Module 6.1 Flashcards
What is Benzene
Benzene is a naturall occuring aromatic compound , which is a very stable planar ring structure with delocalised electrons .
What is a model
a model is a simplified version that allows us to make predictions and understand observations more easily .
what is the empirical formula and moleucular formula of benzene
-emprical formula is CH .
-It has the molecular formula C6H6 .
what state is benzene in
It is liquid at room temperature and is a key ingredient added to gasoline as it increases the efficencey of the car engine .
What is kekule’s model of benzene (1)
-Check page 122 on figure 1
Friedrick Kekule , was a German chemist who in 1865 suggested that benzene was a six membered rign with ALTERNATING SINGLE ND DOUBLE BONDS , between the carbon atoms .
kekule’s model of benzene (2)
He discovered that when one group was added to benzene , onl one isomer was ever made ; but hwen two groups were added , they were always three structural isomers produced . He used this experimental evidence , about hte types of isomers produced when benzene reacted , this supported his theory .
what is the first problem with the kekule model
unlike alkenes benzene is resistant to addition reactions .
what is the second problem with the kekule model .
enthalpy of hydrogenation of benzene shows that benzene is much more stable than was predicted .
what is the third problem with the kekule model
all six carbon bonds in benzene are the same length .
why does benzene tend to undergo substitution of a hydrogen atom rather than addition reactions? why was this unexpected
using the kekule model , you would expect benzene to undergo similar reactions to alkenees .
-ethene will readily undergo addition reactions but benzene tends to undergo substiution of a hdygrogen atom rather than addition reactions .
-kekule tried to explain this by saying the double and single bonds changed position in a very fast equilibrium . check f2 .
what is hydrogenaiton
hdyorgenation is the addition of hdyrogen to an unsaturated chemical .
the enthalpy change for the complete hydrogenation of cyclohexene (this was expected for benzene as well )
using bond enthalpy data , we can calculate the hdyrogenaion of cylohexene , and cyclo1,3,5 hexatriene .
-cyclohexatriene is the kekule model of benzene .
what is the enthalpy change of hydrogenation of benzene
it is actually -208kjmol , which shows it is 152kjmol more energetically stable than predicted .
check figure 3 and figure 4 on page 123
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what are the bond lengths of kekules structrue and what techniqque was used to show this .
This evdience disporved kekule’s strucutre as all six bonds are the same length .
-Xry diffraction techniques were used , and they have shown all six crbon bonds in benzene is 0.140 nm , which are between c-c single bonds 0.14nm and c=c double bond at 0.135 nm .
-Kekule’s structure suggests that there should be three shorer c=c double bonds and three longer c-c single bonds .
This evdience disporved kekule’s strucutre as all six bonds are the same length .
it is now thought that beznene has a delocalised electron strucutre . EXPLAIN
-DOES NOT SUPPORT KEKULE;S STRUCTURE
in the delocalised strucuture , each of the si crbon atoms donate one electron from its p-orbital . These electrons combine to form a ring of delocalised electrons above and below the plane of the moleucle .
-THe electrons in the rings are said to be delocalised as theya re bale to move freely within the ring and do not belong to a single atom . Therefore , UNLIKE kekule;s strucutures ,a ll bonds in this ring are identical so are the same length .
so how many kjmol is benzne due to the delocalised electrons
about 152jkmol-1 . MOre stable than expected when using the kekule model . Beause so much energy is needed to disrupt this decoalisation benzene is ver stable and resistant to addition reaction .
what structure can u use when representing benzene
kekule or delocalised strcture .
what is a substiution reaction
where a group or atom is exchanged for another group or atom in a chemical reaction
benzene derivative
a benzene ring that has undergone a substiution reaction .
what happens when benzene undergoes substiution reactions
as benzene has delocalised electrons it is energetically more stable than initially calculated so it rarely undergoes addition reactions but it will undergoo substituion reacions ..
-In these reactions , a hdyrogen atom is substituted for a different group and a benzene derivative is formed .
subtitute - chlorine atom
-what is the formula
-what is the naming prefix
-cl
-chloro
subtitute - bromine tom
-what is the formula
-what is the naming prefix
-br
-bromo
subtitute - nitro functional group
-what is the formula
-what is the naming prefix
-no2
-nitro
subtitute - alkyl chains
-what is the formula
-what is the naming prefix
e.g - ch3
-e.g methyl
what is a single substiuted benzene derivative
check f1 on page 124
when one hdyorgen is replaced by another group a single substiution reaction has occurred . this compound can be named (check module 4 )
-STEM - the longest chain ofcarbon atom is the aromati ring and has the stem benzene .
PREFIX 0 ther eis one ethyl group on the ring so there is no need to nmber , SO , the prefix is ethyl , the name of this compound is ethylbenzene .
what is a double substituted benzene derivative
two hydorgen toms on different carbon atoms of benzene can be replaed by two groups . If the groups are different , the name is written as a prefix in alphabetical order .
-One group will be added first an dbe given carbon number 1 , the second group will then be given a number to state which carbon atom it is on , with respect to the first substituted group . When naming double substituted benzene dreivatives , the SMALLEST POSSIBLE NUMBERS SHOULD BE USED .
SKIM OVER page 125
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what are multiple substitutions
-further hydorgen atoms , on the aromatic ring can be subsituted . This may take a number of stages in an organic syntsynthesisheiss. For example , benzene can rect to make methylbenzene and then two nitro functiona groups can substitute a further two hydrogen atom .
-TO show there is more than one of the same functional group prefixes such as di and tri are used .
what is an electrophilic substitution
it is a substitution reaction where an electrophile is attracted to an electron-rich atom or part of a molmoleculeeucle and a new covalent bond is formed by the electrophile accepting an electron pair .
what is a reaction mechanism
it is a model with steps to explain and predict a chemical reactin .
recap what makes benzene ring stable
electrons are delocalidelocadelocalizedlizedsed on the benzene ring . This makes the structure stable and unlikely to undergo additional reactins . As the ring , is so electron-dense it is susceptible to electrophilic attack and a hydrogen atom can be substituted by another group.
electrophilic substitution (1)
-electrons above and below the plane of atoms in the benzene ring attract an electrophile .
-electrophile accepts a pair of pi electrons from the delocalised ring and make a covalent bond . This is the slowest step and known as the rate-detrminign step Find out more abt this in 5.1.4
electrophilic subsitiuion (2)
-figure 1 - page 126
- a reactive intermediate is formed where the deloclaied electrons have been disrupted
-the unstable intermediae releases an H+ ion and the stable product is formed . This is a very fast ste[ .
-THIS REACTION MECHANISM can be summarised using curly arrows - where X+ represents a general electrophile .
p.g127 nitration of benzene
this is an electrophilic substiution reaction where a hydorgen atom is exchanged for a nitrogroup (-NO2) . FOor the nitration of benzene the reagent iis CONCENTRATED NITRIC ACID , with concenrated sulfuric acid as a catalyst .
nitration of benzene (3)
-what is added to generate the 2=no2+ electrophile
sulfuric acid is regenerated after the nitration and is thereofre a catalkyst . THe steps of the mechanism is shown below .
nitration of benzene (2)
-how is the reaction set up
- write the balanced equation
-initially the concentrated nitric acid and concentrated sulfuric acid are mixed together ina flask held in an ice bath .
-The benzene is added and a reflux condenser is set up , keeping the mixture at 50 degrees to prevent further subsittiuion reactions occuring .
halogenation of benzene - does it happen directly why/why not
benzene does not directly react with halogen as the aromatic ring is too stable . A halogen carrier such as iron 9which forms an iron halide in situ 0 , iron halides or aluminium halides are used . The halogen carrier will generate a positive halogen ion .
halogenation carriers that can be used for chlorination
ALCL3 , FECL3 , or Fe
halogenation carriers for bromination
ALBR3 , FEBR3 OR FE
bromine can react with iron bromide (III)to form what ?
a positive bromine ion that can act as electrophile . This can be represented by the following balanced chemical equation .
-check sheet ofr equation .
bromine reacting with iron bromie - bromine i egenrated in situ , so what happens next
it can then attack the benzene ring and electrophilic substituion occurs ;
-the halogen carrir is a catalyst and regenerated at the end of the halogenation as the relesed H+ , from the benzene rng forms HBr . the follwoign balanced chemiccl equation illustraed this ;
check equation .
what is a friedel crafts reaction
it is a substitution reaction where hydrogen is exchanged for an alkul or acyl chain .
why is benzene not as reactive as alkenes even thoguh it is aromatic compounds and they are unsaturated
-in 4.1.8 we saw that ethene is the simplest alkene has a pi bond between the two carbon atoms , this is a region of high electron denisty and will readily undergo addition reactions to become saturated .
what does shaking etehene with bromine do
it causes decolourisation as the coloured bromine is used to form the colourless 1,2dibromoethane .
-check sheet f1 128 for bonding ethene .
if cyclohexene is mixed with bromine water what happens
an addition reaction occurs . the first part of the mechanism is the bromine moleucle having an induced diple due to the interaction of the pi bond of the cyclohexene
what happens when benzene is haken with bromine water and why is this suprising
this suprisiing result occurs even thought the electrons in benzene are delocalised in pi bonds . So benzene must have a lower electron density between the crbon atoms than alkene .
why is a hdyorgen carrier useful
when non-polar molecules like bromine approach the benzene ring , there is not enough electron densit betweent he carbon atoms to induce a diple and start the reaction . This is also the case when attempting to substitute alkl halides like haloalkanes . Byusing a halogen carrier a stronger electrophile can be generated and alkylation can occur .
how did friedel crafts come about
in paris during the 19th century , french chemist charles friedel worked with ajmes cradt to develop a technique for aromatic electrophilic subsitions where hdyorgen is subsititued for an alkyl chain
how does friedel crafts work
this occurs by breaking C-H bonds and forming a C0C bond and this is called allkylation . It is very difficult to add alkyl groups to benzene and this was a signifiant breakthrough in organic syntheisis . In all these reactings a strong LEWIS ACID (which acceps a apir of electrons see 5.1.9 is ussed as a catalsyst .
friedel crafts reaction - haloalkanes like chloromethane are mixed with a halogen carrier such as iron (iII) chlochlorideroide , what happens
the halogen carrier acts as a catalysts and is regenerated at the end of the reaction .
-A reactive carbocation is made which undergoes electrophilic subsituion with the benzenne ring .
figure 3 shows he general mechanisms in gigure 3 shows a chloroalkane undergoing a friedel crafts reaction , what is r
R is an alkyl group , This reaction will occur at room temperture .
