Half arrows

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Sarahi Garza 3k
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Half arrows

Postby Sarahi Garza 3k » Mon Mar 06, 2017 11:26 pm

How do you know when to use the half arrows when representing a nucleophilic attack? In the Chapter four practice problems, for the halogens being broken off a Carbon bond (4.24), we've been using the full arrows, why is that?

Lauren Trent 2A
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Re: Half arrows

Postby Lauren Trent 2A » Mon Mar 06, 2017 11:31 pm

Half arrows are used when a single electron is transferred in the attack, compared to a full arrow when a pair of electrons is being transferred.

Sylvia_Cai_3D
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Re: Half arrows

Postby Sylvia_Cai_3D » Thu Mar 09, 2017 1:24 am

You use the full arrows because two electrons from the Carbon bond are going to the halogen. Before the reaction the halogen has 3 lone pairs, which is 6 electrons and the Carbon and the halogen share electrons through a bond. During the reaction the electrons from the bond go to the halogen making it have 8 electrons now and a negative charge. It wouldn't be a half arrow because that would mean only 1 electron is transferred to the halogen which leaves the other electron on the Carbon, but that shouldn't happen since it is unstable.

Nikhil_Madan_3C
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Re: Half arrows

Postby Nikhil_Madan_3C » Thu Mar 09, 2017 3:20 pm

The practice problems deal with pairs of electrons being transferred so that's why mostly full arrows are used. Each bond consists of two shared electrons. The chlorine is more electronegative than the carbon so you can think about it like the chlorine pulling the electrons towards itself from the bond (away from the carbon). The electron rich Nucleophile is then attracted to the now positively charged carbon (electrostatics).

Chem_Mod
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Re: Half arrows

Postby Chem_Mod » Thu Mar 09, 2017 4:49 pm

Single headed arrows are used to show single electron movement which is equivalent to radical chemistry. We will not be doing any mechanisms with radicals in this class so no need to worry about it.


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