carbonyl compounds suggested answers to george facer a2 chaper 7 page 142 31

8/4/2019 Carbonyl Compounds Suggested Answers to George Facer a2 Chaper 7 Page 142 31

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Carbonyl Compounds Chapter 7 - Suggested answers to George Facer p142-143

Q1. Ketones contain a C=O where the C is bonded to alkyl (or aryl groups),whereas in aldehydes there is one alkyl (or aryl group) and one H bonded to theC of C=O.

2 CH3CH2CH2COCH3, CH3CH2COCH2CH3 and what I will now call Dzatils molecule(CH3)2CHCOCH3 are the 3 possible molecules for C5H10O

3) A temp higher than the bpt of the aldehyde is needed to boil off the aldehydefor collection while distilling. It not wanted that the alcohol boils off too (it wouldrender the separation attempt useless!) so a temp lower than the alcohol bpt isused to keep it (mostly) as a liquid {you do get a small bit of alcohol vapourcoming off}

4) In ethanal the hydrophobic part of the molecule is small and insignificanttowards the polarity of the molecule. In pentanal, the greatest proportion of themolecule is hydrophobic hence its the polar CHO group that is insignificant,resulting in low solubility.

5) For molecules of the same (or very similar) numbers of electrons, hydrogenbonding is the strongest of the intermolecular forces followed by dipole-dipoleforces. Butan-1-ol therefore has the strongest intermolecular forces and so thehighest boiling point. Butanone contains a dipole due to the polar C=O group butno hydrogen bonds resulting in its mpt being less than that of butan-1-ol. Pentanehas no hydrogen bonding and no dipole, but van der Waals forces only so it hasthe lowest boiling point.

6) a) HCN is a gas at RT. The reaction would be HCN(g) + CH3COCH3(l)CH3C(OH)(CN)CH3(l)b) Given that Facer is now talking about alkali, I guess he means a solution based

reaction instead of the one in part a. In which case the reaction equation is:HCN(aq) + CH3COCH3(l) CH3C(OH)(CN)CH3(l)

If no alkali is present, there will be no-:CN nucleophile to attack the + C of the

C=O. The alkali makes the reaction progress as it deprotonates the weak acid

HCN yielding the-:CN nucleophile.

7)

8) The reaction contains a single molecule, 2-hydroxypropanenitrile which cannothave a non-superimposable mirror image. The product is therefore not opticallyactive.

9) This reaction produces molecules which are non-superimposable mirror imagesof each other (i.e enantiomers), however, they are present in equal amounts

because in their formation, the:CN nucleophile had equal chance to attack either

face of the flat ( trigonal planar ) carbonyl. The rotation of the plane of rotation of

monochromatic plane polarised light from one enantiomer is cancelled by theother.

Al

H

H HH H3C

O

CH3 H3C

O

CH3H

H3C

O

CH3H

H_

Li +

(1) (2)_

H +stage 1 stage 2:

dissolved in dry ether

H+fromHCl(aq)


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10) a) (NO2)2C6H3NHN=CHCH2CH3 obs: colourless propanal and orange solutionproduces an orange/yellow precipitate.b) CH3CH2COOH(aq) obs: colourless propanal added to a orange solution producesan blue/green solution.c) CH3CH2COO

-(aq) colourless propanal and colourless Tollens reagent produce a

silver mirror on the side of a test tube on warming.d) CH3CH2COO

-(aq) colourless propanal and deep blue Fehlings reagent produce ared ppte on warming.e) No reaction. Not a methyl carbonyl. obs: colourless propanal is added tocolourless mixture of I2(aq) and NaOH(aq). No change occurs.

11) a )The formula C4H10O i.e. compound X, reveals there are no double bonds inthe molecule. Compound Y is an oxidation product of X which undergoes theiodoform test, so must be a methly carbonyl. (CH3-C=O i.e. an acyl group must bepresent) The carbonyl existence of Y is also seen from its reaction with 2,4-DNP. Ymust be a ketone as it isnt oxidised by Fehlings reagent. Y must be CH3COCH2CH3and X must be CH3CHOHCH2CH3.

b) Both would give a yellow precipitate.

c) CH3COCH2CH3 + NaOH(aq) + I2(aq) CHI3 + -OOCCH2CH3(aq) + Na+(aq)

balanced would be: CH3COCH2CH3 + 4 NaOH(aq) + 3 I2(aq) CHI3 +NaOOCCH2CH3(aq) + 3NaI(aq) + 3H2O(l)

d) Y must be a ketone so the alcohol it came from must be a 20 alcohol. Itunderwent the iodoform reaction so it must be a methyl secondary alcohol.

12a) CH2=CHCH2CH2OH

a) CH3CH2CH2CH2OH

H C C C C H

H

H

OH

HH

HH

H

H C C C H

H

H

OH

CH

H

H

HH

butan-2-ol 2-methylpropan-2-ol

Note: will 'do' triiodomethane reaciton Note: will NOT 'do' triiodomethane reaciton


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13) Theres a suggestion in the Q (the two different boiling points), as to what isexpected in your answer i.e. distillation, but it is not necessary (see the seconddiagram). To play along with the clue First assemble the apparatus as shown inthe diagram below.

Add a couple of anti-bumping granules and boil for about 20 minutes {ensurewhen heating that the vapours do not progress too far up the condenser and donot use a Bunsen burner}.

After reaction, set up the apparatus for distillation to separate the butanone fromany unreacted butan-2-ol or any side products.

Collect the fraction that comes off at 80oC. The condensate will contain water. Addanhydrous MgSO4 until clumping ends. Then separate using filtration. {Diagramshould have clamp positions on the neck of the round bottomed flask and halfway along the condenser and support at the bottom of the collection vessel.

Open gapshown

Thermometerbulb level withexit arm ofstill head


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Q14 a) Nucleophile: species with a pair of electrons which can be donated for thepurpose of forming a dative covalent bond. Oxidation is the loss of electrons, gainof oxygen or loss of hydrogen for an atom in a given species.b) i) nucleophilic addition.ii) oxidation

iii) condensation reaction (or nucleophilic addition followed by elimination ofwater)

15. Recrystallise the orange precipitate by dissolving it in the minimum of hotsolvent. Filter when hot using fluted filter paper. Allow filtrate to cool slowly.Vacuum filter the crystals using a Bucher funnel and aspirator. Wash crystals witha few drops of ice cold solvent. Allow to dry in air (warm in oven if necessary). Putrecrystallised crystals in a capillary tube to about 2-3 mm in depth. Use meltingpoint apparatus to determine the melting range. Take the mid point to be themelting point value. Look up the value against reference tables for 2,4-DNPcarbonyl derivatives.

carbonyl compounds suggested answers to george facer a2 chaper 7 page 142 31

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