Nucleophilic Substitution with ammonia help

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JackLeggett
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When it goes to its second stage, why is it that the Nitrogen has a positive charge? Why can it not have a dative covalent bond with the carbon which only has 6 electrons?
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713Wave
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The whole point of a nucleophilic substitution is to form (in this case) a (primary) amine. One thing to remember is that dative bond IS NOT a covalent bond (necessary to form an amine)!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

The intermediate, let's say in the formation of methylamine (CH3NH3), will have a positive charge on the nitrogen as it has effectively lost one electron (which is being used to stabilize the carbon atom). Therefore, a hydrogen falls off (as a stable positively charged hydrogen ion), effectively regenerating the lone pair of electrons that the nitrogen atom originally had, thus stabilizing it.
(i apologise for my earlier mistake).

I hope this helps ,
(btw I am studying chemistry at university so I know all this stuff).
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JackLeggett
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(Original post by 713Wave)
The whole point of a nucleophilic substitution is to form (in this case) a (primary) amine. One thing to remember is that dative bond IS NOT a covalent bond (necessary to form an amine)!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

The intermediate, let's say in the formation of methylamine (CH3NH3), will have a positive charge on the nitrogen as there still is an unpaired electron that can form a bond, (no bond means that the nitrogen is missing out on an electron, thus giving it its 'positive charge'. However as nitrogen is very electronegative, it's easier for a hydrogen ion to fall off, than to form another covalent bond (thus breaking the octet rule), as the nitrogen itself is very sterically hindered.
The electron gained in this process (to form the lone pair of electrons on the nitrogen), acts as the missing bonding electron thus stabilizing the nitrogen atom.

I hope this helps ,
(btw I am studying chemistry at university so I know all this stuff).
Hi thanks for reply, I read your reply a few times and still have a question. Where is the unpaired electron on the nitrogen? I though, as it is bonded to 3 hydrogens, and that the lone pair is then being shared with carbon there is no other electrons to bond with?
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h3rmit
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(Original post by JackLeggett)
When it goes to its second stage, why is it that the Nitrogen has a positive charge? Why can it not have a dative covalent bond with the carbon which only has 6 electrons?
The nitrogen does form a dative covalent bond to the carbon, with its lone pair. Because it's a covalent bond and the electrons are shared, it's essentially lost an electron (though not entirely due to the electronegativity difference), giving it a positive charge.
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713Wave
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(Original post by JackLeggett)
Hi thanks for reply, I read your reply a few times and still have a question. Where is the unpaired electron on the nitrogen? I though, as it is bonded to 3 hydrogens, and that the lone pair is then being shared with carbon there is no other electrons to bond with?
Ok, let's stick with the formation of methylamine from bromomethane.

In the nucleophilic reaction, Ammonia DOESN'T form a dative bond with the carbon atom (once the bromine has fallen off). The carbon atom has one unpaired electron on its outer shell.
The Nitrogen (in ammonia), has a lone pair of electrons and these electrons form a COVALENT bond with the carbon atom forming CH3NH3.

Then follows the process of a hydrogen ion falling off as I have described in my first reply/ post.
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JackLeggett
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(Original post by 713Wave)
Ok, let's stick with the formation of methylamine from bromomethane.

In the nucleophilic reaction, Ammonia DOESN'T form a dative bond with the carbon atom (once the bromine has fallen off). The carbon atom has one unpaired electron on its outer shell.
The Nitrogen (in ammonia), has a lone pair of electrons and ONE of these electrons forms a COVALENT bond with the lone electron on the carbon atom forming CH3NH3.

That leaves one unpaired electron on the nitrogen atom (that once formed part of the lone pair of electrons on the nitrogen in the ammonia molecule).
Then follows the process of hydrogen falling off as I have described in my first reply/ post.
Ok I fully understand what you said then, but surly when the bromine leaves the carbon it takes both electrons with it giving it a negative charge, so that carbon only has 6 electrons in its outer shell?
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713Wave
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(Original post by h3rmit)
The nitrogen does form a dative covalent bond to the carbon, with its lone pair. Because it's a covalent bond and the electrons are shared, it's essentially lost an electron (though not entirely due to the electronegativity difference), giving it a positive charge.
Sorry, you cannot possibly be correct, In the formation of the intermediate molecule (RNH3), a dative bond cannot be formed as that would form an adduct. Instead, a covalent bond is formed between the nitrogen and carbon atoms and one of hydrogens falls off from nitrogen atom to form the final product (RNH2).
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h3rmit
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(Original post by 713Wave)
Ok, let's stick with the formation of methylamine from bromomethane.

In the nucleophilic reaction, Ammonia DOESN'T form a dative bond with the carbon atom (once the bromine has fallen off). The carbon atom has one unpaired electron on its outer shell.
The bromine-carbon bond undergoes heterolytic fission
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h3rmit
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(Original post by 713Wave)
Sorry, you cannot possibly be correct, In the formation of the intermediate molecule (RNH3), a dative bond cannot be formed as the carbon atom itself has an unpaired electron.
Since the carbon-halogen bond's fission is heterolytic, the carbon can't have an unpaired electron left.
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713Wave
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(Original post by h3rmit)
Since the carbon-halogen bond's fission is heterolytic, the carbon can't have an unpaired electron left.
yes, i realized instead, I meant to say an adduct would form, which isn't the case as a covalent bond forms instead.
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h3rmit
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(Original post by 713Wave)
yes, i realized instead, I meant to say an adduct would form, which isn't the case as a covalent bond forms instead.
I'm not sure what you're saying here, can't adducts be formed from covalent bonds anyway?
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713Wave
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(Original post by JackLeggett)
Ok I fully understand what you said then, but surly when the bromine leaves the carbon it takes both electrons with it giving it a negative charge, so that carbon only has 6 electrons in its outer shell?
not quite as the bromine is more electronegative and takes both electrons in the bond to form the stable bromide ion.

(just realized my mistake) What happens is that the lone pairs of electrons on nitrogen forms a covalent bond with the carbon atom, NOT a dative bond (that produces an adduct to the carbon atom.)

However, in donating its 2 electrons, it has effectively lost one of them (as one of them as is stabilizing the carbon atom). The loss of one electron means the loss of a 1- charge giving the Nitrogen atom an overall +1 charge.
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713Wave
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(Original post by h3rmit)
I'm not sure what you're saying here, can't adducts be formed from covalent bonds anyway?
no, adducts are not quite bonds, they are more interactions.
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h3rmit
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(Original post by 713Wave)
no, adducts are not quite bonds, they are more interactions.
I thought they were products of addition reactions?
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Plantagenet Crown
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It has a positive charge because it uses its lone pair to bond to carbon. So previously, both of the electrons in the lone pair would have "belonged" to the nitrogen, but upon making the bond, it shares one of them with carbon, and thus effectively loses some of its electron density, giving it a positive charge.
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713Wave
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(Original post by h3rmit)
I thought they were products of addition reactions?
Ok let me give you an example, magnesium sulfate is used a drying agent. When it comes into contact with water, one of the lone pair of electrons on the oxygen (in the water molecule) forms an adduct with the magnesium sulphate as represented below:

MgSO4 . H2O

The dot does not represent a covalent bond, instead, the water is attached (via strong intermolecular forces) to the Magnesium Sulphate molecule. This is known as an adduct, very different to a covalent bond.
It's the same with ligands forming dative bonds/ adducts with metal ions.
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h3rmit
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(Original post by 713Wave)
Ok let me give you an example, magnesium sulfate is used a drying agent. When it comes into contact with water, one of the lone pair of electrons on the oxygen (in the water molecule) forms an adduct with the magnesium sulphate as represented below:

MgSO4 . H2O

The dot does not represent a covalent bond, instead, the water is attached (via strong intermolecular forces) to the Magnesium Sulphate molecule. This is known as an adduct, very different to a covalent bond.
It's the same with ligands forming dative bonds/ adducts with metal ions.
But the whole compound there would also be called an adduct.
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Plantagenet Crown
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(Original post by 713Wave)
Ok let me give you an example, magnesium sulfate is used a drying agent. When it comes into contact with water, one of the lone pair of electrons on the oxygen (in the water molecule) forms an adduct with the magnesium sulphate as represented below:

MgSO4 . H2O

The dot does not represent a covalent bond, instead, the water is attached (via strong intermolecular forces) to the Magnesium Sulphate molecule. This is known as an adduct, very different to a covalent bond.
It's the same with ligands forming dative bonds/ adducts with metal ions.
You're thinking of an adduct in the classic sense and how it's mostly used. But chemically speaking, an adduct is simply a product that contains all the atoms of the starting materials and thus adducts can indeed have covalent bonds.
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713Wave
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(Original post by h3rmit)
But the whole compound there would also be called an adduct.
Well its seems like our perceptions of adducts are different.

Either my inorganic chemistry lecturer is lying or....... well she isn't.
The compound would be described as water forming an adduct with the magnesium sulphate, and the whole compound would not be called an adduct, instead it would be called by its name as hydrated magnesium sulphate.

Having said that, thanks for correcting me earlier on.
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Plantagenet Crown
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(Original post by 713Wave)
Well its seems like our perceptions of adducts are different.

Either my inorganic chemistry lecturer is lying or....... well she isn't.
The compound would be described as water forming an adduct with the magnesium sulphate, and the whole compound would not be called an adduct, instead it would be called by its name as hydrated magnesium sulphate.

Having said that, thanks for correcting me earlier on.
The product you get from Diels Alder reactions is always an adduct, and it's certainly composed of covalent bonds.
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