aromatics Flashcards
what is the molecular structure of benzene
C6H6
What does Kekule’s structure of benzene suggest
it suggests that it is a cyclo structure with alternating double and single bonds
what are the problems with Kekule’s structure
- does not undergo an addition reaction with bromine even though the model suggests that it should ( it has double bonds)
- the carbon-carbon bond lengths are all the same length
- the enthalpy of hydrogenation value is less exothermic than expected
the first issue with Kekule’s structure
alkenes undergo addition reactions therefore if we react Kekule’s structure with bromine solution, it would go colourless
however, if we carry this reaction in a lab, nothing happens
why does benzene not undergo addition reaction
benzene has delocalised electrons, so it has a lower electron density than alkenes as the electrons spread themselves out
and so, it does not induce a strong dipole in BR2
Therefore, bromine becomes a poor electrophile and the two can’t undergo an addition reaction
the second issue with Kekule’s structure
if benzene has alternating C-C and C=C bonds then we would expect benzene to have different lengths as the double bonds are expected to be shorter than the single bonds
this is because the p-orbitals overlap by the same amount
what is the third issue with Kekule’s structure
you would expect that the overall enthalpy of hydration would be -360
so overall the reaction is exothermic
if we carried out this reaction in a lab, the enthalpy for hydrogenation is -208 which is less exothermic than expected
therefore Kekule’s structure can’t be correct
what is the enthalpy of hydrogenation
when 1 mole of a compound reacts duly with an excess of hydrogen
explain the bonding in benzene
each carbon dorms three covalent bonds (3 sigma bonds)
the pi orbitals overlap with 2 other neighbouring p orbitals forming a pi-system
the electrons in this p- system are delocalised
explain why the enthalpy of hydrogenation is more endothermic/ less exothermic than expected
benzene has a pi- system of electrons which are delocalised across the ring
these delocalised electrons stabilise the molecule
this makes the enthalpy of hydrogenation less exothermic than expected due to the extra energy needed to disrupt the pi-system
what makes a compound stable
if a compound has alternating c-c and c=c bonds, it has delocalised electrons
these delocalised electrons make the compound more stable
why do delocalised electrons make the compound more stable
as the electrons are more spread out, the electron density decreases therefore, it is less likely to attract other molecules
therefore, when a compound has delocalised electrons, its stability increases
what are aromatics
when a compound has 6 carbons in a ring and the p orbitals overlap to form a pi system, we say that compound contains a benzene ring which is an aromatic compound
describe the shape of the benzene
- planar
- the bond angles in benzene are 120
- c-c bonds are equal in length
what are important things to remember when naming benzene
- when benzene is the highest priority group, the suffix is benzene and the prefix is whatever group is attached to it
e. g. ethylbenzene - if benzene is the lowest priority group, the prefix is phenyl and the suffix is whatever group is attached to it
phenylpropanoic acid
- the benzene is never included in the longest chain
what is electrophilic substitution
reacting benzene with Br2 under a different set of conditions the two can react together
in which bromine substitutes for a hydrogen atom
what are the conditions for electrophilic substitution
conditions for this reaction:
1. we use a hydrogen carrier e.g. AlBr3 which is electron deficient as it has 6 electrons in its outer shell
so it will react with Br2 like this:
Br2 +AlBr3 - Br+ + AlBr4-
this Br+ cation is stronger electrophile - strong enough to react with benzene
all hydrogen carriers act in the same way
how is the catalyst reformed at the end
AlBr4- + H+ - HBr + AlBr3
why does benzene undergo an electrophilic substitution reaction
- compared to Br2 (like in electrophilic addition) Br+ much stronger electrophile so benzene can react with it
- the ring is only temporarily broken as it reforms at the end of the reaction
The molecule, therefore, keeps the benzene ring and so it keeps its extra stability
what is Friedel crafts acylation
this is substituting acyl groups onto benzene rings e.g. acyl chlorides
the acyl group substitutes for a hydrogen when we add an acyl group to a compound it is called acylation
This is the same mechanism for electrophilic substitution
what are the conditions for Friedel crafts acylation
on their own, chlorides are not strongenough to react with benzene
to make them strong enough to react we need to use a hydrogen carrier e.g. AlCl3 (this is the same reaction with bromine)
what is the reaction for the regeneration of the electrophile
RCO+ + AlCl4-
what is the overall reaction for friedal craft acylation
C6H5COR +HCL
the alumium chloride is reagenrated
what is nitration
benzene + nitric acid - nitrobenzene + water
what must happen before nitration can occur
we need to form the electrophile which is a strong acidic catalyst in this case it is H2SO4 catalyst
the electrophile is NO2+
generating the electrophile:
-HNO3 +H2SO4 - NO2+ + HSO4 + H2O
what are the reagents for nitration
nitric acid and sulfuric acid needs to be concentrated so we can get as many molecules as possible
why must the reagent be concentrated
the mechanism for the generation of the electrophile is reversible
therefore if our reaction contains a lot of water, the equilibrium will move to the left and we will end up with a lower yield of the electrophile
this is the reason why we need to keep out water in all of our electrophilic substitutions reactions - the electrophile reacts with water as it is very reactive
how do we test for carboxylic acids
we react with a carbonate e.g. sodium carbonate
a positive test will produce bubbles due to the CO2 gas provided
what are the properties of carboxylic acids
completely soluble in water
higher melting points
why are carboxylic acids soluble in water
they can form hydrogen bonds with water molecules
why do carboxylic acids have higher melting points
they can form hydrogen bonds with each other
they then will be considered to be a carboxylic dimer which is twice as big as a single carboxylic acid
therefore, carboxylic acids have greater melting points as they can former dimers
why are carboxylic acids stronger/more stable than alcohols
the carboxylate ion has an extra electron which is delocalised over 3 atoms making the ion more stable when it releases a hydrogen
what are the uses of esters
- plasticizers
- solvents
- perfumes
how are esters formed
alcohol + carboxylic acid - ester + water
what are the conditions required for esters to form
H2SO4 is needed as a catalyst- acidic catalyst
the acid has to be concentrated
The reaction also occurs under reflux
how do we name esters
the name reflects the acid and the alcohol that it is formed
carboxylic acid has the highest priority
so it acts as the main carbon group
the hydrolysis of ester in acidic conditions
ester + water - carboxylic acid + alcohol
this occurs in the presence of a strong acid catalyst/acidic condition
this is a reversible reaction
hydrolysis of esters in alkaline conditions
it is non-reversible and forms
a salt and an alcohol
what is the IUPAC name for glycerol
propane,1-2-3 tiol
it can form many hydrogen bonds as it has 3 alcohol groups
therefore it is very soluble in water
what are the uses of glycerol
- moisturizing agent
- plasiticsizer
- solvent
how do we create biofuel
we react the fat/oil with methanol in the presence of a strong alkali as a catalyst
this produces methyl ester and glycerol. It is the methyl ester that is the biodiedel
what are carboxylic derivatives
esters are formed from carboxylic acids
therefore, they are called acid derivatives
what are acyl groups
an acyl group is an o=c bond
what is the general formula for acyl chlorides
RCOCL
how do we convert carboxylic acids into acyl chlorides
we need to use sulfurous dichloride SOCL2
the products are an acyl chloride
sulfur dioxide
hydrogen chloride gas
the reaction is non-reversible
how do we name an acyl chloride
ethATHOYL chloride
what are acyl anhydrides
it is another carboxylic acid derivative
we simply replace the acid with anhydride
e.g. ethanoic anydride
What is the general formula for acid anhydrides
RCOOCOR
or (HCO)2O
how do acid anhydrides and acyl chloride react in water
they both react to dorm carboxylic acids they were derived from
acyl chlorides react the most vigorously with wat
acid anhydrides: 2 carboxylic acids
acyl chlorides: a carboxylic acid and hydrogen chloride
how do alcohol react with acyl chlorides react
this is another way to form an ester
we can’t do this reaction in an aqueous solution as acyl chloride react rapidly in water
how do anhydrides react
- acid anhydrides + alchol - ester + carboxylic acid
- acid anhydride + water - carboxylic acid
- acid anhydride + ammonia - amide + ethanoic acid/ in excess of ammonia
an ethanoate salt
what is an optical isomer
optical isomerism occurs in carbon compounds with 4 different groups of atoms attached to carbon to a carbon (called an asymmetric carbon
the four different groups are arranged tetrahedrally around the carbon
this causes two different isomers that are non-superimposable mirror images
what happens when we mix two enantiomers together
a mixture containing a 50/50 mixture of the isomers is described as a racemate or racemic mixture
this mixture will not rotate plane polarised light at all
what are the chemical and physical properties of optical isomers
optical isomers have similar physical and chemical properties but they rotate plane polarised light in different directions
one enantiomer rotates plane polarised light in one way and the other rotates it in the opposite way
how are racemates formed
a racemate will be formed in a reaction mechanism when a reactant or intermediate has a trigonal planar group in the molecule is approached from both sides by an attacking species
There is an rqual chances of either enatiomer forming so a racemate forms
how is drug action determined by optical isomers
drug action may be determined by the steriochemisrty of the molecules
Different optical isomers may have very different effects
One enatiomer of thalidomide causes birth detects in unborn children whilst the other had useful sedative problems
Unfortunately it was given in a racemic mixture when first used
what are carbonyls
carbonyls are compounds with a C=O
they can be either aldehydes or ketones
where is the C=O in the ketones
it is attached in the middle
the name will end with a one
Ketones do not like to be on their own so they are in the middle
where is the C=O in the aldehydes
the C=O is on the end of the chain
the name will end in -al
Aldehydes like to hide and so they are at the end
reactions of carbonyls
the C=O bond is polarisd because O is more electronegative than carbon
The positive carbon atom attracts nucleophiles
describe the intermolecular forces in carbonyls
pure carbonyls cannot form hydrogen bonds themselves
but they are attracted instead by permanent dipole forces
what is the solubility of carbonyls in water
the smaller the carbonyls are soluble in water because they can form hydrogen bonds with water
which is stronger, the C=C bonds in alkenes or the C=O bond in carbonyls
in comparison to the C=C bond is alkenes, the C=O is stronger and does not undergo addition reaction easily
what happens when aldehydes and alcohols are oxidised
potassium dichromate K2CR2O7 is an oxidising agent that can causes alcohol and aldehydes to oxidise
e.g.
Primary alcohol - aldehydes - carboxylic acids
Secondary alcohols - ketones
tertiary alcohols - so not oxidise
what is the reagents needed for the reduction of carbonyls
NaBH4 in aqueous ethanol
the conditions need to be room temperature and pressure
can ketones be oxidised
no
what happens when aldehydes are reduced
it forms an alcohol
what happens when ketones are reduced
ketones will be reduced to secondary alcohols
what is the mechanism that occurs during the reduction of an aldehyde/ ketone
nucleophillic addition mechanism
how else cancarbonyls be reduced
carbonyls can be reduced using catalytic hydrogenation
the reagent is hydrogen and nickel catalyst under high pressure
what are the conditions for the addition of hydrogen cyanide to carbonyls to form hydroxycynitriles
Reagent: sodium cyanide (NaCN) and dilute sulfuric acid
Conditions: Room temperature
Mechanism: nucleophilic addition
why were the reagents needed
the NaCN supplies the nucleophilic CN- ions
The H2SO4 acid supplies H+ ions needed in the second step of the mechanism
another adavantage of using KCN or NaCN is that there will be higher concentration of the CN- opm as these compounds will completely ionise
HCN is a weak acid and will only partially ionise (HCN also leads to the production of toxic gas that is difficult to contain - KCN/NaCN are still toxic because of the cyanide ion)
what happens when nucleophillic addition occurs with HCN to aldehydes and ketones (unsymmetical)
when the trigonal planar carbonly is approached from both sides by the HCN attacking species results in the formation of a racemate
This is because there is an equal chance of either enatiomers forming so a racemate forms
No optical activityis seen
how are carboxylic acids stabilised
the carboxylic acid salts are stabalised by delocalisation while makes dissociation more likely
describe the strength of carboxylic acids
upon delocalisation
increasing the chain length pushes electron density on to the COO- ion making it more negative and less stable
This makes the acid less strong
e.g. propanoic acid is less acidic than ethanoic acid
electronegative chlorine atoms withdraw electron density from the COO- ion making it less negative and more stable
This makes the acid more strong
e.g. chloroethanoic acid is more acidic than ehtnaoic acid
describe the oxidation of methanoic acid
carboxylic acids cannot be oxidised by using oxidating agents but methanoic acid is an exception as it has a structure that is effectively an aldehyde
it forms a carbonic acid (H2CO2) whci can decompose to give CO2
what is esterification
a carboxylic acid reacts with alcohols, in the presence of a strong acid catalyst, to form esters and water
the reaction is reversible
- it is quite slow and needs heating under refluc
low yields are achieved
An acid catalyst (H2SO4) is needed
how do we make aspirin
- dissolve the impure componud in a minimum volume of hot (near boiling) solvent
- Hot filter solution through (fluted) filter paper quickly
- Cool the filtered solution by inserting beaker in ice
- suction filtrate with a Buchner flask to separate out crystals
- wash the crystals with distilled water
- dry the crystals between absorbent paper
why did step 1 happen
an appropritate solovent is one which will dissolve both compound and impurities when hot and one in which the compound itself does not dissolve well when cold
The minimum volume us used to obtain saturated solution and to enable crytallisation on cooling
why did step 2 happen
this step will remove any insoluble impurities and heat will prevent crystals reforming during filtration
why did step 3 happen
crystals will reform but soluble impurities will remain in solution form because they are presented in small quantities so solution is not saturated
Ice will increase the yeild of crystals
why did step 4 happen
the water pump connected to the Buchner flask reduces the pressure and speeds up the filtration
why did step 5 happen
to remove soluble impurities
how does an acyl chloride react with water
acyl chlorides react vigorously with cold water, producing carboxylic acid and hydrogen chloride
how do acyl chlorides react with alcohol
acyl chlorides react with alcohols at room temp vigorously forming an ester and hydrogen chloride
how do acyl chlorides react with ammonia
acyl chlorides react with ammonia vigorously at room temp to produce an amide and you guessed it hydrogen chloride
how do acyl chlorides react with a primary amine
acyl chlorides react vigorously with primary amines at room temp producing an N substituted amide
what is the mechanism for nucleophilic addition elimination
what are animal fats and vegetable oils
they are esters*** of ***glycerol and fatty acids
what are fatty acids
fatty acids are long chain carboxylic acids. They combine with glycerol (which is propane-1,2,3,-triol and alcohol) to make fats and oils
what are fats and what are the properties of fats
fats mainly are saturated hydrocarbons chains - they fit together increasing the van der Waal forces between them
This means higher temperatures are needed to melt them and so they are solid at room temp
what are oils and what are the properties of oils
oils have unsaturated hydrocarbons chains - the double bonds mean that the chains are bent and so they don’t pack together well, decreasing the effect of the van Der Waals forced
Therefore they easily melt and are liquids at room temperature
what happens when you hydrolyse a fatty acid/ oil
in a base hydrolysis, you form a carboxylate ion and the alcohol formed is glycerol
If you want you can convert the sodium salt back into a long chain carboxylic acid (fatty acid) by adding an acid such as HCL
The H+ ions from the acid displayed the Na+ ions in the salt to form a carboxylic acid, which releases a free Na+ ion.
