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
what conditions are all Friedel-Crafts reactions performed under?
under reflux and in anhydrous conditions
describe electrophile formation in Friedel-Crafts reactions
the chlorinated group is polarised by the catalyst and generates the electrophile (R+ or R delta+)
what ion acts as an electrophile during nitration of benzene
Nitronium
what are phenols
aromatic compounds that have an alcohols hydroxyl group attached DIRECTLY to the ring
what is the simplest formula for a phenol
C6H5OH
describe the structure of phenols
oxygen bonded to the benzene has two lone pairs of electrons in p-orbitals
as the ring has low electron density, one of the lone pairs of e- in the oxygen p-orbital overlaps into ring structure - becomes delocalised
increasing the ring’s electron density allowing electrophilic substitution to occur more easily
therefore phenol is more reactive than benzene
what are the properties of phenol
- p-orbital overlap into the ring weakens the O-H bond so phenol can donate H+ and form phenoxide
means that phenol solutions are acidic
why are phenols weak acids
because the OH bond is weakened by the overlap of the P-orbital into the ring
what is the name of the negative ion that phenols form?
phenoxide ion
Why is the polarity of the C=O bond in aldehydes and ketones important?
Because it means that they can react with some nucleophiles - nucleophilic addition
What reagent is used to reduce aldehydes and ketones to their original alcohols
NaBH4
What does HCN form when it reacts with an aldehyde or ketone
Hydroxynitrile
How can you detect carbonyls
Brady’s reagent (2,4 - DNP) - produces a yellow or orange precipitate
2,4 - DNP is usually dissolved in methanol and sulfuric acid
How can you distinguish between aldehydes and ketones
Tollen’s reagent - silver nitrate in aq. NH3 - aldehydes produce a silver mirror
What is the conjugate base of a carboxylic acid
The carboxylate ion
- carboxylic acid that has donated a proton
Why do carboxylic acids act as weak acids
Because they can dissociate the hydrogen from the OH bond
What is the smallest carboxylate ion
Methanoate CHOO-
How are esters formed
Reacting an alcohol with a carboxylic acid in the presence of an acid catalyst
How are esters named
Alcohol = prefix e.g. methyl from methanol
Carboxylic acid = suffix e.g. ethanoate from ethanoic acid
What are acid anhydrides
Dehydrated carboxylic acids
What do acid anhydrides react with to form esters
What is required
Alcohols
Only gentle heating is required
Why is the formation of an ester from an acid anhydride and alcohol preferred from the reaction of carboxylic acids + alcohols
Because the reaction is reversible and no catalyst is required - as acid anhydrides are more reactive than carboxylic acids
What type of reaction is esterification
Condensation reaction - elimination
What type of reaction is esterification
Condensation reaction - elimination
What are esters broken down into when hydrolysed
Their constituent alcohols and carboxylic acids act
What is required to hydrolyse esters
Reflux the ester
Under acidic conditions (dilute HCl or H2SO4)
Or
Alkaline conditions - form an alcohol and a carboxylate salt
What do reactions between carboxylic acids and thionyl chloride form
Acyl chlorides
R-C=O-Cl
How are acyl chlorides formed
Carboxylic acids reacted with thionyl chloride (SOCl)
What does acyl chlorides +
Water -
Alcohols/phenol -
Ammonia/ amine -
Produce
Water = carboxylic acid
Alcohols/ phenol = ester
Ammonia/amine = amide
What is an acyl group
A functional group containing an alkyl group and a double bonded oxygen atom
What is NMR used for
To analyse a molecular structure in detail
What isotopes do are frequently used in NMR
H1
C13
F19
P31
What does NMR require
Strong magnetic field - electromagnet
Low-energy radio-frequency radiation
What is a chiral centre
A carbon with four different atoms/ groups bonded to it (so is asymmetrical) that causes optimal isomers
Also known as enantiomers
What are the properties of optical isomers
Rotate plane-polarised light in different directions
What does a racemic mixture contain
Equal proportions of each enantiomer
Will have no effect on plane-polarised light
(Rotations cancel out)
Define bi-functional
Acts as an acid or a base
What are the products from the acid hydrolysis of a secondary amide
A carboxylic acid and an ammonium salt
