Cthode rays Experiment (MOURYA SIR)

Abnormal Reimer Tiemann Reaction (MOURYA SIR)

Imortant for IIT JEE : Abnormal Reimer Tiemann Reaction (MOURYA SIR)

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Paper 1 Solution

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Abhishek Mourya

Preparation of alcohol from Grignard reagent

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SN1 reaction mechanism animation ( MOURYA SIR)

SN2 Reaction mechanism Animation

Diffusion of gases HCl and NH3

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R and S Absolute Nomenclature (Abhishek Mourya Sir) PATANG IIT JEE Forum

Rand S can be used to describe the configuration of a chiral centre

Before going on to talk about single enantiomers of chiral molecules in more detail, we need to
explain how chemists explain which enantiomer they’re talking about. We can, of course, just draw a
diagram, showing which groups go into the plane of the paper and which groups come out of the
plane of the paper. This is best for complicated molecules. Alternatively, we can use the following set
of rules to assign a letter, R or S, to describe the configuration of groups at a chiral centre in the
molecule.

Here again is the enantiomer of alanine you get if you extract alanine from living things.

1 Assign a priority number to each substituent at the chiral centre. Atoms with higher atomic
numbers get higher priority.

Alanine’s chiral centre carries one N atom (atomic number 7), two C atoms (atomic number 6),
and one H atom (atomic number 1). So, we assign priority 1 to the NH2 group, because N has the
highest atomic number. Priorities 2 and 3 will be assigned to the CO2H and the CH3 groups, and
priority 4 to the hydrogen atom; but we need a way of deciding which of CO2H and CH3 takes
priority over the other. If two (or more) of the atoms attached to the chiral centre are identical,
then we assign priorities to these two by assessing the atoms attached to those atoms. In this case,
one of the carbon atoms carries oxygen atoms (atomic number 8), and one carries only hydrogen
atoms (atomic number 1). So CO2H is higher priority that CH3; in other words, CO2H gets
priority 2 and CH3 priority 3.

2 Arrange the molecule so that the lowest priority substituent is pointing away from you.
In our example, naturally extracted alanine, H is priority 4, so we need to look at the molecule
with the H atom pointing into the paper, like this.

3 Mentally move from substituent priority 1 to 2 to 3. If you are moving in a clockwise manner,
assign the label R to the chiral centre; if you are moving in an anticlockwise manner, assign the
label S to the chiral centre.

A good way of visualizing this is to imagine turning a steering wheel in the direction of the
numbering. If you are turning your car to the right, you have R; if you are turning to the left you
have S. For our molecule of natural alanine, if we move from NH2 (1) to CO2H (2) to CH3 (3)
we’re going anticlockwise (turning to the left), so we call this enantiomer (S)-alanine.

You can try working the other way, from the configurational label to the structure. Take lactic
acid as an example. Lactic acid is produced by bacterial action on milk; it’s also produced in your
muscles when they have to work with an insufficient supply of oxygen, such as during bursts of vigorous exercise. Lactic acid produced by fermentation is often racemic, though certain species of bacteria produce solely (R)-lactic acid. On the other hand, lactic acid produced by anaerobic respiration in muscles has the S configuration.

Newman projection of cyclohexane

Newman projection of cyclohexane (chair and boat form)

A closer look at cyclohexane

The heats of combustion data show that cyclohexane is virtually strain-free. This must include strain

from eclipsing interaction as well as angle strain. A model of the chair conformation of cyclohexane

including all the hydrogen atoms looks like this.

The view along two of the C–C bonds clearly shows that there are no eclipsing C–H bonds in the
chair conformation of cyclohexane—in fact, all the bonds are fully staggered, giving the lowest
energy possible. This is why cyclohexane is strain-free.
Contrast this with the boat conformation. Now all the C–H bonds are eclipsed, and there is a particularly

bad interaction between the ‘flagstaff’ C–H bonds.

This explains why the boat conformation is much less important than the chair conformation.
Even though both are free from angle strain, the eclipsing interactions in the boat conformation
make it approximately 25 kJ mol–1 higher in energy than the chair conformation. In fact, as we shall
see later, the boat conformation represents an energy maximum in cyclohexane whilst the chair conformation
is an energy minimum. Earlier we saw how the eclipsing interactions in planar cyclobutane
and cyclopentane could be reduced by distortion of the ring. The same is true for the boat
conformation of cyclohexane. The eclipsing interactions can be relieved slightly if the two ‘side’ C–C

bonds twist relative to each other.

So we can say chair form is the most stable form of cyclohexane, since all bond in chair form are at perfect tetrahedral bond (109.5*) and all are staggered to each other, where as in boat form all bonds are eclipsed to each other hence, is less stable.

Effect of subsituent in Newman projection of cyclohexane (chair form)

If you want to add a substituent then equatorial position is more favorable then axial position, in order to have least steric crowding

Substituted cyclohexanes

In a monosubstituted cyclohexane, there can exist two different chair conformers: one with the substituent

axial, the other with it equatorial. The two chair conformers will be in rapid equilibrium (by

the process we have just described) but they will not have the same energy. In almost all cases, the

conformer with the substituent axial is higher in energy, which means there will be less of this form

present at equilibrium.

For example, in methylcylcohexane (X = CH3), the conformer with the methyl group axial is 7.3

kJ mol–1 higher in energy than the conformer with the methyl group equatorial. This energy difference

corresponds to a 20:1 ratio of equatorial:axial conformers at 25 °C.

There are two reasons why the axial conformer is higher in energy than the equatorial conformer.

The first is that the axial conformer is destabilized by the repulsion between the axial group X and the

two axial hydrogen atoms on the same side of the ring. This interaction is known as the 1,3-diaxial

interaction. As the group X gets larger, this interaction becomes more severe and there is less of the

conformer with the group axial.

The second reason is that in the equatorial conformer the C–X bond is anti-periplanar to

two C–C bonds, while, for the axial conformer, the C–X bond is synclinal (gauche) to two C–C bonds.

Organic question

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