Revision Work Organic (Nov 02)
Name : . Group : .
(a) Explain how you would attempt the following preparations :
i) Ethanal from a named suitable alkene
A1 (i) Ethene is passed through cold conc. H2SO4 to form the ethyl hydrogen sulphate adduct and is then drowned in excess water to form the alcohol which is removed by distillation. The alcohol, ethanol, is then oxidised with acidified potassium dichromate to give the aldehyde - only obtained if the aldehyde is removed from the mixture by immediate distillation
ii) Butanone from 1-bromobutane
(ii) The 1-bromobutane is made to give the alkene by reaction with hot alcoholic KOH and the alkene immediately absorbed into cold conc. H2SO4. Drowning in water (i.e. reaction with excess water) gives butan-2-ol which is easily oxidised with acidified dichromate to give the ketone – butanone in this case.
(b) What tests (one for each) would you carry out to confirm the structures for the products in a(i) and a(ii)?
Starting out with an alcohol in a(i) and ending up with an aldehyde should easily be shown by testing the product for its aldehyde structure. It reduces Tollens reagent to metallic silver. Therefore formation of a silver mirror with product from a(i) confirms successful formation of aldehyde. For reaction in a(ii), a ketone is expected to form. This gives a positive 2,4-DNPH (indicating presence of a carbonyl group) and a negative Tollens test as compound is not an aldehyde but a ketone, giving no reducing properties. Confirmation of part of the structure comes with the Iodoform test (I2/OH-) which gives a pale yellow ppt having an antiseptic smell. ( 2 marks)
Consider the following compounds:
(a) Give suitable structures for the following compounds (i) the hydrazone obtained when A reacts with a named compound of your choice and (ii) an oxime obtained from C
(b) How would compounds E and F to behave when mixed with hot conc. NaOH(aq)? (Emphasise particularly what you expect to see in such a reaction).
The ketone E gives no visible reaction but F, an aldehyde with a-hydrogens, gives a brown resinous material which separates out from the hot mixture ( 3 marks)
(c) What is the Cannizzaro reaction and which of the above compounds can give this?
The Cannizzaro reaction is a simultaneous oxidation and reduction of an aldehyde. For this reaction to occur the aldehyde must not have any a hydrogens. Compounds from the above list that will give this are : A and G. ( 3 marks)
(d) Compound D will react slowly with concentrated aqueous NaHSO3 to give a water-soluble crystalline solid. Give the structure of the product and state what type of reaction takes place.
The type of reaction that takes place here is an Addition reaction (- the bisulphite addition) and is useful for purification of aldehydes. ( 3 marks)
(e) Comment on the stereochemistry of the product(s) in (d) above – indicating the number of stereo isomers possible.
With 2 chiral carbons, shown by the asterisk, four optical isomers are possible for this compound. (Each labelled carbon has four different groups attached to it and this makes it optically active; rotating the plane of polarised light one way or the other. Sine the groups are not the same on both carbons it is not likely that one carbon exactly negates the effect of the other. Hence no inactive meso form is expected here. ( 3 marks)
(f) How could Fehling`s solutions be used to distinguish between compounds E and G.
The aldehyde G has reducing properties as it is an aldehyde and would consequently form the red-brown Cu2O from the Fehling`s solutions on heating while the ketone E gives no reaction with Fehling`s solutions ( 3 marks)
(g) What product is obtained when C reacts with PCl5(s) and how does this behave in the presence of hot dilute aqueous NaOH solution?
2,2-dichloropentane, a gem dihalide is obtained with PCl5. (No HCl fumes are observed). Alkaline hydrolysis of this regenerates the parent ketone C
(h) Give the structure of the product formed when a solution of G in ethoxyethane is bubbled with dry ammonia gas.
Consider the following information relating to compound R (mol. formula C6H10):
(i) R decolorises bromine water and reacts with one mole of Br2 in CCl4 per mole of R.
(ii) Ozonolysis of R, followed by decomposition of the ozonide with water in the presence of Zn powder gives only one compound, S.
(iii) S reacts positively to 2,4-DNPH
(iv) S reacts with Ag(NH3)2+ to give a silver mirror and with I2/NaOH(aq) to give a pale yellow precipitate.
a) What inferences can be drawn from the above reactions :
One must not overlook the fact that the molecular formula given (C6H10) is one which indicates either an alkyne (CnH2n-2), a diene or a cycloalkene (2 hydrogen atoms being removed from the alkane molecular formula for each of the following : each p-bond and cyclisation as the end hydrogens will have to be removed to allow the end carbons to bond to each other).
(i) Decolourisation of Bromine water indicates the presence of an unsaturated compound e.g. an alkene or alkyne. The need of only one mole of Br2 suggests that only one alkenic (C=C) is present in R not an alkyne. (Molecular formula therefore supports a cycloalkene)
(ii) The formation of only one compound by ozonolysis suggests that either the compound is a highly symmetrical one or indeed that it is cyclic in nature opening up to produce one bifunctional molecule
(iii) Reaction of S with 2,4-DNPH indicates presence of a carbonyl group e.g. an aldehyde or a ketone.
(iv) As S reacts positively both to Tollens as well as the Iodoform reagents, the compound S must have both a CH3CO and a CHO group. (4 x 2 marks)
b) Identify R and S by drawing their structures :
(2 x 2 marks)
What is the systematic name for R ?
1-methylcyclohexene ( 1 mark)