(7) Aldehydes and Ketones Flashcards
general formula for aldehyde
CₙH₂ₙO
. O
. ||
R–C–H
carbonyl group on end (terminal carbon)
-al
general formula for ketones
CₙH₂ₙO
. O
. ||
R–C–R’
carbonyl group in middle of carbon chain
-one
bpt
what intermolecular forces do aldehydes and ketones have
permanent dipole of C=O, pd-pd attractions between molecules (stronger than vdW)
bpt
compare bpt of ethanal to ethanol
H-bonds in ethanol > pd-pd in ethanal
describe aldehydes and ketones solubity in water
both soluble in water and will dissolve polar and non-polar solutes
solubility
why are aldehydes and ketones soluble in water
permanent dipole of C=O allows molecules to break H-bonds in water molecules and form H-bonds with water molecules
solubility
what happens when carbon chain increases
regarding C=O
-C=O has less influence on physical properties
- short chain = polar due to C=O
- large chain = non-polar as far from C=O
preparation
oxidising agent used for oxidation of alcohols (+colour change)
acidified potassium dichromate; Kr₂Cr₂O₇/H+
orange –> green
preparation
summarise reactions of alcohols with acidified potassium dichromate (VI)
- primary (1°) alcohol -[O]-> aldehyde -[O]-> carboxylic acid
- secondary (2°) alochol -[O]-> ketone
- tertiary -X-> doesnt oxidise
preparation
what is the practical difference between producing an aldehyde and a carboxylic acid
oxidation of 1° alcohol
aldehyde = distilled and heated
carboxylic acid = prolonged reflux*
*continously heating and cooling (vapour turns back into liquid)
preparation
how are aldehydes, ketones and carboxylic acids produced
practically
aldehyde- distillation(1° alcohol)
carboxylic acid- reflux (1°/ -al)
ketone- reflux (2° alcohol)
reaction with hydrogen cyanide
reaction of aldehydes and ketones with hydrogen cyanide
both produce 2-hydroxynitriles (-CN, -OH)
* aldehyde/ketone + HCN -> cyanohydrin
what is hydrogen cyanide
HCN, toxic gas
reaction with hydrogen cyanide
nucleophillic addition with HCN
- :CN- breaks C=O and attaches to C*
- :O- goes to H
- CN and OH both are attached to C
“goes to” = arrow in flow scheme
*CN has higher priority than OH
what is a nucleophile
ion/molecule with lone pair that attacks low electron density region
attracted to positive charge
what is addition
double covalent bond breaks and something is added
pi bond breaks
reaction with hydrogen cyanide
describe CN- attack on aldehydes
attacks central carbon equally from either side of double bond giving a racemic mixture
apart from methanal as no chiral carbon (optically inactive)
product = 2-hydroxy(no. of carbons)nitrile
reaction with hydrogen cyanide
describe the products in CN- attack on ketones
if ketone symmetrical = product optically inactive
if ketone asymmetrical = product optically active
reaction with 2,4-dinitrophenylhydrazine
how to produce bradys reagent
solution of 2,4-dinitrophenylhydrazine in a mixture of methanol and sulfuric acid
2,4-dinitrophenylhydrazine
how to tell if something is an aldehyde/ketone
- add aldehyde/ketone to bradys reagent
- orange or yellow precipatate shows C=O bond
structure of 2,4-dinitrophenylhydrazine
reindeer with NO₂ antlers
what type of reaction is aldehyde/ketone with 2,4-dinitrophenylhydrazine
condensation reaction- two molecules join together losing a small molecule in the process
water formed
describe reaction between aldehyde/ketone with 2,4-dinitrophenylhydrazine
-al/-one + 2,4-dinitro –
> -al/-one attached to 2,4 dinitro by C≡N + water
water: O comes from C=O and H₂O from tail and back leg of 2,4-nitro
product is called 2,4-dinitrophenylhydrazone for -al and -one
properties of 2,4-dinitrophenylhydrazone
orange, crystalline solid with a well defined mpt
preparation of 2,4-dinitrophenylhydrazone
describe the suction filtration method
- place filter paper in buchner funnel
- place funnel in buchner flask
- attach flask to suction pump and suck air through the flask
preparation of 2,4-dinitrophenylhydrazone
describe diagram for suction filtration
- buchner funnel and filter paper
- buchner flask
- no gaps
- suction indicated with arrow
distinguishing aldehydes and ketones
describe experiment with acidified dichromate (VI) solution
small amount of potassium dichromate (VI) acidified with dilute sulfuric acid, a few -al/-one drops are added. Mixture warmed gently in warm water bath
* aldehyde = orange -> green*
* ketone = no change
*orange dichromate ions reduced to green chromium ions
-CHO oxidised to -COOH
distinguishing aldehydes and ketones
describe fehlings soltution experiment
mixture of fehlings solutions A and B give blue solution with Cu(II) ions, warm in hot bath
* aldehyde = reddish precipitate of copper (I) ions
* ketone = no change
aldehyde reduces Cu(II) to Cu(I)
distinguishing aldehydes and ketones
describe tollens’ agent experiment
tollens’ agent= colourless solution of silver(I)nitrate
* aldehyde = silver mirror on test tube
* ketone = no change
aldehyde reduces Ag(I) ion to Ag
Ag+ + e- -> Ag
reduction
what are aldehydes and ketones reduced to
aldehydes = primary alcohols
ketones = secondary alcohols
reduction
what is lithal (lithium tetrehydridoaluminate)
powerful reducing agent that produces H- ion
* reduces water vigorously so must be in anhydrous conditions
* usually dissolved in dry ether (no water present)
reduction
equations for reduction of ethanal and propanone
- (-al) CH₃CHO + 2[H] -> CH₃CH₂OH (1
- CH₃COCH₃ + 2[H] -> CH₃CH(OH)CH₃
