why special methods are used to prepare hbr & hi
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Answer:
Hydrogen bromide is the diatomic molecular compound with the formula HBr, a hydrogen halide consisting of hydrogen and bromine. In pure form it is a colorless gas.
Hydrogen bromide is very soluble in water, forming hydrobromic acid, which is saturated at 68.85% HBr by weight at room temperature. Aqueous solutions that are 47.6% HBr by mass form a constant-boiling azeotrope mixture that boils at 124.3 °C. Boiling less concentrated solutions releases H2O until the constant-boiling mixture composition is reached.
Both the anhydrous and aqueous solutions of HBr are common reagents in the preparation of bromide compounds.
Uses of HBr
Hydrogen bromide and hydrobromic acid are important reagents in the production of inorganic and organic bromine compounds. The free-radical addition of HBr to alkenes gives alkyl bromides:
RCH=CH2 + HBr → R−CHBr−CH3
These alkylating agents are precursors to fatty amine derivatives. Similar free radical addition to allyl chloride and styrene gives 1-bromo-3-chloropropane and phenylethylbromide, respectively.
Hydrogen bromide reacts with dichloromethane to give bromochloromethane and dibromomethane, sequentially:
HBr + CH2Cl2 → HCl + CH2BrCl
HBr + CH2BrCl → HCl + CH2Br2
Allyl bromide is prepared by treating allyl alcohol with HBr:
CH2=CHCH2OH + HBr → CH2=CHCH2Br + H2O
Other reactions
Although not widely used industrially, HBr adds to alkenes to give bromoalkanes, an important family of organobromine compounds. Similarly, HBr adds to haloalkene to form a geminal dihaloalkane. (This type of addition follows Markovnikov's rule):
RC(Br)=CH2 + HBr → RC(Br2)−CH3
HBr also adds to alkynes to yield bromoalkenes. The stereochemistry of this type of addition is usually anti:
RC≡CH + HBr → RC(Br)=CH2
Hydrogen iodide (HI) is a diatomic molecule and hydrogen halide. Aqueous solutions of HI are known as hydroiodic acid or hydriodic acid, a strong acid. Hydrogen iodide and hydroiodic acid are, however, different in that the former is a gas under standard conditions, whereas the other is an aqueous solution of the gas. They are interconvertible. HI is used in organic and inorganic synthesis as one of the primary sources of iodine and as a reducing agent.
Properties of hydrogen iodide
HI is a colorless gas that reacts with oxygen to give water and iodine. With moist air, HI gives a mist (or fumes) of hydroiodic acid. It is exceptionally soluble in water, giving hydroiodic acid. One liter of water will dissolve 425 liters of HI, the most concentrated solution having only four water molecules per molecule of HI.[4]
Hydroiodic acid
Hydroiodic acid is not pure hydrogen iodide, but a mixture containing it. Commercial "concentrated" hydroiodic acid usually contains 48–57% HI by mass. The solution forms an azeotrope boiling at 127 °C with 57% HI, 43% water. The high acidity is caused by the dispersal of the ionic charge over the anion. The iodide ion is much larger than the other common halides, which results in the negative charge being dispersed over a large space. By contrast, a chloride ion is much smaller, meaning its negative charge is more concentrated, leading to a stronger interaction between the proton and the chloride ion. This weaker H+···I− interaction in HI facilitates dissociation of the proton from the anion and is the reason HI is the strongest acid of the hydrohalides.
HI(g) + H
2O(l) → H
3O+
(aq) + I−(aq) Ka ≈ 1010
HBr(g) + H
2O(l) → H
3O+
(aq) + Br−(aq) Ka ≈ 109
HCl(g) + H
2O(l) → H
3O+
(aq) + Cl−(aq) Ka ≈ 106