Bond fission

A covalent bond is formed when electrons are shared between two atoms. A single bond (sigma bond) is thus formed after sharing of two electrons. Now a chemical reaction takes place when old bonds are broken and new ones are created. There are two ways of breaking a bond as shown below.

Homolytic fission

Figure 1. Homolysis

Homolytic fission is the type of fission in which electrons are equally shared after breaking of covalently bonded atom having same electro negativity. After this type of bond breaking free radical is formed and usually for this purpose light particles or photons is needed. Free radicals are the species of odd electron.

Heterolytic fission

Figure 2. Heterolysis

Heterolytic fission is the type of fission in which electrons are unequally shared after breaking of polar covalently bonded atom having different electro negativity. After this type of bond breaking carbocation (positively charged species) or carbanion (negatively charged species) is formed and usually for these purpose reagents (electrophiles or nucleophiles) is needed. 

Familiar arrow marks, bonds and symbols

Radius Ratio Rule

In the ionic model, the bonding is described as the electrostatic interaction between charged spheres, whose sizes are given by the ionic radius.

In determining an ionic radius, it is necessary to split up the internuclear separation into a contribution from the anion and a contribution from the cation. This is most often done by assuming the value of the radius of one ion, and then calculating the radii of other ions from this basis. This standard ion is generally the oxide ion, as it occurs in combination with many other elements.

Also, it is a relatively unpolarizable ion, and so its size changes little with changing counterion.

The use of ionic radii to predict aspects of crystal structure like lattice parameters, the lengths of the axes of the unit cells, is often useful, but only when the values of the ionic radii are taken form the same source, i.e. they use the same reference ion and so have the correct relative sizes.

It should also be noted that the ionic radius of a given ion changes with coordination number: As the coordination number increases, the ions must get further away from the central ion in order to accommodate more of them, and hence the interionic separation increases, and the short ranged repulsion decreases, and the electron cloud on the central ion can expand, and hence the central ion increases in size.

Hence, ionic radius increases with coordination number.

The sizes of the ions can be used to predict the structure that will be adopted when they are combined. In a cubic close packed array of anions, for example, the octahedral and tetrahedral holes have different sizes, and so cation might be expected to occupy the holes which are just big enough to hold them. This is examined in terms of the radius ratio.

The Radius Ratio

The radius ratio of a given pair of ions is defined at the ionic radius of the smaller ion divided by the ionic radius of the larger ion, ie. ρ = r_s/r_l.

Often the smaller ion is the cation (as the reduced repulsion brought about by the missing electron tends to contract the electron cloud), and the larger ion is the anion (as the extra repulsion from the negative charge tends to make the ion expand).

Consider a simple cubic arrangement of anions, with a cation in the center of the cubic cell (as in CsCl, which is (8,8)-coordinate). As the cation decreases in size, it will reach a point when the anions begin to touch, which unfavourable electrostatically, due to the repulsion between like-charged species. At this point, the structure changes so the anions are again separated by oppositely charged cations, an arrangement which is electrostatically favourable, and the (6,6)-coordinate NaCl structure is adopted. as this trend is continued, there will be a switch to the (4,4)-coordinate ZnS structure. Similar arguments hold for structures of stoichiometry AB₂, and others.

Therefore, as the radius ratio decreases, there is a trend towards structures of lower coordination numbers. The Radius ratio rules are the prediction of structure adopted by a given set of ions based on the radius ration of those ions.

The radius ratio rules are not universally successful. As the degree of covalency in the bonding increases, the deviation from the ionic model increases and the less reliable the choice of structure based on the radius ratio becomes. The rules are least reliable for simple compounds like alkali metal halides and alkaline earth metal oxides, and are most reliable for complex fluorides and the salt of oxoanions: in general, as the degree of ionicity increases, so does the accuracy of the rules.

Limitation of the Radius Ratio Rule

Although radius is a useful concept of determining coordination number and in understanding the structure of ionic compounds yet there are a few limitations to this rule.

(i)    Each ion is considered as a hard sphere for determining the optimum arrangement of ions in the crystal lattice. This is far form reality and serious errors can be made if anions get polarized and the bonding the intermediate (partially covalent) in character.

(ii)    Some compounds may crystallize in more than one modification with different coordination numbers. In such case, anion-anion repulsions and hence intern clear distance would be different.

(iii)    Effective radius of a cation is greatly influenced by the anions with the consequence that the radius ratio changes. For example, A_gF and N_aF crystallize out in N_aCI type of structure with coordination No. 6 and if we assume that the size of F^- remains constant than Ag⁺ is bigger than Na^+. On the other hand, for a given size for a chloride ion in A_gCI and N_aCI, the sizes of cations are reversed, that is, Na⁺ is bigger than Ag^+. It is also true for their bromides. This is understandable because Ag⁺ is softer than N⁺ and introduces relatively more covalent character with Polarizable anions like CI^- and  Br^-  

Decarboxylation of Carboxylic Acids

Specific carboxylic acids eliminate carbon dioxide relatively easily. That is, they decarboxylate. Examples of such carboxylic acids are β-keto acids and 1,3-dicarboxylic acids. The decarboxylation of these compounds is promoted by tautomerism and mesomerism, respectively. Oftentimes, decarboxylation occurs even at room temperature. As the α hydrogen atom is considerably acidic due to the effect of two adjacent carbonyl groups and the second carbonyl group may be easily removed through decarboxylation, these reactions are important in several ketone and carboxylic acid syntheses.

Decarboxylations

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Decarboxylation using soda lime

2O

A carboxylic acid has the formula RCOOH where R can be hydrogen or a hydrocarbon group such as an alkyl group. The hydrocarbon group could equally well be based on a benzene ring. The sodium salt of a carboxylic acid will have the formula RCOONa. In decarboxylation, the -COOH or -COONa group is removed and replaced with a hydrogen atom.

Soda lime is manufactured by adding sodium hydroxide solution to solid calcium oxide (quicklime). It is essentially a mixture of sodium hydroxide, calcium oxide and calcium hydroxide. It comes as white granules. In equations, it is almost always written as if it were simply sodium hydroxide.

It is an easier material to handle than solid sodium hydroxide. Solid sodium hydroxide absorbs water from the atmosphere and you tend to end up with puddles of extremely concentrated (and corrosive) sodium hydroxide solution if you leave it exposed to the air. Soda lime has much less tendency to absorb water.

The solid sodium salt of a carboxylic acid is mixed with solid soda lime, and the mixture is heated. For example, if you heat sodium ethanoate with soda lime, you get methane gas formed:

This reaction can be done with certain carboxylic acids themselves. For example, benzene can be made by heating soda lime with solid benzoic acid (benzenecarboxylic acid), C6H5COOH.

You can think of this as first a reaction between the acid and the soda lime to make sodium benzoate, and then a decarboxylation as in the first example.

Wurtz Reaction

The Wurtz Coupling is one of the oldest organic reactions, and produces the simple dimer derived from two equivalents of alkyl halide. The intramolecular version of the reaction has also found application in the preparation of strained ring compounds:

Using two different alkyl halides will lead to an approximately statistical mixture of products. A more selective unsymmetric modification is possible if starting materials have different rates of reactivity  (seeWurtz-Fittig Reaction).

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Mechanism of the Wurtz Reaction

Side products:

Kolbe Electrolysis

The electrochemical oxidative decarboxylation of carboxylic acid salts that leads to radicals, which dimerize. It is best applied to the synthesis of symmetrical dimers, but in some cases can be used with a mixture of two carboxylic acids to furnish unsymmetrical dimers.

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Mechanism of the Kolbe Electrolysis

Side products:

The formation of side products depends on the ease of the follow-up oxidation which leads to carbenium ions, and their subsequent rearrangement:

Corey–House synthesis:

The Corey–House synthesis (also called the Corey–Posner, Whitesides–House reaction and other permutations) is an organic reaction that involves the reaction of a lithium dialkyl cuprate with an alkyl halide to form a new alkane, an organocopper compound and a lithium halide.^[1][2][3]

R₂CuLi + R'-X → R-R' + RCu + LiX

This reaction occurs in two steps. The alkyl halide is treated with lithium metal, and solvated in dry ether, which converts the alkyl halide into an alkyl lithium compound, R-Li. The starting R-X can be primary, secondary or tertiary alkyl halide:

R-X + 2Li → R-Li + Li-X

The second step requires the alkyl lithium compound to be treated with cuprous iodide (CuI). This creates a lithium dialkyl cuprate compound. These compounds were first synthesized by Henry Gilman of Iowa State University, and are usually called Gilman reagentsin honor of his contributions:

2RLi + CuI → R₂CuLi + LiI

The lithium dialkyl cuprate is then treated with the second alkyl halide, which couples to the compound:

R₂CuLi + R'-X → R-R' + RCu + LiX

If second alkyl halide is not the same as the first, then cross-products are formed.

It is important to note that for this reaction to work successfully, the second alkyl halide must be a methyl halide, benzyl halide, primary alkyl halide or a secondary cyclo alkyl halide. The relative simplicity of this reaction makes it a useful technique for synthesizing organic compounds.