Polar Protic And Polar Aprotic Solvents
There are two types of polar solvents polar protic and polar aprotic.
Polar protic solvents are capable of making hydrogen bonding i.e. they contain a hydrogen connected to an electronegative atom and thus can make intermolecular hydrogen bonding in addition to the dipole-dipole interactions. The most common polar protic solvents are the water and alcohols.
For example, when NaCl is dissolve in water, the sodium ion is solvated, through a dipole-dipole interaction, by the oxygen and the Cl ions are solvated by hydrogen bonding with water molecules:
Polar , on the other hand, are the ones without a hydrogen connected to an electronegative atom and the key difference compared to polar protic solvents is the lack of intermolecular hydrogen bonding.
For example, when the salt is added to a solution of DMSO, only the sodium ions are solvated and the Cl stays now as a naked ion:
To compare the effect of polar protic and parptic solvents, we can say that the protic solvent puts the nucleophile in a cage, thus making it weaker, while the polar aprotic solvent solvates the cation levig the nucleophile free. As a result, in the polar aprotic solvent, it becomes a stroger nucleophile since the couterion does reduce its reactivity.
There is a large group of common polar aprotic solvents such acetone, acetonitrile, DMF, DMSO, HMPA shown below:
Sn1 Vs Sn2 Leaving Groups
Sn1 and Sn2: Both sn1 and sn2 reactions require good leaving groups, so the nature of the leaving group does not impact the type of reaction very much. However, a very poor leaving group may prevent either reaction from occurring at all.
A good leaving group is one that is highly electronegative, because a leaving group needs to be able to take the electrons from its bond to leave. The more electronegative a species is, the greater its ability to attract electrons, especially those of a bonded pair.
Some examples of good leaving groups common to both sn1 and sn2 reactions are: Cl, Br, I, H2O
What Is A Nucleophilic Substitution Reaction
A nucleophilic substitution reaction involves a nucleophilic molecule replacing another atom or group of atoms, called the leaving group, on a molecule. The nucleophilic molecule is rich in electrons, which attack the substrate molecule. The leaving group on the substrate molecule departs with a newly-gained electron pair. The nucleophilic molecule bonds with the substrate molecule. Nucleophilic substitution reactions are common in organic chemistry, and take the general form of: Nuc + LG-R Nuc-R + LG.
This reaction can happen in either one or two steps, depending on the type of reaction. In this tutorial, we will be discussing the two-step version of this reaction: the sn1 reaction.
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What Is The Rate Of Reaction
For SN1 reactions: Rate of reaction = K For SN2 reactions: Rate of reaction = K
The rate of SN1 reactions is usually dependent on the stability of the carbocation, cation, and anion. The rate of SN2 reactions, on the other hand, is mainly dependent on the strength and concentration of the nucleophile undergoing the reaction. A few other differences in the rate of reaction are mentioned below:
- In SN1 reactions, the rate of reaction is independent of the concentration and strength of the nucleophile and dependent on the stability of the carbocation, cation, and anion. 3° > 2° > 1° substrates.
- In SN2 reactions, the rate of reaction is mainly dependent on the strength and concentration of the nucleophile. Another point to note is that, in this case, the rate is inversely proportional to the bulkiness of C atom-attached groups. Also, 1° > 2° > 3° substrates.
What types of solvents prefer SN1 and SN2 reactions?
- SN1 reactions: Polar protic solvents such as alcohols
- SN2 reactions: Polar aprotic solvents such as DMA and DMSO
In which reaction does the Walden Inversion occur?
- Inversion of the chiral center of a molecule during a chemical reaction is termed as Walden Inversion. This is one of the characteristics of SN2 reactions. SN1 reactions do not undergo Walden Inversion.In SN1 reactions, you will see that mirror image product are formed. You will see R-form as well as S-form products after the completion of both the steps.
How do we identify SN1 and SN2 reactions?
Steric Number Calculation Examples
- Methane – Methane consists of carbon bonded to 4 hydrogen atoms and 0 lone pairs. Steric number = 4.
- Water – Water has two hydrogen atoms bonded to oxygen and also 2 lone pairs, so its steric number is 4.
- Ammonia – Ammonia also has a steric number of 4 because it has 3 hydrogen atoms bonded to nitrogen and 1 lone electron pair.
- Ethylene – Ethylene has 3 bonded atoms and no lone pairs. Note the carbon double bond. Steric number = 3.
- Acetylene – The carbons are bonded by a triple bond. There are 2 bonded atoms and no lone pairs. Steric number = 2.
- Carbon Dioxide – Carbon dioxide is an example of a compound that contains 2 sets of double bonds. There are 2 oxygen atoms bonded to carbon, with no lone pairs, so the steric number is 2.
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Tin In The Periodic Table
The element tin, symbol Sn, is atomic number 50 in the periodic table. It lies between indium and the semi-metal antimony. It lies below the semi-metal germanium, and above the metal lead.
Tin is a post-transition metal that has similarities to both germanium and lead. It is in group 14 of the periodic table. Tin has an electron configuration of 5s²4d¹5p² .
What Are Sn1 And Sn2 Reactions
- 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. 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. since steric hindrance increases as we go from primary to secondary to tertiary, the rate of reaction proceeds from primary > secondary > > tertiary .
- The SN1 reaction proceeds stepwise. The leaving group first leaves, whereupon a carbocation forms that is attacked by the nucleophile.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 as well as with resonance. the rate of reaction for the SN1 goes from primary < < secondary < tertiary
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Example Of An Sn1 Reaction
The diagram above shows an sn1 reaction mechanism between methyl tert-butyl ether and hydrogen bromide. Firstly, the oxygen atom in the electrophile bonds with a hydrogen that has dissociated from the HBr. This is because CH3O is a poor leaving group, but CH3OH is a good leaving group. Thus, as the oxygen bonds with this hydrogen, the leaving group improves and can carry out the first step of the sn1 reaction, detaching from the molecule. The electrophile is now left with a carbocation, and the remaining Br, a strong nucleophile, can attack and create a bond to that carbon.
Characteristics Of Sn1 And Sn2 Reactions:
SN1 Reactions: SN1 reactions have several steps it starts with the removal of the leaving group, resulting a carbocation and then the attack by the nucleophile.
SN2 Reactions: SN2 reactions are single step reactions where both nucleophile and substrate are involved in the rate determining step. Therefore, the concentration of the substrate and that of the nucleophile will affect to the rate determining step.
Barriers of the reaction:
SN1 Reactions: The first step of SN1 reactions is removing the leaving group to give a carbocation. The rate of the reaction is proportional to the stability of the carbocation. Therefore, the formation of the carbocation is the greatest barrier in SN1 reactions. The stability of the carbocation increases with the number of substituents and the resonance. Tertiary carbocations are the most stable and primary carbocations are the least stable .
SN2 Reactions: Steric hindrance is the barrier in SN2 reactions since it proceeds through a backside attack. This happens only if the empty orbitals are accessible. When more groups are attached to the leaving group, it slows the reaction. So the fastest reaction occurs in the formation of primary carbocations whereas slowest is in tertiary carbocations .
SN1 Reactions: SN1reactions require weak nucleophiles they are neutral solvents such as CH3OH, H2O, and CH3CH2OH.
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Tin Specific Heat Latent Heat Of Fusion Latent Heat Of Vaporization
Specific heat of Tin is 0.227 J/g K.
Latent Heat of Fusion of Tin is 7.029 kJ/mol.
Latent Heat of Vaporization of Tin is 295.8 kJ/mol.
Specific heat, or specific heat capacity, is a property related to internal energy that is very important in thermodynamics. The intensive properties cv and cp are defined for pure, simple compressible substances as partial derivatives of the internal energy u and enthalpy h, respectively:
where the subscripts v and p denote the variables held fixed during differentiation. The properties cv and cp are referred to as specific heats because, under certain special conditions, they relate the temperature change of a system to the amount of energy added by heat transfer. Their SI units are J/kg K or J/mol K.
Different substances are affected to different magnitudes by the addition of heat. When a given amount of heat is added to different substances, their temperatures increase by different amounts.
Heat capacity is an extensive property of matter, meaning it is proportional to the size of the system. Heat capacity C has the unit of energy per degree or energy per kelvin. When expressing the same phenomenon as an intensive property, the heat capacity is divided by the amount of substance, mass, or volume. Thus the quantity is independent of the size or extent of the sample.
Latent Heat of Vaporization
Latent Heat of Fusion
The temperature at which the phase transition occurs is the melting point.
Mechanism Of Nucleophilic Substitution
The term SN2 means that two molecules are involved in the actual transition state:
The departure of the leaving group occurs simultaneously with the backside attack by the nucleophile. The SN2 reaction thus leads to a predictable configuration of the stereocenter – it proceeds with inversion .
In the SN1 reaction, a planar carbenium ion is formed first, which then reacts further with the nucleophile. Since the nucleophile is free to attack from either side, this reaction is associated with racemization.
In both reactions, the nucleophile competes with the leaving group. Because of this, one must realize what properties a leaving group should have, and what constitutes a good nucleophile. For this reason, it is worthwhile to know which factors will determine whether a reaction follows an SN1 or SN2 pathway.
Very good leaving groups, such as triflate, tosylate and mesylate, stabilize an incipient negative charge. The delocalization of this charge is reflected in the fact that these ions are not considered to be nucleophilic.
Hydroxide and alkoxide ions are not good leaving groups however, they can be activated by means of Lewis or Brønsted acids.
Epoxides are an exception, since they relieve their ring strain when they undergo nucleophilic substitution, with activation by acid being optional:
Under substitution conditions, amines proceed all the way to form quaternary salts, which makes it difficult to control the extent of the reaction.
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How To Find The Steric Number
To determine the steric number, you use the Lewis structure. The steric number gives the electron-pair arrangement for the geometry that maximizes the distance between valence electron pairs. When the distance between valence electrons is maximized, the energy of the molecule is at its lowest state and the molecule is in its most stable configuration.
The steric number is calculated using the following formula:
- Steric Number = +
Here’s a handy table that gives the bond angle that maximizes separation between electrons and gives the associated hybrid orbital. It’s a good idea to learn the bond angle and orbitals since these appear on many standardized exams.
Key Difference Sn1 Vs Sn2 Reactions
The SN1 and SN2 reactions are nucleophilic substitution reactions and most commonly found in Organic Chemistry. The two symbols SN1 and SN2 refer to two reaction mechanisms. The symbol SN stands for nucleophilic substitution. Even though both SN1 and SN2 are in the same category, they have many differences including the reaction mechanism, nucleophiles and solvents participated in the reaction, and the factors affecting the rate determining step. The key difference between SN1 and SN2 reactions is that SN1 reactions have several steps whereas SN2 reactions have only one step.
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What Is The Abbreviated Electron Configuration Of Sn
Tin – Sn, on the periodic table is found in the fourteenth column of the periodic table Group IVB this is the second column of the p block. Tin is in the fifth energy level .
This means that Tin must end with an electron configuration of #5p^2# The total electron configuration would be #1s^2 2s^2 2P^6 3s^2 3p^6 4s^2 3d^10 4p^6 5s^2 4d^10 5p^2#
The noble gas Krypton has an electron configuration of #1s^2 2s^2 2P^6 3s^2 3p^6 4s^2 3d^10 4p^6#
So we can replace this portion of tin’s electron configuration with the noble gas notation
This makes the electron configuration for Tin – Sn # 5s^2 4d^10 5p^2#
I hope this was helpful.SMARTERTEACHER
How Do Sn2 Reactions Take Place
Bimolecular reactions, such as SN2, take place through a transition state in which the two reactants are joined together.
In the following example, the electrophile and the nucleophile react through a pentacoordinate transition state which involves both reactants:
This has different practical implications.
For example, since two of the reactants are involved in the rate-limiting step of the overall process, the rate of the reaction will depend on the concentration of both reactants.
Furthermore, substitution reactions that go through a SN2 mechanism, go through an inversion of configuration in the carbon atom in which the exchange takes place. This is because, as you can see in the scheme above, SN2 reactions go through a backside attack substitution.
But some substitution reactions cannot go through this kind of SN2 mechanism. This is mainly because of steric hindrance.
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The Other Group 14 Elements
- Page ID
- To understand the trends in properties and reactivity of the group 14 elements.
Tin and lead oxides and sulfides are easily reduced to the metal by heating with charcoal, a discovery that must have occurred by accident when prehistoric humans used rocks containing their ores for a cooking fire. However, because tin and copper ores are often found together in nature, their alloybronzewas probably discovered before either element, a discovery that led to the Bronze Age. The heaviest element in group 14, lead, is such a soft and malleable metal that the ancient Romans used thin lead foils as writing tablets, as well as lead cookware and lead pipes for plumbing.
Although the first glasses were prepared from silica around 1500 BC, elemental silicon was not prepared until 1824 because of its high affinity for oxygen. Jöns Jakob Berzelius was finally able to obtain amorphous silicon by reducing Na2SiF6 with molten potassium. The crystalline element, which has a shiny blue-gray luster, was not isolated until 30 yr later. The last member of the group 14 elements to be discovered was germanium, which was found in 1886 in a newly discovered silver-colored ore by the German chemist Clemens Winkler, who named the element in honor of his native country.
The Solvent In Substitution And Elimination Reactions
There are so many factors to consider when choosing between SN1, SN2, E1 and E2 that the solvent is often overlooked. However, you may get asked about the effect of solvent on the nucleophilicity and basicity and that is what todays post is about.
The solvent is what we use to carry out the reaction so, the main requirement for it is to dissolve the reactants. And because the reactants in nucleophilic substitution and elimination reactions are polar molecules, the solvent needs to be polar as well. The dipole-dipole interaction of these polar molecules with the reactants is what helps to solvate them.
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What Are Substitution Reactions
Substitution reactions are chemical reactions in which a functional group on a molecule is replaced by another one. One of the most basic examples of substitution reaction is the Finkelstein reaction.
The Finkelstein reaction is also known as the halogen exchange reaction, because that is basically it:
A carbon-halogen bond is polarized, the carbon atom attached to the halogen is electrophilic, and can be attacked by an external nucleophile.
We have covered this concept of electron distribution in a previous post. Halogens can act as nucleophiles. This results in exchanging one for another, through an SN2 mechanism.
This halogen exchange reaction is actually an equilibrium: it is reversible and it can take place in both directions. However, you can drive this kind of substitution taking advantage of solubility:
In the example above, sodium chloride, which is significantly insoluble in acetone, precipitates out of the reaction mixture. This drives the equilibrium to the right. This a nice visualization of Le Chateliers principle.
This substitution reaction goes through what we call a SN2 mechanism.
Sn1 And Sn2 Reaction Kinetics Mechanism Stereochemistry And Reactivity
Nucleophilic substitution reaction
- Nucleophiles are electron rich atoms or group of atoms which attack on electron deficient centre during chemical reaction. Nucleophiles are the negatively charged species or neutral species having electron rich centre.
- Any substitution reaction that involves replacing of an atom or a functional group by a nucleophile is called nucleophilic substitution reaction.
- During nucleophilic substitution reaction in haloalkanes , the nucleophile attacks the haloalkane and replaces the halogen atom.
- There are two main types of nucleophilic substitution reactions SN2 and SN1 reaction.
SN2 reaction is also known as bimolecular nucleophilic substitution reaction. Such reactions are generally shown by primary haloalkanes. For example, hydrolysis of ethyl bromide with aq.KOH.
Kinetics of SN2 reaction:
Rate of SN2 reaction depends upon the concentration of both substrate and nucleophile. Thus the reaction follows second order kinetics since both the reactants are present in rate determining step.
Mechanism and Stereochemistry of SN2 reaction:
Energy profile diagram of SN2 reaction:
Reactivity of alkyl halides towards SN2 reaction:
The order of reactivities of alkyl halides towards the SN2 reaction is:
SN1 reaction is also known as unimolecular nucleophilic substitution reaction. Such reaction are generally shown by secondary and tertiary haloalkanes. For example, hydrolysis of tertiary butyl bromide with aq. KOH.
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