f5 organic chem

2012-07-30 6:49 am
1.Why ethene is a planar molecule while all other alkene molecules are not
planar?
4.Hexane reacts with bromine(in organic solvent) in sunlight,but bromine is
already a liquid(also hexane),why bromine has to dissolve in organic solvent
before the reaction?
更新1:

For q1,I mean,how do you know that all constituents of ethene lie on the same plane?

回答 (2)

2012-08-02 7:38 am
✔ 最佳答案
Frankly speaking, that "answer" is crappy.

1.
If you've learnt about orbital hybridization, that wouldn't be difficult.
Even not, it's still not difficult. You may use VSEPR theory to predict this.

For ethene CH2=CH2 , on a carbon atom, there're THREE groups of electron pairs:
- One group from C-H bond ;
- One group from another C-H bond ;
- One group from the C=C bond ---- notice that even it's a double bond, it's still counted as one GROUP, as the bonds are closely packed.

Now, there're 3 groups of electrons. To minimize the repulsion between groups of electrons, they're arranged in trigonal planar geometry. Two trigonal plane combines to give a planar molecule (the C=C bond has restricted rotation). [Bond angle = 120*]


For higher alkene, there must be some -CH3 group or -CH2- etc. groups.
On such carbon atoms, there're 4 groups of electrons from 4 single bonds.
To minimize the repulsion, they're arranged in tetrahedral geometry, thus is "3D" and cannot be planar. [Bond angle = 109.5*]


4.
Nothing about reaction rate.
If pure bromine is used instead of bromine in org. solvent, the reaction rate should be even faster and more (un)favourable.

It's more related to safety: pure bromine is highly volatile, corrosive and toxic. To reduce the amount of bromine used and the amount of bromine vapor produced, diluted bromine solution is preferred over pure bromine.

N.B. Bromine is soluble in hexane.
2012-08-01 7:50 am
1.
H H
| |
C =C how can't it be planar without any 4 th molecule
| |
H H


H H H
| | |
C =C - C - H draw it you will know
| | |
H H H

4, Just improve the reaction rate .

2012-08-01 23:37:45 補充:
Each double-bonded carbon of an alkene [ethene included] has three sp2 orbitals that lie in a plane with angles of 120 degrees. Because three points determine a plane, each sp2 carbon and the two atoms singly bonded to it lie in a plane. Furthermore, p orbitals must be parallel to each other in orde

2012-08-01 23:37:58 補充:
Each double-bonded carbon of an alkene [ethene included] has three sp2 orbitals that lie in a plane with angles of 120 degrees. Because three points determine a plane, each sp2 carbon and the two atoms singly bonded to it lie in a plane. Furthermore, p orbitals must be parallel to each other in orde


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