Organic - Aromatic Chemistry Flashcards
What are arenes?
Arenes are hydrocarbons based on benzene (C6H6), which is the simplest one. Although benzene is an unsaturated molecule, it is very stable. It has a hexagonal ring structure with a special type of bonding.
Why are arenes called aromatic compounds?
Arenes were first isolated from sweet-smelling oils, such as balsam, and this gave them the name aromatic compounds.
Arenes are still called aromatic compounds, but this now refers to their structures rather than their aromas. Benzene and other arenes have characteristic properties.
What symbol is given to benzene?
A hexagon with a large circle in the middle. This is a skeletal formula, which does not show the carbon or hydrogen atoms. Benzene consists of a flat (planar, 120 degree angles), regular hexagon of carbon atoms, each of which is bonded to a single hydrogen atom.
An arene can have other functional groups (substituents) replacing one or more of the hydrogen atoms in its structure.
What is the C-C bond length in benzene?
The C-C bond lengths in benzene are intermediate between those expected for a carbon-carbon single bond (0.153nm) and a carbon-carbon double bond (0.134nm). So each bond is intermediate between a single and a double bond (0.140nm).
How can the benzene symbol be explained?
This can be explained by using the idea that some of the electrons are delocalised. Delocalisation means that electrons in the pi system do not belong to any particular carbon atom, they are spread over more than two atoms and a free to move throughout the whole pi system - in this case the six carbon atoms that form the ring.
How are the electrons in benzene arranged?
Each carbon has three covalent, sigma bonds - one to a hydrogen atom and the other two to carbon atoms. The fourth electron of each carbon atom can be found anywhere in a p-orbital (perpendicular to the plane of the ring), and there are six of these - one on each carbon atom.
The p-orbitals overlap with each neighbouring p-orbital to form a pi bond, and the electrons in them are delocalised. They form a region of electron density (negative charge, “electron cloud”) above and below the plane of the ring, hence the circle.
What is aromatic stability?
Overall, each carbon-carbon bond is intermediate between a single and a double bond. The delocalised system is very important in the chemistry of benzene and its derivatives. It makes benzene unusually stable (lower in energy) as the electrons are delocalised and more spread out, so they will repel each other less making the molecule more stable. This is sometimes called aromatic or delocalisation stability.
What is the thermochemical evidence for stability?
- The enthalpy change for the hydrogenation of cyclohexene is -120kJmol-1.
- So the hydrogenation of a ring with alternate double bonds would be expected to be three times this (-360kJmol-1).
- The enthalpy change for benzene is in fact -208kJmole-1.
- If these values are put on an enthalpy diagram, you can see that benzene is 152kJmol-1 more stable than the unsaturated ring structure.
What are the physical properties of arenes?
- Benzene is a colourless liquid at room temperature.
- It boils at 353K and freezes at 279K.
- Its boiling point is comparable with that of hexane (354K) but its melting point is much higher than hexane’s (178K).
- This is because benzene’s flat, hexagonal molecules pack together very well in the solid state.
- They are therefore harder to separate and this must happen for the solid to melt.
- Like other hydrocarbons that are non-polar, arenes do not mix with water but mix with other hydrocarbons and non-polar solvents.
How do you name aromatic compounds?
- substituted arenes are generally names as derivatives of benzene, so benzene forms the root of the name
- if there is more than one substituent, the ring is numbered
- when two or more different substituents are present, they are listed in alphabetical order
- the benzene ring can be regarded as a substituent on another molecule (phenyl group)
- when there is an alcohol group attached, it is known as a phenol
What factors are important in the reactivity of aromatic compounds?
- The ring is an area of high electron density, because of the delocalised bonding, and is therefore attached by electrophiles (either a positive ion or the positive end of a dipole).
- The aromatic ring is very stable. It needs energy to be put in to break the ring before the system can be destroyed. This is called the delocalisation energy. It means that the ring almost always remains intact in the reactions of arenas.
What are most of the reactions of aromatic systems?
electrophilic substitution reactions
What compound was benzene originally thought to have a structure similar to?
cyclohexatriene, with three double bonds and three single bonds
Who was Kekule?
Kekule made a significant breakthrough and was the first chemist to realise that benzene had a ring structure with six carbon atoms each joined to one hydrogen atom. However, he thought that the ing contains three C=C double bonds and three C-C single bonds. This molecule would be a “triene” (“cyclohexa-1,3,5-triene”) with three C=C double bonds rather than a delocalised ring system.
How does the C-C bond length support the delocalised structure?
- All the C-C bonds are the same length - and this length is in between the length of C-C single and C=C double bonds.
- If benzene was a triene, we would expect three longer C-C single bonds and three shortened C=C double bonds, making it asymmetrical.
How do addition reactions support the delocalised structure?
- Benzene does not readily undergo addition reactions (e.g. benzene does not decolourise bromine water).
- If benzene was a triene, we would expect it to readily undergo addition reactions such as this but it doesn’t.
What happens when arenes burn in air?
Arenes burn in air with flames that are noticeable smoky. This is because they have a high carbon : hydrogen ratio compared with alkanes. There is usually unburnt carbon remaining when they burn in air and this produces soot. A smoky flame suggests an aromatic compound.
What reaction does benzene undergo?
Although benzene is unsaturated, it does not react like an alkene.
The most typical reaction is an electrophilic substitution that leaves the aromatic system unchanged, rather than addition which would require the input of the delocalisation energy to destroy the aromatic system.
