6 Flashcards
What were the problems with Kekule’s model of benzene?
It does not react like alkenes (bromine water is not decolourised), Kekule explained this by saying the double and single bonds were quickly changing.
Double bonds are shorter than single bonds but in benzene all the carbon-carbon bonds are of equal length.
The enthalpy of hydrogenation is less exothermic than expected (when compared with cyclohexane) which suggests that benzene is more stable than this structure predicts.
What is a nitro functional group?
-NO2
What is a phenyl functional group?
C6H5
How does benzene react with electrophiles?
The electrophile is attracted to the electron rich π bonding system.
The electrophile accepts a pair of π electrons from the ring.
An unstable intermediate is formed.
H+ leaves putting two electrons back into the ring.
[mechanism needed]
What is needed for the nitration of benzene?
Conc. nitric acid and conc. sulphuric acid. Reflux at 50 degrees C.
HNO3 + H2SO4 –> HSO4^- + H2O + NO2^+
H^+ + HSP4^- –> H2SO4
What is needed for the halogenation of benzene?
Br2 + AlBr3 (or Fe/FeBr3)
The AlBr3 is an electron deficient halogen carrier catalyst.
AlBr3 + Br3 –> AlBr4^- + Br^+
AlBr4^- + H^+ –> AlBr3 + HBr
What are the Friedal Crafts reactions?
Alkylation and acylation of benzene.
What is needed for the alkylation of benzene?
A haloalkane and a halogen carrier catalyst.
What is needed for the acylation of benzene?
A haloalkoyl (contains an acyl group C=O).
Reflux at 30 degrees C for 30 minutes. Under anhydrous conditions.
What are reflux conditions indicative of?
A continuous process of evaporation and condensation to prevent volatile reagents from escaping. (Often heated in a water bath with the top open)
What is a halogen carrier catalyst?
A catalyst which works by taking a halogen atom from the reagent by forming a covalent bond to said halogen.
What is a phenol group?
A benzene ring with a hydroxyl group (OH) directly bonded to it. It is only slightly soluble in water.
What directing effect does OH have?
2, 4
What directing effect does NH2 have?
2, 4
What directing effect does NO2 have?
3
Model answer: describe the bonding in benzene.
Each carbon forms 3 sigma bonds.
This leaves 1 electron on each carbon in a p-orbital.
These p-orbitals overlap sideways to form a delocalised π bonding system. [diagram]
The π electrons are delocalised above and below the ring.
All C-C bond lengths and strengths are the same and the ring is planar.
Model answer: Compare the reactivity of benzene and cyclohexene - benzene in more resistant to bromination.
Benzene is less reactive towards bromination (it needs a catalyst).
The π electrons are delocalised so benzene has a lower electron density than a double bond in which the π electrons are in a localised π bond.
Therefore, benzene is less attractive to electrophiles and not able to polarise them as strongly.
Model answer: Why does benzene undergo substitution and not addition?
The delocalised π bonding means that the electrons are spread out.
This reduces the repulsion between them and gives benzene extra stability.
Substitution allows the delocalised π bonding system to be retained.
Addition would permanently disrupt thew π bonding system.
Model answer: compare the reactivity of benzene and phenol - benzene is more resistant to bromination.
Phenol is more reactive than benzene (does not need a catalyst to form try-substitution.
This product can be seen as a white solid.).
In phenol the lone pair of electrons in a p-orbital on the oxygen overlaps with the p-orbitals on the carbons in the benzene ring.
So, the lone pair is drawn into the ring and becomes part of the delocalised π bonding system (the ring is activated).
This increases the density in the ring.
Making it more attractive and able to polarise more strongly.
A fun fact about phenols?
Phenols are weakly acidic as shown by how they neutralise NaOH but how they do not react with carbonates.
What are amines and why are they basic?
derivatives of ammonia (pyramidal = 107°C).
Amines are weak bases as the lone pair of electrons on the nitrogen form a dative covalent bond with a hydrogen and therefore accept a proton.
How are aliphatic amines prepared?
The lone pair of electrons on the N attack the slightly positive C-Cl bond and the bond between the N-H breaks with the electrons going to the N to form CNH2 and HCl.
Heat a halogenoalkane and excess (because the products are an acid and a base and because further nucleophilic substitution os also possible) ethanolic ammonia in a sealed tube (not at reflux because NH3 is a gas).
A better reaction for this is the reduction of a nitrile as it only produces a primary amine.
What are amino acids?
Amino acids are the building blocks of peptides and polypeptides, proteins. There are 20 different amino acids int he body.
Alpha amino acids have both the NH2 group and COOH group bonded to the same carbon (general formula: RCH(NH2)COOH)
Amino acids (bar glycine) have a chiral carbon so have no plane of symmetry so are not superimposable mirror images so show optical isomerism.
What are enantiomers?
A pair of optical isomers. They rotate plane polarised light by equal amounts in opposite directions.
What is a racemic mixture?
A mixture containing 50% of each enantiomer.
What is a chiral carbon?
A compound which contains 4 single bonds to 4 different R groups.
If a molecule contains n chiral centres, there will be 2^n stereoisomers.
