aromatic chemistry Flashcards
what does aliphatic mean?
straight or branched chain organic substances
what does aromatic/arene mean?
includes one or more ring of six carbon atoms with delocalized bonding
what is the simplest arene?
benzene
the molecular formula of benzene
C6H6
Describe the structure of benzene
- planar cyclic structure
- each C atom bonded to two other C atoms and 1 H atom by single sigma bond - leaves one unused electron on each C atom in a P orbital, perpendicular to the plane of the ring
p-orbitals (on each C atom) combine & form ring of delocalized electrons.
What does the circle on the benzene structure represent?
delocalized ring of electrons
What is the structure of cyclohexa-1,3,5-triene?
ring made up of alternating single and double bonds
describe why you’d expect an enthalpy of hydrogenation of cyclohexa-1,3,5-triene to be -360kJ mol-1 and explain why it’s not
- cyclohexene hydrogenated enthalpy change -120kJ mol-1 so you’d expect hydrogenation of cyclohexa-1,3,5-triene to be -360 but its -208
- energy is put in to break bonds and released when bonds made. so more energy must have been put in to break bonds in benzene than would be needed to break bonds in theoretical cyclohexa-1.,3,5-triene
- difference indicates benzene is more stable than cyclohexa-1,3,5-triene would be. This is due to delocalized ring of electrons (delocalisation energy)
length of the C-C bond in the ring
they’re the same length (140pm)
between single C-C bonds (154) and double C=C bond (135)
what are aromatic compounds?
derivates from benzene
Draw out (1)chlorobenzene, (2) nitrobenzene and (3) 1,3-dimethybenzene, (4) benzenecarboxylic acid/benzoic acid and (5) benzaldehyde, (6) phenol and (7) phenylamine
(1) https://www.google.com/search?q=chlorobenzene&rlz=1C1CHBF_enGB856GB856&oq=chlorobenzene&aqs=chrome..69i57j0l5.3019j1j1&sourceid=chrome&ie=UTF-8
(2) https://www.google.com/search?q=nitrobenzene&rlz=1C1CHBF_enGB856GB856&sxsrf=ACYBGNScEs_-LeAQbEDpEPa6DzqTyfYCRg:1573495547263&source=lnms&tbm=isch&sa=X&ved=0ahUKEwibzZnZ3-LlAhVzpHEKHRBDAZ0Q_AUIEigB&biw=1536&bih=754&dpr=1.25#imgrc=cQCvEdsX_Kz4qM:
(3) https://www.google.com/search?q=1,3-dimethylbenzene&rlz=1C1CHBF_enGB856GB856&sxsrf=ACYBGNQvhYeo6uKODH5PCbHSU_HgRk_i8Q:1573495598274&source=lnms&tbm=isch&sa=X&ved=0ahUKEwi5hcPx3-LlAhVIWxUIHW_pCmoQ_AUIEigB&biw=1536&bih=754#imgrc=MMK1rIGJKf8sGM:
(4) https://www.google.com/search?q=benzenecarboxylic+acid&rlz=1C1CHBF_enGB856GB856&sxsrf=ACYBGNTwYKDmZFXacg151-Soc5Q2BYWy0w:1573495642291&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiKysGG4OLlAhXTtXEKHUq2Dw4Q_AUIEigB&biw=1536&bih=754&dpr=1.25#imgrc=9B5CrBJH_slbPM:
(5) https://www.google.com/search?q=benzaldehyde&rlz=1C1CHBF_enGB856GB856&sxsrf=ACYBGNQEQeiXg0NbKlPlxdQfufJFRANV5w:1573495706725&source=lnms&tbm=isch&sa=X&ved=0ahUKEwi1pZ6l4OLlAhU4SBUIHV5wB1QQ_AUIEigB&biw=1536&bih=754#imgrc=m9y-Gr3bRxLeDM:
(6) https://www.google.com/search?q=phenol&rlz=1C1CHBF_enGB856GB856&sxsrf=ACYBGNQ8nQlUB0Ru9pPyA6U1vIFRcqBi3A:1573495732995&source=lnms&tbm=isch&sa=X&ved=0ahUKEwj94eGx4OLlAhWRsnEKHQEGD0sQ_AUIEigB&biw=1536&bih=754&dpr=1.25#imgrc=PNRcGl07KY8VeM:
(7) https://www.google.com/search?q=phenylamine&rlz=1C1CHBF_enGB856GB856&sxsrf=ACYBGNTCUloWS8BrfA5bSehHxEit8_xlhQ:1573495771363&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjJuYfE4OLlAhVDt3EKHU8DAYoQ_AUIEigB&biw=1536&bih=754#imgrc=HCBpjrqeQCWOFM:
compounds containing a benzene ring are called what?
arenes or aromatic compounds
benzene ring is a region of what?
high electron density so it attracts electrophiles (electron-pair acceptors)
why don’t benzenes undergo electrophillic addition?
benzene ring is very stable
electro add.. would destroy delocalised ring of electrons
- instead, it undergoes electrophilic substitution reactions one of the H atoms (or another functional group on ring) is substituted for the electrophile
what are the two electrophilic substitution mechanisms for benzene?
- Friedel-Crafts acylation
- nitration reaction
useful chemicals that contain benzene rings
dyes
pharmaceuticals
is benzene reactive?
its fairly UNreactive because it’s very stable
What do Friedel-Crafts acylation reactions do?
add an acyl group (RCO-) to the benzene ring
- then side chains modified using further reactions to make useful products
Properties of the electrophile in a friedel-crafts acylation reaction?
- has to have a strong +ve to be able to attack stable benzene ring - most arent polarised enough
- so made into stronger electrophiles using a catalyst called a halogen carrier e.g. AlCl3
electrophile in a friedel-crafts acylation reaction?
acyl chloride and a halogen carrier (AlCl3)
How does the AlCl3 make the acyl chloride electrophile stronger?
- AlCl3 accepts LP of electrons from the acyl chloride
as lone pair of electrons is pulled away, the polarisation in the acyl chloride increases and it forms a carbocation - makes it a much stronger electrophile & it gives it a strong enough charge to react with benzene ring
describe the electrophilic substitution mechanism in benzene
- electrons in benzene ring attracted to +ve charged carbocation
- two electrons from benzene bond with the carbocation (partially breaks delocalized ring & gives it a positive charge)
- negatively charged AlCl4- ion is attracted to +ve charged ring. one Cl- breaks away from aluminium chloride ion and bonds with Hydrogen ion
- this removes hydrogen from the ring forming HCl - also allows the catalyst to reform
conditions of the electrophilic substitution in benzene
- heat under reflux
- non-aqueous solvent (e.g. dry ether)
What do you get when warm benzene with conc. nitric and sulfuric acids ? what does the sulfuric acid act as?
you get nitrobenzene
- sulfuric acid acts as a catalyst ( helps to make the nitronium ion, NO2+, the electrophile)
Describe electrophillic substitution mechanism for nitrobenzenes
- nitronium ion attacks the benzene ring
- unstable intermediate forms
- H+ ion is lost
- H+ reacts with HSO4- to reform catalyst H2SO4
what happens if temp is above 55 degrees Celsius
- if you only want 1 NO2 group added (mononitration), you need to keep the temp below 55 degrees
- above this you’ll get loads of substitutions
why are nitration reactions useful?
- nitro compounds can be reduced to form aromatic amines (used to manafacture dyes & pharaceuticals)
- some nitro compounds can be used as explosives e.g. 2,4,6-trinitromethylbenzene (TNT)
problems with Kekule structure in terms of shape
- C-C single and C=C are different lengths
- means that the hexagon would be irregular if it had the Kekulé structure (alternating shorter and longer sides)
- in real benzene all the bonds are exactly the same - intermediate in length between C-C and C=C
