Tuesday, 27 March 2018

Optical Isomerism

Optical Isomerism
(c) optical isomerism (an example of stereoisomerism, in terms of non- superimposable mirror images about a chiral centre)
(d) identification of chiral centres in a molecule of any organic compound.
Topic 17A: Chirality
1. know that optical isomerism is a result of chirality in molecules with a single chiral centre
2. understand that optical isomerism results from chiral centre(s) in a molecule with asymmetric carbon atom(s) and that optical isomers are object and non- superimposable mirror images
3. know that optical activity is the ability of a single optical isomer to rotate the plane of polarisation of plane-polarised monochromatic light in molecules containing a single chiral centre
4. understand the nature of a racemic mixture
5. be able to use data on optical activity of reactants and products as evidence for SN1 and SN2 mechanisms
3.3.7 Optical isomerism (A-level only)
Compounds that contain an asymmetric carbon atom form stereoisomers that differ in their effect on plane polarised light. This type of isomerism is called optical isomerism.
Optical isomerism is a form of stereoisomerism and occurs as a result of chirality in molecules, limited to molecules with a single chiral centre.
An asymmetric carbon atom is chiral and gives rise to optical isomers (enantiomers), which exist as non super- imposable mirror images and differ in their effect on plane polarised light.
A mixture of equal amounts of enantiomers is called a racemic mixture (racemate).
Students could be asked
• to recognise the presence of a chiral centre in a given structure in 2D or 3D forms. They could also be asked to draw the 3D representation of chiral centres in various species.
• draw the structural formulas and displayed formulas of enantiomers
• understand how racemic mixtures (racemates) are formed and why they are optically inactive.
Optical Isomerism
There is an incredible confusion about optical isomerism in A level courses and students’ thinking at this level. 
The confusion arises because we so often start in the wrong place to try and understand what’s going on here.
So let’s begin where it is best to begin with the experimental behavior of molecules and optical activity.  Then move to the structural explanation.

What is optical isomerism?
Optical Isomerism is a form of isomerism in certain molecules that enables an aqueous solution of an organic compound such as an amino acid to rotate the plane of plane–polarised light in an instrument called a polarimeter.
The rotation can happen in one of two different directions.
Here is a diagram of a polarimeter:

Above you can see a picture of a typical polarimeter and the 10cm sample tube used in the instrument.
As you can see above the light is first polarized so that there is only one plane for the light’s electric vector.
This is plane polarized light.
This plane polarized light is then passed through the polarizing solution under test.
The solution of say the amino acid or sucrose is of a given concentration and length (10cm).  As the light passes through the solution, the molecules of the amino acid or sugar rotate the plane of the electric vector a given number of degrees either to the right or to the left.
This rotation can be measured as the light emerges from the solution and enters the analyser.  You can see this illustrated in the two diagrams below.

If the substance in solution rotates the plane of plane polarised light it is said to be optically active.

The question is now why is it optically active?
What are the structural features of molecules that enable them to rotate the plane of plane polarized light?
There are several features of molecular structures that give rise to optical activity in molecules but only one that comes up at A level.
The one feature of molecules that give rise to optical activity in these A level courses is that the molecule contains at least one chiral centre.
But what is a chiral centre?
Amino acids are a prime example of chiral molecules because they have four different functional groups attached to a central carbon atom. 
Here we see how that works in practice:
Each type of amino acid is called an enantiomer.
An aqueous solution of one will rotate the plane–polarized light to the left and the other to the right if both solutions are of the same length and concentration.
Another thing, these two isomers are non–superimposable mirror images of each other.  (As in topology no number of rotations of objects will resolve mirror images of objects).
How do we tell them apart?
Here is a fool proof method in this illustration.
But note that the molecular structure does not correspond to the rotation direction.  But the structure of the D– form of the acid does correspond to a reading of the functional groups so reading the D– form functional groups clockwise spells CO R N. 
This is a convenient way to remember the structure of a D– amino acid.  However, it does NOT mean that the acid will rotate the plane of polarized light to the right in your polarimeter.  As in the illustration above the D– alanine is laevorotatory—it rotates the plane polarized light to the left!!
R and S are now used to designate these two structures.
Any molecule that contains a carbon atom which has four different functional groups attached will be optically active.  Such a carbon atom is called an asymmetric carbon atom and designated with a star symbol.
If there is a mixture of equal moles of D– molecules and L– molecules then such a mixture/solution is called a racemic mixture and does not rotate plane polarized light.

Optical activity can be used to distinguish Sn1 from Sn2 mechanisms.  The next two images illustrate this effect quite well.

Thursday, 8 March 2018

Amines (3): Nucleophilic reactions


Topic 18B:9.
To understand the reactions of primary aliphatic amines, using butylamine as an example, with:
.    i  water to form an alkaline solution
.    ii  acids to form salts
.    iii  ethanoyl chloride
.    iv  halogenoalkanes
.    v  copper(II)ions to form complex ions
Nucleophilic properties of Amines
Amines are nucleophiles.
The nucleophilic substitution reactions of ammonia and amines with halogenoalkanes to form primary, secondary, tertiary amines and quaternary ammonium salts.
The use of quaternary ammonium salts as cationic surfactants.
The nucleophilic addition–elimination reactions of ammonia and primary amines with acyl chlorides and acid anhydrides.
Students should be able to outline the mechanisms of:
.    these nucleophilic substitution reactions 

.    the nucleophilic addition–elimination reactions of ammonia and primary amines with acyl chlorides. 

Reactions of Amines

Let’s first look at these simple reactions of amines with butylamine  C4H9NH2 as our example:

1. With water

C4H9NH2    +    H2O                    [C4H9NH3]+        +      OH

Like ammonia, butylamine is a water soluble weak base and forms butylammonium ( [C4H9NH3]+ ) ions in aqueous solution.

2. With acids to form salts

C4H9NH2    +    HCl                     [C4H9NH3]+        +      Cl 

3. With ethanoyl chloride

C4H9NH2        +    CH3COCl               C4H9NHCOCH3       +   H+     +   Cl   

This very fast room temperature reaction forms N-butyl ethanamide.  The picture below shows this reaction and in particular the evolution of the white fumes of the amine chloride.

4. With excess halogeno alkanes under pressure and at 120oC

C4H9NH2         +    CH3Cl               [C4H9NH2CH3 ]+     +   Cl   

Then methyl groups successively replace hydrogen atoms in the amine salts:

[C4H9NH2CH3 ]+     +   CH3Cl       [C4H9NH(CH3)2]+     +   Cl   


[C4H9NH(CH3)2]+     +   CH3Cl       [C4H9N(CH3)3]+     +   Cl   

The final product is the quaternary amine salt: trimethyl butyl ammonium chloride.

5. With copper(II) ions to form complex ions

Butylamine acts as if it were ammonia so that the nitrogen lone pair forms a dative covalent bond with the transition metal ion.  4 moles of the amine react with one mole of the copper ion as in the ammonia reaction and produce a square planar complex ion.

4C4H9NH2      +      Cu2+            [Cu(C4H9NH2)4 ]2+

In the equation above, I have removed all state symbols and other unnecessary ions (e.g. sulphate SO42—) in order to show up more clearly the formation of the complex. 

In the photo above of the reaction in aqueous solution the pale blue copper ion solution turns the deeper blue of the copper complex.

Let’s now recap the two significant mechanisms here:

First the mechanism for the reaction between a halogenoalkane and an amine:

The reaction depends on the nucleophilic character of the amine i.e. that it carries a lone pair of electrons on its nitrogen atom.  It is this lone pair that attacks the electropositive carbon atom in the halogenoalkane in step one.  Excess amine is then protonated to leave the free secondary amine and the amine salt: 

In the diagram above note that the amine is shown with a negative charge rather than a lone pair:

Second, the mechanism for the reaction between an amine and an acyl chloride
This again illustrates the nucleophilic character of the amine because of its nitrogen lone pair.

In the illustration below I have kept to the ammonia molecule for simplicity

But you can see that the first step is the nucleophilic attack on the electropositive carbon atom of the —COCl group. 

This attack leaves a species with a charge separation: the oxygen atom carries a negative charge and the nitrogen atom a positive charge. 

Therefore, the second step is the resolution of this charge separation as they come together.  In doing so, a molecule of HCl is eliminated. 
So an addition step is followed by an elimination step.

Hence this is called an addition-elimination mechanism.

This leaves a new molecule where the ammonia or amine group has replaced the labile chlorine atom. 

Here is summary chart for the substitution mechanism:

In the next blog, we’ll look at amides and amino acids.

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