Aromatics 3.3.10 Flashcards
(43 cards)
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
