Halogen acids-strength-addition to alkenes.

Halogen acids strength

When halogen elements are reacts with hydrogen gas, the compound produce, are known as halogen acids or halogen hydracids with general formula HX.

There are four halogen acids or hydracids are known which are, hydrofluoric acid ( HF ), hydrochloric acid (HCl), hydrobromic acid ( HBr ) and hydro iodic acid (HI).

The acidic strength of halogen acid depends on the degree of ionization in aqueous solution as well as bond dissociation energy of halogen acid.

Generally, the degree of dissociation decreases with increasing bond dissociation energy.That is with increasing bond dissociation enthalpy, acidic strength decreases.

Since, the order of bond dissociation energy of halogen acids are, HI < HBr < HCl < HF.Therefore, the increasing acidic order of the above halogen acids are,

HF < HCl < HBr < HI .

Addition of halogen acids to alkenes

Owing to the presence of a double bond, the alkenes undergo a large number of addition reactions. But, under special conditions, they also undergo substitution reaction.

The high reactivity of the alkenes are due to presence of pi-bond, that is, due to presence of loosely bounded olefinic bond electrons.

When alkenes participate in addition reaction , the triangle arrangement of alkene changes to the tetrahedral arrangement in the saturated compound produced .

Addition reaction of alkenes occurs involving an electrophile in the first step . This is the slow as well as rate determining step .

The carbonium ion produced in first step,then reacts with a nucleophile very rapidly.

The more stable the carbonium ion, the faster is the addition reaction.

For example, addition of HCl or HBr to ethylene gives chloro ethane or bromo ethane respectively.

The order of reactivity of the addition of the halogen acids to alkenes are ,

HI > HBr > HCl > HF , this is also the acid strength of halogen acids .

The conditions for the addition are similar to those for the halogens , but the addition of HF is effected only under pressure .

The energy profile diagram of a two step reaction of this type is shown below,

In case of unsymmetrical alkenes like propene, the addition of halogen acids is not similar to that of symmetrical alkene ( ethylene ).

For unsymmetrical alkene ( propene ), the addition of halogen acid to take place in two different ways.

For example, when HI is added to propene, then two possible product should be obtained , propyle iodide or isopropyl iodide.

In such type of cases, the addition of halogen acids to alkenes take places according to an empirical rules, known as Markownikoff’s rule .

According to this rule , when an addendum is added to an unsymmetrical alkenes, the negative part of the addendum adds on to the carbon atom that is joined to the least number hydrogen atoms.

In the case of halogen acids (HI), the negative part of halogen acids that is halide ion (I–) adds on to the carbon that is contain least number of hydrogen atom. So isopropyl iodide or 2-iodo propane is the main product.

The reaction occurs by a polar mechanism. Since the addition of halogen acid is an electrophilic reaction, the proton adding first, followed by the halide ion.

Hence the addition is predominantly trans, and this may be explained in terms of the formation of a bridged carbonium ion and its stability.

Rate of halogen acids addition reaction to alkenes

Addition of halogen acids to alkenes, follow the mechanism, electrophilic addition reaction . The reaction consists of two step method.

In first step, the addition of H+ ion to alkenes occurs slowly and form carbonium ion . In the second step , the carbonium ion is attacted by chloride ion very fast .

Now ,which product will be predominant, depends on the relative stability of carbonium ion produced. For example, addition of HBr to propene ,gives 2-bromo propane as main product.

We know that tertiary carbonium ion is more stable than secondary, and secondary carbonium ion is more stable than primary, due to +I effect of alkyl group or hyperconjugation effect.

Due to this effect,the positive charge on carbon atom spread out all over the molecule.

So the extent of positive charge on carbon atom decrease and hence stability of the carbocation increases.

Halogen acids strength compare.

The acidic strength of halogen acid depends on the degree of ionization in aqueous solution as well as bond dissociation energy of halogen acid.

Generally, the degree of dissociation decreases with increasing bond dissociation energy .

That is with increasing bond dissociation enthalpy , acidic strength decreases.

Since, the order of bond dissociation energy of halogen acids are, HI < HBr < HCl < HF.

Therefore, the increasing acidic order of the above halogen acids are, HF < HCl < HBr < HI.

Consequently, among the halogen acids (, HF , HCl , HBr and HI ), HI is the strongest acid and HF is the weakest acid.

Which acid is stronger HF or HCl ?

HCl is more strongeracid than HF.The acidic strength of acid depends on the degree of ionization in aqueous solution and also bond dissociation energy.

Generally, the degree of dissociation decreases with increasing bond dissociation energy.

That is with increasing bond dissociation enthalpy, acidic strength decreases.

Since H–F bond is shorter as well as stronger than H–Cl bond.

So,HF acid gives lesser number of H+ ion in aqueous solution with compare to HCl acid.Consequently, HCl is more stronger acid than HF and this fact also supported by their pKa value.

With increasing pKa value of an acid,the acidic strength decreases.

Summary