Module 6 key information Flashcards
What is a sigma bond?
head-on overlap of orbitals between bonding atoms
What is pi bond?
sideways overlap of adjacent p-orbitals above and below the bonding atoms
Differences between a sigma bond and a pi bond? ( 3 comparisons)
1) sigma bond is the head-on overlap of orbitals, pi bond is the sideways overlap of orbitals.
2) sigma bonds have their electron density between bonding atoms, pi bonds have their electron density above and below the plane.
3) sigma bonds have a high bond enthalpy whereas pi bonds have a lower bond enthalpy because the electrons are further away from the nucleus.
What is benzene
A colourless liquid found in crude oil. It is a planar molecule with lots of points of contact
What is kekule’s model of benzene?
- 6 membered ring
- alternating single and double bonds
- P-orbitals overlap in one direction
- Pi electrons are localised
- C6H6
- Draw diagram of kekule’s model
Why was Kekule’s model disproved?
1) LACK OF REACTIVITY. COMPARED TO ALKENES- alkenes undergo electrophillic addition and C=C is lost whereas benzene undergoes electrophilic substitution so does not decolourise bromine water
2) THEY HYDROGENATION ENTHALPIES ARE LESS EXOTHERMIC THAN EXPECTED- benzene is more stable
3) LENGTH OF THE CARBON CARBON BONDS ARE ALL THE SAME LENGTH- single carbon bonds and double carbon bonds are of different length.
What is the delocalised model of benzene?
- 6 membered ring
- C6H6
- P-orbitals are perpendicular to the ring structure
- Adjacent P-orbitals overlap sideways creating a system of pi bond spread across the ring
- P-orbitals overlap in both directions
- Ring of electron density above and below the plane
- draw model out
Compare the two models of benzene?
SIMILARITIES: . 6 membered ring - each carbon bonded to 1 H -Sideways overlap of P-orbitals - Electron density above and below the ring DIFFERENCES: -K= p-orbitals overlap in 1 directions -D= p-orbitals overlap in both directions
- K= Pi electrons are localised
- D= Pi electrons are delocalised
-DRAW DIAGRAM OF BOTH
Another word for aromatic compounds?
Arenes
What group on a benzene ring give the name ‘——-benzene’
Alkyl groups up to C6H13
Halogens
Nitro groups
What groups on a benzene ring give the name ‘phenyl——–’
Alkyl groups with more than 7 carbons Alkyl chain with a functional group Amine group (NH2)
Structure of benzoic acid
C6H5(COOH)
Structure of phenol
C6H5(OH)
Structure of benzaldehyde
C6H5(CHO)
Why does benzene undergo electrophilic substitution?
Benzene delocalised electron above and below the plane which makes it stable. It does not want to break the delocalisation. The nucleophile is spread across the ring so requires a catalyst.
What are the conditions of nitration of benzene?
Conc sulphuric acid + nitric acid, 50 degrees (higher than 50 degrees will cause further substitution)
How is the electrophile formed? ( First part of the reaction) N
HNO3 + H2SO4 ———NO2+ + HSO4- + H20
……….
What are the conditions of halogenation of benzene?
Halogen carrier catalyst ( ALBR3 OR FEBR3)
How is the electrophile formed? ( First part of the reaction) H
CL2 + FECL3———————–FECL4- + CL+
What is another name for alkylation and acylation of benzene?
Friedel-crafts
What are the conditions of alkylation and acylation?
Halogen carrier catalyst
How do alkenes react with bromine thats different from benzene?
Alkene: No catalyst, addition reaction, loss of the double bond
Why does benzene require a catalyst?
Electron density more spread out across the ring, less electron density in one place. Need a stronger electrophile
How to name phenols?
The carbon bonded to the OH is always carbon 1