3.6 - Aromatic Compounds And Amines Flashcards
What is benzene?
C6H6. It has a planar cyclic structure. Each carbon atom forms single covalent bonds to the carbons on either side of it and to one hydrogen atom. The final unpaired electron on each carbon atom is located in a p-orbital that sticks out above and below the plane of the ring. The p-orbitals on each carbon atom combine to form a ring of delocalised electrons. All the carbon-carbon bonds on the ring are the same, so they’re the same length - 140pm. This lies in beteeen the length of a single C-C bond and a double C=C bond. Draw it as one mountain on top and bottom, then straight lines. Then circle in the middle. You can also see benzene drawn as alternating double and single bonds, which is wrong but that’s what scientists used to think.
Why is benzene stable?
Benzene is far more stable than the theoretical cyclohexa-1,3,5-triene would be. You can see this by comparing the enthalpy change of hydrogenation for benzene with the enthalpy change of hydrogenation for cyclohexene:
Cyclohexene has one double bond. When it’s hydrogenated, the enthalpy change is -120 kj mol^-1. If benzene had three, you’d expect -360 kj mol^-1. But the experimental enthalpy of hydrog of benzene is -208kj mol^-1 - far less exothermic than expected. Energy is put in to break because bonds and released when bonds are made. So more energy must have been put in to break the bonds in benzene than would be needed to break the bonds in a theoretical cyclohexa-1,3,5-triene molecule. This difference indicates that benzene is more stable than cyclohexa-1,3,5-triene would be. This is thought to be due to the delocalised ring of electrons.
How do you name aromatic compounds?
Compounds containing a benzene ring are called arenes or ‘atomic compounds’. You can name as substituted benzene rings like chloroBenzene, nitrobenzene, 1,3-dimethylbenzene. Others are named as compounds with a phenyl group (C6H5) attached.
What substitution reactions do arenes go through?
The benzene ring is a region of high electron density, so it attracts electrophiles. As the benzene ring’s so stable, it doesn’t undergo electrophilic addition reactions, which would destroy the delocalised ring of electrons. Instead it undergoes electrophilic substitution reactions where one of the hydrogen atoms (or another functional group on the ring) is substituted for the electrophile. You need to know two electrophilic substitutions mechanisms for benzene - friedel-crafts acylation and nitration.
Describe friedel-crafts acylation.
Many useful chemicals such as dyes and pharmaceuticals contain benzene rings. But because benzene is so stable, it’s fairly unreactive - so it can be tricky to make chemicals that contain benzene. Friedel-crafts acylation reactions are used to add an acyl group (RCO-) to the benzene ring. Once an acyl group has been added, the side chains can be modified using further reactions to make useful products. An electrophile has to have a strong positive charge to be able to attack the stable benzene ring - most aren’t polarised enough. But some can be made into stronger electrophiles using a catalyst called a halogen carrier. Friedal-crafts acylation uses an acyl chloride as an electrophile and a halogen carrier. Here’s how the AlCl3 makes the acyl chloride electrophile stronger:
AlCl3 accepts a lone pair of electrons from the acyl chloride. As the lone pair of electrons is pulled away, the polarisation in the acyl chloride increases and it forms a carbocation. This makes it a much stronger electrophile, and gives it a strong enough charge to react with a benzene ring.
What is the mechanism for electrophilic substitution?
Electrons in the benzene ring are attracted to the positively charged carbocation. Two electrons from the benzene bond with the carbocation. This partially breaks the delocalised ring and gives it a positive charge.
Arrow from ring to C+ of carbocation. Attach the carbocation and a H too to the benzene and add a plus where they’re bonded. Draw an arrow from H bond to the plus and and arrow from the AlCl4^- ion (Al-Cl bond) to the H. This forms phenylketone + HCl + AlCl3.
The negatively charged AlCl4^- is attracted to the positively charged ring. One chloride ion breaks away from the aluminium chloride ion and bonds with the hydrogen ion. This removes the hydrogen from the ring forming HCl. It also allows the catalyst (AlCl3) to reform.
The reactants need to be heated under reflux in a non-aqueous solvent (e.g dry ether) for the reaction to occur.
What is nitration? Mechanism?
When you warm benzene with concentrated nitric and sulfuric acids, you get nitrobenzene. Sulfuric acid acts as a catalyst - it helps to make the nitronium ion, NO2^+, which is the electrophile.
HNO3 + H2SO4 -> H2NO3^+ + HSO4^-
H2NO3^+ -> NO2^+ + H2O
Electrophilic substitution:
Arrow from ring to nitronium ion. Unstable intermediate forms. And arrow from H bond to +. H^+ is lost. The ion will react with HSO4^- to reform the catalyst, H2SO4. If you only want one NO2 group added (minimisation), you need to keep the temperature below 55°C. Above this temperature you’ll get lots of substitutions. Nitration reactions are really useful:
Nitric compounds can be reduced to form aromatic amines. These are used to manufacture dyes and pharmaceuticals.
Some nitric compounds can be used as explosives, such as 2,4,6 - trinitromethylbenzene.
What are amines?
If one or more of the hydrogens in ammonia is replaced with an organic group, you get an amine. If one hydrogen is replaced with an organic group, you get a primary amine. If two are replaced, it’s secondary and three is tertiary. The lone pairs of electrons on the nitrogen atom in a tertiary amine can also bond with a fourth organic group - that gives you a quaternary ammonium ion.
Methylamine, di, tri, tetra meth (quaternary ammonium ion with an N^+). Aromatic amine is phenylamine (which is primary).
What are quaternary ammonium salts used for?
Because they’re positively charged, they will hang around with any negative ions that are near. The complexes formed are called quaternary ammonium salts - like tetramethylammonium chloride (CH3)4N^+Cl^-1
Quaternary ammonium salts with at least one long hydrocarbon chain are used as cationic surfactants. The hydrocarbon tail will bind to nonpolar substances such as grease, whilst the cationic head will dissolve in water, so they are useful in things like fabric cleaners and hair products. In addition, the positively charged part (ammonium ion) will hinder to negatively charged surfaces such as hair and fibre. This gets rid of static, so they are often used in fabric conditioners.
Why can amines act as weak bases?
They accept protons. There’s a lone pair of electrons on the nitrogen atom that can form a coordinate bond with a H^+ ion. The strength of the base depends on how available the nitrogen’s lone pair of electrons is. The more available the lone pair is, the more likely the amine is to accept a proton, and the stronger a base it will be. A lone pair of electrons will be more available if its electron density is higher. Primary aliphatic amines are stronger bases than ammonia, which is a stronger base than aromatic amines. Here’s why:
The more available the lone pair of electrons, the stronger the base.
Primary aromatic amine (phenylamine):
The benzene ring draws electrons towards itself and the nitrogen lone pair gets partially delocalised onto the ring. So the electron density on the nitrogen decreases, making the lone pair much less available.
Ammonia has a greater availability of the lone pair.
Primary aliphatic amine: alkyl groups push electrons onto attached groups. So the electron density on the nitrogen atom increases. This makes the lone pair more available. The lone pair of electrons also means that amines are nucleophiles. They react with halogenoalkanes in a nucleophilic substitution reaction, or with acyl chlorides and acid anhydrides in nucleophilic addition-elimination reactions.
How can you produce aliphatic amines?
You can heat a halogenoalkane with ammonia:
Amines can be made by heating a halogenoalkane with excess ammonia.
Ethylamine can be made by reacting ammonia with bromoethane:
2NH3 + CH3CH2Br -> CH3CH2NH2 + NH4Br
Arrow from H3N lone pair to C with delta positive charge. Then arrow from C-Br bond to Br.
Halogen is released and H3N bonds with a + charge.
A second ammonia molecule donates its lone pair of electrons to a hydrogen, which breaks off from the alkylammonium salt. So arrow from NH3 lone pair to H, then arrow from H to H-N bond. N is positive.
<=>
H2N attached + NH4^+Br
The mechanism is similar to the reaction of ammonia with a halogenoalkane - two amine molecules react with the halogenoalkane in succession to form a more substituted amine and an ammonium salt with a similar structure to the original amine.
The h will be replaced with the halogenoalkane.
You can also reduce a nitrile:
You can reduce a nitrile to a primary amine by a number of different methods. You can use lithium aluminium hydride (a strong reducing agent) in a non-aqueous solvent (such as dry ether), followed by some dilute acid. For example:
Nitrile + 4[H] -> (1. LiAlH4 and 2.dilute acid) primary amine.)
This method is fine in the lab, but LiAlH4 is too expensive for industrial use. In industry, nitriles are reduced using hydrogen gas with a metal catalyst such as platinum or nickel at high temperature and pressure. This is called catalytic hydrogenation.
Nitrile + 2H2 (nickel catalyst and high temperature and pressure) -> primary amine.
How are aromatic amines produced?
Reducing a nitro compound, such as nitrobenzene. First you need to heat a mixture of a nitro compound, tin metal and concentrated hydrochloric acid under reflux - this makes a salt. For example, if you use nitrobenzene, the salt formed is C6H5NH3^+Cl^-1
Then to turn the salt into an aromatic amine, you need to add an alkali, such as sodium hydroxide solution.
Nitrobenzene + 6[H] (tin, conc HCl, reflux, NaOH) -> phenylamine + 2H2O
Aromatic amines are useful compounds in organic synthesis - they’re used as the starting molecules for lots of dyes and pharmaceuticals.
What are amides?
-CONH2
The carbonyl group pulls electrons away from the NH2 group, so amides behave differently from amines. You can use N sub to name what is attached to the N.
What are condensation polymers?
Involves two different types of monomer, each with at least two functional groups. Each functional group reacts with a group on another monomer to form a link, creating polymer chains. Each time a link is formed, a small molecule (water) - that’s why it’s called condensation. Examples of condensation polymers include polyamides, polyesters and polypeptides.
What makes polyamides?
Carboxylic groups of dicarboxylic acids react with the amino groups of diamonds to form amide links. They both have functional groups at each end of the molecule, so long chains can form. Nylon is used to make clothing, carpet, rope, and airbags and parachutes.
Kevlar is used in bulletproof vests, boat construction, car tyres and lightweight sports equipment.
