Structure Of Benzene
- Problems with Kekulé structure of Benzene:
-Low reactivity: if C=C bonds were present then benzene would react with Br2, similar to alkenes, decolourising bromine water - which doesn't occur.
-Also Kekulé structure contains both C=C and C-C alternating bond, whereas methods have shown that all bond lengths are equal and with length between both the previous.
-Hydrogenation: the three C=C proposed bonds would have a hydrogenation enthalpy of -360kJmol-1 whereas the actual enthalpy is -208kJmol-1 which is less than expected.
- Therefore a delocalised model of benzene was produced, which has the following features:
-Benzene is a cyclic hydrocarbon with 6C's and 6H's.
-The 6C's form a planar hexagonal ring.
-The shape around each C is trigonal planar with bond angle of 120 degrees.
-Each C has 4 outer shell electrons, 3 of which are bonded and 1 is not. The 3 form sigma bonds and the other 1 resides in a 2p orbital above and below the plane.
-The electrons in the p-orbitals overlap and form a ring of electron density above and below.
-This overlap produces a system of pi-bonds, which spread over all 6C's; forming a delocalised ring.
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- Just to illustrate how the orbitals overlap above and below the plane.
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- Under normal conditions Benzene does not:
-Decolourise bromine water
-React with strong acids such as HCl
-React with halogens
- Instead of taking part in addition reactions, Benzene takes part in substitution reactions; which retains the delocalisation and the maintains stability.
- Typically benzene reacts with electrophiles, taking part in electrophilic substitution.
-To recap an electrophile is an atom or group that is attracted to an electron-rich centre; and accepts a pair of electrons to form a new covalent bond.
- Benzene can be nitrated as such:
-C6H6 + HNO3 -> C6H5NO2 by using a H2SO4 catalyst at 50 degrees C under reflux.
- Methylbenzene can also be nitrated to form TNT. Fun fact woo.
- Although benzene does not react with halogens alone, it can with the presence of a halogen carrier:
-C6H6 + Cl2 -------- FeCl3/AlCl3--------> C6H5Cl + HCl
-This is the same for Br2 etc, and thus the carrier changes accordingly (ie. FeBr3)
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- Electrophilic substitution reactions occur in benzene chemistry because of the delocalised ring above and below the plane:
-The electron-dense ring attracts the electrophile, which accepts a pair of e- and forms a covalent bond.
-The intermediate forms that contains both the electrophile and the H atom that is being substituted; the intermediate is unstable though.
-The unstable intermediate rapidly loses H as H+ and stability is restored.
- Reaction mechanim:
- The curly arrows show the movement of an e- pair to form a new covalent bond.
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- A mixture of concentrated HNO3 and H2SO4 is used.
- The sulfuric acid is needed to generate an electrophile from the nitric acid, which is the nitryl cation and has the formula NO2+.
- HNO3 + H2SO4 -> NO2+ + HSO4- + H2O
and reforms: H+ + HSO4- -> H2SO4
(and thus is a catalyst)
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Reactivity Of Cyclohexene
- Cyclohexene and bromine water:
-Acting as a typical alkene and the Br forms across the double bond.
-The Br2 is non-polar but the pi-bond (C=C) contains localised electrons above and below the 2 C's, producing an area of high electron density.
-This induces a dipole in the Br-Br bond; making it polar.
-The pi-electron pair from the C=C bond is attracted to the positive end of Br2
-Causing the C=C bond to break and a new bond to be formed, forming a carbocation
-Finally the Br- ion is attracted towards the intermediate carbocation; forming 1,2-dibromocyclohexane.
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Benzene And Bromine
- Benzene does not react with just Br, requiring Fe filings to catalyse.
-Benzene has delocalised pi-electrons spread over all 6C's, whereas Cyclohexene has localised pi-electrons above and below the 2C's.
-Thus benzene has a lower electron density.
-When a non-polar molecule such as Br approaches the benzene ring there is insufficient electron density to cause necessary polarisation in Br2.
-A halogen carrier is required to make a more powerful electrophile, Br+
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- Class of hydroxyl, -OH, attached to a benzene ring directly.
- Phenol is a pink solid crystal at room temperature; however is is less soluble than most alcohols due to the presence of a benzene ring.
- Reactions with salts:
-Dissolves in H2O to form a weak acidic solution by losing H+ from the -OH group:
C6H5OH + aq <-> C6H5O- + H+
-Phenol is neutralised by aq. NaOH to form a salt:
C6H5OH + NaOH -> C6H5O-Na+ + H2O
- Reactions with Na:
-Metal effervesces forming H2(g), the organic product is C6H5O-Na+ which is a salt:
2C6H5OH + 2Na -> 2C6H5O-Na+ + H2
- Phenol reacts with (3)Br2 without the need for a halogen carrier at room temperature, this is due to the O, from which a lone pair dissociate into the delocalised ring. This activates the ring and so the increased electron density polarises Br2 upon proximity.
- Phenol is used for: detergents, antiseptics, disinfectants, preparation of aspirin, epoxy resins
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