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A2 Chemistry 4.8

Edexcel chemistry - further organic chemistry

What is isomerism? Where molecules have the same molecular formula but different structural or spatial arrangements of atoms
What types of isomerism are there? Structural: positional-location of fg changes, chain-branching of C chain, fg-different arrangements of atoms give different fgs. Stereo: geometric-restricted rotation of π bond& 2 different substituents attached to each C in C=C, optical
What is optical isomerism? Molecules have no planes of symmetry, have chiral centres, exist as enantiomers which have non-superimposible mirror images and they rotate plane-polarised light in opposite directions but the angle of rotation is equal.
What is a chiral centre? An atom in an enantiomer that is bonded to 4 different substituents. Rules: no double bond, all different substituents, only counts if ring is asymmetrical
What happens if there are 2 asymmetric carbon atoms? 4 enantiomers could form: ++, +-, -+, --. If a molecule has 2 identical asymmetric carbon atoms, then the enantiomer with a plane of symmetry is not optically active
What is plane-polarised light and how do enantiomers influence it? Light that only vibrates in 1 plane due to passing through a polariser. Enantiomers rotate it in opposite directions but angle of rotation is equal. Dextrorotatory = clockwise, laevorotatory = anticlockwise. Angle depends on nature of enantiomer + conc
What is a racemic mixture? An equimolar mixture of enantiomers that doesn't rotate plane-polarised light, because half enantiomers will rotate it clockwise, and the other half will rotate it equally anticlockwise. Overall - not optically active.
Describe a polarimeter Measures the angle of rotation by enantiomers: sodium discharge tube/ light source -> fixed polariser -> plane-polarised light -> reaction tube -> rotated light -> rotatable polariser -> eye.
What can optical activity show you about a reaction? A change in optical activity (measured by a polarimeter) can show a change in concentration of reactants or products, which shows rate of reaction
During nucleophilic substitution, can primary compounds form enantiomers? No, they will form another primary compound with 2 H atoms attached, therefore the central carbon atom can't be a chiral centre
During nucleophilic substitution, can secondary compounds form enantiomers? Yes. SN2 (if there isn't much steric hindrance): only forms 1 enantiomer because molecule isn't planar, nucleophile attacks from opposite side to leaving group-flow of e-s. If the original molecule is an enantimoer, the product will be the opposite 1
Continued SN1 (if there is lots of steric hindrance): the carbocation intermediate is planar so nucleophile can attack from above or below the plane, forms a racemix mix of enantiomers, which is not optically active
During nucleophilic substitution, can tertiary compounds form enantiomers? Yes - SN1 - the carbocation intermediate is planar so nucleophile can attack from above or below the plane, forms a racemix mix of enantiomers, which is not optically active
List and draw a range of compounds containing the carbonyl group Ketone, aldehyde, carboxylic acid, ester, acyl chloride, amide, acid anhydride (ROC=OCOR)
What are the intermolecular forces in aldehydes and ketones? London forces, permanent dipole forces (no hydrogen bonding because they lack the δ+ hydrogen atom)
Describe the boiling points of the aldehydes and ketones Higher than hydrocarbons but lower than alcohols - intermolecular forces. Boiling point increases as no. of c atoms increases: larger molecules have more electrons+ points of contact, larger dipole moment, stronger london forces, requires more heat energy
Describe the solubility of aldehydes and ketones Small carbonyl compounds are MISCIBLE with water because they can form more hydrogen bonds and stronger london forces, energetically favourable - solubility decreases as length of C chain increases, larger proportion is non-polar
Why are carbonyl compounds good solvents? They have polar and non-polar parts so can dissolve polar and non-polar solutes
How do you test for a carbonyl group (in aldehydes and ketones only)? React with 2,4-DNPH - positive test is orange ppt, 2,4-dinitrophenylhydrazone derivative, ppt can be purified by crystallisation to form a solid with a specific melting pt for identification, mechanism: nucleophilic addition - elimination of H2O
How do you distinguish between aldehydes and ketones? Aldehydes can be easily oxidised by oxidising agents due to oxidisable H on carbonyl C, ketones can't. Heat with Tollens': ammoniacal silver nitrate [Ag(NH3)2]+, aldehyde oxidised to carboxylic acid, Ag+ reduced to Ag, silver mirror
Continued Heat Fehling's/Benedict's: blue Cu2+ ions (complex ions), aldehyde oxidised to carboxylic acid, Cu2+ reduced to brick-red ppt Cu(I)2O. Heat acidifed Cr2O72- ions: aldehyde oxidised to carboxylic acid, orange dichromate(VI) -> green chromium(III).
How do you convert carbonyls to alcohols? Reduction: react with LiAlH4 in dry ether (e.g. ethoxyethane dried with anhydrous MgSO4) because it reacts vigorously with water. Aldehyde + 2[H] -> 1° alcohol. Ketone + 2[H] -> 2° alcohol. Carboxylic acid + 4[H] -> 1° alcohol + H2O.
Continued LiAlH4=hydride donor, H- =selective nucleophile & reducing agent for polar substances. AlH4 has 3 covalent bonds and 1 dative covalent. Prepared by reacting lithium hydride and aluminium chloride: 4LiH + AlCl3 -> LiAlH4 + 3LiCl
Which is more reactive and why: LiAlH4 or NaBH4? LiAlH4. 1) Li is more electronegative than Na, bonds easily to carbonyl O increasing δ+ charge on carbonyl C, favouring reduction. 2) Al is less electronegative than B - more electron density on hydride ion in AlH4, more able to reduce carbonyl c
How do you increase the length of a carbon chain in a carbonyl compound? React with HCN: HCN (with some KCN/ alkai) + carbonyl -> hydroxynitrile. KCN/ alkali increases [CN-] as HCN is weak. Mechanism: nucleophilic addition - protonation with another HCN molecule
How is optical isomerism involved in the reaction? HCN + symmetrical ketone -> CN- +1 hydroxynitrile. No optical isomerism as there is no chiral centre. HCN + asymmetrical ketone/ aldehyde -> CN- + racemic mix of enantiomers. Carbonyl group is planar, CN- nucleophile can attack from above or below
Is the reaction faster with aldehydes or ketones and why? aldehydes (1 alkyl) > ketones (2 alkyls). alkyl groups have a positive inductive effect by pushing e-s towards carbonyl C, ketones: charge is less localised, weaker attraction for nucleophile
How do you test for the CH3CO group? Warm compound with iodine solution, add alkali e.g. NaOH, KOH so it goes colourless, if CH3CO group is present, a pale yellow ppt of CHI3 (triiodomethane) will form. Present in ethanal and methyl ketones
What is the equation for the iodoform reaction? CH3COR + 3I2 + 4NaOH -> CHI3 + RCOONa + 3H2O + 3NaI
What is optical activity? The ability of a single enantiomer to rotate the plane of polarisation of plane-polarised, monochromatic light
Why are carbonyls susceptible to nucleophilic attack? The polar, planar C=O bond makes δ+ carbon very susceptible to nucleophile attack - nucleophiles can attack from either side of bond
What is a nucleophile? A species with 1+ lone pairs of electrons that can donate a pair of electrons to an electron deficient species to form a new covalent bond
What are some examples of carboxylic acids? Methanoic acid = formic acid = ant venom. Ethanoic acid = acetic acid = vinegar.
What intermolecular forces are there in carboxylic acids? Hydrogen bonding, London forces. In a pure carboxylic acids, hydrogen bonding produces a dimer.
What are trends in boiling/ melting points? Bp increases as no. of C atoms increases, decreases when C chain branches. Mp decreases as no. of C atoms increases - small molecules have stronger Hbonding (dimers), less in large molecules due to steric hindrance. Small are liquid at RT
Why are the boiling points of carboxylic acids so high? Strong hydrogen bonding - dimers. Dimerisation doubles the size (and number of electrons in ) a molecule, therefore there are larger dipole moments and stronger London forces.
Describe solubility of carboxylic acids first 4 are very water soluble (H bonding, London forces) - enthalpy of hydration > energy required to separate acids. Solubility decreases as length of carbon chain increases
How can you prepare a carboxylic acid via oxidation? Oxidise a primary alcohol: heat under reflux with K2Cr2O7/H2SO4. ROH + 2[O] -> R'COOH + H2O. Oxidise an aldehyde: heat under reflux with K2Cr2O7/H2SO4. RCHO + [O] -> RCOOH
How can you prepare a carboxylic acid via hydrolysis? Esters. a) Heat ester under reflux with dilute, aqueous acid - hydrolysed into carboxylic acid and alcohol. RCOR' + H2O (H+) -> RCOOH + R'OH. b) Heat ester under reflux with aqueous alkali - hydrolysed into carboxylate salt and alcohol...
Continued RCOR' + NaOH -> RCOONa + R'OH. Cool and add excess strong acid to protonate the salt. Larger yield. Nitriles: a) heat nitrile under reflux with dilute, aqueous acid - hydrolysed into ammonium salt and acid. RCN + 2H2O + HCl -> NH4Cl + RCOOH
Continued b) Heat nitrile under reflux with aqueous alkali - hydrolysed into carboxylate salt and ammonia. RCN + H2O + NaOH -> RCOONa + NH3. cool and add excess strong acid to protonate the salt.
How can a carboxylic acid be formed from the iodoform reaction? Warm NaOH, I2 and a compound containing CH3CO - forms carboxylate salt, 1 less C atom than organic reactant. Cool and add excess strong acid to protonate the salt.
How do carboxylic acids react with water? RCOOH + H2O <-> RCOO- + H3O+. Forms a hydronium ion.
Why are carboxylic acids more acidic than alcohols? They donate protons more easily. C=O group pulls electron density away from OH group, H atom has a stronger δ+ charge, more reasily lost as a H+ ion. Carboxylate ion is stable -resonance - negative charge shared between 2 O atoms, overlap of pz orbitals,
What are the steps to working out the concentration of citric acid in lemon juice? Pipette NaOH into conical flask (known concentration), add indicator (phenolphthalein or methyl orange), white tile, titrate juice from burette till colour change (colourless/orange), swirling, repeat for concordant titres, mean titre.
Draw and describe the mechanism of ethanol + methanoic acid 1) conc H2SO4 (catalyst) protonates the acid, which loses a H2O molecule. 2) lone pair on hydroxyl oxygen bonds to +ve carbon, H+ ion is lost 3) H+ + HSO4- -> H2SO4.
Describe the experimental procedure for the formation of methyl butanoate warm H2SO4, butanoic acid + methanol in water bath for few mins, smell pineapple. Pour into separating funnel, let dense aqueous layer run out (acids, H2O, alcohol), add NaHCO3 to neutralise excess acid, remove aqueous layer. Protective gloves
What are some uses of methyl butanoate and ethyl ethanoate? Methyl butanoate smells of pineapple so is used to flavour sweets. Ethyl ethanoate smells of pear so is used to flavour sweets and is a solvent for decaffeinating tea/ coffee and for nail varnish remover.
How can you convert a carboxylic acid into an acyl chloride? React with PCl5: RCOOH, POCl3, HCl (steamy fumes, damp blue litmus paper -> pink, white smoke with NH3). React with PCl3: RCOOH, H3PO3. React with thionyl chloride, SOCl2: SO2, HCl, RCOOH (only acid left in beaker)
How can you convert a carboxylic acid into a halogenoacid? Bubble halogen through boiling carboxylic acid, in presence of sunlight or UV. Free radical substitution. Halogen atom replaces a hydrogen atom on carbon chain
Draw the mechanism for ethanoic acid -> chloroethanoic acid Cl2 -> 2Cl*. CH3COOH + Cl* -> HCl + CH2COOH*. CH2COOH* + Cl2 -> CH2ClCOOH + Cl*
What mechanism do acyl chlorides tend to undergo? Nucleophilic substitution - the carbon is very δ+ because it is bonded to 2 electronegative atoms, and the C-Cl bond is weak so the Cl- ion is readily lost.
Propanoyl chloride + ammonia -> ? word equation + chemical equation Propanamide + ammonium chloride. C2H5COCl + 2NH3 -> C2H5CONH2 + NH4Cl
Ethanoyl chloride + water -> ? word + chemical equation Ethanoic acid + HCl. CH3COCl + H2O -> CH3COOH + HCl
Butanoyl chloride + ethanol -> ? word + chemical equation Ethyl butanoate + hydrogen chloride. C3H7COCl + C2H5OH -> C3H7COOC2H5 + HCl
Ethanoyl chloride + propanamine -> ?word + chemical equation N-propyl ethanamide + propylammonium chloride. CH3COCl + 2C3H7NH2 -> CH3CONHC3H7 + C3H7NH3+Cl-
Describe the reduction of acyl chlorides Acyl chlorides are reduced by LiAlH4 in dry ether e.g. ethoxyethane, to a primary alcohol + HCl, 4[H]
What is a natural ester? A vegetable oil or animal fat, which exist as triglycerides, containing 1 molecule of glycerol and 3 fatty acids. Fatty acids all have even number of C atoms, but can be saturated or mono/poly unsaturated
What determines the state of a natural ester at rt? The more unsaturated a triglyceride is, the lower its melting point. Fats = saturated, solid at RT, e.g. stearic acid. Oils = unsaturated, some C=C double bonds, kink in tail, liquid at RT, e.g. oleic acid
How is soap produced? Saponification: heat a triglyceride with NaOH, forms sodium salts of the 3 fatty acids present in the triglyceride (SOAP) + glycerol
What is transesterification + types + uses? The conversion of an ester to another ester. React w. organic acid - replaces acid e.g. manufacture of low-fat margarine. React w. alcohol-replaces alcohol part e.g. biodiesel-fuel made from natural veg oil
How is biodiesel formed? React a vegetable oil with methanol in the presence of NaOH(catalyst), forms methyl esters of fatty acids present in triglyceride + glycerol;
What are the two ways of forming low-fat, spreading margarine Transesterification and partial hydrogenation with H2/nickel catalyst
How does transesterification work? Combine catalyst, stearic acid (saturated), vegetable oil, stearic acid replaces an unsaturated fatty acid in triglyceride, less unsaturated fatty acids crystallise first during crystallisation and are collected
What are the costs and benefits of transesterification? Adv: doesn't produce trans fats. Disadv: some stearic acid goes into the middle carbon position in the glycerol, which may be harmful to health
How does hydrogenation work + disadvantage? Partially hydrogenate vegetable oils with H2 which saturates some C=C bonds in presence of a nickel catalyst. Prodcues trans fats which are thought to increase cholesterol levels.
What is a condensation polymer? A polymer formed when many monomers join together while eliminating a simple molecule e.g. H2O, HCl
What is a polyester? A condensation polymer formed when a dicarboxylic acid forms ester bonds with a diol, releasing molecules of water.
How is PET formed? Draw + give uses of PET. Benzzene-1,4-dicarboxylic acid + ethane-1,2-diol -> poly(ethylene terephthalate) + H2O. PET is also called Terylene. Use: make plastic bottles for fizzy drinks and water as it doens;'t allow dissolved CO2 to escape and is shatterproof.
What is lactic acid an example of + draw reaction/ repeat unit? A homopolyermic ester - formed from 1 type of monomer. 2-hydroxypropanoic acid -> lactic acid + water.
What are some other uses of esters? Filling duvets - they are thermal insulators. Clothing (along with natural fibres, e.g. cotton) - they give strength and crease resistance
Created by: 11043



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