miscellaneous; mainly ochem Flashcards
why does potassium dichromate need to be acidified
yellow chromate ( CrO4 2-) and orange dichromate (Cr2O7 2-) ions are found at equilibrium. adding H+ ions helps shift the equilibrium onto the dichromate ion.
catalyst poisoning
in catalytic convertors: exhaust has substances (lead) that froms a coating on the surface containing the catalyst. the convertor can no longer carry out its function. unleaded petrol has to be used.
difference between high denisty polythene and low density polythene
high denisty is stronger as its hcains have fewer branches so more numerous intermolecular forces.
what does hybridization of Carbon help to explain?
- C can form multiple bonds of differnet strengths
- accounts for the fact that the four outer e- are in different orbitals and so, in different energy states, even though in compounds like methane, the C-H bonds are the same length and the molecule is symmetrical with equal bond angles.
why is a pi bond weaker than a sigma bond?
the electrons are further from the nuclei of C atoms than those in the sigma bond. this reduces the attraction between the pi electron pair an the nuclei.
what propertis does the presence of a double bond give to an organic compound?
- the double bond is a region of increased electron density compared to a single bond, having 4 e- between 2 C atoms instead of 2, the C=C bond tends to attract electron loving species (electrophiles)
- since the pi bond is relatively weak, it tend sto break and get replaced by a single bond. hi gives rise to addition type of reactions.
use of combustion of alkynes
it is a very exothermic reaction.high C: H ration so very sooty and luminous flame.if the alkyne is burnt in pure oxygen, a very high temp is reached which can be used in wleding and cutting of metals,, referred to as an oxyacetylene flame.
comproportionation
a chemical reaction where two reactnats each containing the same element but with a different oxidation number, will from a product in which the elments involves reach the same oxidation stae
flame test of Li
crimson red
flame test of Na
yellow orange
flame test of K
lilac
flame test of Ca
brick red
explain +I , -I , +M and -M
+I pushes away e-
-I pulls e-
+M donates e-
-M withdraws e-
examples of activating groups
all are 2, 4 directing
-OH, -NH2 , -R , -OR
examples of deactivating groups
-X , -NO2 - COOH , SO3H -COR -CHO -COOR -CONH2 -CN
all 1,3 directing apart from X
why do carboxylic acids evolve CO2 when reacting to Na2Co3, but phenol doens’t?
phenol is acidic enough to ract with Na2CO3 where the carbonate accepts a H+ from phenol to form a hydrogencarbonate and the phenoxide. but due to being a weak acid , the hydrogencarbonate cannot accept another hydrogen ion to form CO2 and H2O.
C6H5OH +Na2CO3 C6H5ONa + NaHCO3
why do 2-nitrophenol and 4-nitrophenol have such differnet boiling points
2-nitrophenol has the lower bp ou tof the 2.
both can form intermolecular bonding due to a polar O and polar H, but in 2 nitrophenol, the NO2 and OH groups are on nieghbouring carbons and so can also engage in intramolecular H bonding. this comes at the expense of intermolecular H bonding. due to fewer H bonds, the 2 nitrophenol has a lower bp
explain the boiling point trend along fluoroethane, chloroethane, bromoethane and iodoethane
the bp increases from F to I, iodoethane has the highets bp. this is because ther is an increase in electrons as well as an increase in mass, which leads to an icnrease in induced dipole induced dipole forces.
even though the permanent dipoles between F and H, and Cl and H are stornger than those between I and H, the id id forces are more numerous and hecne more importanat than the permanent dipole permanent dipole froces.
why cant KI be made in situ by mixing KI with conc H2SO4
H2SO4 will oxideize the I- to I2 even at room temp. H3PO4 is used instead since this acid is not oxidizing
conditions when making HBr in situ by mixing KBr and H2SO4
must not allow the temp to rise above 40 degrees since H2SO4 can partially oxidise Br- to Br2
how is the C-X bond formed and what dictates the strength of the bond.
the C-X bond is formed by an overlap of an sp3 orbital of the C with a p orbital of the halogen. the larger tha halogen, the larger and more diffuse is the p orbital, so the more idstant is the bonding electron pair from the nuclei of the adjacent atoms ;; the weaker the bond.
what must nucleophiles do and mention examples
nust be able to donate an electron pair .
they may be negative or neutral species. eg, OH, H2O, NH3, CN-, RO-, RCOO-
what affects how good of a nucleophile something is?
- the negative charge density eg OH is strongerthan H2O
- the electronegativity; the less electronegative the stronger the nucleophile eg NH3 is stronger than H2O
- if delocalistaion is present, it is a weaker nucleophile, due to a decrease in -ve charge density.
why cant tertiary halogenoalkanes folow the SN2 mechanism
- due to three alkyl groups, the partial positive charge on the C carrying the halogen is decreased highly (by inductive effevt). this makes it much less reactive and less likely to be attacked by a nucleophile.
- due to bulky alkyl groups it is unlikely to form a 5 member transition state, as it would be too crowded.
why can’t primary halogenoalkanes follow the SN1 mechanism
stearic reason: in the SN1 mechanism an intermediate carbocation is formed, to reduce crowding(as C loses a group). this is less likely to occur to a non bulky molecule
electronic reason: with tertiary halogenoalkanes , the intermediate carbocatin is stabilised by the inductive effect of the alkyl groups, this is not the case for primary halogenoalkanes. for primary haolgenoalkanes, the intermediate would be too unsable and not energetically feasible
does SN1 or SN2 mechanism lead to formatino of a racemic micture
SN1 mechanismm. the planar intermediate may be attacked from either side of the plane with equal probability. equal amounts of the two enantionmers form leading to a racemic mixtrue by external compensation.
factors accounted to know if halogenoalkane will undergo substitution or elimination
-type of RX primary ; substitution and tertiary ; elimination
=solvent aq; sub alc; elimination
- conc of reagent;; high conc favors eliminitaion
- temperature;; high temp favurs elimination
naming ethers
largest part gets main nameand the rest is meth , eth prop etc
eg: methoxycyclopentane, ethoxypropane
name esters
part from acarboxylic acid salt or acid takes main name
eg, 1-propyl ethanoate for CH3COOCH2CH2CH3
test for haloalkanes procude
warm halogenoalkane with NaOH in ethanol water mixture to make halogen in an inorganic ofrm.
RX+ OH- –> ROH + X-
the product is acidified using dilute HNO3 and AgNO3 is added. precipitates form. as a confirmatory test, ammonia solution is added.
AgCl- pp dissolves for colourless soln
AgBr - pp almost insoluble in dil ammonia but soluble in conc ammonia
AgI -pp is insoluble
what is the only way that iodobenznene can be prepared in the lab
reacting ice cold bezenediazonium chloride soln with cold KI. effervesenc eof N2 is observedas well as oily droplets of iodobenzene.
reaction:
benzene ring with N+triple bond N + I- —> iodobenzene + N2
characteristics of iodoform
pale yellow solid with antiseptic smell
naming compounds that have aldehyde and ketone group
aldehyde (anal) takes main name and ketone uses oxo,,, oxo can be used for any carbonyl group
what compounds are used for purification of carbonyl compounds? how do they look`
hydrogen sulfate IV compounds such as NaHSO3. they are white crysttaline solids
when does the aldol reaction take place; what happens
when the carbonyl compound has at least 1 alpha hydrogen. the compound dimerises
one molecule will lose an alha hydrogen, and the other will gain an H to from OH
the 2 molecules bond , bone from an alpha carbon to the carbonyl carbon of the other
addidtion of conc alkali and heat to the product of an aldol reaction
loses water to from an enal or enone. this product can polymerise to from a brown resin
when do you use aldol or cannizzaro
aldol- has at least one alpha hydrogen
cannizzaro- has no alpha hydrogens
PCl5 with carbonyl compounds, alcohols and carboxylic acids
alcohols nd carb acid; give off HCl
aldehydes and ketones ; no HCl given odd
which 2 acids can be oxidised further to CO2 and H2O
methanoic and ethanedioic acid
tollen’s reagent
overall Ag+ /NH3
solution of AgNO3 slightly alkaline by the addition of aq NaOH where a brown ppt of Ag2O is observed,, excess aq NH3 is added to from a colourless soln
contains the complex ion [Ag(NH3)2]+.
on warmin gwith aldehyde; Ag+ reduced to Ag;;; silver mirror
aldehyde becomes carboxylate.
tify between
which homologous series can dimerise?
carboxylic acids, they do not form dimers in water
characteristics of benzoic acid
insoluble in cold water but soluble in hot water.
crydtalline solid at room temp with a tendency to sublime. this is used in the purification of benzoic acid.
comment on delocalisation in carboxylic acids and how that affects their acidity
the carboxylate ion has two Carbon oxygen bonds that shift between single and double. these C O bonds are identical, the structure has a certain degree of delocalisation. the negative charge and the pi bond are not fixed in place but there is a delocalised pi system over the COO group. (canonical structure snad resonance hybrid can be drawn)
sinc ethe neagtive charge is shared, it is lower than the phenoxide and alkoxide;;; more stable ;;weaker conjugate base;;; stronger acid
why is benzoic acid a stronger acid than any other aliphatic acid?
In the benzoate ion the delocalistion involves the ring, the negative charge can be delocalised as in carboxylic acids but also by the interaction with the pi electron cloud of benzene. This makes the benzoate is less available to attach to Hydrogens, so a weaker conjugate base than the carboxylate and therefore a stronger acid than aliphatic acids
can an acid chloride be prepared directly from methanoic acid?
no it can’t as methanoyl chloride is very unstable and will break down to form CO and HCl
characteristics of carboxylic acid salts
ionic, white solids at room temp unelss cation is coloured.
test for cation in carboxylic acid salt
flame test
if ammonium; add dilute aq NaOH and NH3 (a pungent gas) is released.
test for amides and primary amines
reaction with nitric III acid . amide forms carboxylic acid,, and both produce N2 gas. HNO2 is unstable so produced in situ
NaNO2+ HCl —> HNO2 + NaCL (cold)
why are amides not basic, but amines are
amines are basic due to the lone pair of the nitrogen. this allows for the attraction of H+ .
in amides, the lone pair on the N overlaps with the p electrons in the pi bond of C=O. the llone pair is involved in delocalised pi bonding involving N,C and O,, and is not avalible to attract H+. this makes amides more stable.
prep of polyester; PET poly(ethene tretraphthalate
prepared from ethane-1,2-diol and benzene -1,4-dioic acid (diol and diacid)
uses of condensation polymer PET
plastic bottles, packagng trays, blister packs, undersea cables
formation of polyester: PLA poly(lactic acid)
hydroxy acid; such as lactc acid
characteristics of PLA
bioplastic derived from biomass
biodegradable
used for bottles, films, biodegradable medial devices, tea bags, cutlery
formation of polyamides
usually diacid and diamine or diacid chloride and diamine
examples of polymamides
nylon, Kevlar,
why are poyesters and polyamides biodegradable
both have linkages that are easily hydrolysed. ester and amide linkages are found in nature, they are natural links that can be hydrolysed by microorganisms.
organic compounds with fishy smell
amines
why are amines more volatile than alvohols of similar mass?
hydrogen bonding in amines is less efficient due to only having 1 lone pair on the N, some slightly poisitive H will not find a pair to hydrogen bond with. alcohols have 2 lone pairs
show amines acting as bases and acids
as a bas: RNH2 + H2O RNH3+ + OH-
as an acid RNH3+ + OH- —> RNH2 + H2O
identifiaction of aldehydes and ketones; solid derivative
react unknown carbonyl compound with 2,4-DNPH. the solid obtained is filtered and washed, and recrystallised to purify. it’s melting point is found and comapred to a table of mp of 2,4-dinitrophenylhydrazones
characterisation of alcohols using solid derivatives
react alcohol with 3,5-dinitrobenzoyl (acid chloride). the ester solid derivative prepared is purified by recrystallisation and its mp is determined. once the structure of the ester is determined the aclohol can be known
characterisation of amines uing solid derivates
unknown amine is reacted with acid chloride. the solid obtained (amide) is purified and its mp is found.
reactivity and stabilisation of diazonium ion
diazonium compounds have a full psoitive charge on the N together with and anoin, showing ionic character. the N2+ group is a very strong electron withdrawing group, making the C it is bonded partially positive and prone to attck by nycleophiles.
aliphatic diazonium compounds are difficult to iolate due to their high reatctivity
aromatic diazonium compounds are less reactive as the positive charge is delocalised over the ring, stabilising it, and making it possible for further reaction
allotropes of carbon
graphite, diamond and fullerenes (buckminsterfullerene -bucky ball)
property of electrophiles
positive species that have the ability to gain electrons
test for water
anhydrous CuSO4 white to blue
CoCl2 from blue to pink
