Aromatic compounds and Amines Flashcards
What is the chemical formulae of benzene?
C6H6
Describe the structure of benzene:
Each carbon atom forms single covalent bonds and the final unpaired electron on each carbon atoms are located in a p-orbital. The p orbital combine to form a ring of delocalized electrons.
How does the delocalized ring of electrons in benzene make it more stable?
Benzenes far more stable than the theoretical compound cyclohexa-1,3,5-triene would be:
- Cyclohexene has one double bond and when its hydrogenated the enthalpy change is -120kjmol-1 (if had three bonds you’d do 120x3=360). But the experimental enthalpy of hydrogenation of benzene is -208 kjmol-1 (less exothermic). Energy is put back to break bonds and released when bonds are made so more energy has 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 different indicates that benzenes more stable than cyclohexa-1,3,5-triene due to the delocalized ring of electrons.
Why do benzenes undergo electrophilic substitution?
The benzene ring has a high electron density and attracts electrophiles. As the benzene ring is stable it doesn’t undergo electrophilic addition reactions as they would destroy the delocalized ring of electrons and it instead undergoes electrophilic substitution reactions where one of the hydrogen atoms is substituted for the electrophile.
Why are nitrification reactions useful?
Nitro-compounds can be reduced to form aromatic compounds which manufacture dyes and pharmaceuticals. Some nitro compounds can be used as explosives such as 2,4,6-trinitromethylbezene (TNT).
How are amines derivates of ammonia?
- If one or more of the hydrogens in ammonia is replaced with an organic group you get an amine.
- If one hydrogens replaced with an organic group you get a primary amine,
- If two hydrogens are replaced its a secondary amine.
- If three hydrogens are replaced its a tertiary amine.
- The lone pair of electrons on the nitrogen atom in a tertiary amine can also bond with a fourth organic group; giving you a quaternary ammonium ion.
What are the uses of quaternary ammonium salts?
Quaternary ammonium ions are positively charged forming quaternary ammonium salts like tetraethylammoniumchloride (CH3)4N+Cl-. Quaternary ammonium salts with at least one long hydrocarbon chain are used as cationic surfactants. The hydrocarbon tails bind to non polar substances such as grease whilst the cationic head will dissolve in water and are useful ion things like fabric cleaners and hair products (their positive charge binds to negatively charged surfaces like hair and fibre which gets rid of static.
How do amines act as weak bases?
They accept protons as there’s a lone pair of electrons on the nitrogen atom that can form a dative covalent (coordinate) bond with H+ ions. The strength of the base depends on how available the lone pair is, the more lies;y the amine is to accept a proton and the stronger the base will be. A lone pair of electrons will be more available if its electron density is higher. The lone pair of electrons means that amines are nucleophiles and react with halogenoalkanes in a nucleophilic substitution reaction or with acyl chlorides/acid anhydrides in nucleophilic addition elimination.
What is the difference in base strength between ammonia, primary aliphatic and primary aromatic amines?
Primary aliphatic amines are stronger bases than ammonia which is a stronger base than aromatic amines because the more available the lone pair of electrons are the stronger the base is.
What are amides?
Amides contain the functional group -CONH2 and the carbonyl group pulls electrons away from the NH2 group so amides behave differently from amines.
