Two organic chemistry questions..... need help.......

[SarahGreenEyesN]

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Hi there everyone...... I have two organic questions:

1) Is there any stereochemistry applied to nucleophilic attack of carbonyls ? I am thinking that since the aldehyde or ketone is sp2 ( 120 degree ) the nucleophile can attack from any posible direction. Is that true ?

2) I remember reading in some o-chem book about adding alcohols to alkenes using acid in water. Do you know the name of this reaction ? Do you agree that you can add alcohols to alkenes with acid in water ?

Thank you !

 

[QofQuimica]

1) Is there any stereochemistry applied to nucleophilic attack of carbonyls ? I am thinking that since the aldehyde or ketone is sp2 ( 120 degree ) the nucleophile can attack from any posible direction. Is that true ?

Since carbonyls are flat, the Nu can attack from either above or below the plane of the carbonyl. So there are only two directions of attack. If your ketone or aldehyde is unsymmetrical, then you can end up with a racemic mixture of products (assuming your Nu is different than either of the substituent groups).

2) I remember reading in some o-chem book about adding alcohols to alkenes using acid in water. Do you know the name of this reaction ? Do you agree that you can add alcohols to alkenes with acid in water ?

I'm a little confused about what you mean here. Do you mean *converting* alkenes to alcohols using acid and water? If so, then yes, you can do this reaction. It's an addition reaction that goes by a two-step mechanism (E1).

 

[Slide]

The second part is a little unclear too. You can convert alcohols into alkenes by simply adding sulfuric acid with heating. However, this process is usually not preferred because charring happens quite often. This reaction is called dehydration.

To add a hydroxyl group to an alkene (I believe this is what she means), you need to use either a mercury acetate compound, or a Sia(BH)3 compound with water, depending on whether u want Mark. or anti-Mark. compound.

 

[SarahGreenEyesN]

The second part is a little unclear too. You can convert alcohols into alkenes by simply adding sulfuric acid with heating. However, this process is usually not preferred because charring happens quite often. This reaction is called dehydration.

To add a hydroxyl group to an alkene (I believe this is what she means), you need to use either a mercury acetate compound, or a Sia(BH)3 compound with water, depending on whether u want Mark. or anti-Mark. compound.

Wait... I know I can make alc. groups with hydroboration or oxymerc. but I really want to make to two alc groups on an alkene then oxidize them into carbonyls. The trick is I cant use osonolysis or KMNO4 or any other dual oxidizing method.

So I recall some rxn from long ago. I think it was represented as just acid in water or H30+/H20 that added two alc groups to an alkene. Am I just halucinating ?

 

[gdbaby]

Hi there everyone...... I have two organic questions:

1) Is there any stereochemistry applied to nucleophilic attack of carbonyls ? I am thinking that since the aldehyde or ketone is sp2 ( 120 degree ) the nucleophile can attack from any posible direction. Is that true ?

2) I remember reading in some o-chem book about adding alcohols to alkenes using acid in water. Do you know the name of this reaction ? Do you agree that you can add alcohols to alkenes with acid in water ?

Thank you !

Q of Qumica is correct, but don't forget Cram's rule which states that the lowest energy conformation of a carbonyl compound has the largest substituent perpindicular to the carbonyl compound. Once you place your molecule in that configuration, the Nu: will want to attack from the side where it goes through the smallest substituents.

You can add alcohols to alkenes via a hydration reaction, adding H-OH across the double bond (Ragents = H3PO4 catalyst and high heat). You can also do Oxymerc-demerc (Rgts=Hg(OAc)2, H2O/THF Step 2=NaBH4). Oxymerc-Demerc will occur in Markovnikov fashion. there is also Hydroboration, but that does not involve an acid as hydrogen acts like hydride in this instance.

 

[Kitra101]

I do remember seeing a product from a reaction making a geminal diol...what you do is take formaldehyde (least stable compared to other aldehydes and ketones) and you add acid to the reaction. This is an equilibrium reaction, so you want to push the forward reaction by decreasing the pH of the reaction solution. This will protonate the carbonyl oxygen and cause the pi bond electrons to move towards the partial positive charge on the protonated oxygen. Now, you have a partial positive charge on the (once) carbonyl carbon and if you make sure that there is enough water to push the equilibrium towards the forward reaction, water will attack this carbon. You next have to increase the pH of this reaction and the water group will deprotonate leaving you with a geminal diol. This is essentially two step wise equilibrium reactions so you need to carefully control it:

Please Ignore the periods...I had to use them as spaces because for some reason, the space bar did not work to space them out.

..O.............OH+..................OH...........OH
..||...H+.....||......................|....-H+....|
HCH <-> H-C-H + H2O <-> H-C-H <-> H-C-H
.........................................|............|
.......................................HOH+........OH

 

[Slide]

I remember this bad-boy reaction. There are a few ways you can make a diol on an alkene. This is probably the simplest way

Add MCPBA to the alkene to form an epoxide. Then, add acid in water to open the epoxide and form a trans diol compound. This is probably what you were referring to earlier. Then, you can use whatever oxidizing agent to form carbonyls.

Another way is to use Br2 to substitute two halogens on there, then use strong basic conditions to attack the halogens. The yield probably is not as good as the first method though.

 

[LabMonster]

How about OsO4 in pyridine to make your diol?

 

[dilated]

Q of Qumica is correct, but don't forget Cram's rule which states that the lowest energy conformation of a carbonyl compound has the largest substituent perpindicular to the carbonyl compound. Once you place your molecule in that configuration, the Nu: will want to attack from the side where it goes through the smallest substituents.

Also, directional attack on a sp2 can be influenced by non-steric factors to get a desirable ee, like with the Sharpless epoxidation. For MCAT-level purposes though you get a racemic mixture of attack from both sides.