Nucleophilic Substitution with ammonia help

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).

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?

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 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.

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.

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.

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.

yes, i realized instead, I meant to say an adduct would form, which isn't the case as a covalent bond forms instead.

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?

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.

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.

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.

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.