CH4 - Isomerism & Aromaticity

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What are stereoisomers?
Stereosiomers have the same molecular and structural formula, but a different spatial arrangement of atoms.
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What are the 2 types of stereoisomerism?
E-Z Isomerism & Optical Isomerism
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What are the two features responsible for E-Z Isomerism?
C=C double bond (restricted rotation) & Both carbon's must be bonded to 2 different groups.
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What causes the occurence of Optical Isomerism?
When a tetrahedral Carbon atom in a molecule is bonded to 4 different atoms/groups of atoms.
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What is the name of the tetrahedral Carbon in an optical isomer?
Chiral carbon (or chiral centre)
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What are optical isomers described as?
Non-superimposable mirror images.
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What is another name for an optical isomer?
Enantiomer
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What is a racemic mixture?
A racemic mixture contains equimolar quantities of each enantiomer.
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What effect would a racemic mixture of enantiomers have on the plane polarised light?
They would have opposite effects - one enantiomer would rotate it clockwise while the other would rotate it anti-clockwise. There is no overall effect because opposite effects cancel eachother out..
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What is resolution?
The separating of optical isomers.
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In nature, how many of the isomers tend to be predominant?
One (l or d)
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When ibuprofen is synthesised, both optical isomers are produced in equal quantities, but what is the difference between the two isomers?
One isomer is effective (pharmacologically active) while the other is ineffective and harmless. This is why the racemic mixture is not separated.
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What is the molecular formula of Benzene?
C6H6
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Who was the first scientist to suggest a cyclic structure for benzene?
Kekule
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Give a brief description of benzene.
It is a planar hexagonal molecule with a very stable structure.
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Kekule believed that Benzene had alternating single and double bonds. With regards to electrophilic addition, give 1 reason why Kekule was inaccurate.
Benzene would readily undergo electrophilic addition reactions if this structure was correct. In actual fact, Benzene reacts in this way far less readily than alkenes.
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What are the differences between the conditions of cyclohexane and benzene when undergoing electrophilic addition reactions.
Adding a few drops of orange Bromine water to cyclohexane at room temperature and shaking will result in decolourisation. To achieve the same with benzene, the benzene must be boiling and in the presence of intense UV light for addition to occur.
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Concerning bond lengths, give a reason why Kekule's structure was inaccurate.
If it were accurate then 2 different bond lengths would be observed in the benzene molecule. However all bond lengths are equal in the benzene molecule.
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Concerning the enthalpy of hydration of benzene and cyclohexane, give a reason why Kekule's structure was inaccurate.
If it were accurate, then multiplying the enthalpy of hydrogenation of cyclohexane (-120 kJmol-1) by 3 would give the value of the enthalpy of hydrogentation of benzene. However, the actual value for benzene is -208kJmol-1 (difference of 152).
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Explain the term 'delocalisation energy'.
The extra stability in the benzene molecule due to electron delocalisation.
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Describe the structure of benzene, concerning p orbitals and pi orbitals.
The pi orbital spreads over 2 carbon atoms. One electron of each of the carbon's p orbitals enters the pi orbital due to the sideways overlap above and below the plane of the ring. These electrons are delocalised.
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The current model of benzene produces 6 equivalent carbon atoms and 6 identical bonds, with each carbon atom bonded to 2 carbons and one hydrogen. What does the delocalised system give to the molecule?
A hugely increased stability, which makes it reluctant to undergo addition reactions.
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Why is benzene more readily attacked by electrophiles (e- pair acceptors)?
Benzene has a high electron density within its pi bond.
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Why does benzene more readily undergo substitution reactions?
Taking part in addition reactions would reduce the energetic stability of benzene.
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What are the typical reactions of benzene?
Electrophilic Substitution
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What is 'mononitration'?
When only one hydrogen is substituted by the NO2 group.
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What is the electrophile in the nitration of benzene?
Nitronium ion - NO2+
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What are the conditions for the nitration of benzene?
Nitrating mixture - concentrated nitric acid (HNO3) and concentrated sulfuric acid (H2SO4) (responsible for producing the NO2+ electrophile). Reacting mixture is refluxed gently at approximately 50 degrees celcius.
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Give the reaction equation for the mononitration of benzene.
C6H6 + HNO3 ------> C6H5NO2 + H2O
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What is the halogenation of benzene?
The substitution of Cl or Br. The X atom is substitued for one of the hydrogen atoms on the benzene ring.
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What is the electrophile in the halogenation of benzene?
X+ ion - either Cl- or Br-
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What are the conditions for the halogenation of benzene?
Room temperature / Anydrous / Absence of light / Halogen carrier (catalyst) - either AlCl3 or AlBr3 (depending on the halogen being substituted.) Fecl3 and FeBr3 are also suitable halogen carriers.
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Why is the chlorine molecule unable to carry out the substitution by itself?
The chlorine molecule is non-polar and cannot act as an electrophile.
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What does the halogen carrier do in the halogenation of benzene?
It reacts with the chlorine molecule to form anhydrous AlCl3 or FeCl3 which are covalent chlorines and are soluble in benzene.
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Give the reaction equation for the halogenation of benzene.
C6H6 + Cl2 ------> C6H5Cl + HCl
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What si the Friedel-Crafts alkylation of benzene?
The substitution of an alkyl group, e.g. CH3 or C2H5, for one of the hydrogen atoms on the benzene ring.
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Name an electrophile used in this reaction and describe how it is produced.
CH3+ ion. It comes from the heterolytic fision of the C-CH3 bond in halogenoalkane.
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Give the conditions for the Friedel-Crafts alkylation of benzene.
Room emperature / Anhydrous / Halogen Carrier - AlCl3
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Give the reaction equation for the Friedel-Crafts alkylation of benzene.
C6H6 + CH3Cl ------> C6H5Cl
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Give the equation for the regeneration of the halogen carrier / catalyst.
AlCl4+ + H+ ------> AlCl3 + HCl
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Why is the addition of Cl2 or Br2 to benzene difficult to achieve?
Addition across a double bond in benzene would result in the delocalisation being lost and the system would lose stability.
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What test is used to detect unsaturation in compounds?
Cyclohexane decolourising orange bomine water.
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Why are far more vigorous reaction conditions needed for an addition reaction with benzene?
Because of the chemical stability of the delocalised pi electron system. A large amount of energy is required to overcome this stabillity.
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Why is chlorobenzene far more resistant to alkaline hydrolysis than halogenoalkanes? (1)
The electrons in the p orbitals of the Cl atom can become part of the delocalised electron system. This strengthens the C-Cl bond in comparison to the bond in halogenoalkanes.
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Why is chlorobenzene far more resistant to alkaline hydrolysis than halogenoalkanes? (2)
The polarity of the C-Cl bond is reduced too, making it less susceptible to nucleophilic attack.
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What would removing the Cl- during nucleophilic substitution cause?
It would result in the loss of the delocalised system and would decrease the energetic stability of the molecule.
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What are the conditions required to achieve nucleophilic substitution of chlorobenzene by OH-?
Extreme conditions - a temperature of 200 degrees celcius and a pressure of 200 atm.
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Card 4

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Card 5

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