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.
