Name : Sadhana Sudhir Warangule Sub : Chemistry PPT (Organic chemistry) Topic : Hukels rules, chirality etc

2. PRESENTATION TOPIC:
Huckles rule& chirality and meso compunds

3. HUCKELS RULE
• 1931, German chemist and physicist Erich Hückel
proposed a rule to determine if a planar ring
molecule would have aromatic properties. This
rule states that if a cyclic, planar molecule has
4n+2π electrons, it is aromatic. This rule would
come to be known as Hückel's Rule.

4. Hückel’s Rule: What Does 4n+2 Mean?
• “4n+2 is not a formula that you apply to see if
your molecule is aromatic. It is a formula that tells
you what numbers are in the magic series. If your
pi electron value matches any number in this
series then you have the capacity for aromaticity.”
– Claire

5. FOUR CRITERIA FOR AROMATICITY
• When deciding if a compound is aromatic, go through the
following checklist. If the compound does not meet all the
following criteria, it is likely not aromatic.
• The molecule is cyclic (a ring of atoms)
• The molecule is planar (all atoms in the molecule lie in the
same plane)
• The molecule is fully conjugated (p orbitals at every atom
in the ring)
• The molecule has 4n+2π electrons (n=0 or any positive
integer)

6. The Huckel 4n + 2 Pi Electron Rule
• A ring-shaped cyclic molecule is said to follow
the Huckel rule when the total number of pi
electrons belonging to the molecule can be
equated to the formula ‘4n + 2’ where n can be
any integer with a positive value (including zero).

7. EXAMPLE
• Examples of molecules following Huckel’s rule
have only been established for values of ‘n’
ranging from zero to six. The total number of pi
electrons in the benzene molecule depicted below
can be found to be 6, obeying the 4n+2 𝛑 electron
rule where n=1.

9. • Thus, the aromaticity of the benzene molecule is
established since it obeys the Huckel rule.
• This rule is also justified with the help of the
Pariser-Parr-Pople method and the linear
combination of atomic orbitals (LCAO) method.
• Generally, aromatic compounds are quite stable
due to the resonance energy or the delocalized
electron cloud. For a molecule to exhibit aromatic
qualities, the following conditions must be met by
it:

11. • Other examples of aromatic compounds that
comply with Huckel’s Rule include pyrrole,
pyridine, and furan. All three of these examples
have 6 pi electrons each, so the value of n for
them would be one.

12. • Stability of Monocyclic Hydrocarbons
• The stability of monocyclic hydrocarbons, their
cations, and their anions can be understood with
the help of the Huckel Rule. A great example of
such a monocyclic hydrocarbon is benzene.

13. • It can be observed that benzene tends to undergo
substitution reactions wherein the number of pi
electrons remains the same in the product.
Benzene does not tend to take part in addition
reactions which would cause it to lose its pi
electrons. However, catalysts are generally a
prerequisite for the benzene molecule to
participate in a substitution reaction. This stability
of the pi electron system belonging to benzene is
often referred to as ‘aromaticity’.

14. • Considering the example of cyclopentadiene, its
corresponding anion (cyclopentadienyl anion) can
easily be generated since it has six pi electrons
and is quite stable. On the other hand, the cation
of cyclopentadiene only has four pi electrons,
which implies that it does not exhibit aromaticity
as per Huckel’s rule. This cation is quite difficult to
generate, especially when compared to its acyclic
counterpart – the acyclic pentadienyl cation.

15. • Thus, Huckel’s rule is very useful in the estimation
of the aromaticity (and therefore the stability) of
ring-shaped molecules of planar structures.

16. CHIRALITY
• Molecules that form nonsuperimposable mirror
images, and thus exist as enantiomers, are said to be
chiral molecules.
• For a molecule to be chiral, it cannot contain a plane
of symmetry. A plane of symmetry is a plane that
bisects an object (a molecule, in this case) in such a
way that the two halves are identical mirror images.
An example of a structure that has a plane of
symmetry is a cylinder.

17. CHIRALITY DEFINITION
• In chemistry, a molecule or ion is called chiral if it
cannot be superposed on its mirror image by any
combination of rotations and translations. This
geometric property is called chirality.
• The terms are derived from Ancient Greek χείρ,
meaning "hand"; which is the canonical example of
an object with this property.

18. • Cutting a cylinder in half lengthwise generates two
halves that are exact mirror images of each other.
• A molecule that possesses a plane of symmetry in
any of its conformations is identical to its own
mirror image.
• Such molecules are achiral, or nonchiral. Butane
is an achiral molecule, while 2‐bromobutane is
chiral.

21. • The van't Hoff rule predicts the maximum number
of enantiomers an optically active molecule can
possess. This rule states that the maximum
number of enantiomers a molecule can have is
equal to 2 raised to the nth power, where n equals
the number of stereogenic centers. The molecule
2‐chlorobutane has one stereogenic center, so two
enantiomers are possible.

23. MESO COMPOUNDS
• A meso compound or meso isomer is a non-
optically active member of a set of stereoisomers,
at least two of which are optically active.
• This means that despite containing two or more
stereogenic centers, the molecule is not chiral.
• A meso compound is "superposable" on its mirror
image.

24. • In general, a meso compound should contain two
or more identical substituted stereocenters. Also,
it has an internal symmetry plane that divides the
compound in half.
• These two halves reflect each other by the
internal mirror. The stereochemistry of
stereocenters should "cancel out"

25. • What it means here is that when we have an
internal plane that splits the compound into two
symmetrical sides, the stereochemistry of both
left and right side should be opposite to each
other, and therefore, result in optically inactive.
• Cyclic compounds may also be meso.

26. IDENTIFICATION
• If A is a meso compound, it should have two or more
stereocenters, an internal plane, and the stereochemistry
should be R and S.
• Look for an internal plane, or internal mirror, that lies in
between the compound.
• The stereochemistry (e.g. R or S) is very crucial in
determining whether it is a meso compound or not. As
mentioned above, a meso compound is optically inactive,
so their stereochemistry should cancel out. For instance, R
cancels S out in a meso compound with two stereocenters.

27. trans-1,2-dichloro-1,2-ethanediol

28. (meso)-2,3-dibromobutane

29. EXAMPLE
• This molecule has a plane of symmetry (the
horizontal plane going through the red broken
line) and, therefore, is achiral; However, it has two
chiral carbons and is consequentially a meso
compound.

31. EXAMPLE 2
• This molecules has a plane of symmetry (the
vertical plane going through the red broken line
perpendicular to the plane of the ring) and,
therefore, is achiral, but has has two chiral
centers. Thus, its is a meso compound.