Substitution problem help

[dirtmagurt]

For this reaction http://i.imgur.com/Y2Y1c.png?1?5611

 

We have to show how it occurs and it has to be subsitution because we haven't "learned" elimination yet. By this I mean it is a practice question for a test coming up where elimination will not be tested. I am able to get one of the products, the product where the CH3 group is not inverted but I don't understand how to get the second product where the methyl is inverted.

 

So I tried it again and I am not sure if this is correct but I wrote that the alcohol group on the Carbon that is also attached to a deterium gets protonated by the H3PO4. This makes h2O and h2Po4 and the H2O gets attacked by the nucleophilic OH (other alcohol) which makes an oxonium ion. The extra H+ then gets taken by the H2PO4 and that is one of the products? Is that correct because that is what my friend told me but I am not sure if it is correct or not and I was wondering if maybe this reaction has a hydride shift?

[CarbonCopy]

well its largely right. you just need to account for the steriochem for the reaction also. The alcholic grp with no deuterium attacks the other alch grp by sn2 and make that carbon inverted in config. hence h and deuterium get inverted. now 2 pdts will be formed depending on whether the alcohol moves forward ( and pushes the ch3 grp back ) or whether the alcohol moves back ( and pushes the ch3 grp forward ) and this accounts for the steriochem of the the 2 ch3 groups of the 2 pdts.

[hypervalent_iodine]

well its largely right. you just need to account for the steriochem for the reaction also. The alcholic grp with no deuterium attacks the other alch grp by sn2 and make that carbon inverted in config. hence h and deuterium get inverted. now 2 pdts will be formed depending on whether the alcohol moves forward ( and pushes the ch3 grp back ) or whether the alcohol moves back ( and pushes the ch3 grp forward ) and this accounts for the steriochem of the the 2 ch3 groups of the 2 pdts.

 

This is incorrect for a number of reasons.

• S_N2 reactions result in complete inversion of stereochemistry, not a racemate. That means that if this reaction were S_N2, you would only see one stereoisomer and not the other one.
• The sterochemistry of the carbon with the deuterium attached doesn't change its configuration. It's ® configuration in both starting material and product. The carbon that actually underwent substitution, however, is racemic. See:

 

 

 

• The carbon with the deuterium attached to it is not the carbon that gets attacked. If it were, you would not expect the other carbon centre to undergo racemization.
• You need to write more clearly. I couldn't understand a word of the last part of your post.

For this reaction http://i.imgur.com/Y2Y1c.png?1?5611

 

We have to show how it occurs and it has to be subsitution because we haven't "learned" elimination yet. By this I mean it is a practice question for a test coming up where elimination will not be tested. I am able to get one of the products, the product where the CH3 group is not inverted but I don't understand how to get the second product where the methyl is inverted.

 

One of the key differences between an S_N1 and an S_N2 reaction is the way they effect stereochemistry. In an S_N1 reaction, your leaving group is eliminated fromt he carbon undergoing attack to generate a carbocation intermediate. These types of intermediates are flat structures and attack from either face will result in a different steroisomer:

 

Image from http://www.organic-chemistry.org/namedreactions/nucleophilic-substitution-sn1-sn2.shtm

 

 

Because they are flat, an incoming nucleophile will attack either face of the carbocation without any preference, thereby resulting in an equimolar mixture of both ® and (S) isomers (i.e. a racemate).

 

S_N2 reactions do not undergo the same mechanism and result in an inversion of stereochemistry. The analogy I was taught in undergrad is to think of it as being similar to flipping an umbrella inside out.

 

Image from http://www.organic-chemistry.org/namedreactions/nucleophilic-substitution-sn1-sn2.shtm

 

So I tried it again and I am not sure if this is correct but I wrote that the alcohol group on the Carbon that is also attached to a deterium gets protonated by the H3PO4.

 

You need to think about this some more. Why would the carbon act as a nucleophile and pick up a proton? Since you know that this is a substitution reaction, the first things you should be trying to identify are:

 

1. The electrophilic carbon centre and whether it is a primary, secondary or tertiary centre.
2. The nuclophile.
3. The leaving group.

In this case, the electrophilic carbon (i.e., the carbon being attacked by the nucleophile and which undergoes substitution) is one of the benzylic carbons attached to the aromatic ring. Both are secondary centres, which you would typically say could undergo either an S_N1 or S_N2 reaction. To figure out which of these is occurring, you only need look at your products. Since you have a racemate, it must have undergone an S_N1. The reason for this is because the carbocation intermediate is able to be resonance stabilised by the adjacent ring.

 

Looking at your products, you should be able to see which carbon has changed in its stereochemistry and therefore, which carbon was attacked by your nucleophile.

 

This makes h2O and h2Po4 and the H2O gets attacked by the nucleophilic OH (other alcohol) which makes an oxonium ion. The extra H+ then gets taken by the H2PO4 and that is one of the products? Is that correct because that is what my friend told me but I am not sure if it is correct or not and I was wondering if maybe this reaction has a hydride shift?

 

This is not in any way correct. You are dealing with a simple substitution reaction, certainly not a hydride shift or anything else. You need to break this down and look at it step by step. Firstly, can you identify what your leaving group was? You're right to assume that the acid has done something, but you have not correctly identified what that is. Have a look at your starting material and try to identify which atoms are capable of donating a pair of electrons to pick up an extra proton. Hint: it's not the carbons.

[khadega]

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hypervalent_iodine i have learned alot thank u

[mooeypoo]

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