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A2 chemistry 5.3.3

Edexcel chemistry - organic synthesis + analysis

QuestionAnswer
What are the steps for working out a molecular formula? 1) find % composition by mass of each element by combustion analysis. 2) find empirical formula with table (element, mass, mass/ RAM, ratio) -> round up to give integers. 3) molecular formula = empirical formula x molar mass/ empirical mass
What is combustion analysis? Burn a known mass of compound in excess dry O2, then measure mass of H2O produced (adsorb onto CaCl2) + mass of CO2 (by react Ca(OH)2). Find moles of CO2, mass of C. Find moles of H2O, mass of H. Mass of O. Find % composition by mass of H,C, O
Why must O2 be dry? So that the only water that becomes adsorbed onto the CaCl2 has been produced by combustion of the compound, otherwise: overestimate of % composition of H
How can you find the molar mass? Molar mass = mass/ amount. Mass/ charge ratio of molecular ion (M+) peak on mass spectrum
What does combusting a compound tell you about it? Clean flame = saturated. Smoky flame = unsaturated e.g. alkene, arene
Add water: steamy fumes turn damp blue litmus paper pink + form white smoke with NH3. What is the chemical + reaction? Acyl chloride, hydrolysis/ nucleophilic substitution -> carboxylic acid + HCl
How would you test for an aldehyde? Add 2,4-DNPH= orange ppt. Warm with Tollens' reagent= silver mirror. Warm with Benedict's/Fehling's reagent= brick red ppt. heat with K2Cr2O7/ dilute H2SO4= orange -> green. KMnO4: acidic (pruple -> colourless), neutral ( brown ppt), alkaline ( green)
What are the possible results from the bromine water test? Yellow -> colourless = alkene -> bromoalcohol, electrophilic addition. Yellow -> colourless + white ppt = phenylamine, electrophilic substitution. Yellow -> colourless + white ppt + antiseptic smell = phenol, electrophilic substitution
How do you distinguish between 1/2/3 alcohols? Warm with K2Cr2O7/ dilute H2SO4 - colour change from orange to green = primary or secondary. Distil the product off as it forms (keep T above bp of aldehyde/ ketone and below bp of alcohol), warm with Tollens' reagent: silver mirror means primary alcohol
What compounds react with Na, and what is the observation? Water, phenol, alcohols, carboxylic acids: Na dissolves, effervescence, white solid forms
How do you test for alcohols? Add Na: effervescence, Na dissolves, white solid forms. Add PCl5: steamy fumes HCl. Add NaHCO3: no reaction. Also, warm with carboxylic acid and conc H2SO4, then pour into dilute NaHCO3, estery smell. Distinguish between 1/2/3 with K2Cr2O7/ H2SO4, Tollens
How do you distinguish between alcohols and carboxylic acids? NaHCO3: carboxylic acids effervescence, CO2 forms white ppt of CaCO3 in limewater, alcohols = no reaction. Heat with carboxylic acid/ alcohol and conc H2SO4, pour into dilute NaHCO3, ester smell
What tests give the same results for phenol and alkenes? Combustion: smoky flame. Bromine water: yellow -> colourless. KMnO4 (can be oxidised ): acidic (purple -> colourless), neutral (brown ppt), alkaline (purple -> green)
What tests give the same result for phenol and alcohols? Na: effervescence, Na dissolves, white solid forms
What tests give the same results for alkenes and aldehydes? KMnO4: acidic (purple -> colourless), neutral (brown ppt), alkaline (purple -> green)
What must be added at the end of an esterification reaction and why? Dilute NaHCO3 to neutralise excess H2SO4 and carboxylic acids, removes smell of carboxylic acid so ester can be smelt to detect esterification reaction e.g. fruity, glue-like, nail varnish
How do you distinguish between chloro/bromo/iodoalkanes? HEAT with AQUEOUS alkali, add excess dilute HNO3/ AgNO3. White ppt, dissolves in dilute NH3 = AgCl, RCl. Cream ppt, dissolves in excess dilute NH3/ conc NH3 = AgBr, RBr. Yellow ppt, insoluble in NH3 = AgI, RI.
What are the products of these reactions converted to? silver halides are decomposed to metallic silver by visible light
Warm with I2, add NaOH -> pale yellow ppt: what can you infer? contains CH3CO group (or even CH3CH(OH) group). Ethanal, methyl ketones, ethanol (oxidised to ethanal first), methyl secondary alcohols (oxidised to ketones first)
How do you test for aromatic amines + reactions taking place? NaNO2, excess HCl, then phenol + NaOH, in ice bath, 0-10 degrees. yellow/ orange ppt of azo dye. Aromatic amine -> benzene diazonium chloride -> azo dye (electrophilic substitution, nitrogen coupling)
Why is the temperature of this reaction so important? If T> 10 degrees c, benzene diazonium chloride decomposes and reacts with water to form phenol
How do you test for acyl chlorides? What are the mechanisms? Add water/ alcohol, steamy fumes of HCl turn damp blue litmus paper pink/ form white smoke with NH3. Hydrolysis with water/ nucleophilic substtution. Forms carboxylic acid/ ester
What other technique can be used to identify functional groups? IR spectroscopy, identify functional group signals above 1500cm-1
Define molecular, empirical, structural formulae Molecular = exact number of atoms of each element in a molecule. Empirical = simplest whole number ratio of atoms of each element present in a molecule. Structural = list of conventional groups
How can you work out the structure of the whole molecule once you know the functional groups? NMR, fingerprint region of IR spectrum (<1500cm-1) is analysed by a computer and compared to a database, Rf values in chromatography
How would you test the purity of a compound? Melting/ boiling point analysis
Which method is preferable? Melting point analysis: boiling point changes with variations in atmospheric pressure, the boiling points of 2 similar substances may differ by less than experimental error
How do you measure boiling point? Place liquid in boiling/ ignition tube, thermometer +elastic band, clamp in beaker of water, capillary tube in liquid open end down, heat water, record T when bubbles stream out of capillary tube, record T when liquid sucks back up, avg of recorded Ts
Before measuring melting point, what must be done? 1) convert to a solid derivative e.g. aldehyde/ ketone + 24 DNPH. 2) filter. 3) purify by recrystallisation
How do you measure melting point? 1) place solid in capillary tube, attach to thermometer with elastic band, clamp in liquid, heat till melts, compare to databook. 2) heat in boiling tube in beaker of liquid until molten, thermometer + stirrer inside, record T when crystals form, compare
Why would you heat under reflux? Increase rate, increase yield, prevent flammable + volatile organic vapours from escaping
What is washing + where is it done? In separating funnel. Wahs with NaHCO3 to neutralise acidic impurities, let dense aqueous layer run out + discard. Wash with H2O to remove sodium salts/ soluble organic compounds, let dense aqueous layer run out + discard
What is drying + why is it done? To remove water dissolved in organic liquid. Add a dyring agent: anhydrous inorganic salt e.g. CaCl2 (not form AMINES/ ALCOHOLS), K2CO3, MgSO4. Drying is complete when solution is clear + not cloudy
What is the function of distillation? Isolate a VOLATILE liquid that is IMMISCIBLE with water and does NOT decompose at its boiling point. Collect the sample that boils at +- 2 degrees of bp of substance
When should fractional/ simple distillation be used? Simple: non-volatile impurity, a little bit of volatile impurity. Fractional: similar boiling points, lots of volatile impurity.
What is steam distillation? Isolate VOLATILE liquid, IMMISIBLE with water, DECOMPOSES at bp. Heat water in a flask, pass steam into reaction mixture, distil off water+ volatile organic liquids, CONDENSE (!!) in Liebig condenser, continue until pure water. Separating funnel, dry
What is an advantage of steam distillation? Prevents decomposition
What is solvent extraction + when should it be done? Liquids have similar boiling points/ lots oil dissolved in water. Add a solvent that only the desired liquid dissolves in, separate in a separating funnel, distil off solvent with rotary evaporator
How do you purify a solid? Filter, recrystallize: dissolve IMPURE SOLID in minimum amount of hot solvent, filter through paper in warm funnel -> flask discard solid, cool, filter under reduced pressure in Buchner funnel, discard filtrate, wash with cold solvent, dry w.filter paper
Give 4 examples of solids + suitable solvents for recrystallisation 2,4 -dinitrophenylhydrazone derivatives = ethanol. Benzoic acid = water. methyl 3-nitrobenzoate = ethanol. Cholesteryl benzoate = ethyl ethanoate
What is a disadvantage of recrystallization? reduces yield: some sample may remain in filtrate (from 2nd filtration) or on filter paper
What are the methods of increasing the carbon chain length? Halogenoalkane + KCN -> nitrile + KX, heat under reflux. Aldehyde/ ketone + HCN (KCN/ alkali) -> hydroxynitrile. Friedel crafts: benzene + halognoalkane -> alkyl benzne + HX/ benzene + acyl chloride -> phenyl aldehyde/ ketone + HCl. AlCl3, ether
How do you decrease the length of a carbon chain? Iodoform reaction: warm compound containing CH3CO group with I2, NaOH, pale yellow ppt of CHI3, forms salt of cabroxlic acid with 1 less carbon than organic reactant. Cool + add excess strong acid to protonate
How do you form methyl benzoate? Heat methanol + benzoic acid with conc H2SO4 -> methyl benzoate + H2O
How do you isolate the ester? Wash with NaHCO3 in separating funnel to neutralise H2SO4/ benzoic acid, let dense aqueous layer run out + discard. Wash with H2O to remove sodium benzoate + methanol, let dense aqueous layer run out + discard. Dry with anhydrous CaCl2
How do you nitrate it? Add 10cm3 conc. H2SO4 to methyl benzoate in ice bath. Add 10cm3 of mixture equal parts conc. H2SO4 and conc. HNO3 dropwise, T<15. Leave to stand for 20 mins. Pour onto crushed ice, methyl3-nitrobenzoate crystallises out. Ice has melted=filter with Buchner
How do you purify methyl 3-nitrobenzoate? Purify by recrystallization with minimum amount of ethanol
How do you form cholesteryl benzoate + what is it? Benzoyl chloride + cholesterol, purify by recrystallisation with ethyl ethanoate, a liquid crystal.
What are the products of organic synthesis? Drugs (paracetamol, aspirin), antiseptics (phenols), polymers, flvaourings (esters), azo dyes, catalysts
Why are sensitive methods of analysis needed to accurately plan and monitor organic syntheses? To determine reaction stereospecificity + detect minute amounts of impurity e.g. drugs/ forensics
Why is it important to understand stereoisomerism in a reaction when producing drugs? Many receptors in body are stereospecific so only 1 isomer will be pharmacologically active, 1 could be harmful e.g. thalidomide. May be necessary to use a stereospecific catalyst (heterogeneous)/ separate + discard wrong enantiomer, reducing atom economy
Why is it useful if a drug is not chiral? Doesn't exist as enantiomers so doesn't exist as a racemic mixture. No need to use a stereospecific catalyst. No need to isolate wrong isomer + discard, so atom economy is high. No danger of a dangerous incorrect isomer
What reactions involve stereoisomerism? HCN + aldehyde/ asymmetric ketone = racemic mixture. nucleophilic substation of 2 halogenoalkanes by SN2: opposite enantiomer to reactant. nucleophilic substitution of 2/3 halogenoalkanes by SN1: racemic mixture. 2,4DNPH: geometric isomers
How do you calculate % yield? Actual yield/ theoretical yield x 100
What factors decrease % yield? Incomplete reaction, competing side reaction, reactants/ products lost in practical preparation e.g. recrystallization (sample left in filtrate/ on filter paper), distillation (sample remains in distillation apparatus/ doesn't reach condenser), many steps
Define hazard + examples Intrinsic danger associated with using particular apparatus/ chemicals. Benzene = flammable, carcinogen. Phenol = corrosive.Dichromate = toxic
Define risk The likelihood that hazard will cuase harm, lowered by using safety precuations
What methods are there to reduce risk? Protective gloves for toxic/ corrosive/ irritant substances, fume cupboard for toxic gases, prevent suckback, safety screen, keep lids on volatile substances, heat volatile/ flammable organic vapours under reflux + with electric heater, smaller quantities
What is combinatorial chemistry + uses? Synthesise a large number of similar compounds in short period of time, immediately screen for useful properties. Drugs: library of compounds that are analogues of a biologically active molecule, more effective drug/ fewer side effects. Catalysts
Describe the method for combinatorial chemistry Adsorb compounds separately onto RESIN BEADS/ POLYMER SUPPORTS (covalent bonding). Mix + randomly split into 2 groups, add different chemicals to each. Mix + randomly split into 2 groups, add different chemical to each. Exponential increase, 2^n
Created by: 11043