Organic - 3.10 Aromatic Chemistry Flashcards
What is the molecular formula of benzene?
C6H6
Does benzene decolourise bromine water?
No
What is the Mr of benzene?
78
What is the catalyst used in the hydrogenation of benzene?
A nickel catalyst
What does benzene form when it is hydrogenated?
Cyclohexane
What did Kekulé’s structure suggest about the bonds in benzene?
Alternating single and double carbon bonds
How was Kekulé’s structure proved incorrect?
In 1922, bond angles were measured and it was found that all the carbon carbon bonds were the same length, at an intermediate value between that of C-C and C=C, suggesting Kekulé was incorrect.
Kekulé’s Benzene structure has three C=C bonds so it would be expected to have an enthalpy change of -360 however when benzene is hydrogenated the enthalpy change is only -208. This means that the actual structure of benzene is more stable than the Kekulé structure .
What did Kathleen Lonsdale found about the structure of benzene?
She found that all the carbon carbon bonds were the same length (an intermediate length between C-C and C=C). Suggests that Kekules structure was incorrect.
What is the enthalpy of hydrogenation?
The enthalpy change when one mole of an unsaturated compound reacts with an excess of hydrogen to become fully saturated
What is the shape of todays model of benzene?
Planar hexagonal
What is the length of the bonds in todays model of benzene?
Intermediate length between that of C-C and C=C
How does the ring of delocalised electrons form in the benzene ring?
Each carbon uses three of its outer electrons to form three sigma bonds to two other carbon atoms and one hydrogen atom. This leaves each carbon at with one electron in a P orbital
What is the bond angle in todays model of benzene?
120
What does the circle represent in the structure of benzene?
The ring of delocalised electrons
What are arines?
Aromatic hydrocarbons that contain one or more benzene rings
What is the state of arines at room temperature?
Colourless liquid
Why is benzene immiscible in water?
It is non-polar
Why does benzene not decolourise bromine water?
It doesn’t undergo addition reactions due to stable delocalised electron ring
Why does cyclohexene decolourise bromine water?
Cyclohexene contains a reactive double bond which undergoes addition reaction.
What type of reaction does benzene undergo?
Electrophilic substitution
How is a benzene derivative formed?
If one of the hydrogen atoms on a benzene ring is replaced by an atom or group
How is an alkyl group formed?
Alkyl groups are formed by removing a hydrogen atom from an alkane
How is a phenyl group formed?
Removing a hydrogen group from a benzene ring
Why is benzene attacked by electrophiles?
There is a region of higher electron density above and below the plane of the molecule
Why do substitution reactions occur rather than addition reactions?
Substitution reactions occur rather than addition reactions as this preserves the stability of the benzene ring
What happens in the nitration of benzene?
The hydrogen atom on the ring is replaced by a nitro group
What is the electrophile in the nitration of benzene?
Nitronium ion
What are 3 uses of nitrated arines?
1) manufacture of explosives
2) nitrobenzene can be reduced to phenylamine(dyes)
3) organic synthesis
What is acylation?
The process of replacing a hydrogen atom in certain molecules by an Acyl group
What is the catalyst used in benzene acylation?
Aluminium chloride
What is the catalyst used in benzene nitration?
Concentrated sulfuric acid
What is formed from the acylation of benzene?
An aromatic ketone
What is the reagent in the nitration of benzene?
Concentrated sulfuric acid and concentrated nitric acid
What are the conditions for the nitration of benzene?
50°C
What happens in the nitration of benzene?
Hydrogen atom on the ring is replaced by a nitro group
What are the products in the nitration of benzene?
Aromatic nitro compounds
What is the overall equation for the nitration of benzene?
C6H6 + HNO3 -> C6H6NO2 + H2O
What is the equation for the generation of the electrophile in the nitration of benzene?
HNO3 + 2H2SO4 -> NO2+ + 2HSO4- + H3O+
Forms a nitronium ion
What is the reagent in the acylation of benzene?
Acyl chloride or acid anhydride & AlCl3
What are the conditions for the acylation of benzene?
Anhydrous
What happens in the acylation of benzene?
Hydrogen atom on the ring is replaced by an acyl group
What are the products in the acylation of benzene?
Aromatic ketones
What is the overall equation for the acylation of benzene?
C6H6 + RCOCl -> C6H5COR + HCl
What is the equation for the generation of the electrophile in the acylation of benzene? (Acylium)
CH3COCl + AlCl3 -> CH3CO+ + AlCl4-
What is the equation for the regeneration of the catalyst in the acylation of benzene? (Acyl Chloride)
AlCl4- + H+ -> AlCl3 + HCl
What is an addition reaction?
A reaction in which molecules combine to form a single product
What is an aromatic compound?
A compound containing at least one benzene ring
What is an electrophile?
An electron pair acceptor
What is electrophilic substitution?
A reaction in which an electrophile replaces an atom/group of atoms in a compound.
What is Friedel-Crafts Acylation?
An important synthetic reaction involving an electrophilic aromatic substitution reaction between benzene and Acyl chlorides or anhydrides, used to form monoacylated benzene rings. AlCl3 is used as a catalyst.
What is nitration?
Benzene undergoes nitration via an electrophilic substitution reaction, using concentrated nitric acid and a concentrated sulfuric acid catalyst at 50 degrees.
What is a substitution reaction?
A reaction in which one atom/group of atoms is replaced by another atom/group of atoms.
