(yellow) Carbonyl compounds Flashcards
(yellow) Carbonyl compounds
why does solubility decrease as you increase chain length
As chain lengths increase, the hydrocarbon “tails” of the molecules (all the hydrocarbon bits apart from the carbonyl group) start to get in the way. By forcing themselves between water molecules, they break the relatively strong hydrogen bonds between water molecules without replacing them by anything as good. This makes the process energetically less profitable, and so solubility decreases.
does propanal have hydrogen bonding
no
why does alcohols have higher boiling points than alderhydes
alcohols have hydrogen bonding while alderhydes dont therefore
Will a Hydrogen bond form between the water and propanone molecule?
explain your answer
yes
On propanone delta- O: has lone pairs of electrons.
Water has dealta+ H atoms due to electronegativity difference between H and O.
Lone pairs on C=O enter H orbital –> H-bond
The lower members of the series are all soluble in water. As the chain length increases the solubility in water decreases. Explain this trend.
The energy required to overcome the two H-bonds between water molecules is compensated by formation of the single H-bond a carbonyl compound forms with water.
Larger carbonyl compounds have more e-, stronger London forces the overcoming of which are not compensated when a single H-bond forms with water.
what things can carbonyl compounds hydrogen bond to
therefore carbonyl compounds cannot H bond with themselves (hense the low bp) but can H bond to water (hence the high solubility)
how would you tern a alderhyde to a carboxilic acid
reagents: acidifided potassium dichromate
under reflux
reduction of the carbonyl group
how would you reduce ketone, caboxilic acid and alderhyde
reagents : lithium aluminium hydride (LiAlH4)
in dry ether
by either distillation and or reflux
The H- ions from the LiAlH4 are strong reducing agents, however they cannot reduce a C=C to a C-C. Why not?
AlH4- acts as a source of H- ions which will act as a nucleophile. They therefore attach delta positive carbons. The electron rich C=C bond would repel the H- ion and no reaction would occur.
HCN is a weak acid so how do we make more CN-
HCN is a weak acid and produces very few CN− ions in solution, so a small amount of base is added to shift the equilibrium to the RHS, producing more CN− ions. With KCN this makes KOH.
HC≡N + OH− ⇌ :C≡N− + H2O
What feature does the C in the CN− have that makes it a nucleophile?
Lone pair on C (and N, but N is more electronegative so ‘keeps’ its lone pair)
nucleophilic addition reaction with HCN
alderhyde/ketone —> hydroxy nitrile
The reaction of HCN (in KCN) with an aldehyde or ketone produces a compound called a hydroxynitrile.
Conditions: Room temperature and pressure, small amount of alkali
The conditions are very important for this reaction. If the pH is too low (acidic), there are not enough CN− ions, if it is too high (alkaline) there are not enough H+ ions.
how would you tern Propionitrile into a carboxilic acid give general formula
CH3CH2CN + HCl + 2H2O —-> CH3CH2COOH + NH4Cl
Hydrolysis of CN group
how do you tern a nitrile to a carboxilic acid
reagents : HCl , water and warm
If the nitrile group is further reacted with HCl the nitrile group is hydrolysed into a carboxylic acid. The nucleophilic addition of HCN followed by a hydrolysis can be used to add a carbon onto a chain and then turn it into a carboxylic acid is a very useful tool in the synthesis of compounds
CH3CH2CN + HCl + 2H2O —> CH3CH2COOH + N
what is a test for C=O groups
hint : what kind of ppt does it give off
also what type of reaction is it
2,4-DNPH
yellow /orange ppt
Method:
Add a few drops of aldehyde or ketone to a few cm3 of DNPH solution.
If a precipitate does not form immediately, add a few drops of dilute sulphuric acid and warm in a water bath. Then allow it to cool.
condensation reaction
what is the definition of condensation reaction
Condensation reaction : Two molecules covalently bond together to form a bigger molecule, with the elimination of a molecule of water (or other small molecule)
how would you identify a carbonyl compound after using 2,4-DNPH
Aldehydes and ketones are mainly liquids but the DNPH product forms a crystalline precipitate; simple aldehydes form yellow precipitates, aromatic aldehydes or ketones form orange precipitates.
The identity of the carbonyl compound can be found by measuring the melting point of the DNPH derivative as this is more reliable than measuring the boiling point of the liquid alone.
Method
* Filter off the precipitate
* Dissolve in the minimum quantity of hot ethanol, filter, cool and allow it to recrystallise.
* Filter off the crystals, dry and measure their melting point.
* Compare the melting temperature with known 2,4-DNPH derivatives in a data book. (data sheet in online resources folder)
what is the main diffrence between ketones and alderhydes
We can identify the two series based on the fact that aldehydes are reducing agents. They can therefore be oxidised
what are the three tests that are used to distinguish between ketones and alderhydes . and whihc one does it give a positive test value to
reacts with aldehydes but not ketones
fehlings solution
tollens reagent / silver mirror test
potassium dichromate (VI) and sulfuric acid (VI) acid (orange to green solution )
how would you know if fehlings solution had reacted
Fehling’s solution is a solution of Cu2+ (blue) ions dissolved in an aqueous alkali and complexed with salts of tartaric acid. When an aldehyde is warmed with the solution it changes from blue and forms a red precipitate of Cu2O, copper(I) oxide. As there are OH- ions present the carboxylic acid formed reacts to produce the carboxylic acid salt.
CH3CH2CHO(aq) + 2Cu2+ + 5OH- —-> CH3CH2COO- + Cu2O + 3H2O
copper(I) oxide red ppt.
how would you know if silver mirror reagent had reacted
Tollens’ reagent contains the Ag+(aq) ion; this can be reduced by the aldehyde to produce Ag(s) (metallic silver). A silver mirror will form on the inside of the test tube.
CH3CHO + 2Ag+(aq) + 3OH- —-> CH3COO- + 2Ag(s) + 2H2O
Write out the equation for propanal with ammoniacal silver nitrate:-
CH3CH2CHO + 2Ag+(aq) + 3OH- —–> CH3CH2COO- + 2Ag(s) + 2H2O
what kind of group does iodoform test react with
CH3C=O group (and also CH3CH(OH) groups).
what is the yellow ppt in the iodoform test
CHI3
why are hydroxide ions needed for the iodoform test
- The reagents are iodine in the presence of alkali, this results in the formation of iodate(I) ions:-
I2 + 2OH- –> IO- + I- + H2O
what do IO- in the iodoform test
- The IO- substitute into the CH3 next to the C=O forming CI3C=OR
what is the general equation for the iodoform test for a ketone or CH3=O
CH3OR + 3I2 + 4NaOH —-> Na+O(-) C=OR + 3NaI + 3H2O + CHI3
what is the general equation for the iodoform test for a 2 * alcohol or CH3CHOH
CH3CH(OH)R + 4I2 + 6NaOH —-> Na+O(-)=C(R)–O-Na+ + 5NaI + 5H2O + CHI3
is a the grinyard reagent a reducing or oxidising agent
reducing agent
