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What is Markownikoff’s/ Markovnikov’s Rule?

Markovnikov’s rule was proposed by Russian chemist Vladimir Markovnikov in the year 1869. It is the condition that applies to the unsymmetrical alkene or alkyne predicting the regiochemistry of the addition of hydrogen halide to give alkyl halideAs per the Markovnikov rule,

‘For an unsymmetrical alkene or alkyne, when treated with hydrogen halide HX, the negative part of the reagent attaches to carbon having less number of hydrogen across the double bond giving alkyl halide.’

Regiochemistry of Markovnikov's Rule

In short, the negative ion attacks the most substituted carbon with less hydrogens across the double bond. A mnemonic or shortcut to remember this is-

‘Markovnikov says - NO Member Shall Cheat.'

Where,

NO stands for Negative iOn

Member Shall Cheat for Most Substituted Carbon.

Mnemonic Markovnikov Rule
The negative ion comes from the hydrogen halide (HX) that is polar in nature. The electronegative halogen carries a partial negative charge and the hydrogen a partial positive charge. Amongst the hydrogen halides, the reactivity order is-

H-I>H-Br>H-Cl>H-F.

The acidity of HI is highest in the group as it is easy to break the HI bond. The conjugate base I- is more stable and nucleophilic than HBr, HCl and least is for HF. 

Order of Reactivity HaloAcid

The addition of haloacid (HX) to the double bond is a two step process and goes via the electrophilic addition mechanism.

electrophilic addition mechanism double bond HBr

In the first step, the π bond of the nucleophilic alkene first picks up the positively charged proton to give a carbocation intermediate. As the 2o carbocations are stabilized by hyperconjugation and induction it is more stable than 1o hence when a double bond picks up a proton; it tends to form more stable 2o carbocation than 1o.

After the formation of the carbocation at most substituted carbon, the attack of the negatively charged halogen takes place giving the alkyl halide. The loss of one π bond and formation of two σ bonds makes the reaction exothermic and energetically favorable.

If two carbon atoms of the double bond are equally substituted by hydrogen, then two products would be formed in equal ratios.

Unsymmetrical Alkene Markovnikov Addition

When hydrogen halide adds to the alkyne, it gives vinyl halide. In the presence of an excess of Hydrogen halide, a second addition of HX results in geminal dihalides. The mechanism is similar to the alkene addition forming a stable carbocation followed by the attack of the bromide ion.

Markovnikov Addition To Alkynes

Let’s look at few examples of alkenes following Markovnikov's Rule for the addition of HBr wherein the bromide will add to the most substituted carbon or the carbon carrying less hydrogens across the double bond-

Examples Markovnikov's Rule

From 1-Propene after Markovnikov’s addition of HBr, we get 2-bromopropane.

2-methyl-prop-1-ene on the addition of HBr according to the rule gives 2-bromo-2-methyl-propane.

2- butene  is a symmetrical alkene (equally substituted) and it need not follow a rule for the addition of HBr across the double bond. Addition from either side would give the same product.

The symmetrical nature of 2-butene is lost on substituting a methyl at the 3rd position to give 3-methyl-but-2-ene and it now it obeys Markovnikov’s rule to give 2-bromo-2-methylbutane.

Remember, the formation of most stable carbocation is preferred by Markovnikov’s rule.

The Markovnikov reaction rule for addition to the unsymmetrical alkene is not only followed by the hydrogen halide but also by other reagents such as water, halogen, and water, Water and H2SO4, Iodine monochloride, with the more electronegative atom carrying the negative charge.

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Cleavage Property of Crystalline Solids

Differentiation between Crystalline and Amorphous solids based on the Cleavage Property



If a crystalline solid is cut with a sharp object, it would always give parts with smooth edges whereas an amorphous solid would cut into surfaces with rough, uneven edges. Crystalline solids are therefore said to have cleavage property, and amorphous solids do not show cleavage property.

Do amorphous solids show cleavage property

Imagine you are passing a raw potato through a slicer blade; what you get is thin slices of potato with smooth edges. But if you pass a boiled potato through a slicer, it would crumble and fall apart giving small uneven pieces with rough edges. The raw potato represents a crystalline solid and the boiled potato acts like an amorphous solid.

Why Crystalline Solids show Cleavage Property?

The cleavage property is shown by crystalline solids because they possess cleavage planes. In a crystalline solid, the cells are neatly stacked. The cleavage planes are areas where the crystal structure is the weakest. It is only along these planes that a crystalline solid can be cut. Therefore, a cut from a sharp object would give two smooth parts. Amorphous solids do not show any cleavage planes.

crystalline solid cleavage planes

If you look at the crystal structure, you will notice a constant arrangement of the unit cells. In the two-dimensional diagram, it is seen as a lattice. Usually, the cuts made in the direction of the linear sequence of points shown in green are preferred over other cuts. The cuts along the direction of red are not allowed. These are the cuts that can shatter or disintegrate the crystal. In other words, the cuts that preserve the arrangement of the particles are preferred in a crystalline solid.

cleavage planes crystalline solid
An example is of the diamond, a crystalline solid that when cut along the cleavage planes gives small diamonds with smooth edges having the same arrangement of the particles as that of the parent diamond.

cleavage planes in crystals

Let me give you another example. A NaCl crystal structure has a cubic arrangement. Its cleavage plane is parallel to the cube faces. If cut along any other plane as shown in red, the crystal structure will shatter.

NaCl cleavage plane
Every crystal has a unique cleavage plane depending on the arrangement of the particles. For example, crystals gypsum, feldspar and calcite have one, two and three cleavage planes.
The amorphous solid quartz does not have well-defined cleavage planes. It does not show cleavage property and breaks unevenly giving rough edges.

examples cleavage planes crystalline amorphous solids

In summary, crystalline solids show cleavage property and is the reason why crystalline solids are smoother than the amorphous solids. Cut along the cleavage plane results in getting crystalline solid with smooth edges. Cleavage planes exist due to the ordered arrangement of the atoms thereby giving smaller crystalline solids of same geometric arrangement as the parent. On the other hand, the constituent particles of the amorphous solids are randomly arranged and do not show cleavage property. They break into uneven pieces with rough edges when cut.

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Crystalline Versus Amorphous Solids- Anisotropy and Isotropy

Anisotropy and Isotropy

Amorphous solids are said to be isotropic, and crystalline solids are anisotropic for their physical property measurements.

Isotropy comes from the Greek word; iso means same and tropos means direction. The name rightly indicates that for the amorphous solids; the physical property measurements are same in all the directions. The same correlation applies for anisotropy that means no same direction. It means that for the crystalline solids, the physical property measurements are not same in all the directions.

The physical properties that depend on direction for taking measurements and therefore affected by the nature of the solid are- refractive index, electrical conductivity, thermal conductivity, photoelasticity and many more.

For the crystalline solids; the arrangement of particles is ordered and periodic, then why does the physical property measurement changes? How does it become anisotropic?

isotropic and anisotropic crystals

What it implies is that, if physical property measurements are taken along the x-axis, its reading will be different than if measured from another axis, say y-axis or the z-axis. Different direction will give different measurements. But this is not the case with amorphous solids. For the amorphous solids, same values will be obtained irrespective of the direction of the measurement.

An example will help us to understand this better. A refractive index measurement is taken for crystal calcite and amorphous solid glass at a single wavelength keeping the direction fixed at the x-axis. For the crystal calcite, the values ranged from 1.4 to 1.6 but for the glass the values ranged from 1.50-1.52 only. When the direction of the measurement changed, the values changed drastically for calcite but remained the same for glass at 1.50-1.52.

isotropic and anisotropic solids examples

Anisotropy is observed in crystalline solids because the concentration of the atoms is different in different directions of the unit cell. If you look along the X-axis, the concentration of particles around it is different than the distribution of atoms around the y-axis and same is for the z-axis. As the concentration of the particles in a particular direction of the crystal changes, therefore, the measurement of physical property changes depending on the direction of the measurement.

example anisotropy in crystalline solids

Amorphous solids have a tightly packed random arrangement of the constituent particles, unlike the crystalline solids that have a fixed arrangement of atoms in a crystal lattice. Due to the random arrangement, the distribution of particles would be widely different along each axis. Hence, an average value of the measurement is taken.

arrangement particles crystalline and amorphous structure of matter

But what if the crystalline solid has an equal perfect distribution of atoms in a unit cell like in a cubic structure, then would it be isotropic for all the properties? The answer is no. A perfectly arranged cubic crystal structure would be isotropic for some properties like refractive index but would be anisotropic for other properties like photoelasticity.

Example of anisotropy in crystalline solids

Therefore, in general, we can say that all crystalline solids are anisotropic for some of their physical properties and all amorphous solids are isotropic.  

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Difference Between Crystalline and Amorphous Solids

Difference between crystalline and amorphous solids

compare crystalline and amorphous solids

Let us take a look at the table to side by side compare the differences between crystalline and amorphous solid.

1) The first point of differentiation is appearance. While crystalline solids are orderly arranged in a regular fashion, amorphous solids do not have any regular arrangement.

2) Due to this crystalline solids have both long range and short range order whereas, amorphous solids only have short range order.

3) Crystalline solids have sharp melting point whereas amorphous solids melt over a range of temperature.

4) Crystalline solids show a definite heat of fusion whereas amorphous solids do not have a definite heat of fusion.

5) Crystalline solids undergo a clean cleavage when cut with a sharp object such as a knife. Amorphous solids, on the other hand, cut irregularly.

6) Crystalline solids show anisotropy in physical property measurements whereas amorphous solids are isotropic.

7) All these properties make crystalline solids, true solids and amorphous solids are called pseudo-solids or supercooled liquids.

The following statement can serve as a mnemonic to help you memorize these differentiation points;

Overeating Created Many New Health Problems for Alice

Where, the every first alphabet in the sentence stands for- 

Arrangement, Order, Cleavage property, Melting point, Nature, Heat of Fusion, and Physical property.

how do crystalline solids differ from amorphous solids

Crystalline solids behave like the good guy displaying positive behavior for all the properties. They are true solids, regularly arranged, show cleavage property, exhibit definite heat of fusion and a sharp melting point. But as every good guy has a weakness, for the crystalline solids, it is anisotropy for physical property measurements. On the other hand, amorphous solids behave like the bad guy but have one good quality and that is isotropy.

Each differentiation point mentioned here will be explained in detail in the following videos so that you understand the concept better.

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Solid State Chemistry (Class 12 Videos)

A) Solid State- Introduction

A matter can exist in four states- solid, liquid, gaseous and plasma. The main property that differentiates the solid from the other states of matter is that it possesses rigidity or hardness. The liquids and gases are not hard in nature and have an ability to flow, therefore, possesses fluidity.

characteristics of solid matter (Class 12 Chemistry)

Solids have a definite mass, shape and volume whereas Liquids have a definite volume but no definite shape. It takes the shape of the container it occupies. Gases have no definite shape and no definite volume.

Solids being hard in nature, it cannot be compressed or pressed into smaller shapes. In short, the solids are incompressible. They can only be cut into smaller shapes. The constituent particles that form the solid structure do not move from its place and occupy fixed positions. It is the strong intermolecular forces that are responsible for holding these molecules in place. The distance between these particles that is their intermolecular distance is also less. 

Although these molecules occupy fixed positions, but these molecules are not stationary. They show motion known as the vibrational motion. These particles oscillate or vibrate about their mean position like the pendulum going from side to side constantly. This vibrational energy that solids possess is less. But if this energy is increased by supplying the solid with thermal or the heat energy, in that case, the increase in temperature can increase the vibrations and weaken the intramolecular forces. The particles will then become free and go into the liquid and the gaseous state. Therefore, low thermal and vibrational energy preserves the solid state. 

To summarize; the characteristic properties that define the solid state are:

1) Solids have definite shape, mass and volume

2) They are rigid and incompressible

3) The constituent particles (that is atoms, ions or molecules) occupy fixed positions

4) Strong intermolecular forces hold the constituent particles in one place

5) The intermolecular distance between the particles is less

6) The particles do not remain stationary but oscillate about their mean positions. In other words, the constituent particles have low vibrational energy and low thermal energy.

All these features are responsible for a substance to exist as a solid.

B) Types of Solids- 

i) Introduction to Crystalline and Amorphous Solids
Solids are made up of a vast number of particles. Depending on how the particles are arranged, solids can be divided into two categories- crystalline solids and amorphous solids. The particles may be in the form of atoms, ions or molecules. 
For example; gold atoms, NaCl ions, and sucrose molecules give solids- gold, salt, and sugar respectively. 

Types of solids-Crystalline solids-Amorphous solids

ii) Formation and Arrangement- If a solid is heated to a particular high temperature and the molten liquid is gradually cooled, the constituent particles get sufficient time to arrange themselves into a highly ordered crystalline solid structure. This process of heating a solid to a particular high temperature followed by the gradual cooling is called annealing, and it can affect the crystalline property of a solid. 
Examples of crystalline solids are Diamond, NaCl, and Quartz

If the molten liquid is not cooled gradually but is cooled rapidly or vapors of the liquid are frozen suddenly then, the particles do not get enough time to organize. The arrangement is then disordered forming an amorphous solid. Some of the examples of amorphous solids are Rubber, Glass, Tar, Plastic and Quartz glass.
In short, it is the cooling process of the molten liquid to solid that decides whether a crystalline solid is obtained or an amorphous. 

It is important to note that an ideal, perfect crystalline solid can only be obtained at 0K because particles do not have any thermal energy at this low temperature. There is no disorder, the entropy of the entire system is zero, and the particles can arrange themselves in perfect order. At all other temperatures, small irregularities always exist in the crystal structure and we do not obtain a perfect crystalline solid.

The smallest building block of a crystal is called a unit cell. In a three-dimensional diagram, a unit cell is shown as a box with corners. The atoms, ions or molecules that make up the crystalline solid structure occupy these corners of the box. The atoms may also be present at the centre of the unit cell, on the faces of the unit cell or at the edges of the unit cell in addition to the corners. These places that atoms occupy in a unit cell are called the lattice points. Multiples of unit cells repeat to give a crystal lattice. It is called a lattice because, in the two dimensions, a crystal looks like a series of points with criss-cross lines like a network or mesh. 

When the entire crystal is shown as lines and points in the three dimensions, it's called a crystal lattice. As the unit cells extend in all the three dimensions in space, therefore, a crystal lattice is also known as a space lattice. These repeating structures of the unit cell in all the three dimensions give an entire crystalline solid. 

Let me summarize this concept for you in brief. The unit cell is the basic unit of a crystalline solid. When the unit cells repeat periodically in all the direction maintaining the order of the arrangement of the particles, it gives a crystalline solid. A crystal lattice shows a part of the crystalline solid as lines and points denoting the three-dimensional arrangement of the unit cells. 

We can understand this better by comparing a crystalline solid with a living being. The biological cell is the basic unit of all the living organisms. Similarly, the unit cell is the basic unit of a crystalline solid. As the biological cells comes together to give multi-cellular organism. Similarly, many unit cells come together in all the three dimensions to give a crystalline solid. 

iii) Order- Crystalline solids are therefore said to have both short range and long range order that is the arrangement of the constituent particles are ordered and regularly repeat over a smaller region and also over an extended area. Due to this repeating arrangement, we can predict the structure of the entire crystal if the structure of one of the unit cell is known.

Amorphous solids do not have long-range order but may have short range order. It means that there may be small parts in the solids which may have an ordered arrangement of the constituent particles. There may exist small scattered regions having tiny crystals similar to the crystalline solids. Such crystals present in an amorphous solid are called crystallites.

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