Aromatics 3.3.10 Flashcards
what shape is benzene
Planar molecule
what’s the formula for benzene
C6H6
how many valent electrons does carbon have and how are they bonded to the carbon atom
4
- each carbon is bonded to 2 other carbons and 1 hydrogen
- the final lone electron is in a p-orbital which sticks out above and below the planar ring
what do the lone electrons in the p orbital combine to form
a delocalised ring of electrons
why are all the C-C bonds in the molecule of benzene the same
Due to the delocalised electron structure the C-C bonds have the same bond length
what type of bond length is the C-C bond in benzene
lies between a single bond and a double bond
What did kekule say about his structure of benzene
He thought there was alternating double and single bonds
what is benzene more stable than
the theoretical alternative cyclohexa-1,3,5-triene
how do we measure stability of benzene
By comparing the enthalpy change of hydrogenation in benzene and cyclohexa-1,3,5-triene
Why would we expect benzene to have an enthalpy change of hydrogenation of -360kjmol-1
•Because cyclohexane has 1 double bond and its enthalpy change of hydrogenation is -120kjmol-1
• If benzene had 3 double bonds
- 3 x -120 = -360
Does benzene or cyclohexa-1,3,5-triene require more energy to break bonds
Benzene requires more energy to break the bonds
Why is benzene more stable than the theoretical cyclohexa-1,3,5-triene with 3 double bonds
- Less exothermic so
-More energy is required to break bonds in benzene than in cyclohexa-1,3,5-triene - The stability is due to the delocalised electron structure
- All have the same bond length
what are arenes
Also known as aromatic compounds .
They are molecules that contain a benzene ring
how are aromatic compounds named
1) Name with “benzene” at the end .
- With a halogen , alkyl group or nitro
2) Named as if phenyl is a functional group ( C6H5) . Benzene ring can be regarded as a substituent side group on another molecule , like alkyl groups are
E.g alkene , alkane , ketones , aldehyde , carboxylates
Only if 7 or more carbons are attached to benzene ring
what’s the chemical formula for a phenyl group
C6H5
what are the specific examples for naming aromatic compounds
1) Phenol ( a benzene ring with OH attached )
2) Phenylamine ( a benzene ring with NH2 attached )
3) benzoic acid ( carboxylic acid group attached to benzene ring )
4) benzaldehyde ( benzene ring with aldehyde group attached )
what are the two mechanisms you need to know for reactions of arenes
1) friedel-crafts acylation
2) nitration reaction
how do we make a powerful electrophile in friedel-crafts acylation
Use halogen carrier AlCl3 ( aluminium chloride ) and react it with an acyl chloride
what is the change in functional group of friedel crafts acylation
Benzene => phenyl ketone
what reagents and conditions are needed for friedel crafts acylation
Reagent :
Acyl chloride in the presence of anhydrous AlCl3 catalyst
Condition :
Heat under reflux (50 C)
Anhydrous AlCl3
why is using friedel craft acylation useful
Can help benzene to react as it is difficult due to its stability
why is nitration of benzene useful
Allows us to make dyes for clothing and explosives such as TNT (trinitrotoluene)
what is the reaction needed to make an electrophile for nitration of benzene
HNO3 + 2H2SO4 => NO2^+ + 2HSO4^- + H3O^+
what do we do after we make an electrophile for nitration of benzene
Use nitronium ion ( NO2+) ( electrophile ) and react with benzene to produce nitrobenzene
what’s the electrophile needed in nitration of benzene
NO2+ (nitronium ion )
what is the change in functional group for nitration of benzene
benzene => nitrobenzene
what are the reagents and conditions needed for nitration of benzene
reagents :
Concentrated HNO3
Concentrated H2SO4 ( catalyst )
Conditions :
50 C
how are dyes and pharmaceuticals formed from nitrobenzene compounds
Can be made by reducing nitrobenzene’s to aromatic amines
What is the overall reaction for nitration of benzene
C6H6 + HNO3 => C6H5NO2 + H2O
What’s the reaction for the generation of electrophile using acyl chloride in friedel crafts acylation
CH3COCl + AlCl3 => CH3C^+=O + AlCl4^-
What’s the reaction for the generation of electrophile using acid anhydrides in friedel crafts acylaction
CH3COOCOCH3 + AlCl3 => CH3CO^+ + [ Cl3AlCOOCH3]-
What is the reaction for the regeneration of AlCl3 (catalyst) using the acyl chloride in friedel crafts acylation
AlCl4 ^- + H^+ => AlCl3 + HCl
What’s the reaction for the regeneration of AlCl3 ( catalyst) using the acid anhydrides in friedel crafts acylation
[ Cl3AlCOOCH3 ] ^ - + H^+ => AlCl3 + COOHCH3
what’s the reaction mechanism called for aromatics
electrophilic substitution
EQ :
explain the bonding in and the shape of a benzene molecule . Compare the stability of benzene with that of the hypothetical cyclohexa-1,3,5-triene molecule . Use data in your answer (6)
Bonding :
• Each C has 3 bonds
• Spare electrons in a p orbital overlap
• delocalisation
Shape :
• planar
• 120 bond angle
• C-C bonds equal in length
Stability :
• Expected Enthalpy change of hydrogenation of cyclohexa-1,3,5-triene = 360kJmol-1
• hydrogenation of benzene is less exothermic by 152kJmol-1
• Benzene is more stable than cyclohexa-1,3,5-triene
EQ : the enthalpy of hydrogenation of cyclohexa- 1,3-diene is not exactly double that of cyclohexane
Suggest a value for the enthalpy of hydrogenation of cyclohexa-1,3-diene and justify your value . (3)
•Between -239 to -121
• Alternating double and single bonds
• some delocalisation
what’s the reaction to form benzene ring with NH2 and OH from benzene ring with NO2 and OH
benzene ring (OH) and NO2 + 6[H] => benzene ring (OH) and NH2 + 2H2O
EQ : Benzene => cyclohexane
Identify a reducing agent for this
State the empirical formula of the product
state the bond angle between the carbon atoms in the starting material and the bond angle in the products (4).
- H2
- CH2
- in benzene = 120
- in cyclohexane = 108
