A-LEVEL CHEMISTRY, ORGANIC CHEMISTRY III. Flashcards
WHAT IS BENZENE?
BENZENE IS A CYCLIC, PLANAR MOLECULE WITH THE FORMULA C6H6.
BENZENE STRUCTURE.
IN BENZENE THE CARBON HAS FOUR,4, VALENT ELECTRONS.
EACH CARBON IS BONDED TO TWO,2, OTHER CARBONS AND ONE,1, HYDROGEN ATOM.
THE FINAL LONE ELECTRON IS IN A P-ORBITAL WHICH STICKS OUT ABOVE AND BELOW THE PLANAR RING.
BENZENE LONE ELECTRON.
THE LONE ELECTROSN IN THE P-ORBITALS COMBINE TO FORM A DELOCALISED RING OF ELECTRONS.
DUE TO THE DELOCALISED ELECTRONS TRUCTURE ALL OF THE C-C BONDS IN THE MOLECULE ARE THE SAME.
THEY HAVE THE SAME BOND LEGNTH, 139pm.
CARBON TO CARBON BOND LENGTH IN BENZENE.
THE C-C BOND LENGTH IN BENZENE LIES BETEWEEN 154pm, SINGLE BOND, AND 134pm, DOUBLE BOND.
HOW IS BENZENE NORMALLY DRAWN?
BENZENE IS NORMALLY DRAWN IN THE SKELETAL FORMULA.
WE DO NOT SHOW THE HYDROGENS IN THE SKELETAL FORMULA.
KEKULE’S STRUCTURE.
THIS REFERS TO THE STRUCTURE OF BENZENE.
THIS STRUCTURE SHOWS BENZENE WITH DOUBLE BONDS.
IT IS CALLED KEKULE’S STRCTURE NAMES AFTER AUGUST KEKULUE WHO DISCOVERED IT.
HE THOUGHT THERE WAS ALTERNATING DOUBLE AND SINGLE BONDS.
DRAWING BENZENE TO SHOW DELOCALISED ELECTRONS.
THIS STRUCTURE IS THE HEXAGON WITH THE CIRCLE IN THE MIDDLE.
THE CIRCLE IS REFERD TO AS THE DELOCALISED RING.
THIS STRUCTURE SHOWS THE DELOCALISED ELECTRON SYSTEM AND YOU WILLL BE MORE LIKELY TO SEE THIS THAN THE KEKULE STRUCTURE, HOWEVER BOTH CAN BE USED.
THIS IS KNOWN AS THE DELOCALISED MODEL.
BENZENE STABILITY.
BENZENE IS ACTUALLY MORE STABLE THAN THE THEORETICAL ALTERNATIVE CYCLOHEXA-1,3,5-TRIENE.
THE ALTERNATING SINGLE AND DOUBLE BOND MODEL.
HOW DO WE MEASURE THE STABILITY OF BENZENE?
WE MEASURE THE STABILITY OF BENZENE BY COMPARING THE ENTHALPY CHANGE OF HYDROGENATION IN BENZENE AND CYCLOHEXA-1,3,5-TRIENE.
IF WE HYDROGENATE CYCLOHEXENE IT HAS THE ENTHALPY CHANGE OF -120kLmol^-1, CYCLOHEXENE HAS ONE,1, DOUBLE BOND.
IF BENZEE HAS THREE,3, DOUBLE BONDS WE WOULD EXPECT AN ENTHALPY CHNAGE OF HYDROGENATION OF 360kJmol^-1.
BENZENE, REAL ENTHALPY CHANGE OF HYDROGENATION.
BENZENE HAS A PREDICTED ENTHALPY CHANGE OF HYDROGENATION OF -360kJmol^-1.
HOWEVER, WHEN WE MEASURE THE ENTHALPY CHANGE OF HYDROGENATION FOR BENZENE IT IS FAR LOWER AT -208kJmol^-1.
THIS IS THE EXPERIMENTAL VALUE.
ENERGY IS REQUIRED TO BREAK BONDS AND ENERGY IS RELEASED TO FORM BONDS.
THIS SUGGETS MORE ENERGY IS REQUIRED TO BREAK BONDS IN BENZENE THAN CYCLOHEXA-1,3,5-TRIENE.
LESS EXOTHEMIC, AS LESS HEAT IS GIVEN OUT FROM BOND FORMATION.
AS MORE ENERGY IS TAKEN IN FOR BOND BREAKING.
THIS SUGGESTS THAT BENZENE IS MORE STABLE THAN THE THEORETICLA CYCLOHEXA-13,5-TRIENE WITH THREE,3, DOUBLE BONDS.
THE STABILITY IS DUE TO THE DELOCALISED ELECTRON STRUCTURE.
WHAT IS BENZENE?
BENZENE IS A HYDROCARBON.
BENZENE BURNING WITH OXYGEN.
BENZENE BURNS READILY IN OXYGEN.
BENZENE BURNS IN OXYGEN TO PRODUCE CARBON DIOXIDE AND WATER IF BURNED COMPLETELY.
THIS IS NO DIFFERENT TO BURNING A STANDARD HYDROCARBON.
THE REACTION FOR THE COMPLETE COMBUSTION OF BENZENE IS:
2C6H6 + 15O2 -> 12CO2 + 6H2O.
INCOMPLETE COMBUSTION.
IN REALITY CARBON DOES NOT BURN COMPLETELY AS THERE IS NOT ENOUGH OXYGEN IN THE AIR.
AS A RESULT WE GET A LOT OF UNREACTED CARBON ATOMS, SOOT, AND A BLACK SMOKY FLAME IS OBSERVED.
ADDING BROMINE TO AN ALKENE.
ALKENES HAVE A DOUBLE BOND AND UNDERGO ELECTROPHILIC ADDITION.
ADDING BROMINE WATER TO AN ALKENE CAUSES A COLOUR CHANGE FROM BROWN-ORANGE TO COLOURLESS.
BROMINE, BROWN-ORANGE, IS THE ELECTROPHIE AND ADDS TO THE ALKENE FORMING A DIBROMOALKANE, COLOURLESS.
THIS IS BECAUSE BROMINE IS A DIATOMIC MOLECULE, Br2.
ADDING BROMINE TO AN ALKENE THE MECHANISM.
Br2 IS POLARISED AS THE ELECTRONS IN THE DOUBLE BOND REPELS ELECTRONS IN Br2.
AN ELECTRON PAIR IN THE DOUBLE BOND IS ATRACTED TO THE DELTA POSITIVE BROMINE AND FORMS A BOND.
THIS BREAKS THE Br-Br BOND.
A CARBOCATION INTERMEDIATE IS FORMED AND Br- IS ATTRACTED TO C+.
COLOURLESS DIBROMOALKANE FORMED.
ELECTROPHILE IN THE ADDITION OF BROMINE TO AN ALKENE.
BROMINE IS THE ELECTROPHILE AS IT ATTRACTED TO THE REGION OF HIGH ELECTRONDENSITY IN THE DOUBLE BOND.
ARENES REACTION.
WHEN ARENES REACT THEY UNDERGO ELECTROPHILIC SUBSTITUTION REACTIONS.
WHY IS BENZENE ATTRACTED TO ELECTROPHILES?
BENZENE HAS A HIGH ELECTRON DENSITY AS IT HAS A DELOCALISED RING OF ELECTRONS.
THIS IS ATTRACTIVE TO ELECTROPHILES, ELECTRON LOVING SUBSTANCES.
BENZENE HIGH ELECTRON DENSITY.
BENZENE HAS A HIGH ELECTRON DENSITY AS IT HAS A DELOCALISED RING OF ELECTRONS.
WHY CAN BENZENE NOT UNDERGO ELECTROPHILIC ADDITION REACTIONS?
AS WE HAVE SEEN BENZENE IS STABLE SO UNLIKE TRADITIONAL AKENES THEY DO NOT UNDERGO ELECTROPHILIC ADDITION REACTIOS, UNLIKE THE BROMINATION OF ALKENES, AS THIS WOULD DISRUPT THE STABLE RING OF ELECTRONS.
ELECTROPHILIC SUBSTITUTION OF BENZENE.
INSTEAD OF UNDERGOING ELECTROPHILIC ADDITION THEY UNDERGO ELECTROPHILIC SUBSTITUTION REACTIONS, WHERE A HYDROGEN OR A FUNCTIONAL GROUP ON THE BENZENE RING IS SUBSTITUTED FOR THE ELECTROPHILE.
HOW MANY MECHANISMS ARE THERE FOR THE ELECTROPHILIC SUBSTITUTION REACTION OF ARENES?
THERE ARE FOUR,4, MECHANISMS YOU NEED TO KNOW:
FRIEDEL-CRAFTS ACYLATION,
FRIEDEL-CRAFTS ALKYLATION,
HALOGENATION REACTION,
NITRATION REACTION.
AROMATIC COMPOUNDS.
AROMATIC COMPOUNDS ARE MOLECULES THAT CONTAIN A BENZENE RING, THEY ARE ALSO KNOWN AS ARENES.
NAMING AROMATIC COMPOUNDS.
THEY ARE NAMED IN TWO,2, WAYS.
WE CAN NAME THE BENZENE AT THE END.
FOR EXAMPLE, BROMOBENZENE.
WE CAN USE THE WORD PHENYL TO NAME THEM.
HERE WE USE PHENYL AS IF IT IS A FUNCTIONAL GROUP, C6H5.
FOR EXAMPLE.
C6H5OH,
PHENOL.
C6H5NH2.
PHENYLAMINE.
ELECTROPHILIC SUBSTITUTION BENZENE, CONDITIONS.
THE DELOCALISED ELECTRONS IN THE BENZE RING ARE ATTRACTED TO THE CARBOCATION.
TWO,2, ELECTRONS MOVE TO FORM A BOND WHICH BREAKS THE RING AND A POSITIVE CHARGE DEVLOPS.
THE ELCTRONS IN THE C-H BOND THEN MOVE TO NEUTRALISE THE POSITIVE CHARGE AND RE-FORM THE RING.
HYDROGEN IS SUBSTITUTED.
ELECTROPHILE TYPE, ELECTROPHILIC SUBSTITUTION BENZENE.
BENZENE RINGS ARE STABLE MOLECULES SO REACTIONS ARE DIFFICULT.
WE NEED A VERY STRONG ELECTROPHILE TO REACT.
THESE CAN BE CREATED BY USING A HALOGEN CARRIER CATALYST.
HALOGEN CARRIERS, BASE.
HALOGEN CARRIERS ARE USUALLY ALUMINIUM HALIDES, IRON AND IRON HALIDES.
FOR EXAMPLE, AlCl3.
BENZENE USE.
BENZENE IS USED WIDELY IN PHARMACEUTICALS AND DYE STUFFS HOWEVER DUE TO THE STABILITY OF BENZENE IT IS DIFFICULT TO REACT.
FRIEDEL-CRAFTS REACTIONS CAN HELP TO SOLVE THIS PROBLEM.
FRIEDEL-CRAFTS DISCOVERIES.
CHARLES FRIEDEL AND JAMES CRAFTS CAME UP WITH A REACTION WHERE AN ACYL GROUP, RCO-, OR ALKYL GROUP, R-, IS ADDED ONTO A BENZENE MOELCULE.
AFTER THE ACYL OR ALKYL GROUP IS ADDED THE BENZENE STRUCTURE IS WEAKER AND IT MAKES IT EASIER TO MODIFY IT FUTHER TO MAKE USEFUL PRODUCTS.
ADDING THE ACYL OR ALKYL GROUP ONTO THE BENZENE MOLECULE.
IN ORDER TO ADD ONTO THE BENZENE RING THE ELECTROPHILE MUST HAVE A VERY STRONG POSITIVE CHARGE.
ACYL GROUPS HAVE A POSITIVE CHARGE HOWEVER IT IS NOT POSITIVE ENOUGH.
THE USE OF A HALOGEN CARRIER.
WE CAN USE A HALOGEN CARRIER TO ACT AS A CATALYST, FOR EXAMPLE AlCl3, WHICH WILL PRODUCE A MUCH STRONGER ELECTROPHILE WITH A STRONGER POSITIVE CHARGE.
FRIEDEL-CRAFTS ACYLATION OR ALKYLATION.
IN THE FRIEDEL-CRAFTS ACYLATION OR ALKYLATION WE HAVE TO REACT AN ACYL CHLORIDE OR HALOGENOALKANE WITH THE HALOGEN CARRIER, AlCl3, TO CREATE A STRONGLY POSITIVE ELECTROPHILE.
FRIEDEL-CRAFTS ACYLATION REACTION, MAKING THE ELECTROPHILE.
THE HALOGEN CARRIER, ALCl3, ACCEPTS A PAIR OF ELECTRONS AWAY FROM THE ACYL GROUP.
AS A RESULT, THE POLARISATION INCREASES AND A CARBOCATION IS FORMED AND SO IS AlCl4-.
A STRONGER ELECTROPHILE IS PRODUCED WHICH CAN NOW REACT WITH BENZENE.
FRIEDEL-CRAFTS ACYLATION, REACTING THE ELECTROPHILE WITH THE BENZENE.
NOW WE HAVE MADE THE EELCTROPHILE WE NEED TO REACT IT WITH BENZENE TO MAKE A LESS STABLE PHENYLKETONE UNDER REFLUX AND A DRY ETHER SOLVENT.
THE ELECTROPHILE IS ADDED TO THE BENZENE RING.
THE DELOCALISED ELECTRONS ARE ATTRACTED TO THE CARBOCATION, OF THE ELECTROHPILE.
TWO,2, ELECTRONS MOVE TO FORM A BOND WHICH BREAKS THE RING AND A POSITIVE CHARGE DEVELOPS.
THE NEGATIVE, AlCl4-, IS THEN ATTRACTED TO THE POSITIVELY CHARGED RING AND ONE OF THE CHLORINE ATOMS BREAKS AWAY TO FORM A BOND WITH THE HYDROGEN.
THE ELECTRONS IN THE C-H BOND MOVE TO NEUTRALISE THE POSITIVE CHARGE AND RE-FORM THE RING.
WE ALSO FORM HCl AND AlCl3 CATALYST IS REFORMED.
MAKING THE ELECTOPHILE, FOR FRIEDELS-CRAFTS ALKYLATION REACTION.
TO MAKE THE POWERFUL ELECTROHPILE WE USE AlCl3 AS THE HALOGEN CARRIER JUST LIKE IN THE ACYLATION REACTION.
AlCl3 ACCPETS A PAIR OF ELECTRONS AWAY FROM THE HALOGENOALKANE.
AS A RESUST, A CARBOCATION IS FORMED.
A STRONGER ELECTROPHILE IS PRODUCED WHICH NOW REACT WITH BENZENE.
WE ALSO FORM AlCl4- AS A PRODUCT.
THE ELECTROPHILES REACTION WITH BENZENE, FRIEDELS-CRAFTS REACTION.
NOW WE HAVE MADE THE ELECTROPHILE WE NEED TO REACT IT WITH BENZENE TO MAKE A LESS STABLE ALKYLBENZENE UNDER REFLUX AND DRY ETHER SOLVENT.
THE DELOCALISED ELECTROSN ARE ATTRACTED TO THE CARBOCATION.
TWO,2, ELECTRONS MOVE TO FORM A BOND WHICH BREAKS THE RING AND A POSITIVE CHARGE DEVELOPS.
THE NEGATIVE HALOGEN CARRIER, AlCl4- IS THEN ATTRACTED TO THE POSITIVELY CHARGED RING AND ONE THE CHLORINE ATOMS BREAKS AWAY TO FORM A BOND WITH THE HYDROGEN.
THE ELECTRONS IN THE C-H BOND MOVE TO NEUTRALISE THE POSITIVE CHARGE AND REFORM THE RING.
WE ALSO FORM HCl AS A PRODUCT AND AlCl3 CATALYST REFORMED.
ALCOHOL ADDED TO A BENZENE RING.
ALCOHOL BASED GROUPS CAN ALSO BE ADDED TO A BENZENE RING TOO.
IF WE USE AN ELECTROPHILE THAT CONTAINS AN ALKYL CHAIN WITH OAlCl3- THEN THIS CAN ADD AN ALCOHOL BASED GROUP TO THE BENZENE RING.
THIS WORKS IN A SIMILAR WAY TO FRIEDEL-CRAFTS REACTIONS AS THE OXYGEN GROUP HAS A LONE PAIR OF ELECTRONS WHICH IT TO ACT AS A NUCLEOPHILE.
WE ALSO FORM AlCl3.
NITRATING BENZENE USE.
NITRATING BENZENE IS USEFUL AS IT ALLOWS US TO MAKE DYES FOR CLOTHING AND EXPLOSIVES.
HOW DO WE NITRATE BENZENE?
IF WE HEAT BENZENE WITH CONCENTRATED NITRIC ACID, HNO3, AND SULFURIC ACID, H2SO4, WE FORM NITROBENZENE.
HOWEVER LIKE WE HAVE SEEN BEFORE WE HAVE TO MAKE A POWERFUL ELECTROPHILE FIRST.
ELECTROPHILE, NITRATING BENZENE.
TO MAKE THE ELECTROPHILE WE REACT SULPHURIC ACID WITH NITRIC ACID.
HNO3 + H2SO4 -> H2NO3 + + HSO4-.
THE H2NO3 + DECOMPOSES TO FORM THE ELECTROPHILE, NITRONIUM ION NO2 +.
H2NO3 + -> NO2+ + H2O.
USING THE ELECTROPHILE TO NITRATE BENZENE.
THE NITRONIUM ION IS ATTACKED BY THE BENZENE RING FORMING AN UNSTABLE, POSITIVELY CHARGED RING.
THE ELECTRONS IN THE C-H BOND MOVE TO REFORM THE DELOCALISED ELECTRON RING.
NITROBENZENE IS FORMED AND A H+ IS FORMED WHICH REACTS WITH HSO4- TO MAKE H2SO4 AGAIM.
H2SO4 IS A CATALYST.
NITRATING BENZENE, TEMPERATURE.
A TEMPERATURE BELOW 55 DEGREES CELSIUS WILL ENSURE A SINGLE NO2 SUBSTITUTION.
ABOVE THIS WILL RESULT IN MULTIPLE SUBSTITUTIONS.
PHENOLS.
PHENOLS HAVE A HYDROXYL GROUP, -OH, ATTACHED TO A BENZENE RING.
THE CARBON WITH THE, -OH, GROUP IS THE CARBON ONE,1.
WE NUMBER OTHER GROUPS FROM THIS.
PHENOLS REACTIVITY.
PHENOLS ARE REACTIVE THAN BENZENE DUE TO THE ELECTRON DENSITY IN THE RING BEING HIGHER.
ELECTROPHILIC SUBSTITUTION REACTIONS ARE MOR ELIKELY TO OCCUR WITH PHENOL THAN WITH BENZENE DUE TO THE -OH GROUP AND ORBITAL OVERLAP.
THE ELECTRONS IN THE P-ORBITAL OF THE OXYGEN OVERLAP WITHT HE DELOCALISED RING STRUCTURE AND SO THEY ARE PARTIALLY DELOCALISED INTO THE PI-SYSTEM.
THE ELECTRON DENSITY INCREASES WITHIN THE RING STRUCTURE AND SO IS MORE SUSCEPTIBLE TO ATTACK FROM ELECTROPHILES.
ASPIRIN.
ASPIRIN IS AN ESTER AND IS MADE BY REACTING ETHANOIC ANHYDRIDE OR ETHANOYL CHLORIDE AND SALICYLIC ACID.
MAKING ASPIRIN USING ETHANOIC ANHYDRIDE.
ETHANOIC ANHYDRIDE + SALICYLIC ACID -> ASPIRIN + ETHANOIC ACID.
THE USE OF ETHANOIC ANHYDRIDE COMPARED TO THE USE OF ETHANOYL CHLORIDE.
ETHANOIC ANYHDRIDE IS USED INSTEAD OF ETHANOYL CHLORIDE IN INDUSTRY BECAUSE:
IT IS SAFER AS IT IS LESS CORROSIVE.
IT IS CHEAPER.
IT IS SAFER AS IT DOES NOT PRODUCE HARMFUL HCl GAS.
IT IS SAFER AS IT DOES NOT REACT VIGOROUSLY WITH WATER.