module 6 - organic chemistry and analysis Flashcards
benzene is a major feedstock used in which industries?
polymers
pharmaceuticals
dyes
pesticides
explosives
what is the empirical and molecular formula of benzene
empirical - CH
molecular = C6H6
what did kekule propose?
that benzene was a cyclic structure containing three alternating carbon-carbon double bonds
what did kekule think was the reason for benzene’s low reactivity
due to the rapid equilibrium between two isomers
- changing position of double bonds make reactions difficult
what were the three problems with kekule’s model
- resistance to reaction
- bond length
- enthalpy change of hydrogenation
problems with kekule’s model:
resistance to reaction
addition does not occur - product no longer contains a benzene ring
- substitution occurs - still contains benzene ring
problems with kekule’s model:
bond lengths
C-C and C=C have different bond lengths so benzene hexagon would be irregular
benzene has a different bond length (between c=c and c-c)
problems with kekule’s model:
enthalpy change of hydrogenation
benzene can be saturated with hydrogen to form cyclohexane
- cyclohexene = -120kJmol-1
- kekule’s model (cyclohex-1,3,5-triene) would expect to have an enthalpy of -360kJmol-1
benzene has an enthalpy of -208kJmol-1
more stable bonds than alkenes
how do we know benzene has more stable bonds than alkenes
because it has a higher bond enthalpy of hydrogenation
describe the delocalised model of Benzene
- each carbon creates 3 covalent sigma bonds
- the remaining electron is found in a p-orbital at a right angle above and below the carbon atom
- the p-orbitals overlap (sideways) evenly creating a ring of delocalised pi- electrons above and below the structure
naming benzene - when is it a “benzene” root name
when there is:
- an alkyl group (methyl etc)
- a halogen (bromo etc)
- a nitro group (NO2)
- a carboxylic acid (COOH)
naming benzene -
when is it the prefix phenyl
most other functional groups
(apart from alkyls, halogens, nitro groups and carboxylic acids)
define electrophile
an electron pair acceptor
why does benzene resist electrophilic addition
because it would disrupt the delocalised Pi- ring
- product would be less stable
what reaction does benzene undergo instead of electrophilic addition and why
electrophilic substitution by replacing a hydrogen for another group
because it retains the energetically stable delocalised Pi-electron ring
describe the process of electrophilic substitution
- electrophile attracted to ring due to high e- density
- a pair of e- from the ring are donated to electrophile to form covalent bond - an unstable intermediate forms with incomplete ring structure and positive charge
- bond with hydrogen undergoes heterolytic fission to donate electrons to the ring structure - the stable ring structure returns leaving the substituted product and a H+ ion
what are the prefix and suffix given to benzene containing compounds
prefix = phenyl
suffix = benzene
why is the electrophile attracted to the benzene ring
attracted to the delocalised Pi-electrons due to the high electron density
what does the nitration of benzene do to the structure
adds an -NO2 to the ring
what are the reagent, catalyst and conditions for nitration of benzene
reagent = conc. nitric acid (HNO3)
catalyst = conc. sulfuric acid (H2SO4)
conditions = 50-55 degrees Celsius
what temperature does nitration of benzene occur at and why
50-55 degrees Celsius
because multiple substitutions occur at higher temps
why is sulfuric acid a catalyst in the nitration of benzene?
because it is used up during electrophile formation and then reforms at the end of reaction
what are the 3 stages of nitration of benzene
- Nitronium ion (NO2+) formation
- electrophile attack
- catalyst reformation
how do Friedel-Crafts reactions work?
use halogen carriers (Alx3, FeX3 or Fe) as catalysts to substitute R-Cl groups to rings via
- halogenation - sub a halogen
- alkylation - sub an alkyl using haloalkanes
- acylation - sub a ketone using acyl chlorides
