Original post by morphah
Ketones can be reduced by NaBH4 (with HCl added). Why is this a nucleophilic addition reaction? Because the H- that attacks the delta positive C is not a nucleophile.
Unless I am mistaken or misunderstanding?
Unless I am mistaken or misunderstanding?
I assume this is for A level (or equivalent), since the idea of there being H^- ions produced is a bit of an oversimplification (in reality the bonded pair of electrons in one of the B-H bonds is what is donated, along with the hydrogen to the carbon in question).
Well, let’s assume H^- is produced. It has to be nucleophilic. It has a lone pair (and a negative charge), for starters and you have also correctly identified that it is attacking a δ+ carbon, which is to say it is drawn to positive charges. As such, it meets the criteria for being a nucleophile (has a pair of electrons it can donate and is attracted to +ve charges) and due to the negative charge, it should be quite a good one at that.
(edited 3 weeks ago)
Original post by Pigster
The OCR A spec states that NaBH4 is a source of H^- ions.
If you added HCl to that, you'd also have H^+ ions floating around.
Surely... H^- + H^+ -> H2(g) and the ketone wouldn't get a look in?!?
If you added HCl to that, you'd also have H^+ ions floating around.
Surely... H^- + H^+ -> H2(g) and the ketone wouldn't get a look in?!?
Indeed it does, hence I specified my assumption that this an A level question rather than an undergraduate level question where I’d need to correct the OP’s understanding of the reaction taking place.
I think they meant to say you do an acid work-up afterwards. This is what is done in practice, even though aqueous NaBH4 is used and water can act as a proton source (especially considering the levelling effect as the resulting alkoxides are stronger bases than OH^-, but this is also a post-A level detail).
Original post by morphah
Ketones can be reduced by NaBH4 (with HCl added). Why is this a nucleophilic addition reaction? Because the H- that attacks the delta positive C is not a nucleophile.
Unless I am mistaken or misunderstanding?
Unless I am mistaken or misunderstanding?
Hello morphah!
To the best of my memory, the reduction of ketones with NaBH4 in an acidic media with hydrochloric acid present is a hydride reduction reaction, a specific type of electrophilic addition reaction, rather than a nucleophilic addition reaction.
The acid (HCl) in this reaction protonates the borohydride ion (BH4^-) to generate a more reactive species, which then targets the carbonyl carbon to create a new bond. The protonated borohydride ion, the electrophile, attacks the carbonyl carbon, the nucleophile, in this instance of an electrophilic addition process.
Errors and omissions excepted.
If you were to work with NaBH4 in an organic synthesis laboratory, I would advise you to be very careful.
NaBH4 is very dangerous and toxic; I am enclosing the relevant signs of chemical risks:
Bye,
Sandro
(edited 3 weeks ago)
Original post by Nitrotoluene
Hello morphah!
To the best of my memory, the reduction of ketones with NaBH4 in an acidic media with hydrochloric acid present is a hydride reduction reaction, a specific type of electrophilic addition reaction, rather than a nucleophilic addition reaction.
If you were to work with NaBH4 in an organic synthesis laboratory, I would advise you to be very careful.
NaBH4 is very dangerous and toxic; I am enclosing the relevant signs of chemical risks:
Bye,
Sandro
To the best of my memory, the reduction of ketones with NaBH4 in an acidic media with hydrochloric acid present is a hydride reduction reaction, a specific type of electrophilic addition reaction, rather than a nucleophilic addition reaction.
If you were to work with NaBH4 in an organic synthesis laboratory, I would advise you to be very careful.
NaBH4 is very dangerous and toxic; I am enclosing the relevant signs of chemical risks:
Bye,
Sandro
You don’t do the reduction itself in acidic solution. Although NaBH4 is fairly stable under neutral (and I believe basic) conditions in water, it rapidly decomposes to give boric acid and hydrogen upon acidification. The acid is added afterwards to quench leftover reactants / protonate the unprotonated product at the end of the reaction.
It is also a nucleophilic addition reaction - mechanistically, the “H^-” donated uses its “lone pair” to form a bond to the δ+ carbon (hence nucleophilic), which causes the oxygen to withdraw one of the electron pairs in the C=O bond, leading to saturation (hence addition). Note that quotation marks have been used since this is the simplified mechanism taught in some pre-university courses and that in reality it’s a bonded pair of electrons that is used rather than there being naked H^- ions.
(edited 3 weeks ago)
Original post by TypicalNerd
You don’t do the reduction itself in acidic solution. Although NaBH4 is fairly stable under neutral (and I believe basic) conditions in water, it rapidly decomposes to give boric acid and hydrogen upon acidification. The acid is added afterwards to quench leftover reactants / protonate the unprotonated product at the end of the reaction.
It is also a nucleophilic addition reaction - mechanistically, the “H^-” donated uses its “lone pair” to form a bond to the δ+ carbon (hence nucleophilic), which causes the oxygen to withdraw one of the electron pairs in the C=O bond, leading to saturation (hence addition). Note that quotation marks have been used since this is the simplified mechanism taught in some pre-university courses and that in reality it’s a bonded pair of electrons that is used rather than there being naked H^- ions.
It is also a nucleophilic addition reaction - mechanistically, the “H^-” donated uses its “lone pair” to form a bond to the δ+ carbon (hence nucleophilic), which causes the oxygen to withdraw one of the electron pairs in the C=O bond, leading to saturation (hence addition). Note that quotation marks have been used since this is the simplified mechanism taught in some pre-university courses and that in reality it’s a bonded pair of electrons that is used rather than there being naked H^- ions.
Hello TypicalNerd!
Take a look at this very interesting and comprehensive link: https://www.chemistrysteps.com/lialh4-and-nabh4-carbonyl-reduction-mechanism/
At the beginning of the chapter you will find the donors of H.
Sandro
Original post by Nitrotoluene
Hello TypicalNerd!
Take a look at this very interesting and comprehensive link: https://www.chemistrysteps.com/lialh4-and-nabh4-carbonyl-reduction-mechanism/
At the beginning of the chapter you will find the donors of H.
Sandro
Take a look at this very interesting and comprehensive link: https://www.chemistrysteps.com/lialh4-and-nabh4-carbonyl-reduction-mechanism/
At the beginning of the chapter you will find the donors of H.
Sandro
The page you have linked agrees with the mechanism I have outlined. In fact the drawn mechanisms shown are nucleophilic additions.
HCl is not used in the reduction- when it describes hydrogen bonding to the carbonyl oxygen, it’s the solvent that is forming the hydrogen bonds, since a protic solvent is typically used.
I suspect much of your apparent confusion arises from here:
“Sodium borohydride reduces aldehydes and ketones by a similar mechanism with some important differences that we need to mention.
First, NaBH4 is not so reactive and the reaction is usually carried out in protic solvents such as ethanol or methanol. The solvent has two functions here:
1) It serves as the source of a proton (H+) once the reduction is complete
2) The sodium ion is a weaker Lewis acid than the lithium ion and, in this case, the hydrogen bonding between the alcohol and the carbonyl group serves as a catalysis to activate the carbonyl group”
The solvent acts as a proton donor at the end of the reaction because it is an exceptionally poor acid and so it requires an incredibly strong base like an alkoxide to deprotonate it. An alkoxide is precisely what forms once the addition has taken place. So there is no protonation of BH4^- to give a more reactive species and the acid is added afterwards (in the work-up). There are, however, hydrogen bonding interactions between the solvent and carbonyl that activate it. I will admit that this was even something I forgot to bring up in earlier posts.
Original post by Nitrotoluene
Hello morphah!
To the best of my memory, the reduction of ketones with NaBH4 in an acidic media with hydrochloric acid present is a hydride reduction reaction, a specific type of electrophilic addition reaction, rather than a nucleophilic addition reaction.
The acid (HCl) in this reaction protonates the borohydride ion (BH4^-) to generate a more reactive species, which then targets the carbonyl carbon to create a new bond. The protonated borohydride ion, the electrophile, attacks the carbonyl carbon, the nucleophile, in this instance of an electrophilic addition process.
If you were to work with NaBH4 in an organic synthesis laboratory, I would advise you to be very careful.
NaBH4 is very dangerous and toxic; I am enclosing the relevant signs of chemical risks:
Bye,
Sandro
To the best of my memory, the reduction of ketones with NaBH4 in an acidic media with hydrochloric acid present is a hydride reduction reaction, a specific type of electrophilic addition reaction, rather than a nucleophilic addition reaction.
The acid (HCl) in this reaction protonates the borohydride ion (BH4^-) to generate a more reactive species, which then targets the carbonyl carbon to create a new bond. The protonated borohydride ion, the electrophile, attacks the carbonyl carbon, the nucleophile, in this instance of an electrophilic addition process.
Errors and omissions excepted.
If you were to work with NaBH4 in an organic synthesis laboratory, I would advise you to be very careful.
NaBH4 is very dangerous and toxic; I am enclosing the relevant signs of chemical risks:
Bye,
Sandro
Just quoting this so it’s easier to find the mechanism I was commenting on
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