Sn1 and Sn2 Reactions

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Let’s compare the SN1 and the SN2

The Mechanism:

  • The SN2 reaction is concerted. That is, the SN2 occurs in one step, and both the nucleophile and substrate are involved in the rate determining step. Therefore the rate is dependent on both the concentration of substrate and that of the nucleophile.
  • The SN1 reaction proceeds stepwise. The leaving group first leaves, whereupon a carbocation forms that is attacked by the nucleophile.

The Big Factor– this is the most important thing to understand about each reaction. What’s the one key factor that can prevent this reaction from occurring?

  • In the SN2 reaction, the big barrier is steric hindrance. Since the SN2 proceeds through a backside attack, the reaction will only proceed if the empty orbital is accessible. The more groups that are present around the vicinity of the leaving group, the slower the reaction will be. That’s why the rate of reaction proceeds from primary (fastest) > secondary >> tertiary (slowest)
  • In the SN1 reaction, the big barrier is carbocation stability. Since the first step of the SN1 reaction is loss of a leaving group to give a carbocation, the rate of the reaction will be proportional to the stability of the carbocation. Carbocation stability increases with increasing substitution of the carbon (tertiary > secondary >> primary) as well as with resonance.

The dependence of rate upon the substrate

  • For the SN2, since steric hindrance increases as we go from primary to secondary to tertiary, the rate of reaction proceeds from primary (fastest) > secondary >> tertiary (slowest).
  • For the SN1, since carbocation stability increases as we go from primary to secondary to tertiary, the rate of reaction for the SN1 goes from primary (slowest) << secondary < tertiary (fastest)

Remember that SN1 and SN2 reactions only occur for alkyl halides (and related compounds like tosylates and mesylates). If the leaving group is directly attached to an alkene or alkyne, SN1 or SN2 will not occur! The Nucleophile Factor:

  • The SN2 tends to proceed with strong nucleophiles; by this, generally means negatively charged nucleophiles such as CH3O(-), CN(-), RS(-), N3(-), HO(-), and others.
  • The SN1 tends to proceed with weak nucleophiles – generally neutral compounds such as solvents like CH3OH, H2O, CH3CH2OH, and so on.

The Solvent Factor:

  • The SN2 reaction is favored by polar aprotic solvents – these are solvents such as acetone, DMSO, acetonitrile, or DMF that are polar enough to dissolve the substrate and nucleophile but do not participate in hydrogen bonding with the nucleophile.
  • The SN1 reaction tends to proceed in polar protic solvents such as water, alcohols, and carboxylic acids. These also tend to be the nucleophiles for these reactions as well.

Stereochemistry Factor:

  • Since the SN2 proceeds through a backside attack, if a stereocenter is present the SN2 reaction will give inversion of stereochemistry.
  • By contrast, if the SN1 leads to the formation of a stereocenter, there will be a mixture of retention and inversion since the nucleophile can attack from either face of the flat carbocation.

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Elimination Reaction E1 and E2 Reaction

Elimination Reaction Definition

Elimination reactions can be defined as the chemical reactions which involve elimination of leaving group to form unsaturated compound. A general elimination reaction can be written as given below.

 Dehydration of alcohols or dehydrohalogenation (-HX) of haloalkanes are some typical examples of elimination reactions. In an elimination reaction, there are 3 main steps. First step is removal of proton that for a carbocation that form C-C pi bond in the molecule including breaking of the bond of the leaving group.

Elimination Reaction Mechanism

In an elimination reaction, loss of the leaving group results the formation of carbocation as an intermediate.

In the second step, the carbocation losses a proton in the presence of base to form pi-bond in the molecule.

The reaction influences with the stability of carbocation formed during the reaction.The mechanism of elimination reaction can be two types; E1 and E2.

E1 mechanism involves the formation of carbocation as an intermediate during the reaction. Therefore the rate of reaction depends on the stability of carbocation formed during the reaction. The bond of carbon atom and leaving group breaks in rate determining step. So a better the leaving group will increase the rate of reaction. Since base is not the part of rate determining step so it does not affect the E1 mechanism but influences the E2 mechanism as it is part of rate determining step in that mechanism. So we can say that E1 mechanism needs good leaving group, a stable carbocation and weak bases.

Elimination Reaction Examples

One of the most common examples of elimination reaction is formation of alkene from haloalkane in the presence of base.The elimination reaction proceeds with E2mechanism in the presence of base. The attack of base and elimination of leaving group occur simultaneously to form alkene. The hydrogen atom that is removed by the base must be at anti-position of the leaving group. 

of alcohol in the presence of base like ethoxide ion (C2H5O) is also an example of elimination reaction.

Beta Elimination Reaction

Beta elimination reaction can be defined as the chemical reaction in which the leaving group and H are placed at neighbor carbon atoms. This is called as beta elimination. For example; dehydration of alcohol is betaelimination. Heating of alcohol in the presence of strong acid like H2SO4
causes 1,2 – elimination or betaelimination. It results the formation of an alkene and water. Since water is removed during the reaction so it is also called as dehydration reaction. The reaction proceeds with the formation of carbocation therefore the reactivity order of alcohol is tertiary > Secondary > Primary alcohol. The elimination reaction follows the Zaitsev’s rule according to which the more highly substituted alkene forms as the major product. Out of cis and trans-isomers, trans-isomer is more stable therefore trans-isomer is always preferred over cis form. The carbocation that is formed during reaction can rearrange by 1, 2- hydride or 1, 2-alkyl shift to form the stable product. Since primary carbocations are not so stable so they follow the E2 mechanism and reaction proceeds with the formation of transition state as an intermediate.

Example of Beta Elimination Reaction

Elimination reaction removes certain groups from the reactant molecule and forms an unsaturated compound as a product. There are several examples of beta elimination reactions.

  • Reaction of alcohol with mineral acid:

An alcohol reacts with mineral acid at high temperature to form alkene with elimination of water molecule. The reaction can proceed with E1 or E2mechanism. E1 mechanism occurs with formation of carbocation whereas E2mechanism proceeds with a concerted path in single step with the formation of transition state.

  • Another example of beta elimination is formation of alkene from alkyl halide in the presence of base.

The rate determining step involves the formation of transition state due to reaction of alkyl halide with base like ethoxide ion.The presence of base eliminates betahydrogen atom with leaving group that results the formation of alkene. In transition state, the bond between base and H forms whereas bond between C and leaving group and C-H is about to cleave. It results the formation of pi-bond between both carbon atoms. For beta elimination reaction betahydrogen and the leaving group should be on the same plane. Because reaction involves;

  • Formation of C-C pi-bond
  • Cleavage of C-H and C-X bond

Since pi-bond is formed by the sideways overlap of two p-orbitals therefore for maximum overlapping both orbitals must be parallel to each other. Hence beta-H and leaving group should be on the same plane. Bothbeta-H and leaving group should preferably be anti-coplanar that results the anti-elimination. Reaction involves the formation of more stable more substituted alkene as major product.

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PINACOL PINACOLONE REARRANGEMENT

Pinacol is a compound which has two hydroxyl groups, each attached to a vicinal carbon atom. It is a solid organic compound which is white in colour. The IUPAC name of Pinacolone is 3,3-dimethyl-2-butanone. Pinacolone is a very important ketone.

Pinacol and Pinacolone

Pinacol is a compound which has two hydroxyl groups, each attached to a vicinal carbon atom. It is a solid organic compound which is white in colour.

The IUPAC name of Pinacolone is 3,3-dimethyl-2-butanone. Pinacolone is a very important ketone. It has a peppermint like or camphor like odour and appears to be a colorless liquid.

Pinacol Pinacolone Reaction

Pinacol Pinacolone Rearrangement Reaction
PINACOL PINACOLONE REARRANGEMENTS

The pinacol pinacolone rearrangement proceeds through the formation of an intermediate which is positively charged. The methyl group in this intermediate proceeds to migrate from one carbon to another. This reaction can be given by:

CHEMQUEST DURGAPUR

Pinacol Pinacolone Rearrangement Mechanism

The Pinacol Pinacolone rearrangement mechanism proceeds via four steps. Each of these steps are explained below.

Step 1: Since the reaction is carried out in an acidic medium, the hydroxide group of the pinacol is protonated by the acid.

Step 2: Water is now removed from the compound, leaving behind a carbocation. This carbocation is tertiary and therefore stable.

Step 3: The methyl group shifts to the positively charged carbon in a rearrangement of the compound.

Step 4: The oxygen atom which is doubly bonded to the carbon is now deprotonated, giving rise to the required pinacolone.

This reaction mechanism can be illustrated as:

Pinacol Pinacolone Rearrangement Mechanism
MECHANISM OF PINACOL PINACOLONE REARRANGEMENTS

Thus, the required Pinacolone product is generated. It is important to note that this rearrangement is regioselective in nature. The rearrangement of the more stable carbocation yields the major product.

Pinacol-Pinacolone Rearrangement

CHEMQUEST DURGAPUR

Pinacol-Pinacolone Rearrangement

A 1,2-methyl shift generates an even more stable carbocation in which the charge is delocalized by heteroatom resonance. Indeed, this new cation is simply the conjugate acid of the ketone pinacolone, which is the product of repeated rearrangements catalyzed by proton transfer.

HYBRIDIZATION AND MOLECULAR STRUCTURE

What are the different types of hybridization?

Based on the nature of the mixing orbitals, the hybridization can be classified as,

  • sp hybridization (beryllium chloride, acetylene)
  • sp2 hybridization (boron trichloride, ethylene)
  • sp3 hybridization (methane, ethane)
  • sp3d hybridization (phosphorus pentachloride)
  • sp3d2 hybridization (sulfur hexafluoride)
  • sp3d3 hybridization (iodine heptafluoride)

More s character :

Due to the spherical shape of s orbital, it is attracted evenly by the nucleus from all directions. Therefore, a hybrid orbital with more s-character will be closer to the nucleus and thus more electronegative. Hence, the sp hybridized carbon is more electronegative than sp2 and sp3.

CHEMQUEST DURGAPUR

HYBRIDISATION

We know orbitals of last shell overlap with each other.  The overlapping is of two types:

  • Head to head(sigma bond)
  • Sidewise(pi bond)

Overlapping of orbitals takes place between which has same energy. If in case, the orbitals have different energy they can’t overlap. 

Hybridization is the intermixing of orbitals of slightly different energies, so as to redistribute their energy and give rise to new set of orbitals that are similar in shapes and energy.

CHEMQUEST DURGAPUR

Characteristics

  • Number of hybrid orbitals is equal to number of atomic orbitals that combine.
  • The hybrid orbitals are always equal in shape and energy.
  • The hybrid orbitals are more effective in forming bonds as compared to pure atomic orbitals.
  • The hybrid orbitals are directed towards specific directions in space.
  • The type of hybridization gives us the shape of molecule

Now there is a question that can all participate in hybridization

Conditions of hybridization

  • The only valence orbitals participate.
  • The atomic orbitals that participate should have almost same energy.
  • Promotion is not always necessary.
  • The unpaired as well as fully filled orbitals can also participate.

MOLECULAR ORBITAL THEORY

Salient features of MO THEORY: (i) Molecular orbitals are formed by the linear combination of atomic orbitals having nearly the same energies. (ii) Molecular orbitals are associated with the nuclei of the bonded atom in a molecule. (iii) The number of molecular orbitals formed is equal to the number of combining atomic orbitals.

AO VS MO: Orbitals can hold a maximum of two electrons. The main difference between atomic and molecular orbital is that the electrons in an atomic orbital are influenced by one positive nucleus, while the electrons of a molecular orbital are influenced by the two or more nuclei depending upon the number of atoms in a molecule.

LCAO : A linear combination of atomic orbitals, or LCAO, is a quantum superposition of atomic orbitals and a technique for calculating molecular orbitals in quantum chemistry.

In a mathematical sense, these wave functions are the basic functions that describe the a given atom’s electrons.

Antibonding orbitals are higher in energy because there is less electron density between the two nuclei. It takes energy to pull an electron away from a nucleus. Thus, when the electrons in an antibonding orbital spend less time between the two nuclei, they are at a higher energy level.

The BMO has lower energy (i.e. is more stablethan the two isolated atomic orbitals. The ABMO has higher energy (i.e. is less stablethan the two isolated atomic orbitals.

SUPRATIM DAS SIR

HYBRIDIZATION AND MOLECULAR STRUCTURE

Hybridization is the idea that atomic orbitals fuse to form newly hybridized orbitals, which in turn, influences molecular geometry and bonding properties. 

Hybridization is the concept of mixing atomic orbitals into new hybrid orbitals (with different energies, shapes, etc., than the component atomic orbitals) suitable for the pairing of electrons to form chemical bonds in valence bond theory.

Now that carbon has four unpaired electrons it can have four equal energy bonds. The hybridization of orbitals is also greatly favored because hybridized orbitals are lower in energy compared to their separated, unhybridized counterparts. This results in more stable compounds when hybridization occurs in methane, sp3 hybrid orbitals.

The term “sp3 hybridization” refers to the mixing character of one 2s-orbital and three 2p-orbitals to create four hybrid orbitals with similar characteristics. In order for an atom to be sp3 hybridized, it must have an s orbital and three p orbitals.

“It is called as sp3 because it is formed by intermixing of one s and three p orbitals which results in the formation of four sp3 hybrid orbitals which have same shape and energy”.

Hybridization

Hybridization is defined as the concept of mixing two atomic orbitals with the same energy levels to give a degenerated new type of orbitals. This intermixing is based on quantum mechanics. The atomic orbitals of the same energy level can only take part in hybridization and both full filled and half-filled orbitals can also take part in this process provided they have equal energy.

During the process of hybridization, the atomic orbitals of similar energy are mixed together such as the mixing of two ‘s’ orbitals or two ‘p’ orbital’s or mixing of an ‘s’ orbital with a ‘p’ orbital or ‘s’ orbital with a ‘d’ orbital.

Key Features of Hybridization

  • Atomic orbitals with equal energies undergo hybridization.
  • The number of hybrid orbitals formed is equal to the number of atomic orbitals mixing.
  • It is not necessary that all the half-filled orbitals must participate in hybridization. Even completely filled orbitals with slightly different energies can also participate.
  • Hybridization happens only during the bond formation and not in an isolated gaseous atom.
  • The shape of the molecule can be predicted if hybridization of the molecule is known.
  • The bigger lobe of the hybrid orbital always has a positive sign while the smaller lobe on the opposite side has a negative sign.

CHEMQUEST DURGAPUR

What are the different types of hybridization?

Based on the nature of the mixing orbitals, the hybridization can be classified as,

  • sp hybridization (beryllium chloride, acetylene)
  • sp2 hybridization (boron trichloride, ethylene)
  • sp3 hybridization (methane, ethane)
  • sp3d hybridization (phosphorus pentachloride)
  • sp3d2 hybridization (sulfur hexafluoride)
  • sp3d3 hybridization (iodine heptafluoride)

COVALENT BONDS (CHEMICAL BONDING AND MOLECULAR STRUCTURE)

A covalent bond, also called a molecular bond, is a chemical bond that involves the sharing of electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs, BP.

The three types as mentioned in the type of covalent are polar covalent, nonpolar covalent, and coordinate covalent. The first, polar covalent, is formed between two nonmetals that have a difference in electronegativity.

Covalent bonds can be single, double, and triple bonds. Single bonds occur when two electrons are shared and are composed of one sigma bond between the two atoms. Double bonds occur when four electrons are shared between the two atoms and consist of one sigma bond and one pi bond.

Examples of compounds that contain only covalent bonds are methane (CH4), carbon monoxide (CO), water (H2O). 

Covalent bonding between hydrogen atoms: Since each hydrogen atom has one electron, they are able to fill their outermost shells by sharing a pair of electrons through a covalent bond, called sigma bond.

Covalent and ionic bonds are both typically considered strong bonds. However, other kinds of more temporary bonds can also form between atoms or molecules.

Nonpolar covalent bonds are a type of chemical bond where two atoms share a pair of electrons with each other. Polar covalent bonding is a type of chemical bond where a pair of electrons is unequally shared between two atoms.

IONIC BONDS (CHEMICAL BONDING AND MOLECULAR STRUCTURE)

Ionic bonding is a type of chemical bonding that involves the electrostatic attraction between oppositely charged ions, and is the primary interaction occurring in ionic compounds.

It is one of the main types of bonding along with covalent bonding and metallic bonding. 

Ionic bonding is the complete transfer of valence electron(s) between atoms. It is a type of chemical bond that generates two oppositely charged ions.

In ionic bonds, the metal loses electrons to become a positively charged cation, whereas the nonmetal accepts those electrons to become a negatively charged anion.

The definition of ionic bond is when a positively charged ion forms a bond with a negatively charged ions and one atom transfers electrons to another.

An example of an ionic bond is the chemical compound Sodium Chloride.

Ionic bonding is the complete transfer of valence electron(s) between atoms and is a type of chemical bond that generates two oppositely charged ions.

Similarly, nonmetals that have close to 8 electrons in its valence shell tend to readily accept electrons to achieve its noble gas configuration.

Ionic bond to be stronger than covalent bonds due to the coulombic attraction between ions of opposite charges. To maximize the attraction between those ionsionic compounds form crystal lattices of alternating cations and anions.

Ionic bond, also called electrovalent bond, type of linkage formed from the electrostatic attraction between oppositely charged ions in a chemical compound. Such a bond forms when the valence (outermost) electrons of one atom are transferred permanently to another atom.

BOHR’S THEORY – ATOMIC STRUCTURE

The Bohr model shows the atom as a small, positively charged nucleus surrounded by orbiting electrons. 

Bohr was the first to discover that electrons travel in separate orbits around the nucleus and that the number of electrons in the outer orbit determines the properties of an element.

Postulates of Bohr’s atomic model : According to Bohr’s theory , electrons revolve in definite circular orbits around the nucleus and these orbits are designated by the letters K, L, M, N or by the numbers 1, 2 ,3, 4 and so on. … The angular momentum of aln electron is an integral multiple of h/ 2π mvr = nh/ 2π

The Bohr model can be summarized by the following four principles: Electrons occupy only certain orbits around the nucleus. Those orbits are stable and are called “stationary” orbits. Each orbit has an energy associated with it.

In 1913 Bohr proposed his quantized shell model of the atom to explain how electrons can have stable orbits around the nucleus. … The energy of an electron depends on the size of the orbit and is lower for smaller orbits. Radiation can occur only when the electron jumps from one orbit to another.

Main Points of the Bohr Model Electrons orbit the nucleus in orbits that have a set size and energy. The energy of the orbit is related to its size. The lowest energy is found in the smallest orbit. Radiation is absorbed or emitted when an electron moves from one orbit to another.

The Bohr model works only for hydrogen because it considers only the interactions between one electron and the nucleus. The Bohr model is based on the energy levels of one electron orbiting a nucleus at various energy levels. Any other electrons in the atom will repel the one electron and change its energy level.