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
