Topic B6 - Organics (Benzene and derivatives)
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Benzene can be described as…
an aromatic hydrocarbon or arene
What was Kekule’s model?
benzene was based on a 6 membered ring of carbon atoms, joined by alternate single and double bonds
How does the lack of reactivity of benzene disprove Kekule’s model?
Benzene does not undergo electrophilic addition reactions (does not decolourise bromine under normal conditions) Suggests there is no C=C bond
How does the length of the bonds in benzene disprove Kekule’s model?
X-ray diffraction proved that all the bonds in benzene were between the length of a single and double bond
How does the hydrogenation enthalpies show that Kekule’s model is false?
enthalpy of hydrogenation is less exothermic than expected, hence benzene must be more stable than kekule’s model
What is the delocalised model of Benzene?
-planar, cyclic, hexagonal hydrocarbon containing 6 carbo atoms ad 6 hydrogen atoms -each carbon uses 3 of its 4 available electrons to bond to 2 adjacent carbons and 1 hydrogen -each carbon has 1 electron in p-orbital at right angles to plane of bonded carbon and hydrogen atoms -adjacent p-orbitals overlap sideways (above and below plane of carbon atoms to form a ring of electron density) -overlapping of p-orbitals creates a system of pi-bonds -6 electrons in the system of pi-bonds are delocalised
What is the molecular formula of benzene?
C6H6
Are delocalised electrons in benzene shared between more than 2 atoms?
Yes
What does the circle in a benzene skeletal structure demonstrate?
a delocalised pi bond
How does the bonding arrangement of benzene come about?
-p-orbitals overlap to form a pi bond -pi bond extends over all 6 carbon atoms of the ring -pi bond is a double ring of electron density above and below the carbon atoms
What are the reactants required for nitration of benzene?
concentrated sulfuric acid concentrated nitric acid
What can be synthesised from nitrobenzene?
-dyes -pesticides -pharmaceuticals
What catalyst is used for the acylation/ alkylation of benzene?
anhydrous AlCl3
Why is benzene susceptible to attacks by electrophiles?
due to the high electron density of the delocalised pi bond
What is a phenol?
an organic compound with a hydroxyl group directly bonded to an aromatic ring (benzene)
Why is a phenol ring more reactive than a simple benzene ring?
-lone pair of electrons from oxygen p-orbital is donated into the pi system of delocalised electrons of benzene -electron density of benzene ring increases -electrophiles are attracted more strongly -aromatic ring is therefore more susceptible to attack from electrophiles
Why has phenol got a higher MP than methyl benzene?
-phenol ring can form hydrogen bonds with adjacent molecules, due to lone pair of electrons on oxygen and polar O-H bond -methyl benzene can only form weak London Forces -as hydrogen bonds are stronger than London Forces, mor energy is required to overcome hydrogen bonds -hence, phenol has a greater MP
Is phenol more or less soluble than alcohols?
less soluble - due to presence of non-polar benzene ring
What happens when phenol dissolves in water?
phenol partially dissociates - classified as a weak acid forms phenoxide ion + H+
What conditions are required for the nitration of phenol?
-dilute nitric acid -room temp
What are the products of bromination of phenol?
-2,4,6-tribromophenol -hydrogen bromide
What are the conditions for the bromination of phenol?
-room temp -no catalyst
What are the observations of the bromination of phenol?
-orange —> colourless (bromine water is decolourised) -white ppt forms (products are insoluble)
How can you test for a phenol vs carboxylic acid?
-react with sodium carbonate -carboxylic acid reacts to produce carbon dioxide (gas)