Topic 17 - PPQs Flashcards
What does brady’s test for?
C=O associated with ONLY aldehydes and ketones
product of a positive test of 2-4 dnph contains which bond?
C=N
reagents and conditions for converting ethanal (by two step synthesis) to 2-hydroxypropanoic acid
HCN in presence of KCN
dilute HCL, under reflux
What does H- have
a lone pair
mechanism for reduction of propanone to propan-2-ol
step 1: H- has lone pair, curly arrow form H- to C in C=O, arrow from C=O bond to O, dipoles
step2: dipoles on water, curly arrow from O- to H in water, curly arrow from O-H bond to O in water, lone pair on O-
test to distinguish between carboxylic acid and alcohol
sodium carbonate - fizzing if carboxylic acid (production of CO2)
explain why the order of boiling points is this
ethanoic acid > ethanol > ethanal [3]
mp1 - ethanal has no H bonds
mp2 - H bonds in ethanol are stronger than london forces in ethanal
mp3 - ethanoic acid forms dimers
what intermolecular forces are present in:
- ethanal
- ethanol
- ethanoic acid
- london forces
- london forces, ,H bonds
- london forces, , H bonds, form dimers
XXXXX
Fehlings or…
benedicts
what do u do w fehlings/benedicts
warm with
Explain why it is necessary to use HCN and KCN in the carbonyl + HCN mechanism?
the KCN provides the :CN- nucleophile
explain why the product of the carbonyl + HCN is not optically active
- bonds around C=O planar
- nucleophile attacks from both sides (above and below)
- forming a racemic mixture
other than spectroscopy how would you use ppt from a chemical test to identify sm
- measure melting point
- compare against known data
technique used to purify product of test with 2-4dnph
recrystallisation
explain why old stocks of aldehydes often react with sodium carbonate solution
they contain carboxylic acids which have formed by oxidation of the aldehydes (from the air)
carboxylic acid + sodium carbonate
salt + water + CO2
butanoic acid as an acyl chloride
butanoyl chloride
two conditions for
alcohol + carboxylic acid —-> ester + water
- H2SO4
- HEAT
identify another chemical that could be added to methanol to make methyl butanoate
butanoyl chloride
because alcohol + acyl chloride –> ester + HCl
state 2 advantages of and 1 disadvantage
acyl chloride + alcohol —> ester + HCl
VS
carboxylic acid + alcohol –> ester + water
advantages of acyl chloride + alcohol —> ester + HCl
1) goes to completion
2) no catalyst needed
disadvantage
1) acyl chloride difficult to store and more expensive
Naming carboxylic acids - which C is the first C
the carbon in the C=O bond
where is the hydrogen bond from and to
from the O-H bond to the IIIIIIIIII between the H and lone pair on oxygen
Describe how london forces are formed
instantaneous dipole
induced dipole
explain boiling point differences between straight chain and branched chain alcohols
- straight chain higher
- because branching weakens london forces
- because less SA so molecules can’t get as close
the oxidation of an alcohol with sodium dichromate (VI) could involve either reflux or distillation - why
the alcohol can only be oxidised to a ketone/ cannot be further oxidised/ cannot be oxidised to a craboxylic acid
what functional group reacts with acidified potassium manganate (VII) and what is produced
C=C (alkene)
a diol
why might u not be able to oxidise an alcohol with acidified potassium manganate (VII)
any C=C bonds will react to form a diol instead
in water why is hexan-1-ol less soluble than ethanol
london forces between hexan-1-ol molecules are stronger than those between ethanol molecules
think about what heptan-2-one would look like
good job
iodoform test, remember to state
alkali is NaOH
what to remember about drawing HCN in the carbonyl + HCN mechanism
include the triple bond and do the curly arrow from the H-C bond to the triple bond
explain why the solution produced does not rotate plane polarised light?
- :CN- can attack w equal probability from either side
- produces a racemic mixture
- bonds surrounding the carbonyl group are planar
equation for iodine and propanone in presence of acid
CH3COCH3 + I2 –> CH3COCH2I + HI
Why does the rate of reaction increase as the recation between iodine and propanone proceeds
HI produced which catalyses the reaction
identify organic product formula from reaction between iodine and propanone in alkali conditions
- CHI3
- CH3COONa
iodine + propanone in alkali conditions
two observations
- yellow precipitate
- antiseptic smell
iodine + propanone in alkali conditions is essentially the same as
iodine and alkali reaction to test for methyl ketones
explain why water is an unsuitable solvent for reduction reactions
LiAlH4 is very reactive with water
state two uses of 2-4dnph
- prepare a derivative
- test for C=O associated with ketones and aldehydes
if sm is going towards sm in a mechanism dont forget
the bloody SECOND CURLY ARROW
alcohol + KOH
type of reaction
elimination
Name all of the intermolecular forces between glucose molecules
london forces between all atoms
permanent dipole - permanent dipole forces between delta positive C and delta negative O
H bonding between delta positive H and another oxygen
explain why glucose is very soluble in water (3)
- forms hydrogen bonds with water
- has lots of OH groups
- energy released forming new H bonds makes up energy used to break H bonds in water
equation for formation of ethanol from ethene
C2H4(g) + H2O(g) —> C2H5OH(aq)
explain why propanal has a lower boiling point than propanol
propanal has london forces and permanent dipole-permanent dipole
propanol has london forces, permanent dipole-permanent dipole and H bonds
H bonds are strongest
if it says describe a CHEMICAL test what doe sit mean
like another chemical like carbonyl or sm NOT AND INDICATOR
describe a CHEMICAL test to distinguish between propan-1-ol and propanal
positive test for propan-1-ol
- add PCl5
- misty white fumes
what to remember about writing oxidation or reduction equations
BALANCE THE [O] or [H]
describe how reflux ensures an aldehyde initially produced is further oxidised to a carboxylic acid
aldehyde condensed
explain why H bonds do not form between aldehyde molecules
they have no H bonded to an oxygen atom
How to test for an -OH group without using PCl5
Na
effervescence
difference between 2-hydroxypropanoic acid in sour milk and muscles that gives rise to difference in optical activity
- 2-hydroxypropanoic acid formed in muscles is a single enantiomer
- 2-hydroxypropanoic acid formed in sour milk is a racemic mixture
X
- racemic mixture not formed
- bc nucleophile attacks from only one side of the molecule
- so reaction is Sn2
explain why the 2-hydroxypropanoic acid formed from ethanal exhibits no optical activity
- racemic mixture
- bc aldehyde group was planar
- so CN attacked from either side w equal probability
test that tests positive for ethanal but not 2-hydroxypropanoic acid
fehlings, red ppt
test that tests positive for test that tests positive for 2-hydroxypropanoic acid but not ethanal
sodium carbonate
CO2 produced
how can u tell sulfuric acid is in excess when added to salicylic acid to precipitate it out of a solution
no more precipitate formed
why is salicylic acid sparingly soluble in water
because of
- london forces between the rings
- H bonds between salicylic acid and water
bonds around C=O are planar means
no optical activity and racemic mixture is produced
suggest the type of reaction if a nitrile is being converted to a carboxylic acid?
acid hydrolysis
nitrile + water + acid —> carboxylic acid + NH4Cl
give the wave number, shape and bond its due to of the peak present in a carboxylic IR but not an aldehyde
3750-2500
broad
due to the -OH bond
in a carboxylic acid spectrum what is the tall thin peak due to
the C=O
why might a peak in a carboxylic acid due to C=O not be used to distinguish between a carboxylic acid and an aldehyde?
- the C=O aldehyde range is 1740 - 1720
- the C=O carboxylic acid range is 1725 - 1700
so peaks cannot distinguish between two compounds because they overlap (IF THEY OVERLAP - CHECK DIAGRAM OF Q GIVEN)
mass spec of carboxylic acid P has m/z = 43, draw the structures of the two species that could give this peak
- draw ions (must be able to be formed from being broken off from original molecule) which rfm add up to 43
what bonds are broken and what bonds are made in an esterification reaction
bonds broken - C-O, O-H
bonds made - C-O, O-H
if
bonds broken - C-O, O-H
bonds made - C-O, O-H
why might the change in enthalpy still not be EXACTLY zero?
bond enthalpies differ
carbonyl + HCN mechanism
low pH:
high pH:
low pH - very few CN-
high pH - very few HCN
explain why the presence of an alcoholic -OH group cannot be confirmed in the IR for a carboxylic acid
the -OH absorptions for alcohol and carboxylic acid overlap
quick proton NMR recap
number of peaks = number of H environments
look at each environment and count how many H’s attached to the adjacent carbon (no. H’s + 1 = splitting pattern)
why is it a singlet
no H atom on the adjacent C
when would you expect sm to NOT have optical isomers
if it doesn’t have a chiral centre
