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Topic: racemic mixture  (Read 7017 times)

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HongKongALevelboy

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racemic mixture
« on: August 28, 2004, 02:27:18 PM »
i need help , how can racemic mixture be formed ?

Offline Donaldson Tan

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Re:racemic mixture
« Reply #1 on: August 29, 2004, 12:57:55 AM »
hydrolysis of trichlorobromomethane should yield a racemic mixture.

the reaction mechanism is as follows:
CHBrCl(CH3) -> CHBr(CH3)+ + Cl-
CHBr(CH3)+ + OH- ->CHBr(CH3)(OH)

CHBr(CH3)+ is a trigonal planar intermediate.

Such a molecular shape allows the hydroxide ion to attack either above or below the intermediate. Each has a 50% chance of occurring. Attacking from above form D optical isomer whereas attacking from below form L optical isomer. Given the probability, an equimolar amount of D and L optical isomers are formed.

In this way, a racemic mixture is formed.

PS: Been modified to give u a chiral molecule
« Last Edit: August 31, 2004, 09:51:45 PM by geodome »
"Say you're in a [chemical] plant and there's a snake on the floor. What are you going to do? Call a consultant? Get a meeting together to talk about which color is the snake? Employees should do one thing: walk over there and you step on the friggin� snake." - Jean-Pierre Garnier, CEO of Glaxosmithkline, June 2006

HongKongALevelboy

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Re:racemic mixture
« Reply #2 on: August 29, 2004, 11:03:04 AM »
oh yeah , thx a lot , so a trigonal compound with a chiral carbon could give a racemic mixture , right ? but can u give an example of a organic compound which has a chiral carbon , but does not form a racemic mixture

Offline movies

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Re:racemic mixture
« Reply #3 on: August 29, 2004, 01:22:50 PM »
Geodome, your example of how a racemic mixture is formed is good, but the product of your example isn't chiral, so it wouldn't be a racemic mixture.

Racemic mixtures can be formed in several ways, depending on the substrate molecule.  The simplest way would be to react an achiral molecule with another achiral molecule in a way that makes a chiral carbon center somehow.  A good example would be the reduction of acetophenone with sodium borohydride to form sec-phenethyl alcohol.  Since the reagent can't distinguish between the two faces of the carbonyl group of acetophenone, you get a 50/50 mixture of reduction from each face.  That gives you a racemic mixture.

Geodome's example is right on in that trigonal planar carbon can be attacked from either side, it just turns out that attack from either side gives the same molecule product.  To have a chiral carbon you have to have four _different_ groups on the carbon.

Offline AWK

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Re:racemic mixture
« Reply #4 on: August 30, 2004, 02:35:20 AM »
CCl3+ + OH- -> C(OH)Cl3
The last compound does not exist - it decompose immediately to phosgene (COCl2)

Racemisation take place during exchanging of substituens at chiral centre through SN1 mechanism (first order nuclephilic substitution). Mechanism involves carbociaton that can be attacked from both sides forming racemate.
The other possibility is elimination followed by addition.

AWK

Offline Donaldson Tan

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Re:racemic mixture
« Reply #5 on: August 31, 2004, 01:53:24 PM »
apology for using an inappropriate example. I only wanted to highlight that a racemic mixture can be obtained via a planar complex.


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"Say you're in a [chemical] plant and there's a snake on the floor. What are you going to do? Call a consultant? Get a meeting together to talk about which color is the snake? Employees should do one thing: walk over there and you step on the friggin� snake." - Jean-Pierre Garnier, CEO of Glaxosmithkline, June 2006

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