organic chem and analysis Flashcards
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 cyclohexanol 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
explain the term homologous series
series of organic compounds with the same functional group
each successive/subsequent member differs by CH2
explain why nucleophilic addition of carbonyls involves heterolytic fission
O receives both/2 electrons
Breaking of a covalent bond
state the reagents and conditions for oxidation of primary alcohol to carboxylic acid
acidified potassium dichromate
reflux
How does gas chromatography separate compounds in a mixture
Relative solubility in the stationary phase
give 2 reasons why small quantities of reagents are used in organic preparations
reagents can be toxic/ harmful/ dangerous so small quantities minimise the risk of injury
reagents can be expensive so it is less wasteful to use small quantities
why is heating under reflux used
allows continuous/ prolonged boiling without loss of any volatile liquids/ solvents
explain the purpose of filtration under reduced pressure
fast filtration
obtain a drier solid product
state 2 pieces of apparatus that are needed for filtration under reduced pressure
Buchner flask
Buchner funnel
pressure tubing
filter paper
filter pump/ vacuum pump/ aspirator
explain the purpose of recrystallisation in the preparation of an organic solid
purify an organic solid
why is the melting point of an organic solid lower before recrystallisation
before recrystallisation the sample is impure/ contaminated
impurities lower the melting point
explain why a melting point determination is necessary following the preparation of an organic solid
product may not be pure/ melting point can indicate if the product is pure
outline why adding NaCN is a particularly useful reaction in organic synthesis
increases the length of the carbon chain
allows u to synthesise diff members of a homologous series
state the properties of a suitable solvent for recrystallisation
solute must be soluble in hot solvent
and
insoluble in cold solvent
reagents for CN to COOH
aqueous acid
explain why amines can react w acids
N e- pair/ lone pair
accepts a proton/ H+
suggest a reason why a polyester is biodegradable
ester bond can be hydrolysed
describe 2 pieces of evidence to support the delocalised model of benzene
all C-C bond lengths are the same
enthalpy change of hydrogenation less exo than expected
benzene doesn’t decolourise bromine water
alcohol to alkene reagent
acid
haloalkane to amine reagent
NH3 and ethanol
COOH to COCl reagent
SOCl2
suggest 2 limitations of using radical substitution in organic synthesis
further substitution
substitution at different positions along chain
suggest a use for an ester
perfume/ fragrance/ flavouring
suggest 2 benefits of using single stereoisomers in the synthesis of drugs
no/ fewer side effects
increases the activity/ effecttiveness
reduces/ stops the need for/ cost/ difficulty in separating stereoisomers/ optical isomers
in the delocalised model of benzene, are the carbon- carbon bond lengths all the same
yes
what does less branching to do the boiling point of compounds
boiling point increases
more surface contact
more London forces
more energy to break London forces
conditions for initiation of free radical substitution
UV light
explain the term optical isomerism
non superimposable mirror images
state 1 example of an organic reaction in which sulphuric acid is a catalyst
elimination of H2O from alcohols
nitration of benzene
esterification
hydrolysis of esters/ amides
suggest 1 way of processing a waste addition polymer other than landfill and recycling
use as organic feedstock
explain why bromine reacts more readily with phenol than with benzene
lone pair of electrons on O is partially delocalised into the rings
electron density increases
Br2/ electrophile is polarised
give a chemical explanation for why the carbon- carbon bonds in benzene are all the same length
pi bonds in benzene are delocalised
describe the bonding in the updated model of benzene
p orbitals overlap to form pi bonds
electrons are delocalised
is RCl or RBr hydrolysed faster
RBr
because the C-Cl bond enthalpy is greater than C-Br
what is the number of sigma bonds in a molecule of methylbenzene
15
