ARENES Flashcards
(22 cards)
What are arenes? ()
arenes are hydrocarbons based on benzene
describe the structure of benzene. (4)
-unsaturated molecule, its very stable
-consists of a flat, regular hexagon of carbon atoms
-each bonded to a single hydrogen
-the C-C bond lengths are intermediate between those expected in a single bond and double bond
-each carbon makes three bonds
explain why the symbol of benzene looks the way it does. (1)
by using the idea that some of the electrons are delocalised.
what does delocalisation mean? (1)
means that electrons are spread over more than two atoms
explain the electron configuration in a benzene molecule. (1)
the fourth electron of each carbon atom is in a p-orbital, and there are six of these. The p-orbitals overlap and the electrons in them are delocalised, they form a region of electron density above and below the ring.
what is aromatic stability? (1)
the intermediate length between a single and a double bond makes benzene unusually stable, this is sometimes called aromatic stability.
what are the physical properties of arenes? (3)
-colourless liquid at room temperature
-boiling point similar to hexane but melting point a lot higher
-do not mix with water but mix with other hydrocarbons and non-polar solvents
why does benzene have a similar boiling point to hexane but a much higher melting point? (1)
because of 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.
hat is the root name of benzene? (1)
benzene
what factors impact the reactivity of aromatic compounds? (2)
-the ring is an area of high electron density because of the delocalised bonding and is therefore attacked by electrophiles
-the aromatic ring is stable, it needs energy to put in to break the system
what are the reactions of arenes? (2)
-combustion
-electrophilic substitution reactions
how do arenes react in combustion? (1)
arenes burn in air with flames that are smoky, this is because they have a high carbon: hydrogen ratio compared to alkanes, usually unburnt carbon remaining in air causing soot.
how do arenes react in an electrophilic-substitution reaction? (1)
the reaction usually leaves the aromatic system unchanged which does not require an input of energy which would destroy the aromatic system.
explain the mechanism of an electrophilic substitution reaction. (2)
the delocalised system of the aromatic ring has a high electron density which attracts electrophiles, the electrons are attracted towards the electrophile as well. a bond forms between a carbon and the electrophile. To do this the carbon has to use electrons from the delocalised system, this destroys the aromatic system, in order to gain stability, the carbon loses an H+ ion with the electron in the C=H bond returning the delocalised system.
what is nitration? (1)
the substitution of a NO2 group for one of the hydrogen atoms on an arene ring, the electrophile is generated in the reaction mixture of concentrated nitric and concentrated sulfuric acid.
explain the mechanism to produce a nitration electrophile. (2)1
sulfuric acid is a stronger acid than nitric acid and donates a proton, H+ to HNO3. H2NO3+ then loses a molecule of water to give NO2+ which is called a nitronium ion or nitrul cation.
explain the mechanism of nitration? (1)
the over product of the nitronium ion with benzene is nitrobenzene. the H+ then reacts with the HSO4- to regenerate H2SO4 making sulfuric acid a catalyst.
what are the uses of nitrated arenes? (1)
nitration is an important step in the production of explosives, it is also the first step in making aromatic amines which in turn make industrial dyes.
what are Friedel-crafts acylation reactions? (1)
these reactions use aluminium chloride as a catalyst, mechanism for acylation is a substitutionwith RCO substituting for a hydrogen on the aromatic ring
what provides the RCO group in Friedel-crafts acylation reactions? (1)
acyl chlorides, they react with AlCl3 to form AlCl4- and RCO+
how is the friedel-crafts acylation reaction able to take place? (1)
because the aluminium atom in aluminium chloride has only six electrons in its outer main level and readily accepts a lone pair from the chlorine atom of RCOCl
MECHANISM IS ON PAPER
