Topic 15: Carbonyls, carboxylic acids and chirality Flashcards
What are stereoisomers?
- compounds with the same structural and molecular formulae
- have atoms/groups arranged differently in 3-D
What is an optical isomer?
- non-superimposable
- have a mirror image
What product does a SN1 mechanism produce?
- racemic mixture
- nucleophile can attack from either side of the carbonyl group, because the reaction site is planar about the C=O bond
What product does a SN2 mechanism produce?
- optical isomer
- can only attack from one side of the C=O bond
What are the reactions carbonyl compounds undergo?
- Oxidation (aldehydes only)
- Reduction
- Nucleophilic addition
- Iodoform reaction
- 2,4 - DNPH
What are the four oxidation reactions of aldehydes?
- Potassium dichromate (VI) and dilute sulfuric acid
- heat under reflux
- orange —> green (+ result)
- Tollen’s reagent
- colourless —> silver mirror (+ result)
- Fehling’s solution
- blue solution —> red ppt
- reduction of Cu(II) to Cu(I) ions
- Benedict’s reagent
- blue solution —> red ppt
- water bath
What reducing agent is used for reduction?
- LiAlH4 / Lithium aluminum hydride
- in dry ether
What are the steps for nucleophilic addition of carbonyls?
- KCN added
- HCN added
- hydroxy nitrile formed
What is Brady’s reagent (2,4-DNPH) used for?
- used for making derivatives
- to compare carbonly compounds that may have similar boiling/melting points
- with brady’s they have different melting points
- positive result: yellow-orange ppt
What is the iodoform reaction?
- NaOH used
- secondary alcohol or -CH3CO group must be present
- positive result: yellow ppt due to CHI3 formed
How are carboxylic acids prepared?
-
Oxidation of alcohols/aldehydes
- potassium dichromate (VI) & sulfuric acid -
Hydrolysis of nitriles
- acid: NH4+ produced, one step
- alkali: NH3 produced, two steps
BOTH HEAT UNDER REFLUX
What reactions do carboxylic acids undergo?
- Neutralisation
- carboxylate salt produced
- Halogenation
- PCL5 used
- acyl chloride forms
- Reduction
- using LiAlH4
- primary alcohol formed
- Esterification
- use conc. acid catalyst (H2SO4) and heat under reflux
- ester forms
What reactions do acyl chlorides undergo?
- Water
- carboxylic acid produced
- Alcohols
- ester produced
- Ammonia
- amine produced
- Amines
- N-substituted amide produced
ALL produce HCl gas (white misty fumes)
What reactions do esters undergo?
- Acid hydrolysis:
- H2SO4 catalyst
- ester + water —> carboxylic acid + alcohol
- one step, reversible reaction
- Alkali hydrolysis:
- NaOH used
- ester + NaOH —> sodium salt + alcohol
- acid added to convert salt to carboxylic acid
- two steps, complete reaction
- Saponification
- alkali hydrolysis of triglyceride using 3NaOH
- glycerol and sodium stearate (used in soaps) produced
What is thin-layer chromatography?
- same method as paper, however a glass sheet is used instead
- silica used as stationary phase
What is column chromatography?
- consists of:
- stationary phase: liquid/solid that doesn’t move
- mobile phase: liquid that moves through the stationary phase
- Advantage: much larger quantities of material can be separated
What is HPLC?
- High performance liquid chromatography
- solvent forced through tube under high pressure
- stationary phase: silica, a polar solid
- mobile phase: hexane, a non-polar liquid
What affects retention time?
- nature of solvent
- pressure
- temp. inside column
the weaker the attraction to the stationary phase, the shorter the retention time
- retention time is the time take from injection to detection
What is gas chromatography?
- stationary phase: solid/liquid coated inside tube
- mobile phase: INERT gas, e.g; N2, He, CO2
What is the standard used in NMR spectra?
- tetramethylsilane (TMS)
- unreactive, symmetrical, gives a strong signal of zero
- easy to indentify
