The hydrobromic acid It is an inorganic compound that results from the aqueous dissolution of a gas called hydrogen bromide. Its chemical formula is HBr, and it can be considered in different equivalent ways: as a molecular hydride, or a hydrogen halide in water; that is, a hydracid.
In chemical equations it should be written as HBr (ac), thus indicating that it is the hydrobromic acid and not the gas. This acid is one of the strongest known, even more so than hydrochloric acid, HCl. The explanation for this lies in the nature of its covalent bond.
Why is HBr such a strong acid, and even more so dissolved in water? Because the H-Br covalent bond is very weak, due to the poor overlap of the 1s orbitals of H and 4p of Br.
This is not surprising if you look closely at the image above, where clearly the bromine atom (brown) is much larger than the hydrogen atom (white).
Consequently, any disturbance causes the H-Br bond to break, releasing the H ion+. So, hydrobromic acid is a Brönsted acid, since it transfers protons or hydrogen ions. Its strength is such that it is used in the synthesis of several organobrominated compounds (such as 1-Bromo ethane, CH3CHtwoBr).
Hydrobromic acid is, after hydroiodic, HI, one of the strongest and most useful hydracids for the digestion of certain solid samples.
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The image shows the structure of H-Br, whose properties and characteristics, even those of gas, are closely related to its aqueous solutions. That is why there comes a point where there is confusion regarding which of the two compounds is referred to: HBr or HBr (ac).
The structure of HBr (ac) is different from that of HBr, since now the water molecules are solving this diatomic molecule. When it is close enough, the H is transferred+ to a molecule of HtwoOr as indicated in the following chemical equation:
HBr + HtwoO => Br-- + H3OR+
Thus, the structure of hydrobromic acid consists of Br ions-- and H3OR+ interacting electrostatically. Now, it is a little different than the covalent bond of H-Br.
Its high acidity is due to the voluminous Br anion- can barely interact with the H3OR+, unable to prevent you from transferring the H+ to another surrounding chemical species.
For example, Cl- and the F- although they do not form covalent bonds with H3OR+, they can interact through other intermolecular forces, such as hydrogen bonds (which only F- is able to accept them). Hydrogen bonds F--H-OHtwo+ "Hinder" the donation of the H+.
It is for this reason that hydrofluoric acid, HF, is a weaker acid. in water than hydrobromic acid; since, the ionic interactions Br- H3OR+ do not bother the transfer of the H+.
However, although water is present in HBr (aq), its behavior is ultimately similar to that of a H-Br molecule; that is, an H+ is transferred from HBr or Br-H3OR+.
HBr.
80.972 g / mol. Note that, as mentioned in the previous section, only the HBr is considered and not the water molecule. If the molecular weight were taken from the formula Br-H3OR+ would have a value of approximately 99 g / mol.
Colorless or pale yellow liquid, which will depend on the concentration of the dissolved HBr. The more yellow it is, the more concentrated and dangerous it will be.
Pungent, irritating.
6.67 mg / m3.
1.49 g / cm3 (48% w / w aqueous solution). This value, as well as those corresponding to the melting and boiling points, depend on the amount of HBr dissolved in the water..
-11 ° C (12 ° F, 393 ° K) (49% w / w aqueous solution).
122 ºC (252 ºF. 393 ºK) at 700 mmHg (aqueous solution 47-49% w / w).
-221 g / 100 ml (at 0 ° C).
-204 g / 100 ml (15 ° C).
-130 g / 100 ml (100 ºC).
These values refer to gaseous HBr, not to hydrobromic acid. As can be seen, increasing the temperature decreases the solubility of HBr; behavior that is natural in gases. Consequently, if concentrated HBr (aq) solutions are required, it is better to work with them at low temperatures..
If working at high temperatures, the HBr will escape in the form of gaseous diatomic molecules, so the reactor must be sealed to prevent its leakage.
2.71 (relative to air = 1).
-9.0. This negative constant is indicative of its great acidity strength..
29.1 kJ / mol.
198.7 kJ / mol (298 ºK).
-36.3 kJ / mol.
Not flammable.
Its name 'hydrobromic acid' combines two facts: the presence of water, and that bromine has a valence of -1 in the compound. In English it is somewhat more obvious: hydrobromic acid, where the prefix 'hydro' (or hydro) refers to water; although, actually, it can also refer to hydrogen.
Bromine has a valence of -1 because it is bonded to a hydrogen atom less electronegative than it; but if it were bonded or interacting with oxygen atoms, it can have numerous valences, such as: +2, +3, +5 and +7. With the H it can only adopt a single valence, and that is why the suffix -ico is added to its name.
While HBr (g), hydrogen bromide, is anhydrous; that is, it has no water. Therefore, it is named under other nomenclature standards, corresponding to that of hydrogen halides..
There are several synthetic methods for preparing hydrobromic acid. Some of them are:
Without describing the technical details, this acid can be obtained from the direct mixing of hydrogen and bromine in a reactor filled with water..
Htwo + Brtwo => HBr
In this way, as the HBr is formed, it dissolves in the water; this can drag it in the distillations, so solutions with different concentrations can be extracted. Hydrogen is a gas, and bromine is a dark reddish liquid..
In a more elaborate process, sand, hydrated red phosphorus and bromine are mixed. Water traps are placed in ice baths to prevent HBr from escaping and forming hydrobromic acid instead. The reactions are:
2P + 3Brtwo => 2PBr3
PBr3 + 3HtwoO => 3HBr + H3PO3
Another way to prepare it is to react bromine with sulfur dioxide in water:
Brtwo + SWtwo + 2HtwoO => 2HBr + HtwoSW4
This is a redox reaction. The BRtwo it reduces, gains electrons, by bonding with hydrogens; while the SOtwo oxidizes, loses electrons, when it forms more covalent bonds with other oxygens, as in sulfuric acid.
Bromide salts can be prepared by reacting HBr (aq) with a metal hydroxide. For example, the production of calcium bromide is considered:
Ca (OH)two + 2HBr => CaBrtwo + HtwoOR
Another example is for sodium bromide:
NaOH + HBr => NaBr + HtwoOR
Thus, many of the inorganic bromides can be prepared.
And what about organic bromides? These are organobrominated compounds: RBr or ArBr.
The raw material to obtain them can be alcohols. When they are protonated by the acidity of HBr, they form water, which is a good leaving group, and in its place the bulky Br atom is incorporated, which will become covalently bonded with carbon:
ROH + HBr => RBr + HtwoOR
This dehydration is carried out at temperatures above 100 ° C, with the aim of facilitating the breaking of the R-OH bond.two+.
The HBr molecule can be added from its aqueous solution to the double or triple bond of an alkene or alkyne:
RtwoC = CRtwo + HBr => RHC-CRBr
RC≡CR + HBr => RHC = CRBr
Several products can be obtained, but under simple conditions, the product is primarily formed where the bromine is bound to a secondary, tertiary or quaternary carbon (Markovnikov's rule).
These halides are involved in the synthesis of other organic compounds, and their range of uses is very extensive. Likewise, some of them may even be used in the synthesis or design of new drugs..
From the ethers, two alkyl halides can be obtained simultaneously, each carrying one of the two side chains R or R 'of the initial ether R-O-R'. Something similar to the dehydration of alcohols happens, but their reaction mechanism is different.
The reaction can be outlined with the following chemical equation:
ROR '+ 2HBr => RBr + R'Br
And water is also released.
Its acidity is such that it can be used as an effective acid catalyst. Instead of adding the anion Br- to the molecular structure, makes way for another molecule to do it.
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