module 6 Flashcards
what is an arene?
hydrocarbon containing at least one benzene ring
what is a condensation reaction?
2 compounds join to form a small molecule such as water
hydrolysis
the reaction with water to break down a compound
what is an amino acid?
have an amine and carboxyl group attached to the same carbon with formula RCH(NH2)COOH
what are optical isomers?
stereoisomers with non-superimposable mirror images
chiral carbon
- carbon attached to 4 different groups
retention time
-time taken from injection ton detection
what is the formula for total number of optical isomers?
2^n (n is number of chiral carbon atoms)
what is the kekule model for benzene?
-molecular formula C6H6 empirical formula CH
- 6-membered ring for benzene with alternating single and double bonds
what is the new evidence for disproving kekule?
- all carbon-carbon bond lengths are the same
- actual benzene doesn’t undergo electrophilic addition with bromine
- enthalpy of hydrogenation for benzene is less exothermic than predicted
describe delocalised model
- each carbon has 3 sigma bonds and 1 pi bond
- the 4th electron on each carbon is in a p-orbital
- p-orbitals overlap sideways in both directions above and below the plane of the ring to form a pi bond
- 6 pi electrons delocalised over 6 carbon atoms
- planar molecule with trigonal planar shaped carbon atoms and bond angle 120
alkene vs benzene similarities and differences
- both have sideways overlap between p-orbitals
- both form pi bond above and below ring
- alkene has 3 pi bonds which are localised, benzene has delocalised ring
- alkene has 2 electrons in each pi bond, benzene has 6 electrons in one pi bond
- overlap in one direction in alkene, benzene has overlap in both directions
compare reactivity of benzene with bromine and reactivity of alkene with bromine
- alkene more reactive than benzene
- alkene has pi electrons which are localised between 2 carbons, so high electron density so can polarise and attract bromine in electrophilic addition reaction
- in benzene the pi electron is delocalised over 6 carbons and so has a lower electron density so benzene can’t polarise and attract bromine
- benzene only undergoes electrophilic substitution reaction with presence of halogen carrier catalyst
why is phenol partially soluble?
- OH forms hydrogen bonding with water
Why is phenol a weak acid?
Partially dissociates in water
Phenol + NaOH
sodium phenoxide, water
Phenol + Na
Sodium phenoxide + hydrogen
phenol + NaCO3
no reaction as phenol not strong enough to react with carbonates
why can phenol react with bromine without halogen carrier catalyst
as O has lone pair in p orbital which is delocalised into benzene ring so electron density increases and so ring is activated so phenol can polarise and attract bromine
- bromine turns colourless and forms 2,4,6-tribromophenol , a white ppt
what are electron donating groups?
- increase electron density in benzene ring, activating it
- direct electrophiles to position 2 and 4
- OH and NH2 are 2,4 directing
what are electron withdrawing groups?
- withdraw density from benzene ring, deactivating it
- direct electrophiles to position 3
- NO2 is 3 directing
how to test for presence of carbonyl
- 2,4-DNPH
- a red ppt indicates presence of aldehyde/ketone
how to determine actual compound present from carbonyl
- add 2,4-DNPH
- red ppt
- filter ppt, recrystallise, dry and determine melting point
- compare melting point to known values from database
test for aldehyde
- tollens
- oxidise aldehyde to carboxylic acid but doesn’t react with ketone
- silver mirror ppt
- tollens doesn’t oxidise alcohol as too weak
why are carboxylic acids weak acids?
partially dissociate in water
carboxylic acid + metal
salt and hydrogen
carboxylic acid + metal hydroxide
salt and water
out of alcohol, phenol and carboxylic acid what can react with sodium?
all 3
out of alcohol, phenol and carboxylic acid what can react with sodium hydroxide?
phenol and carboxylic acid
out of alcohol, phenol and carboxylic acid what can react with sodium carbonate
carboxylic acid
what is use of esters?
food flavourings
why are amine bases?
lone pair on N accepts a proton forming dative covalent bond to give ammonium ion
why are condensation polymers biodegradable?
- amide or ester bond can be hydrolysed
describe recrystallisation
- dissolve solid in minimum volume of hot solvent
- cool, scratch flask with glass rod until crystalline solid seen, filter
- wash solid with cold solvent and dry further
- measure melting point
- match melting point to database
- pure compounds= sharp melting point over 2 degree range
- impurities have lower melting point and melting= large range
what is name of process of separation in TLC
adsorption
what does it mean when spot is higher up in tlc
- larger rf= more soluble and stronger intermolecular forces with stationary mobile phase, less polar compound
limitations of TLC
- similar compounds= similar Rf values
- new compounds= no database value
what is separation by in Gas chromatography if stationary phase is solid
adsorption
what is separation by in Gas chromatography if stationary phase is liquid
solubility
how to identify compound in gc
match retention time to database
what does number of peaks in gc mean?
number of compounds in mixture
what does relative peak area mean in gc
area is proportional to concentration
limitations of gc
- similar compounds have similar retention times
- new compounds wont have database values
how to use calibration curve to know concentration from peak height
- make standard solutions at varying concentrations of compound
- obtain gas chromatogram for each concentration
- plot calibration curve of peak area against concentration
- obtain gas chromatogram of compound
- use calibration curve to determine concentration
how does NMR work
- interaction of nucleus with strong magnetic field
- low energy frequency
what is reference used in NMR
TMS = 0ppm
why use TMS?
- non-toxic, volatile
- easily removed and single intense peak of 12 H
number of different peaks indicates …
number of different environments
how to remove OH or NH
add D2O as causes proton exchange
use of NMR
MRI imaging
what are reagents for haloalkane to nitrile
- NaCN
- Ethanol
