NMR?

  1. Sample is placed in a strong magnetic field and exposed to a range of different frequencies of radio waves
  2. Nuclei of certain atoms within the molecule absorb energy from radio waves
  3. Amount of energy nucleus absorbs will depend on the environment that it is in
  4. Pattern of absorptions gives information on positions of certain atoms within the molecule and how many atoms of that type are contained
  5. Can use there to work out the structure of the molecule What are the two types of NMR?
  • Carbon - 13 NMR
  • High resolution proton NMR What is Carbon - 13 NMR? Information on the number of carbon atoms that are in the molecule and the environment they are in What is high resolution proton NMR? Information about the number of hydrogen atoms that are in the molecule and environment they are in

Relative area underneath each peak tells you the relative number of H atoms in each environment. Areas can be shown using numbers above the peaks or with an integration trace What is the difference between carbon - 13 and interpreting proton NMR? Peaks in the spectrum split according to how the hydrogen environments are arranged. What is spin-spin coupling? Caused by influence of hydrogen atoms that are bonded to neighbouring carbons, only hydrogen nuclei on adjacent carbon atoms affect each other. these split peaks are called mutliplets and always split into one more that the no. of H atoms on neighbouring C atoms. What is the n + 1 rule? these split peaks are called multiplets and always split into one more that the no. of H atoms on neighbouring C atoms. How can you work out the number of neighbouring hydrogens? By looking at how many the peaks split into:

  • If a peak split into two (doublet) then theres one hydrogen on neighbouring C atoms
  • If a peak split into three (triplet) then theres two hydrogens on neighbouring C atoms
  • If a peak split into four (quartet) then theres three hydrogens on neighbouring C atoms Why are deuterated solvents used in proton NMR spectroscopy? Ordinary solvents would mean hydrogen nuclei would add peaks to spectrum so they are replaced with deuterium (D) - an isotope of H with one proton and one neutron, don't absorb radio wave energy so therefore don't add peaks. Most common is deuterated chloroform (CDCl3) How to identify OH and NH protons Run two spectra of the molecule, one with added deuterium dioxide (D2O) If OH or NH is present it will swap with the deuterium and the peak will disappear How does elemental analysis help to work out structure? In elemental analysis, experiments are used to determine the masses or percentage compositions of different elements in a compound Data can help work out the empirical and molecular formula of a compound and can then be used to work out the structure of the compound What is benzenes formula?

C6H6, cyclic structure. Kekule model and delocalised model What are the 3 key points that do not support the kekule model?

  1. Benzene is resistant to addition reaction due to the delocalised pi system that makes it more stable
  2. Enthalpy of hydrogenation doesn't follow expected pattern
  3. All 6 carbon atoms are the same length. All are 0.140nm which is between the 0.147nm of C-C bonds and 0.135nm for a C=C bond. What are 3 key points to explain the delocalised model?
  4. Delocalised model says the p-orbitals of all six carbon atoms overlap to create a pi system. All bonds are same length as they are the same
  5. Pi system is made of two ring shaped cloud of electrons above and below the plane of six carbon atoms
  6. All electrons in ring are delocalised because they don't belong to a specific carbon atom. Represented as a circle inside ring of carbons How are aromatic compounds derived from benzene? Compounds containing a benzene ring are called arenes or aromatic compounds How do you name aromatic compounds? 2 ways If there is more than one functional group attached to benzene ring you to number the carbons to show the groups
  • If all functional groups are the same, pick any group to start from and count round the way that gives the smallest numbers
  • If functional groups are different, start from whichever functional group gives the molecule its suffice and continue to count round the way that gives the smallest numbers Why does benzene prefer to be reacted by electrophilic substituation? Alkenes have a high electron density meaning they are more willing to undergo addition reactions whereas benzene has a reduced electron density due to the spread out negative charge of delocalised electrons. What reactions do arenes undergo? Electrophilic substitution, hydrogen is substituted by electrophile What is a halogen carrier used for? Can make some compounds that aren't polarised enough stronger as it acts as a catalyst. It accepts a lone pair of electrons from a halogen atom on an electrophile. How are Friedel-Crafts reactions carried out?

How does phenol react with bases and sodium? Weak acid so will undergo typical acid base reactions

  • phenol reacts with sodium hydroxide solution in a neutralisation reaction to form sodium phenoxide and water.
  • doesn't react with sodium carbonate solution - not a strong enough acid Why is the choice of solvent for recrystallisation important? Only work is solid is very soluble in hot solvent but nearly insoluble when solvent is cold. How do you accurately measure melting point? Put a small amount of solid in a capillary tube and place in a beaker of oil with a very sensitive thermometer. slowly heat with constant stirring until solid just melts and read the temp on the thermometer. Examples of a carbonyl compound Aldehydes and ketones and contain carbonyl functional group C=O. Aldehydes have carbonyl group at end of carbon chain, name ends in - al ketones have carbonyl group in middle of carbon chain, name ends in - one How do you form a carboxylic acid and / or an aldehyde? Oxidising primary alcohols
  • Gently heat a primary alcohol with acidified potassium dichromate (VI) in distillation apparatus to form an aldehyde
  • If it is refluxed with acidified potassium dichromate the aldehyde will be oxidised further to form a carboxylic acid What effect does a reducing agent have on aldehydes and ketones? Reduces an aldehyde to a primary alcohol Reduces a ketone to a secondary alcohol Typical reducing agent is NaBH4 (or sodium borohydride) dissolved in water with methanol. reducing agent supplies hydride ions (H-) which has a lone pair of electrons. What is a nucleophilic addition reaction? A nucleophile attacks the molecule and an extra group is added to it. What impact does HCN have on carbonyls? Reacts with carbonyls to produce hydroxynitriles (molecules with a CN or OH group) (nucleophilic addition reaction) What is the test for aldehydes and ketones? 2,4-dinitrophenylhydrazine (2,4-DNP / Bradey's reagent) dissolved in methanol and conc. sulfuric acid will react with carbonyl groups to form a bright orange precipitate and only reacts with C=O groups. Orange precipitate is a derivative of carbonyl compound, produces a crystalline derivative with a different melting point. Measure

melting point of crystals and compare it against known melting point to identify carbonyl compounds. What is the test to determine whether something is an aldehyde or ketone? Tollens' reagent is colourless solution of silver nitrate dissolved in aq ammonia. When heated in a test tube, aldehydes are oxidised and a silver mirror is formed from the silver ions being reduced to silver. Ketones show no change Prefix / suffix and example of Alkanes -ane propane - CH3CH2CH Prefix / suffix and example of branched alkanes alkyl-(-yl) methylpropane - CH3CH(CH3)CH Prefix / suffix and example of alkenes -ene propene - CH3CH=CH Prefix / suffix and example of haloalkanes chloro - / bromo - / iodo - chloroethane - CH3CH2Cl Prefix / suffix and example of alcohols

  • ol ethanol - CH3CH2OH Prefix / suffix and example of aldehydes
  • al ethanal - CH3CHO Prefix / suffix and example of ketones
  • one propanone - CH3COCH Prefix / suffix and example of cycloalkanes cyclo - .... ane- cyclohexane - C6H Prefix / suffix and example of carboxylic acids
  • oic acid ethanoic acid - CH3COOH Prefix / suffix and example of esters alkyl - ... - anoate methyl proanoate - CH3CH2COOCH What are aromatic compounds? compounds that contain a benzene ring What are aliphatic compounds? Compounds that contain carbon and hydrogen joined together in straight chains, branched chains or non-aromatic rings.

places will absorb at different frequencies IR spectrometre produces a spectrum that shows what frequencies of radiation is being absorbed by molecules, can be used to identify functional groups How do you use a mass spectrometer to find relative molecular mass? To find relative molecular mass of a compound you look at molecular ion peak (M peak). mass / charge value of molecular ion peak is molecular mass mass / charge = m / z What are fragments on a mass spectrum? Ions of a molecular ion, can be used to identify molecules and their structure. Only ions show on mass spectrum as other free radicals are lost How can you use a mass spectrometer to work out structural formula?

  1. Identify fragment
  2. Piece them together to form molecule with correct Mr (same value as m/z peak) What are 4 common fragments and their molecular masses? CH3+ - 15 C2H5+ - 29 CH3CH2CH (or) - 43 CH3CHCH3+ OH. - 17 How can these fragments be used to combine techniques to identify a compound?
  3. Use composition to work out molecular formula of compound
  4. Work out functional group in compound with IR spectrum
  5. Use mass spectrum to work out structure of the molecular What are the two types of polymerisation? addition and condensation How are condensation polymers formed? Normally formed from two types of monomer each of which has at least two functional groups. Each time a link is formed a small molecule is lost (usually water). include polyesters (ester link -COO-) and polyamides (amide link -CONH-) How do polyamides hydrolyse? Hydrolyse more easily with an acid than a base to form a dicarboxylic acid and diamine How do polyesters hydrolyse?

Hydrolyse more easily with a base than an acid to form dicarboxylic acid salt and diol. How are polyamides formed? Dicarboxylic acids and diamines Carboxyl (-COOH) groups react with amino (-NH2) to form amide (-CONH). H2O is lost each time amide link it formed (condensation reaction) and forms polyamide How are polyesters formed? Dicarboxylic acids and diols Carboxyl (-COOH) groups react with hydroxyl (-OH) to form ester links (-COO-). Another condensation reaction and polymer formed is a polyester. How do you find the monomer of a condensation polymer? First find the amide (HN-CO) or ester (CO-O) link and break it down the model then add an H or OH to both ends of both molecules to find the monomers What happens to a molecule that contains both an amine and an alcohol group? Can react with dicarboxylic acid in condensation polymerisation reactions and the polymers they form contain both amine and ester links. What happens if a molecule contains a carboxylic acid group and either an alcohol or amine? Can polymerise with itself to form a condensation polymer with only one monomer