L21/22 - Redox

• Define oxidation

  
  

  - what happens to an oxidising agent?

  Loss of electrons/Hydrogen

  
  

  Gain of oxygen.

  
  

  An oxidising agent is itself reduced
• What enzymes are involved in oxidation?

  Dehydrogenase or Oxygenase enzymes

  
  

  (Eg cycloxygenase (inhibited by ibuprofen (COX-2 inhibitor))
• Define Reduction

  
  

  - what happens to a reducing agent?

  Reduction is gain of electrons/hydrogen.

  
  

  Loss of oxygen

  
  

  A reducing agent is itself oxidised
• How do we work oxidation level?


  Ask: how many bonds to an electronegative element are there?

  
• Give an example of a mild oxidising agent?

  
  

  Solvent?

  PCC - Pyridinium Chloro-Chromate

  
  

  Solvent = Ch2Cl2 (dichloromethane)

  
• Give an example of a strong oxidising agent?

  
  

  Solvent?

  KMnO4, or K2Cr2O7 (acidified potassium dichromate)

  
  

  Solvent = acetone
• Oxidising a primary alcohol

  - What product(s) would you get?- what reagents would you use for both steps? Solvents?- What would you use if you only want the first oxidation product?1) primary alcohol oxidised to aldehyde

  - use mild oxidising agent eg PCC

  - Ch2Cl2 solvent

  - mild agent prevents it from being oxidised further if you want the aldehyde as a product.

  
  

  2) further oxidised to carboxylic acid

  - use strong oxidising agent eg KMnO4 or acidified K2Cr2O7

  - acetone solvent

  
• Oxidising a secondary alcohol


  - What product(s) would you get?- what reagents would you use? solvents?- ketone

  - PCC in Ch2Cl2 OR KMnO4/K2Cr2O7 in acetone

  - doesn't matter whether you use a mild or strong oxidising agent as it won't be overoxidised - can't be further oxidised to a COOH

  
• Oxidation of Alkenes


  - what two reagents/solvents can you use?- what kind of reaction is this?- what products would you get when using each reagent?- osmium tetroxide

  - H2O2 solvent

  - di-ol product

  - dihydroxylation reaction

  
  

  OR

  
  

  - mCPBA (meta-chloro-per-benzoic acid)

  - product = epoxide

  
• Ozonolysis


  - reagent?- what happens?- Ozone (O3) reacts with alkenes to break the C=C double bond

  - splits C=C bond in half to form two ketones

  
• Benzyllic and Allylic Carbocations/Radicals


  - what are they?- what are the resonance forms for allylic?Allylic = carbocations/radicals next to a double bond

  Benzyllic = carbocations/radicals next to a benzene

  
  

  - for radicals, only draw a half arrow as moving 1 e-, not a pair

  
  

  - Hyperconjugation occurs

  
• Why do we not breathe 100% oxygen?

  - Oxygen is toxic bc it can exist as a diradical

  
• What happens if you leave an alkene in air?

  
  

  Eg Ethyl Oleate in cream products - why creams have a 12 month expiry!


  - oxygen forms a diradical

  
  

  - can form a radical at allylic position

  
  

  - peroxide can form, then this can be oxidised further to an alpha-beta unsaturated ketone (enone) which is toxic due to many electrophilic regions which interact with body's nucleophiles

  
• Reduction of COOH/Aldehydes/Ketones


  - what products form?- what reagents can be used?- COOH reduced to aldehyde reduced to primary alcohol

  - LiAlH4 (strong reducing agent)

  - use NaBH4 (weak agent) if just want to obtain aldehyde

  
  

  - Ketone reduced to secondary alcohol

  
• Give an example of a mild reducing agent.

  
  

  Solvent?

  
  

  How can it be used to reduce an aldehyde? (mechanism)

  NaBH4

  
  

  solvent = ethanol/water

  
  

  1. NaBH4 in ethanol

  2. Add acid to protonate

  
• Give an example of a strong reducing agent.

  
  

  Solvent? Why use this?

  
  

  How can it be used to reduce COOH/ a derivative? (mechanism)

  LiAlH4

  
  

  solvent = Et2O (diethyl ether). No protons, so won't make it explode unlike water/ethanol bc Lithium reacts with water

  
• Rank Order of Carbonyls

  - which reagents can reduce which groups?- NaBH4 is a weak reducing agent, so can reduce the more reactive groups up to aldehydes and ketones

  - LiAlH4 can reduce all groups

  
• Reduction of Nitriles


  - reducing agents? solvent?- product?- LiAlH4 in Et2O

  - product = amine

  
• Reduction of Alkenes


  - why can't you use H-?- reagent?- product?- can't use NaBH4 or LiAlH4 bc H- and alkenes/alkynes are both nucleophilic

  
  

  - Use H2, and Palladium on Carbon/Nickel (Pd/C)

  
  

  - product = alkane

  
• What's a Lindlar Catalyst?

  
  

  - what's it used for?

  - Pd/C catalyst which has been poisoned eg by lead or sulfur

  
  

  - used to reduce alkynes to alkenes/alkanes

  
• Reduction of Nitro Compounds


  - reagents? products?

  (Eg in paracetamol synthesis)- H2 and Pd/C OR Ni/Sn

  
• How does Redox occur in nature?

  Using NADH as a reducing agent, or NAD+ as oxidising agent

  
  

  (Or NADPH/NADP+)