organic chem and analysis Flashcards
(60 cards)
what is the structure and bonding of a benzene molecule?
planar
120º bond angle
all c-c the same length
p orbitals overlap above and below the ring to form pi bonds
electrons are delocalised
sigma bonds between c-c and c-h
what mechanism does benzene react by?
electrophilic substitution
what are 2 uses of phenols?
disinfectants
detergents
plastics
paints
aspirin
why is phenol more reactive than benzene?
a lone pair from the oxygen p orbital is delocalised with the electron cloud of the benzene ring
electron density increases
attracts electrophiles more strongly
how do you purify an impure solid?
recrystallisation
dissolve in minimum volume of hot water/ solvent
cool solution and filter solid
wash with cold water/ solvent and dry
how do you check the purity of a solid?
obtain mp and compare w known values, pure sample will have similar sharp value
run a TLC, compare Rf to known data, pure sample will have a similar value
describe in terms of orbital overlap, the similarities and differences between the bonding in the kekule model and the delocalised model of benzene
both have sideways overlap of p orbitals
both have pi system above and below C ring
kekule has alternating pi bonds and delocalised has pi ring system
what reacts w ethanoic acid and phenol?
aqueous potassium hydroxide
explain the relative resistance to chlorination of benzene compared with cyclohexene?
cyclohexene has localised electrons but benzene has delocalised
cyclohexene has higher electron density and is more susceptible to electrophile attack
what are the reagents required for the conversion of benzene to nitrobenzene?
conc H2SO4
conc HNO3
what happens when bromine water is added to phenol at room temp?
electrophilic substitution
phenol decolourises the orange bromine solution to form a white ppt of 2,4,6 tribromophenol
why is the enthalpy change of hydrogenation of benzene lower than expected?
the delocalisation of electrons in the benzene ring increases the stability
describe the structure of cyclohexene
each C has 4 covalent bonds
4 of the 6 carbons attempt to have a tetrahedral shape w a bond angle of 109.5
2 of the 6 carbons attempt to have a trigonal planar shape w a bond angle of 120
there is a mix of single and double bond lengths
why won’t benzene react w bromine but cyclohexene will react to form dibromohexane
pi bond in hexene contains localised electrons/ area of high electron density
pi bond in hexene induces a partial positive and negative in Br2
partial positive Br attracted to pi pond, leaving a carbocation intermediate
carbocation intermediate attracts partial neg Br
delocalised electrons w no areas of high electron density in benzene so can’t polarise br2
describe a method to obtain a pure sample of organic liquid from a distillate?
shake and leave to settle in a separating funnel
separate layers by tapping off
add anhydrous magnesium sulfate to organic layer
redistill organic layer
how can you use a TLC to monitor the course of a reaction?
take samples from reaction mixture at regular intervals
run on a TLC plate alongside controls
suggest a suitable test to confirm that a compound contains an unsaturated carbon chain
bromine water decolourised
suggest a suitable test to confirm that a compound contains an aldehyde functional group
Tollens’ reagent
silver mirror ppt
which compound is used as a standard for NMR chemical shift measurements?
Si(CH3)4
explain the use of 2 deuterated compounds in NMR spectroscopy
CDCl3 used as a solvent
D2O used to identify OH/NH
explain how TLC separates compounds in a mixture
adsorption onto the stationary phase
explain how GC-MS enables compounds to be identified
separates compounds
compare them w database
why is (CD3)2SO used as a solvent rather that (CH3)2SO in proton NMR
doesn’t give a peak as there is no H
in electrophilic addition, explain why one of the organic products forms in much greater quantity than the other
major product forms from more stable carbocation
carbocation bonded to more c atoms
