Organic Chemistry - 1

• Describe the structure of benzene

  Benzene is composed of a hexanol ring that contains 6 carbon atoms. Benzene has the molecular formula C6H6

  -> It is the simplest arene (arenes are aromatic hydrocarbons that contain a benzene ring) and has a planar ring structure.

  
• The Kekulé model


  In 1865, the German Chemist Friedrich August Kekulé suggested that the structure of benzene was based on a six membered ring of carbon atoms joined by alternate single and double bonds
• What is the experimental evidence to disprove of Kekulé's model?

  The lack of reactivity of benzene

  The lengths of the carbon-carbon bonds in benzene

  Hydrogenation enthalpies
• The lack of reactivity of benzene

  If benzene contained C=C bonds, it should decolourise bromine in an electrophilic addition reaction.

  -> However, benzene does not undergo electrophilic addition reactions and benzene does not decolourise bromine under normal conditions

  This has therefore led scientists to believe benzene cannot have C=C bonds in its structure

  
• The lengths of the carbon-carbon bonds in benzene

  Using X-ray diffraction, it is possible to measure bond lengths in a molecule. When benzene was examined in 1929, it was found that all bonds in benzene were 0.139nm in length. This bond length was between the length of a single bond (0.153nm) and a double bond (0.134nm)

  
• Hydrogenation enthalpies


  The Kekulé structure (which contains alternate single and double bonds) could be given the name cyclohexa-1,3,5-triene to indicate the positioning of the double bonds

  -> If benzene did have the Kekulé structure, then it would be expected to have an enthalpy change of hydrogenation that is 3 times that of cyclohexene. When cyclohexene is hydrogenated, one double bond reacts with hydrogen. The enthalpy change of hydrogenation is -120kJmol-1

  -> As the Kekulé structure is predicted to contain three double bonds the expected enthalpy change for reacting three double bonds with hydrogen would be -360kJmol-1 (3 x -120)

  -> The actual enthalpy change of benzene is only -208kJmol-1. This means that 152kJmol-1 less energy is produced than expected. The actual structure of benzene is therefore more stable than the theoretical Kekulé model of benzene.

  
• What are the main features of the delocalised model of benzene?

  -> Benzene is a planar, cyclic, hexagonal hydrocarbon containing six carbon atoms and six hydrogen atoms

  -> Each carbon atom uses 3 of its available 4 electrons in bonding to two other carbon atoms and to one hydrogen atom

  -> Each carbon atom has one electron in a p-orbital at right angles to the plane of the bonded carbon and hydrogen atoms

  -> Adjacent p-orbital electrons overlap sideways (in both directions) above and below the plane of the carbon atoms to forma rind of electron density

  -> This overlapping of the p-orbitals creates a system of pi-bonds (π-bonds) which spread over all six of the carbon atoms in the ring structure

  -> The six electrons occupying this system of pi-bonds (π-bonds) are said to be delocalised

  
• Naming aromatic compounds


  Aromatic compounds with one substituent group are monosubstituted

  -> In aromatic compounds, the benzene ring is often considered to be the parent chain. Alkyl groups, halogens and nitro groups are all considered the prefixes to benzene

  -> When a benzene ring is attached an alkyl chain with a functional group or to an alkyl chain with 7 or more carbons, benzene is considered to be a substituent. Instead of benzene, the prefix phenyl is used.

  -> REMEBER: benzoic acid, phenylamine and benzaldehyde

  
• Electrophilic substitution reactions of benzene

  Nitration of benzene

  Halogenation of benzene

  Alkylation reactions

  Acylation reactions

  
• Nitration of benzene


  Benzene reacts slowly with nitric acid to form nitrobenzene.

  Conditions -> Catalysed by sulfuric acid (H2SO4) and heated to 50°C to obtain a good rate of reaction.

  In nitration, one of the hydrogen atoms on the benzene ring is replaced by a nitro, -NO2, group.

  -> The electrophile is nitronium ion, NO2+, produced by the reaction of concentrated nitric acid with concentrated sulfuric acid.

  -> If the temperature of the reaction rises above 50°C, further substitution reactions may occur leading to multi -NO2 substituted onto benzene ring

  
• Mechanism for nitration of benzene

  NO2+ is the electrophile and is produced by the reaction of concentrated nitric acid with concentrated sulfuric acid

  HN03 + H2SO4 --> NO2+ + HSO4- + H20

  -> The electrophile (NO2+) accepts a pair of electrons from the benzene ring to form a dative covalent bond.

  -> The organic intermediate formed is unstable and breaks down to form the organic product, nitrobenzene, and the H+ ion. A stable benzene ring is reformed

  -> Finally, the H+ ion formed reacts with the HSO4- ion to regenerate the catalyst H2SO4.

  
• Halogenation of benzene


  Halogens do not react with benzene unless a catalyst (called a halogen carrier) is present.

  Common halogen carriers include: AlCl3, FeCl3, AlBr3, FeBr3 which can be generated in situ (in the reaction vessel) from the metal and the halogen

  Examples include the bromination and chlorination of benzene

  
• Bromination of benzene


  Benzene reacts with bromine in an electrophilic substitution reaction.

  -> In bromination, one of the hydrogen atoms on benzene ring is replaced by a bromine atom

  -> The electrophile is the bromonium ion, Br+ which generated when the halogen carrier catalyst reacts with bromine in the first stage of the mechanism

  -> The bromonium ion accepts a pair of electrons from the benzene ring to form a dative covalent bond. The organic intermediate is unstable and breaks down to form the organic product bromobenzene, and an H+ ion

  
• Chlorination of benzene


  Chlorine will react with benzene in the same way a bromine (same mechanism). The halogen carrier used is FeCl3 or AlCl3

  
• Alkylation of benzene (Friedel-Crafts alkylation)

  The alkylation of benzene is the substitution of hydrogen atom in the benzene ring by an alkyl group.

  -> Reaction is carried out by reacting benzene with a haloalkane in the presence of AlCl3, which acts as a halogen carrier catalyst, generating the electrophile.

  -> Conditions: Akyl chloride (RCl), halogen carrier catalyst, e.g. AlCl3, Anhydrous conditions

  
• Acylation of benzene


  When benzene reacts with acyl chloride in the presence of an AlCl3 catalyst, an aromatic ketone is formed.

  -> Conditions: Acyl chloride (RCOCl), halogen carrier catalyst, e.g. AlCl3, Anhydrous conditions

  
• Reaction between cyclohexene and bromine

  Alkenes decolourise bromine by an electrophilic addition reaction

  -> The π-bond in the alkene contains localised electrons above and below the plane of the two carbon atoms in the double bond. This produces an area of high electron density

  -> The localised electrons in the π-bond induce a dipole in the non-polar bromine molecule making one bromine atom of the Br2 molecule slightly positive and the other bromine atom slightly negative

  -> The slightly positive bromine atom enables the bromine molecule to act like an electrophile

  
• Comparing the reactivity of alkenes with arenes

  Benzene does not react with bromine unless a halogen carrier catalyst is present (unlike alkenes). This is because benzene has delocalised π-electrons spread above and below the plane of the carbon atoms in the ring structure. The electron density around any two carbon atoms in the benzene ring is less than that in a C=C

  -> When a non-polar molecule such as bromine approaches the benzene ring, there is insufficient π-electron density around any two carbon atoms to polarise the bromine molecule. This prevents any reaction taking place.
• Phenol


  Phenol has the formula C6H5OH. Phenol is an aromatic benzene ring attached to a hydroxyl (-OH) group

  
• Why is phenol more reactive than benzene?

  The electron-donating -OH group activates the aromatic ring, making it more susceptible to electrophilic attack.

  -> One of the lone pairs of electrons in a p-orbital of the oxygen atom overlaps with delocalised π system of electrons in the benzene ring

  -> This allows the lone pair to partially delocalise into the aromatic π-system, increasing the electron density within the ring.

  -> A higher electron density makes the ring more reactive towards electrophiles

  
• Phenol as a weak acid

  Phenol is less soluble in water than alcohols due to the presence of the non-polar benzene ring. When dissolved in water, phenol partially dissociates forming the phenoxide ion and a proton. Because of this ability to partially dissociate to produce protons, phenol is classified as a weak acid. Similarly, other phenols act as weak acids.

  -> Phenol is more acidic than alcohols but less acidic than carboxylic acids - use the acid dissociation constant, Ka of an alcohol with a phenol and a carboxylic acid.

  
• Give an example

  -> Ethanol does not react with sodium hydroxide (a strong base) or sodium carbonate (a weak base)

  -> Phenols and carboxylic acids react with solutions of strong bases such as aqueous sodium hydroxide

  -> Only carboxylic acids are strong enough acids to react with the weak base, sodium carbonate
• How do you distinguish between a phenol and carboxylic acid?

  When the carboxylic acid reacts with sodium carbonate, it produces carbon dioxide, which is evolved as a gas. Whereas, phenols do not produce carbon dioxide

  
• Reaction of phenol with sodium hydroxide (phenol + bases)?

  Phenol reacts with sodium hydroxide to form the salt, sodium phenoxide, and water in a neutralisation reaction

  
• Give examples of electrophilic substitution reactions of phenol

  Bromination of phenol

  Nitration of phenol

  
• How do phenols react with bromine water (bromination of phenol)?

  Phenol reacts with an aqueous solution of bromine water to form a white precipitate. The substitution occurs at the 2- and 4- positions, producing 2,4,6-tribromophenol. The reaction decolourises the bromine water (orange to colourless)

  -> Remember: With phenol, a halogen carrier catalyst is not required

  
• How do phenols react with dilute nitric acid (nitration of phenol)?

  Direct nitration of phenol yields two main isomers, 2-nitrophenol and 4-nitrophenol, at the 2- and 4- positions respectively

  
• Comparing the reactivity of phenol and benzene

  The increased reactivity of phenol is caused by a lone pair of electrons from the oxygen p-orbital of the -OH group being donated into the π-system of phenol. The electron density of the benzene ring in phenol is increased. The increased electron density attracts electrophiles more strongly than with benzene

  -> The aromatic ring in phenol is therefore more susceptible to attack from electrophiles than in benzene. For bromine, the electron density in the phenol ring structure is sufficient to polarise bromine molecules and so no halogen carrier catalyst is required

  
• RECAP: Phenol can under electrophilic substitution reaction with

  Nitric acid to form two isomers, 2-nitrophenol and 4-nitrophenol.
• Further substitution


  Like phenol, many substituted aromatic compounds can undergo a second substitution (disubstitution).
• Activation and deactivation


  -> (1) Bromine requires a halogen carrier catalyst to react with benzene, whereas bromine will react rapidly with phenylamine.

  -> (2) Nitrobenzene reacts slowly with bromine, requiring both a halogen carrier catalyst and a high temperature. The benzene ring in nitrobenzene is less susceptible to electrophilic substitution than benzene itself

  -> (1) The -NH2 group activates the ring as the aromatic ring reacts more readily with electrophiles.

  -> (2) The -NO2 deactivates the aromatic ring as the ring reacts less readily with electrophiles

  -> The -NH2 group directs the second substituent to positions 2 or 4

  -> The -NO2 group directs the second substituent to position 3

  
• Directing effects


  Different groups can have a directing effect on any second substituent on the benzene ring

  -> All 2- and 4-directing groups are activating groups, with the exception of the halogens

  -> All 3-directing groups are deactivating groups

  
• How are aldehydes and ketones carbonyls?

  They contain the carbonyl functional group which is a carbon-oxygen double bond.
• Aldehydes


  In aldehydes the carbonyl functional group is found at the end of a carbon chain.

  It ends in -al and in its structural formula, it is written as CHO

  -> Always count from the carbon atom with the C=O group

  
• Ketones


  In ketones, the carbonyl functional group is joined to two carbon atoms in the carbon chain.

  It ends in -one and in its structural formula, CO

  -> Ketones often have a number indicating the position of the carbon with the C=O.

  
• Oxidation of aldehydes


  Primary alcohols are oxidised to aldehydes and then further oxidised to carboxylic acids using acidified potassium dichromate (VI), usually as a mixture of sodium/potassium dichromate and dilute sulfuric acid.

  
• What happens to the orange dichromate (VI) ions (Cr2O72-) during oxidation reactions?

  They are reduced to green chromium (III) ions (Cr3+)

  
• How is alcohol oxidation to aldehydes/carboxylic acids controlled for?

  -> Gently heating the alcohol with acidified K2Cr2O7 in a distillation set-up stops at the aldehyde product.

  -> Refluxing the alcohol (or aldehyde) with acidified K2Cr2O7 continues the oxidation to the carboxylic acid.
• Oxidising secondary alcohols to ketones

  Ketones are produced when secondary alcohols are oxidised with K2Cr2O7.

  -> Ketones do not undergo oxidation reactions.

  
• Reduction of aldehydes and ketones to alcohols

  Aldehydes are reduced to primary alcohols

  Ketones are reduced to secondary alcohols

  This is done by using reducing agents such as sodium borohydride (NaBH4) dissolved in water and methanol